Course 2: Water Quality and Pollution

UNESCO-IHE Institute for Water Education Course 2 OLC Water Quality Assessment

Course 2: Water Quality and Pollution Table of Contents

Unit 1 Introduction to Water Quality and Pollution

Unit 2 Organic matter

Unit 3 Nutrients

Unit 4 Micropollutants

Unit 5 Aquatic ecosystems

Unit 6

Water quality modelling (optional)


Unit 1 – Introduction to Water Quality and Pollution

1.1. Water uses and functions

In this unit we will give a short overview of:

1. Water uses and functions, and especially the water quality demands related to

various uses;

2. Translation of water quality demands into water quality standards;

3. Natural water quality, or in other the background concentrations of certain

substances that occur in rivers and lake even without human interference;

4. Sources of pollution, also in relation to environmental compartments;

5. Spatial and temporal variations of water quality and pollution.

We have earlier defined pollution as “the introduction by man, directly or indirectly, of

substances or energy which result in deleterious effects" such as: (1) harm to living

resources/aquatic ecosystems; (2) hazards to human health; (3) hindrance to aquatic

activities including fishing, boating, energy production; (4) impairment of water quality

with respect to its use in agricultural, industrial and often economic activities; and (5)

reduction of amenities.

From this definition of pollution, we can easily derive a definition for water quality:

“Water quality expresses the suitability of water for various uses or processes: drinking

water; irrigation water; nature conservation, etc”. A more complete list of uses was

given in Unit 4 of Course 1.

Thus, based on its intended use, we can define different water quality requirements:

− No or hardly any specific requirements: typical examples are navigation water or

power generation

− Defined “minimum standards”: typical examples are irrigation water, fisheries,

recreation water

− “Undisturbed quality”: needed for good ecosystem functioning.

1.2. Water quality standards

Water quality requirements are usually specified as water quality standards (WQS). In

the past WQS mostly consisted of physicochemical requirements such as pH levels,

nitrogen concentrations etc. In the past 10-15 year however, there has been an

evolution to interpret water quality more holistically by not only looking at the chemistry,

but also at the biology and ecology: which algae are present; which fish are present

etc?

We can illustrate this idea with a very simple example: many fish species require

specific habitat conditions for spawning (plants to attach the eggs to, or muddy

sediment to bury the eggs into). Many rivers however have been channelized in the

past, and instead of having natural banks they are now contained within a pair of

concrete walls. Even though the physico-chemical water quality can be excellent in


these rivers, fishes will be absent because of the lack of plants needed for

reproduction.

A similar example is the story of the salmon. This is a so-called anadromous fish,

meaning that it lives part of its life in the sea, but that it migrates upstream into

freshwater for spawning. Because of severe water pollution in the 1960s and 1970s,

salmon disappeared from many European rivers. However, now that many wastewater

treatment plants have been put into operation and that the river water quality has

improved a lot, the population of salmon is not recovering as well as you would expect.

This can be attributed to the multitude of “fish migration barriers” such as dams,

sluices, weirs, etc., which form, in many cases, an impenetrable barrier for the fish

(www.eaa-europe.eu/fileadmin/templates/uploads/Positions/Salmon_and_Migration_Barriers-

final.pdf). The fact that salmon is absent in a certain river can thus point not only to

chemical pollution, but also to structural problems.

To come back to water quality: looking only at the chemical water quality is not

sufficient to tell you whether or not a river is “healthy”; one should also look at the biota.

This idea is the key concept of the recent European Water Framework Directive

(Directive 2000/60/EC), (EUWFD), which demands that all water bodies within the EU

must have reached, in 2015, a good ecological status. Specific monitoring instructions

are given not only with regards to the physical and chemical parameters to be

monitored, but also with respect to the biological parameters (fish, plants, plankton, ...).

Ecological monitoring and monitoring in the EUWFD will further be discussed in Course

3; for now we will focus on physico-chemical standards because many countries all

over the world still solely make use of this type of WQS. On pages 92-94 of the book of

Chapman (1996), you can find a general overview of WQS applied in different parts of

the world, and according to different water uses.

Note that in many cases, there are not only specifications about the desired or

maximum concentration, but also requirements on the number of samples to be taken

each year (sampling frequency), the number of samples that should fulfil the standard

(all samples or 90% of the samples or ...); in specific cases also a difference can be

made between average values and absolute values. In Flanders (Belgium) for instance,

the average concentration of NH4-N in a river (average of one year’s worth of data)

should be less than 1 mg N/L, but every single sample has to meet the absolute

standard of 5 mg N/L.

The water quality standards may be quite different for above different water uses.

Irrigation water, for example, hardly has requirements for dissolved oxygen and

nutrients. On the other hand, there will be specific requirements for chloride, boron etc,

in view of reduced crop yields at elevated concentrations. For recreation water, the

standards are especially related with aesthetic quality (colour, algae) and bacterial

pollution (expressed with the indicator E-coli).

Water fit for ecosystems need virtually undisturbed water quality; much emphasis is

given to the micropollutants levels (heavy metals, pesticides, PCBs, etc.). Industries

should use “Best Available Technologies” (BAT) to minimise waste loads, or even

apply zero discharges. For the other water uses, often the “ALARA principle” is

applied: “As low as reasonably achievable”.


Assignment WQA-3: Water quality standards of your own country or region

Please look up the water quality standards of your country or region. Select maximally

20 parameters and report on their standards for different uses. If available, also specify

sampling frequencies etc. Submit your assignment as a table in Word or Excel via the

WQA-3 Forum on the site (as you have done before with the GEMS data).

1.3. Natural water quality

It is important to realise that even without any human interference, there is no "distilled

water" flowing through a river. There will always be non-zero “background”

concentrations of many substances, resulting from natural processes. For instance,

eruption of volcanoes can bring huge amounts of pollutants into the water. Another

example is mountain rivers which are continuously eroding the rocks over which they

flow, and as such carry some of the eroded materials with them; see PowerPoint 2.1. of

this Course.

Natural water quality varies from place to place, with the seasons, with climate, and

with the types of soils and rocks through which water moves. When water from rain or

snow moves over the land and through the ground, the water may dissolve minerals in

rocks and soil, percolate through organic material such as roots and leaves, and react

with algae, bacteria and other microscopic organisms. Water may also carry plant

debris and sand, silt, and clay to rivers and streams making the water appear “muddy”

or turbid. When water evaporates from lakes and streams, dissolved minerals are more

concentrated in the water that remains. Each of these natural processes changes the

water quality and potentially the water use.

The most common dissolved substances in water are minerals or salts that, as a group,

are referred to as dissolved solids. Dissolved solids include common constituents such

as calcium, sodium, bicarbonate and chloride; plant nutrients such as nitrogen and

phosphorus; and trace elements such as selenium, chromium, and arsenic. Also

dissolved gases such as oxygen and radon are common in natural waters.

A good further overview is given in Chapman (1996). Please read sections 6.3.1 and

6.3.2 on pages 256-263.

1.4. Sources of pollution

Taking into account the concept of natural water quality, we can also expand our

definition of pollution, as follows: “pollution is any deviation from the natural water

quality caused by human interference”.

Man can in many ways cause pollution of the aquatic environment; an overview is

given in Table 1 (taken from the book of Chapman) and details of the sources of each

pollutant will be given in the subsequent units. For now, it is important to realise that

pollution not only comes from the classical sewer pipes discharging into rivers and


lakes (“point sources”), but also from urban and agricultural runoff (“non-point or diffuse

sources” and “mixed sources”) and even from the atmosphere. As an illustration of the

latter, Fig. 1 from the Millennium Ecosystem Assessment is given below, which shows

the evolution of worldwide nitrogen deposition (past, present and projected future).

Atmospheric deposition currently accounts for roughly 12% of the reactive nitrogen

entering terrestrial and coastal marine ecosystems globally, although in some regions,

atmospheric deposition accounts for a higher percentage (about 33% in the USA).

TABLE 1

1.5. Spatial and temporal variations

The water quality of a river or lake is not is difficult to represent in a single value, not

only because of the range of pollution sources and polluting compounds, but also

because there are important variations both in space and in time.

