Water Quality 2

Lecture series outline• Lecture 1a

– Water uses, water quality guidelines; physical, chemical and biological parameters

• Lecture 1b– The origins of the constituents of water quality through the hydrological

cycle• Constituents of rain• Influences of hydrological routing• Acid rain and critical loads

• Lecture 2a– Aquatic ecology

• habitats and niches• Stratifcation• Eutrophication• Influences of reservoirs

• Lecture 2b– Different land uses:

• Forestry• Agriculture• Urbanisation

Composition of water draining catchments

• Determined by:• Rainfall• Geology • Hydrological

characteristics• Aquatic processes• Terrestrial

ecosystems • Human interference

Dust and Sea spray

Chloride (mg/l) concentration affected by distance from the sea

Hydrological Characteristics

• Determine whether rainfall runs off or percolates into the ground.

• Susceptibility to Runoff determined by– Steepness, roughness of slope– Vegetation (interception)– Soil texture. – Ground porosity– Drainage pattern– Storm duration and intensity, direction of storm.

Hydrological Routing

The effect of rainfall

• Throughflow– Initially brief increase in the concentration of

TDS and SS.– At end of rains, decrease in concentration

weathered material washed out of the soil. • Within Rivers

– Greater the flow, the greater the erosion rate and SS, which will encourages dissolved ions into suspension

Seasonal variation of TDS

Variation of TDS during storm flow

The effect of increased periods between rains

• Allows greater time for:– Accumulation of organic substances and

dusts on the vegetation and land surfaces, – Build up of weathering and biological decay

products • Hence greater the concentration of the

above in river waters when it does eventually rain.

Effect of increased runoff on stream waters

• The greater the amount of surface water flow the less the interaction with the local geology and soils and the less dissolved ions in solution.

The effect of increased Throughflow

Dissolved ion concentrations and organic matter concentrations will reflect that of the soils

• Nutrient poor soils with little organic matter – Waters likely to reflect the character of the rain– Sands, Podzols, Oxilsols, Ultisols– Acid waters

• Nutrient rich-– water likely to reflect the character of the sediments: – Clays, Inceptisols, Alfisols, Andosols, Mollisols: – Slightly Acid or Alkaline waters

Acidity and soils• As soils and rock weathers,

– major cations released and neutralise the acids in waters. pH is then raised

• Insufficient buffering – Al released below pH 4.2, – Silicic acid released below pH 4.0 - precipitates as

clay when pH rises. Concentration of Silica is an indication of the weathering rate of the rocks.

– Phosphates • can be released but are only soluble at near neutral pH• Very strongly retained by soils, complexed by Fe and Al in

acid soils and Ca in alkaline

The effects of increased Baseflow/ Groundwater flow

• The greater the ratio of groundwater from deep percolation to that of runoff, the:– More likely stream water will reflect local

geology rather than rain water– Dominated by Na, Ca, Mg, Si, SO4, Cl, HCO3

Effect of local geology• Major source of TDS.• Effects of weathering

– Igneous rocks and metamorphic rocks– resistant > sedimentary rocks. – abundance of cations Mg>Ca>Na>K. – Metal sulphates - mineral rich veins .

– Sedimentary rocks• Less resistant• more fractured • Water acid soluble sulphates, carbonates, phosphates and Iron

– Limestones and Basic igneous rocks• high Ca levels will impart a large levels of Ca to waters

– Sandstones and acid igneous rocks • low Ca contents, little Ca in waters.

Some General Relationships Between Rock Types and Water Quality (Davisand DeWiest, 1966)

Variable; deep aquifers may yield soft water with high sodium and bicarbonate contents

SandstoneLow silica; high calcium and magnesium; pH >7.0LimestoneHigh amounts of iron and fluoride; pH 5.5-7.0ShaleVariable; salinity increases with depthSedimentary

Good to excellent; exceptions near hot springs;tends to be "calcium-magnesium-bicarbonate" water, or when acidic "sodium-bicarbonate" with high silica content

Volcanic

Slightly alkaline, higher total dissolved solids and hardnessGabbro, diorite, andesite, hornblende and gneiss

Slightly acid, lower total dissolved solids and hardnessGranite, gneiss, rhyolite, and mica schist

Dissolved silica <30 ppmQuartzite, marble, slateDissolved silica 25-55 ppmDiorite, syenite

Hardness due more to magnesium than to calcium concentrations

Serpentine, dolomite, gabbro, and amphibolite

Moderate to high hardness (calcium, magnesium)Dolomite, marble

Almost always excellent; exceptions in arid regions and coastal areas; generally high silica content; low total dissolved solids

Metamorphic and plutonic igneous

Water qualityRock type

Solubility dependant upon pH• Weathering – H+ ions replace metal ions (Ca2+, Na+, K+)

in rocks and hence resistance of different rock typesS

olub

ility

(mm

ol/l)

pH independent reactions

• Ionically bonded minerals such as halite, and sulfate minerals, such as gypsum, have simple solubility relationships which are (almost) independent of pH.

• the reaction:• NaCl = Na+ + Cl-

• Describes the solubility of sodium chloride.