Typical spatial patterns or differences in rivers occur between:

− source and mouth (length profile)

− surface layer and deepest layer (depth profile)

− river banks and centre of the stream (width profile)


UNEP/GRID-Arendal, 'Estimated total reactive nitrogen deposition from the atmosphere (wet and dry) in 1860, early 1990s, and projected for 2050', UNEP/GRID-Arendal Maps and Graphics Library, 2005, <http://maps.grida.no/go/graphic/estimated-total-reactive-nitrogen-deposition-from-the-atmosphere-wet-and-dry-in-1860-early-1990s-and>

Fig. 1. World-wide nitrogen deposition from the atmosphere

Similarly, for lakes one can observe spatial patterns as differences between:

− inlet and outlet side (if present)

− surface layer and deepest layer (depth profile)

− lake shores and centre of the lake

− isolated creeks (e.g. protected from wind) and centre of the lake

Temporal patterns for both rivers and lakes can occur as:

− day/night variations (e.g. higher dissolved oxygen levels due to

photosynthesis during the day-time)

− seasonal variations (temperature)

− annual variations (e.g. El Niño effect)

− changes over centuries (e.g. slow accumulation of sediments, global

change)

One of the important temporal factors is the discharge regime of rivers (often higher

during winter time and/or rainy season; lower during summer time and/or dry season).

The discharge Q (m3/second) of a river already determines, to a large, extent the

concentration c (kg/m3) of dissolved substances, due to dilution.

Assuming a constant load: Q x c = Load (kg/sec); the concentration will generally show

an increasing trend with decreasing discharges, and vice versa (see Fig. 2).


In contrast, the total suspended solids (TSS) in a river often show an increasing trend

with higher discharges (see Fig. 3). This can be ascribed to the largely increased

erosion rates connected to rain storm events. Short-term peaks in TSS are often

missed because of the short duration of these rain storms.

From the above overview, we can already make a nice link to Course 3 – "Water

quality Monitoring", because some of the challenges of monitoring are not only the

“What?” questions (Which pollutants?) but also the “Where?” and “When?”. For now

however, we will remain with the “What?” question and discuss the various groups of

pollutants in the next units.

Action List for Unit 1 • Look and listen to the PowerPoint presentation available under “Lecture”. • Complete the assignment WQA-3 and put on the platform.

Assignment WQA-3: Water quality standards of your own country or region

Look up the water quality standards of your country or region. Select maximum 20

parameters and report on their standards for different uses. If available, also specify

sampling frequencies etc. Present your assignment as a table in Word or Excel.

Fig. 2

Fig. 3


Unit 2 – Organic Matter

2.1. Forms of organic matter (OM)

There are millions of different organic compounds. This means it is almost impossible

to analyse all of them (technical and financial limitations). Therefore, very often

“lumped” parameters are used, which analyse a part of or all organic components at

once. Common lumped parameters are:

− BOD: Biochemical Oxygen Demand

− COD: Chemical Oxygen Demand

− TOC: Total Organic Carbon

− (N)OM: (Natural) Organic Matter

BOD and COD are both based on the concept of oxygen demand, which starts from

the assumption that OM can be oxidized by oxygen into simple components such as

CO2 and H2O. The difference between both is that for BOD the reaction uses the

"natural" bacteria, whereas for COD it is a purely chemical reaction.

Theoretical oxygen demand, ThOD

The ThOD of a component can be derived from the stoichiometry of the oxidation

reaction. An example is given below, for ethanol, C2H5OH :

Oxidation reaction: C2H5OH + 3 O2 --> 2 CO2 + 3 H2O, so breahk-down of 1 mole of

ethanol requires 3 moles of oxygen. With molecular weights (M.W.) of C2H5OH: 46

g/mole; O2 : 32 g/mole) --> 46 g. C2H5OH requires 96 g. O2 .

So 1 g. ethanol requires 2.08 g. oxygen: ThOD (C2H5OH) = 2.08 mg O2 per mg ethanol.

Alternatively, knowing its concentration, the ThOD can also be expressed in mg/L. E.g.

for a wastewater with 100 mg/L ethanol, the ThOD will be: 208 mg/L.

Applying ThOD can only be done if you know exactly which components are present,

and at which concentrations. Also, not every compound can be completely oxidised;

some resist degradation and will have an actual oxygen demand which is (often much)

lower than the theoretical one. Therefore, in practice COD and BOD are used, rather

than ThOD.

Chemical Oxygen Demand, COD

COD is measured by oxidation of the samples in the laboratory, using strong chemical

oxidants (mostly K2Cr2O7) and strong acidic conditions (pH around 0) and boiling for 2

hours. Usually a "recovery" >95% is attained, meaning that more than 95% of the OM

is oxidised and thus accounted for. The advantage of COD is that the analysis takes

relatively little time: around 2.5 hours, and even less for a "rapid COD test". A

disadvantage of COD is that the parameter does not really mimic the actual

degradation processes in nature. For that the following parameter is more suitable.

Biochemical Oxygen Demand, BOD

BOD is measured by microbial decomposition in the laboratory under standardized

conditions (temperature, time): BOD205 i.e. BOD during 5 days at 20 ºC. This means

that only biodegradable OM will be taken into account. Since degradation may take


even a month or more, the value is usually around 40% of the COD value and often (in

case of non-degradable materials in the water) much lower.

t is important to realise that during a BOD or COD test, not only carbon-related

compounds can be oxidized, but also e.g. ammonia.. This extra oxygen demand is thus

not representative for the amount of organic material (or organic carbon) in the water.

The most common, and also most important one, is the nBOD or Nitrogen Biochemical

Oxygen Demand, originating from the nitrification reaction:

NH4+ + 2 O2 --> NO3

- + 2 H+ + H2O , or:

1 mole of NH4-N requires 2 moles of O2, so 1 g. NH4-N requires 4.57 g O2 (footnote 1)

So the nBOD can give a large contribution to the total BOD! In estimating the cost of

wastewater treatment, for which "aeration" (oxygen supply) is the largest contributor,

this nBOD is certainly taken into account. To find out the cBOD term only, a

“nitrification inhibitor” can be added in the lab. In that case one uses the term cBOD or

carbon(aceous) BOD. Without nitrification inhibitor, the term tBOD or total BOD is used.

Organic matter mainly comes from domestic and industrial sources. Concentrations

can vary greatly (for example from 430 mg/L of COD in medium-strength domestic

wastewater up to 150,000 mg/L of COD in landfill leachates; the COD values thus

have to be examined on a case-by-case base.

In the Netherlands, the standards for acceptable surface water quality are set at:

BOD205 < 3.0 mg/L

COD < 20 mg/L

Common effluent standards (domestic wastewater after treatment) in Europe are:

BOD205 < 25 mg/L

COD < 125 mg/L

1 Note that in water quality monitoring, charges are often left out; similarly: mg PO4 /L, mg NO3 /L

INTERMEZZO: NH4-N or NH4?

In water quality monitoring we can express the concentration of the constituent:

• Based on the molecule, so as mg NH4/L (M.W. = 14 + 4 =18) (rounded off)

• Based on the atom(s), so as mg NH4-N/L (Atomic weight A.W. = 14) Thus a water quality of 1.0 mg NH4/L corresponds to 0.78 mg NH4- N/L . Similarly: the Worlds Health Organization, WHO, guideline for nitrate in drinking water = 50 mg NO3/L, equivalent to (14/62)*50 = 11.3 mg NO3-N/L .

It is highly recommendable to follow the "Atoms system", e.g. because only then,

mass balances and flows can be estimated.

Be very aware, in water quality data interpretation as well as in your own data

reporting, of the way the results are expressed!


Total organic carbon

Total organic carbon (TOC) is the amount of carbon bound in an organic compound 2

and is often used as a non-specific indicator of water quality. It can further be

subdivided into dissolved and particulate components (DOC and POC).

Natural organic matter

Natural organic matter (NOM) is broken down organic matter that comes from plants

and animals in the environment. Examples are "humic acids" (see Chapman, p. 100).

10-35% of the carbon present forms aromatic rings; these rings are very stable due to

"resonance stabilization", so they are difficult to break down. The main concern of

having NOM in water is that it may react with chlorine and other disinfectants during

drinking water production, producing disinfection by-products, many of which are either

carcinogenic or mutagenic.

2.2. Impacts

Following a BOD discharge, the dissolved oxygen (DO) concentration in a river

typically shows a decrease due to the BOD degradation (oxygen demand), followed by

a recovery caused by the re-aeration from the atmosphere: “oxygen sag curve”; see

Fig.4.

Fig.4. Oxygen sag curve for low-high BOD inputs in a river

The oxygen minimum may be reached some 50 kilometres from the BOD discharge;

thus the effect of BOD discharges is often an interregional or even international

problem. The minimum DO can be found with the help of the following formula3:

2 "Organic compounds" entail all those with the atom C present, except CO2, Carbonates, CN

- , and a few

more 3 For details, see Unit 2.6.: Water quality Modelling


In which:

− L0 = BOD concentration in the river directly downstream of the waste discharge,

after mixing (mg/L)

− cmin = O2 minimum (mg/L)

− cs = O2 saturation concentration (mg/L), usually around 10 mg/L, but lower for

higher water temperatures

− k1 = BOD decay rate constant (day-1); its value is usually 0.2-0.4 day-1

− k2 = re-aeration rate constant (day-1); its value lies usually between 0.2 and 2 day-1,

with high values for shallow, fast-flowing rivers

− tc = travel time of the river to reach the DO minimum (days)

The above formula shows that the “oxygen deficit” [cs – cmin] is roughly proportional to

the BOD load onto the river. This offers a way of solving BOD problems, by reducing

the BOD waste discharges. Be aware that at lower temperatures, more oxygen can

dissolve into the water and microbial degradation processes will be slower, so the

impact of wastewater discharge during winter is smaller than during summer.

See also examples of calculations in Unit 2.6.: "Water quality Modelling".

A nice illustration of these principles can be found at:

http://techalive.mtu.edu/meec/module02/WWTCost.html

Unit 3 – Nutrients

In this unit, we will discuss the influence of nutrients on the aquatic environment. We

will learn about their sources, their cycles and some of the consequences their

presence has on the water and the biota in it.

PART 1 - NITROGEN

3.1. Forms of N

There are four basic forms of nitrogen that can be found in aquatic environments -

nitrogen gas, organic nitrogen, ammonium and nitrate/nitrite. All forms of nitrogen

together are called total Nitrogen (TN) which can have concentrations between 0.4 µg/l

to several mg/l and is often measured to gain an insight into the nutrient status of the

waters (eutrophication status; we will hear more about this later on). Particulate

nitrogen is also distinguished from dissolved forms based on size (by filtration; see

Course 3.6.: dissolved nitrogen is < 0.45 microns). Particulate nitrogen is generally

organic nitrogen and is composed of detritus (e.g. leaf litter, animal faeces, etc.), and

other living algae, plants and animals.

ctk

s ek

Lkcc 1

2

01min

−

=−


N2 gas makes up 78% of our atmosphere, is soluble in water but also biologically inert.

Only specialized organisms have developed the ability to use nitrogen gas directly and

use it as a nitrogen source through the process of biological nitrogen fixation (BNF).

Another component is made up by organic nitrogen compounds, which are largely

made up of amino acids, nucleotides and excretory products.

Further, ammonia (NH3; especially toxic for fish) is largely present in water as

ammonium (NH4+ ; hardly toxic), as a function of pH (see the PowerPoint

presentation 4 ) and is produced by the decomposition of organic matter, such as

proteins, by bacteria as well as a product of N fixation. It is also the preferred form of N

uptake for phytoplankton and plants. When oxygen is present, ammonium also easily

oxidizes to nitrate through the process of nitrification; we have just discussed this

reaction. Since ammonia is largely present in animal wastes, its introduction into water

ways easily leads to oxygen depletion in the water column. Lastly, Nitrate (NO3-) and

Nitrite (NO2-) are common forms of nitrogen found in water when oxygen is present.

Nitrate can be taken up by plants, algae and microbes.

3.2. Sources of Nitrogen (based on: http://www.ext.colostate.edu/pubs/crops/00550.html)

As mentioned before, the earth's atmosphere consists of 78% nitrogen and is the

ultimate source of nitrogen. In most areas of the world, the nitrogen found in soil

minerals is negligible. Manure contains an appreciable amount of nitrogen. Most of this

nitrogen is in organic forms: proteins and related compounds. Cattle manure contains

about 5 to 20 kg. of nitrogen per tonne. About half of this nitrogen is converted to forms

available to plants during the first growing season. Lower amounts are converted

during succeeding seasons. Commercial fertilizer nitrogen is effective when properly

applied. Fertilizer nitrogen is subject to the same transformations as other sources of

nitrogen. Both ammonium and nitrate enter the plant from commercial fertilizer, the

same way as that produced from natural products such as manure, crop residues or

organic fertilizers.

3.3. The Nitrogen Cycle

Different pathways of nitrogen transformation are described in Fig. 5. In the "complete"

N cycle, we distinguish point sources and non-point sources. Examples of the first

category are municipal and industrial wastewater; of the second: agricultural run-off.

The transformations in Fig. 5 are all mediated by microbes (mainly bacteria) and are

thus subject to control by ecological factors (e.g. dissolved oxygen level, pH, microbial

food web dynamics, etc.). The nitrogen cycle begins with nitrogen fixation; this is

incorporated into the organic pool (e.g. enzymes, proteins) This organic N eventually

becomes mineralized, i.e. turned into non-organic forms, most commonly as

ammonium (NH4) or nitrate (NO3). Nitrifiers transform NH4+ to NO2

- (nitrite) and to

nitrate (NO3-) in a two step oxidation process; denitrifiers change NO3

- and NO2- back

into N2 and N2O gas, thus completing the nitrogen cycle.

4 pH = -

10 log [H

+]; at neutral conditions, the values are around 7. For "acidic" waters, pH < 7; for

"alkaline" conditions, pH >7. Especially in eutrophic lakes, the pH can go up to 9-10 during the day-time.

One effect is then the conversion of NH4+

--> NH3 , which is toxic for fish. A low [H+] is related with a

high OH- = hydroxide content, and v.v.


Fig.5. Nitrogen cycle (source: http://sci.waikato.ac.nz/farm/images/Nitrogen_Cycle.jpg)

The scheme is simplified, since NO3- also comes directly from manure, fertilizers and

also because N point sources (domestic and industrial wastewaters) are not included

a) Nitrification

NH4 + + 1 1/2 O2 --> 2 H + + NO2

- + H2O and NO2 - + 1/2 O2 --> NO3

-

The term nitrification refers to the conversion of ammonium to nitrate. This is brought

about by the autotrophic (not needing a carbon source) nitrifying bacteria, with

Nitrosomonas species for the first conversions step to NO2-, and Nitrobacter species,

for the second, to NO3-. The ammonium ion (NH4

+) has a positive charge and so is

readily adsorbed onto the negatively charged clay colloids and soil organic matter,

preventing it from being washed out of the soil by rainfall. In contrast, the negatively

charged nitrate ion is not adsorbed or precipitated on soil particles and so can easily be

washed down in the soil ("leaching"). In this way, valuable nitrogen can be lost from the

soil, reducing the soil fertility. The nitrates can then accumulate in groundwater, and

ultimately in drinking water. We will discuss cases of nitrate groundwater pollution in

Course 3.

Nitrates can be reduced to highly reactive nitrites; these nitrites can be bound by the

blood-haemoglobin, thus reducing its oxygen-carrying capacity. In babies, this can lead

to the "blue baby syndrome". Nitrite in the gut can react with amino compounds,

forming highly carcinogenic nitrosamines. Nitrate also causes eutrophication problems

in ecosystems, especially in estuarine ecosystems where nitrogen limits algae growth

(see Unit 2.5.).


b) Denitrification

NO3- --> NO2

---> N2O --> N2

Denitrification is one of the only ways in which N is permanently lost from ecosystems

because it converts nitrate to gaseous forms (N2O and N), whereas other processes

convert N to other biologically available forms. (Burial is the other way in which nitrogen

can be “lost” from ecosystems.) Several types of bacteria perform this conversion, and

they are all anaerobic heterotrophs, thus requiring a source of organic carbon.

For an overview of the nitrogen cycle, please refer to:

http://bcs.whfreeman.com/thelifewire/content/chp58/5802004.html. Note: they also

have a quizzing section at the end that you may find useful.