Types of Weathering

• Dissolution– Congruent - Dissolution of chemicals into

constituent anions and cationse.g. Na Cl → Na+ + Cl-

• Incongruent – Formation of a new mineral as a result of

water interaction e.g. Albite to Montmorillonite3 NaAlSi3O8 + Mg2+ + 4H2O →

2Na0.5Al1.5Mg0.5Si4O10(OH)2 + 2Na+ + H4SiO4

• Hydration – Incorporation of water into the chemical compound

(water of crystalization)e.g. formation of rust

Fe2O3 + 3H2O → Fe2O3.3H2Oe.g. Gypsum

CaSO4 + 2H2O ↔CaSO4.2H2O

• Hydrolysis– Reaction of water with chemical compounds

Fe2(SO4)3 + 6H2O → 2Fe(OH)3 + 3H2SO4

CaO + H2O →Ca(OH)2 → Ca2+ +2OH-

• Carbonation– Limestone

CaCO3 + H2CO3 ↔ Ca2+ 2 HCO3-

– FeldsparsKAlSi3O8 + H2CO3 ↔ Al2Si2O5(OH)4 + K2CO3 + 4SiO2

Vulnerable to pollution Less vulnerable to pollution

Groundwater pollution.ppt

Zonation in aquifers• The upper zone –

– characterized by rain flushing solutes through the zone of aeration in to the zone of saturation

– typically water in the zone is HCO3- rich (Soil CO2 and calcite)

and is low in TDS. • The intermediate zone –

– slower rate of groundwater flow and higher TDS. – Sulfate becomes the dominant anion (Dissolved from minerals

gypsum (CaSO4. 2H2O) and anydrite (CaSO4). • The lower zone

– with very slow rates of groundwater migration. – large amounts of soluble minerals- very little groundwater

flushing has occurred. – Typified by high Cl- concentrations and high TDS.

Chebotarev Sequence

Chebotarev Sequence

• Proceeds in groundwaters with – Increasing distance– Increasing residence time or

age

• Mineral solubility • Mg/ CaCO3 < CaSO4 < (NaCl, KCl)

HCOHCO33--

↓↓HCOHCO33

-- + SO+ SO22--44

↓↓SOSO22--

44+ HCO+ HCO33--

↓↓SOSO22--

44+ + ClCl--

↓↓ClCl-- + SO+ SO22--

44↓↓ClCl--

Typical concentrations of elements in dilute

oxygenated groundwater at pH 7 and their significance in terms of health and

environmental protection.

Acid Rain• Source Atmospheric Pollution, wet and dry

deposition• NOx and SO2 – burning of fossil fuels, fertilizers

and smelting• S compounds 65 million t /yr released• NOx pollution increasing and currently accounts

for half all NOx inputs• Combined effect is to reduce rain, pH 2.1 has

been recorded in Europe.• Snow worsens the problem

Causes

Effects

Acid Flushes

External sources of Acid Rain

External acid rain source % of total acid rain sources

pH scale in relation to ecosystems

Aquatic organisms tolerances to acidity

Interferes with biochemical pathways of organisms

Acid Rain Politics

• 1979, Convention and resolution on long range transboundary air pollution,– 34 EU and N. American Countries– BATNEEC used

• 1983, 21 European countries committed to reduce SO2 emissions by 30% by 1993. – UK not a signatory but did declare to meet

this target before 2000

1988 EC large combustion plants directive

• Reduction of SO2 1990 levels by 58% by 2003

• Reduction of NOx and particulates by 40% by 1998.

• Each EU state had separate targets.

Critical Loads

• Load that can be applied to an ecosystem without resulting in deleterious effects

• Next set of atmospheric emissions negotiations concentrate on the critical loads approach

http://www.environment-agency.gov.uk/yourenv/eff/pollution/acid_rain/?lang=_ehttp://www.nbu.ac.uk/negtap/finalreport.htm

Critical loads Calculation• CL(A) = ANCw – ANC le(crit)

WhereCL(A) = critical loads of acidity (Keq/ha/yr)ANCw = Acid neutralising capacity produced by

weathering (Keq/ha/yr)ANCle = critical leaching of ANC (Keq/ha/yr)

• ANC = sum of cations mainly Ca2+

• Exceedence of critical loads in 1995- 1997 covered 71% of sensitive ecosystems in the UK and is expected to be 46% in 2010

• NOx deposition will be the main problem in 2010

• Some evidence of recovery in UK upland soils and rivers

The effect of Organisms, vegetation and soils

The effect of Organisms, vegetation and soils

Vegetation – organic compounds from leaves roots and decaying material.

Uptake by plants of nutrients – Storage of minerals and nutrients in plant biomass, will have a

seasonal effect on the concentrations of some nutrients. . – Entrapment of atmospheric dusts– Fixation of N– Binding of sediments– The litter layer effectively protects the underlying soils from

rainfall splash and erosion.– Mediating Temperature

• Organisms– Faeces, urine and other excretions from animals– Anaerobic bacterial activity may reduce some metals and make

them more likely to leach into water bodies.

Ecological sucession

Establishment -biomass of catchment builds, retention of elements begins

Climax - as much of an element that enters the system through rainfall or weathering leaves the catchment in stream water.

Disturbances Man, fire, earthquakes, volcanoes, hurricanes. A pulse of dissolved components and sediments

TDS

(log

sca

le)

Time

Hubbard Brook experiment.

• Catchment; granite with some shales• Lots of individual catchments considered

to be impermeable• Catchments covered in natural broadleaf

forest• Nutrient loss via streams

http://lternet.edu/sites/hbr/http://www.hubbardbrook.org/

Reference watershed

Undisturbed forested catchments

• Weathering supplies most of the Ca, Mg, Na, K and P. • Rain and snow bring large amounts of Na, N, and S.• Most Ca, Mg, Na and S is washed out in dissolved form-

plants have little restraining effect on these elements.• K and N are mostly retained by the ecosystem, though

20-30% are lost in stream water.• P is very strongly held in the ecosystem, less than 1%

lost in stream water.• Comparing the rain water input to the drainage water

there is a net gain of Na, K, Mg and Ca. but a loss of N, P and S in stream water.

Comparison of forest and moorland

Other Vegetation effects on water quality

• Some vegetation may produce pollutants e.g.– Bracken- spores are a stomach irritant– Conifers - phenols are leached which leave

an unpleasant taste – Peaty moors produce coloured waters which

may require treatment.