PART 2 - PHOSPHORUS

3.4. Sources and forms of Phosphorus

Common sources of phosphorus are phosphate-rich rocks and guano (bird droppings;

many islands have metres-thick layers). Phosphate rock in commercially available form

is called apatite. Huge quantities of sulphuric acid are used in the conversion of the

phosphate rock into a fertilizer product called "super phosphate". Since sources for

phosphate rock are limited and at the same time extensively used for agricultural

purposes (with about 2% increase per year), quite some investigators state that these

sources will only last for the coming 50-100 years.

Just as for N, we distinguish P point sources and non-point (diffuse) sources.

Examples of the first category are municipal and industrial wastewater; of the second:

agricultural run-off. Total phosphorus (TP) consists of all P (organic and inorganic) in

dissolved and particulate forms. TP is measured on a sample of unfiltered water and is

generally monitored because it is rather constant, in contrast to inorganic P, which is

taken up by algae; see Unit 2.5.). Typical concentrations in lakes range between 10 -

80 µg P/L.

Orthophosphate 5 (PO43-, HPO4

2- ,H2PO4-, H3PO4) is a major component of the

phosphates present; it gives an indication on the amount of phosphorus that is

available to the biota. The term is roughly equal to "Soluble reactive phosphorus"

(SRP). Other P fractions are e.g. dissolved organic phosphorus (DOP) and Particulate

organic phosphorus (POP) which is the P in organic chemicals of living or dead matter.

3.5. Phosphorus cycle (Fig. 6)

For an introduction to the cycle please refer to the following movie:

http://www.youtube.com/watch?v=5bqynn3EWoY&feature=related

5 The "hydrogen phosphate" forms are dominant at neutral pH; all forms together are indicated as

"PO43- " or "PO4"; in phosphate analysis they are all measured together


Fig. 6. The phosphorus cycle

The type of phosphorus mainly present in the water column is the ortho-phosphate,

PO43-. However, the availability of this form of phosphorus is often highly limited, due

to the uptake by plants and animals in the water column, its rapid deposition and

fixation in the sediments (especially with metal ions). Phosphorus release from

sediments is an important aspect of the phosphorus cycle which is mediated through

e.g. bacterial decomposition mechanisms. Most importantly however, as you can see

from Fig. 6, that once the phosphorus is in the system (lake, river, reservoir, etc.), there

is no "real way out". Thus, although phosphorus can be a "limiting nutrient" in

eutrophication (see Unit 2.5.), it is very hard to get rid of the P excess.

The PowerPoint of this Unit 2 gives an example of the P and N sources for the

Chesapeake Bay, USA. For both nutrients, non-point sources, especially agriculture,

form a larger factor than point sources (domestic, industrial discharges).

Action List for Unit 3

− Look and listen to the PowerPoint presentation available under “Lecture”.

− Read chapter 3.4 in Chapman et al.

− Check the WHO guidelines for drinking water:

http://www.who.int/water_sanitation_health/dwq/GDW12rev1and2.pdf


YouTube illustrations

− Watch the two movies about the Nitrogen cycle and the Phosphorus cycle:

http://bcs.whfreeman.com/thelifewire/content/chp58/5802004.html (don’t forget to check

out their quizzing section) and

http://www.youtube.com/watch?v=5bqynn3EWoY&feature=related

Further reading

Stanley Dodson, Introduction to Limnology, 2005, McGraw Hill

Robert Wetzel, Limnology, 2nd Edition 1983, Saunders College Printing


Unit 4 – Micropollutants

Part 1 - inorganic and organic micropollutants

In this unit we will focus on micropollutants, substances that often occur at very low

concentrations, viz. in the pg/L to ng/L range. More than one-third of the Earth's

accessible renewable freshwater is used for agricultural, industrial, and domestic

purposes, and most of these activities lead to water contamination with numerous

synthetic and natural compounds. It therefore comes as no surprise that chemical

pollution of natural waters has become a major public concern in almost all parts of the

world. The assessment of whether or not a particular chemical is a pollutant and is

harmful, is based upon an understanding of its exposure, i.e., its input, distribution and

fate in a defined system, and of the effects that the chemical has on organisms,

including humans, due to its presence in the system. We will discuss this shortly in the

part "Aquatic Ecotoxicology". Also, aquatic sediments, often serving as a "storage" for

micropollutants, will be discussed.

4.1. Inorganic micropollutants - Trace metals

"Heavy metals" are stable metals whose density is > 4.5 * 103 kg/m3, for example lead,

copper, nickel, cadmium, chromium, zinc, mercury and platinum. "Trace metals" is a

more general term, also including e.g. iron (Fe), manganese (Mn), and also metalloids

such as vanadium and arsenic. These metals are natural constituents of the Earth's

crust; they are stable and cannot be degraded or destroyed, and therefore they tend to

accumulate in soils and sediments. Human activities have drastically altered the

biochemical and geochemical cycles and balance of some trace metals.

Environmental anthropogenic contamination with trace metals began with the discovery

of fire and continued in e.g. the Roman empire, and with metal smelting, in the 16 th

century and later. Between 1900 and 1980, the trace metal emissions world-wide

increased with a factor 10-50. Major sources of trace metals are given in the

PowerPoint. It should be realised that trace metals in the environment do not only

originate from anthropogenic (man-made) sources; natural sources such as volcanoes

and forest fires may substantially contribute to the global input of trace metals, or may

even form a dominant part of it (e.g. for arsenic). In small quantities, certain trace

metals are nutritionally essential for a healthy life. These elements, or some form of

them, are commonly found naturally in foodstuffs, in fruits and vegetables, etc. Large

amounts of any of them may cause acute or chronic toxicity (poisoning) such as

physical, muscular, and neurological degenerative processes that mimic Alzheimer's

disease, Parkinson's disease, muscular dystrophy, and multiple sclerosis.

Heavy/trace metals are dangerous because they tend to bioaccumulate.

Bioaccumulation, in general, means an increase in the concentration of a chemical in a

biological organism over time, compared to the chemical's concentration in the

environment. Compounds accumulate in living things any time they are taken up, and

are stored faster than they are broken down (metabolized) or excreted. This is

discussed in some more detail in part 2 of this unit.


Some important disasters with heavy metals:

1952 - Minamata Syndrome

Sewage containing mercury was released by Chisso's chemicals works into Minamata Bay in

Japan and this mercury accumulated in organisms like fish. In 1952, the first incidents of mercury

poisoning appeared in the population around the Minamata Bay, caused by consumption of fish

polluted with mercury, bringing over 500 fatalities. Since then, Japan has had the strictest

environmental laws in the industrialised world. A short (and be warned - rather cruel!) movie with

a report on the case can be seen at: http://www.youtube.com/watch?v=ihFkyPv1jtU.

1986 - Sandoz

Water used to extinguish a major industrial fire brought about 30 tonne of fungicide-containing

mercury into the Upper Rhine in Switzerland. Fish was killed over a stretch of 100 km. More info

can be found in the BBC news story of that time:

http://news.bbc.co.uk/onthisday/hi/dates/stories/november/1/newsid_4679000/4679789.stm

1998 - Spanish nature reserve contaminated after environmental disaster

Toxic chemicals in water from a burst dam belonging to a mine contaminated the Coto de Doñana

nature reserve in southern Spain. Ca. 5 million m3 of mud containing sulphur, lead, copper, zinc

and cadmium flew down the Rio Guadimar. Experts estimate that Europe's largest bird sanctuary,

as well as Spain's agriculture and fisheries, will suffer permanent damage from the pollution.

When metals are released into water, they will very often be adsorbed onto particles

(suspended solids) and when these particles settle, metals end up in the sediments

which are normally the most important sink of metals in aquatic ecosystems.

Countries have different standards, which are among others based on natural

background concentrations. In the EU (European Union) there has been an attempt for

harmonisation (Directive 2008/105/EC on environmental quality standards in the field of

water policy). Maximum allowable concentrations for inland surface waters are as

follows (note the relationship with "hardness", because of carbonate precipitation):

• Cd: 0.45 µg/L (low hardness) - 1.5 µg/L (high hardness)

• Pb: 7.2 µg/L (as an annual average concentration)

• Hg: 0.07 µg/L

• Ni: 20 µg/L (as an annual average concentration)

More details as well as standards for other micropollutants, can be found in the EU

Directive (available on the platform under additional reading).

4.2. Organic micropollutants - PCBs

Polychlorinated biphenyls (PCBs) are a group of organic chemicals which can be

odourless or mildly aromatic solids or oily liquids. They are built up by two benzene

rings with various chlorine substitutions (see Fig. 7 for chemical structure). In this way,

some 200 different PCB components are known, which are commonly indicated by a

number (e.g. PCB-28). They were formerly used as hydraulic fluids, plasticizers,


adhesives, fire retardants, de-dusting agents, inks, lubricants, cutting oils, in heat

transfer systems, carbonless reproducing paper, etc. PCBs in ecosystems have well-

known effects such as reproduction disorders, and hormone deregulation.

Fig.7. General structure of PCBs

(source: Wikimedia Commons)

PCBs are very persistent in soil and water,

with no known break-down processes other than slow degradation by microbes. They

adhere to soils, and so will not usually leach to ground water. Since a partial global ban

in the 1980s, the levels in the environment have only very gradually decreased

because of the high stability of the PCB components.

4.3. Organic micropollutants - Hormones, endocrine disrupting chemicals and

pharmaceutical residues

(http://www.epa.gov/ppcp/basic2.html)

Hormones are used in livestock growth to enhance growth, milk and egg production

and similar economic activities. In humans, hormones are mainly used through the

female anti-conceptive pills. Residues of these hormones can then be found in the

urine of the users which will then make its way into the waterways. Traditional sewage

treatment plants were not designed to take these leftovers into account and little is

known on the effects on biota. Note that many organic chemicals have a structure that

is similar to human or animal hormones and hence these chemicals also exert a similar

effect as hormones (endocrine disrupting chemicals). Below you can find the case on

the American alligator of the Everglade Marshes in Southern Florida. Here the

introduction of domestic waste and agricultural runoff has created a suite of problems,

including the abnormal development of male alligators due to the residues of hormones

in the water.

Alligators and Endocrine Disrupting Contaminants: A Current Perspective

Louis J. Guillette, Jr., D. Andrew Crain, Mark P. Gunderson, Stefan A. E. Kools, Matthew R. Milnes,

Edward F. Orlando, Andrew A. Rooney and Allan R. Woodward

Many xenobiotic (=not normally found) compounds introduced into the environment by human

activity have been shown to adversely affect wildlife. Reproductive disorders in wildlife include

altered fertility, reduced viability of offspring, impaired hormone secretion or activity and

modified reproductive anatomy. It has been hypothesized that many of these alterations in

reproductive function are due to the endocrine disruptive effects of various environmental

contaminants. The endocrine system exhibits an organizational effect on the developing

embryo. Thus, a disruption of the normal hormonal signals can permanently modify the

organization and future function of the reproductive system. We have examined the

reproductive and developmental endocrinology of several populations of American alligator

(Alligator mississippiensis) living in contaminated and reference lakes and used this species as

a sentinel species in field studies. We have observed that neonatal and juvenile alligators living

in pesticide-contaminated lakes have altered plasma hormone concentrations, reproductive tract


anatomy and hepatic functioning. Experimental studies exposing developing embryos to various

persistent and non-persistent pesticides, have produced alterations in gonadal steroidogenesis,

secondary sex characteristics and gonadal anatomy. These experimental studies have begun to

provide the causal relationships between embryonic pesticide exposure and reproductive

abnormalities that have been lacking in pure field studies of wild populations. An understanding

of the developmental consequences of endocrine disruption in wildlife can lead to new

indicators of exposure and a better understanding of the most sensitive life stages and the

consequences of exposure during these periods.

Source: American Zoologist 2000 40(3):438-452; doi:10.1093/icb/40.3.438

Pharmaceuticals and personal care products were first called "PPCPs" only a few

years ago, but these bioactive chemicals have been around for decades. Their effect

on the environment is now recognized as an important area of research. PPCPs

include e.g. therapeutic drugs, veterinary drugs and sun-screen products. PPCPs in the

environment illustrate the immediate connection of the actions/activities of individuals

with their environment. Individuals add PPCPs to the environment through excretion

and bathing, and disposal of unwanted medications to sewers and trash.

4.4. Organic micropollutants - PAHs

PAHs (polycyclic aromatic hydrocarbons) are aromatic hydrocarbon compounds with

two to eight fused benzene rings (see Fig. 8). PAHs may also contain sulphur, oxygen

or nitrogen in their aromatic ring structure, in that case they are called heterocyclic

PAHs. The main sources of PAHs are combustion processes (automobiles, coal, oil,

stacks, etc.). Especially the higher PAH forms are very stable in the environment.

PAHs are carcinogenic at already low concentrations. They have a large tendency to

accumulate in sediments and in the aquatic food chain.

4.5. Pesticides

(from http://www.fao.org/docrep/W2598e/w2598e07.htm)

A fundamental contributor to the Green Revolution has been the development and

application of pesticides for the control of a wide variety of pests that otherwise would

diminish the quantity and quality of food produce. The term "pesticide" is a composite

term that includes all chemicals that are used to kill or control pests. In agriculture, this

includes e.g. herbicides (weeds), insecticides (insects), and fungicides (fungi). There is

overwhelming evidence that agricultural use of pesticides has a major impact on water

quality and leads to serious environmental consequences. In addition to chemical and

photochemical reactions, there are two principal biological mechanisms that cause

Fig.5.4. Examples of polycyclic aromatic hydrocarbons (PAHs) Fig. 8


degradation of pesticides. These are (1) microbiological processes in soils and water

and (2) metabolism of pesticides that are ingested by organisms as part of their food

supply. While both processes are beneficial in the sense that pesticide toxicity is

reduced, metabolic processes do cause adverse effects in, for example, fish. Energy

used to metabolize pesticides and other xenobiotics is not available for other body

functions and can seriously impair growth and reproduction of the organism.

PART 2 – BIOACCUMULATION

Bioaccumulation is the increase in

concentration of a substance in living

organisms as they take in contaminated air,

water, or food. As bigger animals eat smaller

animals, the level of contamination in the food

is added to the level of contamination already

in their body, and we end up with

biomagnification. Thus:

• Bioaccumulation: increase in

concentration of a pollutant from the

environment to the first organism in a

food chain (e.g. in algae)

• Biomagnification: increase in

concentration of a pollutant from one

link in a food chain to another

The figure to the left shows an example of

bioaccumulation of PCBs in the food chain.

Initially, phyto- and zooplankton take up the

pollutants from the water and are then

consumed by their predators. As these latter

consume large volumes of their prey which by

themselves may only contain small

concentrations of the toxins, the concentration

in the large animals becomes greater. In this

way, the large predators (here Herring gull) end

up having the largest concentrations of

micropollutants in them. Often, these

substances (in particular the organics) tend to be lipophilic (soluble in fat) and therefore

tend to particularly build up in the fat reserves of animals.

Rachel Carson with her book "The Silent Spring" was one of the first ones to make a

connection between the negative impacts of pesticides and ecosystems, and

specifically showed the accumulation in the fat reserves of the larger animals (in

particular birds). A bit of background information is given in the box on the next page.

Note that several organo-chlorine chemicals (PCBs etc.) have even been encountered

in high concentrations in the fat tissue of polar bears and ringed seals in the Arctic

region, showing how widespread pollution of these chemicals is.


Rachel Carson’s Silent Spring and the Beginning of the Environmental Movement

(http://classwebs.spea.indiana.edu/bakerr/v600/rachel_carson_and_silent_spring.htm)

Introduction

When Rachel Carson's Silent Spring was published in 1962, it generated a storm of controversy

over the use of chemical pesticides. Miss Carson's intent in writing Silent Spring was to warn

the public of the dangers associated with pesticide use. Throughout her book are numerous

case studies documenting the harmful effects that chemical pesticides have on the environment.

Along with these facts, she explains how in many instances the pesticides have done more

harm than good in eradicating the pests they were designed to destroy. In addition to her

reports on pesticide use, Miss Carson points out that many of the long-term effects that these

chemicals may have on the environment, as well as on humans, are still unknown. Her book as

one critic wrote, "dealt pesticides a sharp blow" (Senior Scholastic 1962). The controversy

sparked by Silent Spring led to the enactment of environmental legislation and the

establishment of government agencies to better regulate the use of these chemicals.

Miss Carson first became aware of the effects of chemical pesticides on the natural environment

while working for the U.S. Bureau of Fisheries. Of particular concern to her was the

government’s use of chemical pesticides such as DDT. She was familiar with early studies of

DDT and knew of its dangers and lasting effects on the environment. According to Miss Carson,

"the more I learned about the use of pesticides, the more appalled I became. I realized that here

was the material for a book. What I discovered was that everything which meant most to me as

a naturalist was being threatened, and that nothing I could do would be more important." Thus,

Silent Spring was written to alert the public and stir people to action against the abuse of

chemical pesticides (Time 2000).

Impact of Silent Spring

When excerpts of Silent Spring first began appearing in The New Yorker magazine in June

1962, they caused uproar and brought a "howl of indignation" from the chemical industry.

Supporters of the pesticide industry argued that her book gave an incomplete picture because it

did not say anything about the benefits of using pesticides. An executive of the American

Cyanamid Company complained, "if man were to faithfully follow the teachings of Miss Carson,

we would return to the Dark Ages, and the insects and diseases and vermin would once again

inherit the earth." Chemical manufacturers began undertaking a more aggressive public

relations campaign to educate the public on the benefits of pesticide use. Monsanto, for

example, published and distributed 5,000 copies of a brochure "parodying" Silent Spring entitled

"The Desolate Year," which explained how chemical pesticides were largely responsible for the

virtual eradication of diseases such as malaria, yellow fever, sleeping sickness, and typhus in

the United States and throughout the world, and that without the assistance of pesticides in

agricultural production millions around the world would suffer from malnutrition or starve to

death (NRDC 1997).


PART 3 – ECOTOXICOLOGY

The term ecotoxicology was coined by René Truhaut in 1969 who defined it as "the

branch of toxicology concerned with the study of toxic effects, caused by natural or

synthetic pollutants, to the constituents of ecosystems, animal (including human),

vegetable and microbial, in an integral context”. Ecotoxicology is the integration of

toxicology and ecology or, as the book of Chapman suggests: ecology in the presence

of toxicants”. It aims to quantify the effects of stressors upon natural populations,

communities, or ecosystems. Ecotoxicology incorporates aspects of ecology,

toxicology, physiology, molecular biology, analytical chemistry and many other

disciplines. The ultimate goal of this approach is to be able to predict the effects of

pollution so that the most efficient and effective action to prevent or remediate any

detrimental effect can be identified. In ecosystems that are already impacted by

pollution, ecotoxicological studies can inform as to the best course of action to restore

ecosystem services and functions efficiently and effectively.

An ecotoxicological assessment usually consists of two main parts, as shown in Fig. 8

(taken from Schwarzenbach et al., 2006, Science Vol 313):

• Exposure assessment: based on the environmental fate of chemicals; one can

calculate the bioavailability of substances (what is the concentration of a

chemical that is really available to an organism?)

• Toxicological assessment: what is the effect of the bioavailable concentration

on an organism?

Drinking water and food guidelines are then usually determined using a risk-based

assessment. Generally, Risk = Exposure (amount and/or duration) × Toxicity.

We cannot go into too many details because ecotoxicology is a whole science domain

on its own, but below follows some general information about both steps.

Steps taken during exposure assessment:

• One will calculate the distribution or so-called partitioning over different

compartments (solid matter, water, gas, biota). For example: how much Cd

ends up in the water, how much ends up in the sediment, how much is taken up

by the water plants, etc.? This will depend on phenomena such as precipitation,

settling of solids onto which metals might be adsorbed, if chemicals dissolve

easily in water (hydrophylic) or not (hydrophobic), etc.

• One will also look into the degradational process of chemicals. "Degradates"

may have greater, equal or lower toxicity than the parent compound. As an

example, DDT degrades to DDD and DDE. The persistence of chemicals is

usually measured as their half-life (time required for the ambient concentration

to decrease by 50%). Persistence is determined by biotic and abiotic

degradational processes. Biotic processes are biodegradation and metabolism;

abiotic processes are mainly hydrolysis (reaction with water), photolysis

(degradation in the light), and oxidation.


Fig. 8. Steps taken in ecotoxicological assessment

• In toxicology, the median lethal dose, LD50 (abbreviation for “Lethal Dose, 50%”),

and LC50 (Lethal Concentration, 50%) of a toxic substance is the dose required

to kill half the members of a tested population. Typically, the LD50 of a substance

is given in milligrams per kilogram of body weight. Stating it this way allows the

relative toxicity of different substances to be compared, and normalizes for the

variation in the size of the animals exposed. Note that some pollutants can be

protagonistic, meaning in combination they enhance toxicity towards the

organism, or they can be antagonistic, where their toxic effects decrease in

combination, cause less threat to the organism.

Please refer to Chapter 5.7 in Chapman for further insight into methods for assessing

toxic pollution.


PART 4 – MICROPOLLUTANTS IN AQUATIC SEDIMENTS

Aquatic sediments can have different sources:

• Allogenic (or allochthonous) material, deriving from outside the system. These

include mainly sand, silt and clay transported by rivers;

• Endogenic minerals, from processes in the water system (sedimentation of

carbonates, algae, etc.), and

• Authigenic (or authochtonous) minerals, which are formed within the sediment by

physico-chemical (oxidation, precipitation, etc.) and microbiological processes.

Any aquatic sediment in a river, lake, estuary or sea consists of a complex mixture of

discrete minerals and organic compounds to which a number of ions are more or less

tightly bound. For different aquatic ecosystems, the composition of sediments can be of

a highly variable nature, with organic carbon fractions ranging from 0.1% in deep

oceans up to >20% in dredged materials, and grain sizes between 1 and 10-7 mm.

A major fraction of the sediment minerals consists of quartz and feldspars; these are

almost exclusively to be found in the sand (> 63 µm) and silt (between 2 and 63 µm)

fraction. These non-clay minerals are chemically rather inert. In contrast, clays (< 2µm)

are very active in the adsorption of dissolved compounds. This is due to their large

specific surface area and their specific chemical layer structure. Clays are capable to

adsorb large quantities of both cations and anions, as well as organic compounds.

Sediments as “storage reservoirs” for micropollutants

Aquatic sediments commonly form the largest reservoir for micropollutants.

Micropollutants in aquatic ecosystems have a relatively large tendency to be adsorbed

onto suspended matter and sediments. This tendency can quantitatively be expressed

with the so-called Partition coefficient between water and sediment. The Partition

coefficient (L/kg) is defined as the µg micropollutant/kg particulate matter divided by the

µg/L present in the water phase. Some typical values for micropollutants are presented

in Table 2.

Table 2. Partition coefficients P (L/kg) of some chemicals

Heavy metals 104 - 105

Benzo(a)pyrene (PAH) 104 – 105

PCBs 105 – 106

Methoxychlor 104

Naphthalene 103

Thus, for chemicals such as heavy metals and PCBs, more than 50% can easily be

adsorbed onto particulate matter (see Fig. 9), and thus be "lost" due to sedimentation if

the river flow decreases. This mechanism is responsible for the deposition of pollutants

in lakes and stagnant rivers, which then serve as sedimentation basins. The above

mechanism is also a major reason for the potential harm of micropollutants, i.e. by

accumulation in the fatty tissue (liver, kidney, etc.) of organisms, and also of

biomagnification in the food chain. Here, higher organisms (fish, birds, men) feeding on

lower organisms (phyto- and zooplankton) may build up toxic, high levels.


Fig. 9 Fate of dissolved zinc, chromium cadmium and copper in the Dutch IJssel Lake

(Salomons and Förstner, 1984, Metals in the hydrocycle, Springer Verlag)

The adsorption qualities of sediments with respect to micropollutants are much related

to the grain sizes of the individual particles; clay particles (<2 µm) possess a much

higher specific surface area than the silt and sand particles. In a study on river

Rhine/Main (Germany) sediments, it was found that 95 to 99% of all adsorbed heavy

metals were present in the grain size fraction < 63 µm, occupying only 30 % of the

sediment material (see Fig. 10).

Fig. 10. Cd contents (mg/kg) on particle size fractions of river Rhine and Main

sediments (WHO, 1982. Micropollutants in river sediments. Euro Reports and studies,

nr. 61)

Sediments are by no means static media; under the influence of waves and currents,

they may be transported to deeper zones. These deeper zones then act as a kind of

sedimentation basins for especially small-sized particulate material. Here also the most

polluted sediments can often be found.

Undisturbed vertical sediment cores may show a clear picture of historical pollution

events. In the PowerPoint presentation of this Unit, we see mercury (Hg) contents in

sediments in Lake Ontario (Canada) and Lake Windermere (UK), and Cu and Pb

contents in Lake Ontario sediment. In all cases, a clear increase in the heavy metal

sand silt

clay


contents can be observed since around 1900, connected with increased

industrialisation in the areas. The natural background values, without human influences,

will be determined by the geological conditions of the areas. In the last decades, as

indicated by the uppermost sediment layers, the heavy metal contents show a

decrease again, especially due to recent sanitation measures by industries. An

important requirement for the determination of above data is the extraction of

undisturbed vertical sediment cores (see an illustration on the last slide of the

PowerPoint presentation of this Unit) as well as accurate dating techniques. For the

latter, radioactive dating with 210Pb or 137Cs is commonly used.

Even strongly adsorbed micropollutants are not bound irreversibly to the sediment. Organic micropollutants may microbially be degraded or undergo all kinds of other transformation processes, e.g. under the influence of light or pH. Important factors promoting remobilisation of heavy metals from sediments are:

• Elevated salinity, such as in estuaries, where a transition from fresh to marine conditions takes place;

• Lowering of pH, generally leading to metal desorption due to replacement by H+ ions (“ion exchange”);

• Changes in the redox potential Eh. Under low Eh the metals are generally precipitated as metal sulphides. Under aerobic conditions, the sulphides will be oxidised to (the more soluble) SO4

2-, leading to the release of the heavy metals;

• Microbial conversions such as in methyl mercury, which can easier be taken up by fish.

• Biological activities like disturbance caused by bottom-dwelling fish or burrowing animals (often referred to as “bioturbation");

• Resuspension, due to waves and currents, of settled sediment materials, or dredging. In the overlying water, an oxidation of the metal-sulphide bindings will take place, again leading to metal release.

Table 3. Pollution classes with respect to micropollutants contents (mg/kg) for polluted

sediments in the Netherlands. Classes range from class 1 (unpolluted) to class 4

(heavily polluted)


Sediment quality guidelines

In the Netherlands, sediment quality has been divided into four classes; see Table 3.

More recently, Consensus based Sediment Quality Guidelines for Freshwater are used,

based on ecotoxicological guidelines; see Table hereunder.


− Look and listen to the Powerpoint presentation available under “Lecture”.

− Read chapter 3.8 Heavy metals, 3.9 Organic micropollutants and 5.7 Chemical

monitoring in Chapman (1996)

− Go to http://cfpub.epa.gov/ecotox/ from the EPA (US Environmental Protection

Agency) and perform a quick query on a chemical of your choice. You will see the

vast amount of information that has been reported.

− You can read more about pesticides under

http://www.fao.org/docrep/W2598e/w2598e07.htm

Further reading

Stanley Dodson, Introduction to Limnology, 2005, McGraw Hill


Unit 5 – Aquatic ecosystems

Aquatic ecosystems are features in the landscape that participate in the processing

and transport of materials (sediments, pollutants, nutrients, organic matter) as these

materials move downstream from continents to oceans. This ability to process, retain,

or export nutrients, sediments, or pollutants is controlled in part by the physical

structure of the ecosystems (e.g. flowing vs. still water, stratification), by food web

structure and terrestrial inputs, and by nutrient dynamics.

In this unit, we will examine these things: 1. Processing of terrestrial inputs and food

web structure and 2. Nutrient processing, especially as it relates to eutrophication.

5.1. Introduction: Definition of aquatic systems and types

Watch the following movie on aquatic food webs:

http://www.youtube.com/watch?v=roRQQZlGIvM

An aquatic ecosystem is any watery environment, from small to large (i.e. from ponds

to oceans), running or still (rivers or lakes) that contain a group of interacting organisms

(plants, animals, microbes) that are dependent on one another and their water

environment for energy and food (carbon), nutrients (e.g., nitrogen and phosphorus)

and shelter.

Familiar examples are ponds, lakes and rivers, but aquatic ecosystems also include

areas such as floodplains and wetlands, which are flooded with water for all or only

parts of the year, highly polluted waters, and even thermal springs. Our health, human

livelihoods, recreation, and many of our activities are dependent on aquatic

ecosystems. Most of the water that we drink is taken from lakes or rivers, as well as

many food sources such as fish and shellfish. These activities are impaired when these

systems are unhealthy. Here, we will examine some ways in which ecosystems

function ecologically, in food web structure and dynamics, and in nutrient pathways

involved in eutrophication.

5.2. Food webs

Aquatic ecosystems usually contain a wide variety of life forms including autotrophs

and heterotrophs. Autotrophs are organisms that create their own energy from

inorganic sources (e.g. carbon dioxide (CO2) and water); this "photosynthesis" is one of

the dominant forms of autotrophy on Earth. Primary producers are organisms that

photosynthesize; they include algae, cyanobacteria, and higher plants such as sea

grass, bulrushes, cattails, reeds and other macrophytes.

Heterotrophs are organisms that depend on carbon that was previously fixed by

autotrophs. They respire this carbon, consuming oxygen to oxidize organic carbon

molecules back into carbon dioxide. Heterotrophs include a wide variety of organisms

including: small zooplankton that generally feed on phytoplankton, fish, birds, and man.

The food web (depicted in the PowerPoint presentation) is a simplified way of

understanding the process by which organisms in higher trophic levels gain energy by

consuming organisms at lower trophic levels. Because photosynthesis captures light


and converts it into useable energy, the

driving force for nearly all food webs is

sunlight. The concept of the food web

explains how some persistent

contaminants accumulate in an

ecosystem and become biologically

magnified (as we saw in Unit 4).

Each trophic transfer in a food web (e.g.

from primary producer to primary

consumer) results in a 90% energy loss!

Thus, only 10% of the sun's energy is

passed from plants up the food chain for

each trophic level. Food webs have

traditionally focused on food webs that

start with the primary producers and

continue to the grazers, and secondary

consumers. Let us now have a closer

look at the different strata within the food

web.

a) Primary producers

At the bottom of a food chain, we find the primary producers (algae) who take their

energy from sunlight and use it in photosynthesis; simplified:

... CO2 + ...H2O (+P, N,.) ----> .. CH2O (P.., N,..) + .. O2

Light algae

The reverse reaction, respiration "wins" at night and can lead to very low O2 contents in

the water.

Since algae require light to carry out photosynthesis, they must live in the euphotic

zone, as discussed in Course 1. Phytoplankton refers to the algae that live in the

water column. Algae attached to rocks or other hard substrate such as plant stems,

are called periphyton. Algae account for half of all photosynthetic activity on Earth;

they are responsible for much of the oxygen present in the Earth's atmosphere.

(If you have time, check out this fascinating lecture

http://www.youtube.com/watch?v=CB2XlpD-Ld4&feature=channel; on everything you always

wanted to know about algae).

b) Consumers

In the water column, zooplankton feeds on phytoplankton. Zooplankton forms a broad

category of organisms. Through their consumption of phytoplankton (and other food

sources), zooplankton play an important role in aquatic food webs, both as a resource

for consumers on higher trophic levels, including fish. Since they are typically of small

size, zooplankton can respond relatively rapidly to increases in phytoplankton

abundance and can therefore be important in food web dynamics, especially as food

responds to increased nutrient inputs


c) Predators

These can be fish, birds, or even higher

up in the food chain (such as the polar

bears). These can be planktivorous (either

phyto or zoo) or piscivorous (meaning

they eat smaller fish). The type of predator

that is present in the water will determine

largely the species composition and

abundance of the prey and others. In Lake

Victoria, for instance, many fish species

have been identified, with the Nile Perch

being the one most used commercially

(almost 80,000 tons of catch/year).

However, the Nile Perch is an exotic

species to the lake and is responsible for serious depletion in the populations of native

cichlads. (Source: Lake Victoria by J. Awange & O On’gan’ga, 2006 Springer).

d) Top-Down and Bottom-up control of food webs.

The phrase "top-down" and "bottom-up" control refers to the question of whether

primary production (or a prey species) is controlled by consumers (top-down control) or

by nutrients or light (bottom-up control). This question has been especially important in

understanding and mitigating eutrophication and overfishing.

In aquatic systems, nutrient availability is the most commonly considered factor, though

researchers also consider light as an important "bottom-up" factor, especially as

excess nutrients can cause algal blooms which shade benthic macrophytes. Nutrient

availability can determine the presence or absence of species. Further, the diversity

and abundance of phytoplankton determines the abundance and diversity of

zooplankton. "Top-down" control refers to situations where the abundance, diversity

or biomass of lower trophic levels are controlled by consumers at higher trophic levels.

In a "trophic cascade", predators induce effects that cascade down the food chain and

affect biomass of organisms at least two links away. For instance, fish predation of

zooplankton can increase phytoplankton biomass indirectly by eliminating zooplankton

grazing pressure on phytoplankton, thus exerting top-down control on phytoplankton

abundance.

5.3. Eutrophication - or why we worry about nutrients

We have already been talking a lot about pollution and problems with nutrients. Now let

us have a closer look at one of these problems: eutrophication. This term generally

refers to an overgrowth of algae and water plants, due to high inputs of the nutrients N

and P, which are essential for algae and plant growth. In order to understand

eutrophication, we need to look at the so called trophic states (see PowerPoint for

photographs). Lakes with low nutrient contents are usually oligotrophic; they are

characterized by high light visibility (see Course 3; for field measurements) and low

chlorofyll-a contents (the cells active in the primary production of algae and plants).

Lake Baikal (Russia) is an example of such a lake; high alpine lakes and many tropical

lakes such as the East African Rift lakes are other examples. Mesotrophic systems

have slightly higher nutrient contents, but are fairly rare, since they usually represent a


transitional situation from oligotropic towards eutrophic systems. Eutrophic and

hypereutrophic systems are the ones we are mostly concerned about where large

amounts of nutrients cause tremendous algal blooms and excessive macrophyte

growth, causing anaerobic conditions in the water at night, leading to massive fish kills

(see Fig. 11).

Eutrophication was first evident in lakes and rivers as they became choked with

excessive growth of rooted plants and floating algal scums, prompting intense study in

the 1960's-70's and culminating in the scientific basis for banning phosphate

detergents (a major source of P, the most frequent culprit in eutrophication of lakes)

and upgrading sewage treatment to reduce wastewater N and P discharges to inland

waters. Symptoms of eutrophication in estuaries and other coastal marine ecosystems

(where N is the most frequent contributor to eutrophication) were clearly evident by the

1980's, as human activities

doubled the transport of N and

tripled the transport of P from

Earth's land surface to its

oceans. Eutrophication has

emerged as a key human

stressor on the world's coastal

ecosystems.

Fig. 11. Fish Kill in the Salton Sea as a result of eutrophication

Some phytoplankton species produce toxic chemicals that can impair respiratory,

nervous, digestive and reproductive system function, and even cause death of fish,

shellfish, seabirds, mammals, and humans. The economic impacts of harmful algal

blooms can be severe as tourism is lost and shellfish harvesting and fishing are closed

across increasingly widespread regions. Proposed solutions to the eutrophication

problem are multidimensional and include actions to restore wetlands and riparian

buffer zones between farms and surface waters, reduce livestock densities, improve

efficiencies of fertilizer applications, treat urban runoff from streets and storm drains,

reduce N emissions from vehicles and power plants, ban P-containing detergents, and

further increase the efficiency of N and P removal from wastewaters.

5.4. The importance of N:P Ratio

The nutrient requirement necessary for phytoplankton growth is called the Redfield

ratio (taken from research by Redfield in 1958). This ratio relates nutrient uptake by

phytoplankton to primary production. The C:N:P ratio for phytoplankton is generally

considered to be 106:16:1. This means that for 106 moles of carbon fixed by

photosynthesis, the algal cell also requires 16 moles of N and 1 mole of P, or in grams,

about 10 g N per 1 g P; the algae cells will take this up from the water environment, or

in the case of nitrogen may also fix it from the atmosphere.

The key implication of this ratio is that the rates at which nutrients are added to a

system affect the limiting nutrient. The limiting nutrient is the one that is in shortest


supply, so that when added, it stimulates growth in phytoplankton. If the Redfield ratio

of the nutrient inputs is higher than 10:1 (N:P > 10:1), the ecosystem is considered to

be limited by phosphorus. This is the case in most fresh water lakes and rivers. In

contrast, if the ratio of the nutrient loads to an ecosystem is below the Redfield ratio

(N:P < 10:1), the system is nitrogen limited, e.g. in many coastal waters and estuaries.

Eutrophication abatement thus often concentrates on P limitation, also because for N,

there can be alternative sources: biological N fixation.

Whole-lake experiments have been carried out for experimental lakes in Canada; this

is illustrated in the PowerPoint presentation. A curtain divided the two sides of the lake

and fertilizer was added in different proportions to each side of the lake.

Read more about Canada’s whole lake experiments:

http://www.sciencemag.org/cgi/reprint/322/5906/1316.pdf?ck=nck


• Read chapter 6.5 and 7 in Chapman et al.

• Browse through your local newspaper and search for articles related to

eutrophication. See if you can find out the economic impact of eutrophication (i.e. the

cost to remove excessive sea weeds from harbour regions) and report it on the

discussion board.

YouTube illustrations

Watch the following movie on aquatic food webs:

http://www.youtube.com/watch?v=roRQQZlGIvM

Further reading

Planning and Management of Lakes and Reservoirs: An Integrated Approach to

Eutrophication:

http://www.unep.or.jp/ietc/publications/techpublications/techpub-11/index.asp#1

Please refer to the comprehensive report on the current state of lake eutrophication by

the UNEP:

http://www.unep.or.jp/ietc/publications/short_series/lakereservoirs-3/2.asp


Unit 6 – Water quality modelling

As we have seen in Unit 1 of this course, characterising the water quality of for

instance a lake is a challenging task, because there are:

• Temporal variations

• Spatial variations

• Many different water quality variables and standards

• Relations between water quality variables such as BOD/DO, N/phytoplankton, ...

One tool which can help us to get a more comprehensive view of the water quality of

lakes and rivers is mathematical modelling. Once a reliable model has been developed,

it is capable of predicting modelled water quality variables for different times and

locations.

A general approach can be seen in Fig. 12. Developing a water quality model usually

consists of two separate exercises:

• Hydrodynamic modelling: this means modelling the transport of water and

substances (by flow, advection, diffusion, ...)

• Water quality modelling: modelling conversion processes of substances (e.g.

nitrification, oxygen consumption when BOD is being degraded, ...)

One will usually start by developing a conceptual model. This includes setting the

(physical) boundary conditions (e.g. are you only interested in the lake water or do you

also want to include sediment processes), the level of detail that you are interested in

(1D, 2D or 3D; monthly average values or hourly predictions) and a selection of

relevant water quality variables (only interested in BOD-DO or also nutrients,

phytoplankton, ...). Once this has been done, the hydrodynamic and water quality

models can be developed. These cannot be used at once, but first need to be

calibrated. This means that model predictions will be fitted to real measured values by

fine-tuning model equations and/or parameter values. Once that has been done, a

validation/verification is usually required, meaning that the model results are checked

against another, independent set of measured data (i.e. different from the one used for

calibration). Then the model is ready and can be used for scenario analyses, i.e. what

happens if we reduce the nutrient load, what will happen with phytoplankton

concentrations when the temperature rises because of global warming, etc.

This Unit is an optional unit, meaning that you are free to continue looking into this

matter or not. There are also no lecture notes; only a Power Point presentation (without

audio) by dr. Kelderman and a set of seven selected peer-reviewed journal papers: six

on river water quality modelling and one on lake modelling. They are available under

Unit 6 - Supplementary Materials. There is a logical sequence in the papers, so if you

are interested, please follow the numbering.


Fig. 12. General approach to water quality modelling

Course 2: Water Quality and Pollution - un-ihe.org

Documents

Transcript of Course 2: Water Quality and Pollution - un-ihe.org