Eutrophication - Michigan Technological University · 2011-09-28 · Eutrophication Eutrophication:...
Transcript of Eutrophication - Michigan Technological University · 2011-09-28 · Eutrophication Eutrophication:...
Eutrophicationh f b i b i d hiEutrophication: the process of becoming or being made eutrophic
Eutrophic: the state of being enriched in nutrients or food sources
In aquatic ecosystems, eutrophication is caused by excessive inputs of nutrients, both N & P. Generally, freshwaters are P-limited and coastal estuarine waters are N-limited. The nutrients enhance algal growth, and this, in turn, may have a cascade of effects on thegrowth, and this, in turn, may have a cascade of effects on the ecosystem. These effects may include: algal blooms, growth of undesirable algal species, oxygen depletion or anoxia in bottom waters loss of cold water fish species abundance of “rough fish”waters, loss of cold-water fish species, abundance of rough fish , fish kills, unpleasant tastes and odors.
Sources of nutrients
• Point sources– Sewage treatment plant discharges– Storm sewer discharges– Industrial discharges
• Non-point sources– Atmospheric deposition– Agricultural runoff (fertilizer, soil erosion)– Septic systems
Solution: Reduce nutrient inputs
• Agriculture– Reduce animal density, restrict timing of manure spreading,
buffer strips by streams, reduced tillage, underground fertilizer application, wetland preservation and construction
• Watershed management– Buffer zones, wetland filters
• Storm runoff• Storm runoff– Eliminate combined sewer systems (CSO’s)– Stormwater treatment required (holding ponds, alum, etc.)
d i d f ili i– Education on yard fertilization• Erosion from construction, forestry
– Erosion barriers, soil cover, road and bridge stabilizationErosion barriers, soil cover, road and bridge stabilization• Septic systems
– Distance from lake, adequate drainfields
Mitigation strategies
Often there is pressure for quick actions that will reduce the severity of the symptoms. y y pNumerous options exist. To understand these options and choose among them, one p g ,should understand the nutrient cycle within the aquatic system (lake).q y ( )
P Cycle
The P cycle may beInorganic
POrganic
P
Epilimnion
Settio
n
Uptake
The P cycle may be manipulated in several ways to p
InorganicP
OrganicP
tling
Dis
pers
Mineralization
reduce the regeneration of inorganic P and its
SedimentHypolimnion
P PS
ettlingg
transport to the epilimnion or to reduce the algalSediment
InorganicP
OrganicP
Bu
Mineralization
Bu
reduce the algal uptake of P.
urial
urial
Within-lake actions
• Reduce algal growth– Apply algicide– Biomanipulation
• Reduce mineralization– Remove organic P before it is mineralized
• Dredging• Macrophyte harvesting• Macrophyte harvesting
• Reduce transport of inorg. P to epilimnion– Hypolimnetic water withdrawalHypolimnetic water withdrawal
In-lake strategies cont.
• Reduce P release from sediments– Sediment amendments (NO3
-, Fe oxides, alum)Sediment amendments (NO3 , Fe oxides, alum)– Hypolimnetic aeration– Artificial circulationArtificial circulation
P release from sediments is greatly enhanced by anoxic conditionsP release from sediments is greatly enhanced by anoxic conditions under which iron oxides dissolve and release all P sorbed to their surfaces. Maintaining oxic bottom waters not only retards P
l f di b l h l i i b hi d fi hrelease from sediments but also helps maintain benthic and fish species.
Useful references
• http://www.aquatics.org/pubs/madsen2.htm• McComas S 1993 LakeSmarts: the firstMcComas, S. 1993, LakeSmarts: the first
lake maintenance handbook, Terrene Inst., Washington D CWashington, D.C.
Macrophyte harvestingMacrophyte harvesting
Macrophyte harvesting
Harvesting
Lake aeration
Below: Bubbles rising to f i i h lsurface in winter when large-
diameter air bubbles are released from diffusor. Lake Sempach, Switzerland.
Above: 10-m diameter diffusor being lowered to bottom (87 m) of L. Sempach, Switzerland.
Aeration continued
http://www.northstarfishhatchery.com/html/equipment.htmly q p
htt // itt h /http://www.rittenhouse.ca/
http://www.windmillaeration.com/
Models of P CycleAtmosphericdeposition
Inorganic Organic
p
River inputs Riveroutflow
Point-source InorganicP
OrganicP
Epilimnion
Settling
Dis
pers
ion
UptakeTotal PPoint sourceinputs
Nonpoint-sourceinputs
Hypolimnion
InorganicP
OrganicP
Settling
MineralizationSettling
DISSOLVED
SedimentHypolimnion
InorganicP
OrganicP
B
Mineralization
B
Inorganic(DIP orSRP)
InputsBacteria
PARTICULATE
urial
urial
Phytoplankton(POP)
Outflow
Organic(DOP)
non-living
Settling Resus-pension
non living
Eutrophicaton modelingg
dC ?dCV W QCdt
= − −Atmosphericdeposition
sdCV W QC v ACdt
= − −Total P
River inputs Riveroutflow
Point-source Total P
Settling
inputs
Nonpoint-sourceinputs
Settling
Steady State Solutiony
dCV W QC v AC= − −
W WCQ A
= =+
sV W QC v ACdt
J J Jsa Q v A+
W s flushing ss
H
J J JC Hq v H k v vτ
= = =+ ⋅ + +
?s
AC Q vA
= =+
Hτ
sA
( )( ) HJ C q v C Hk v C v⎛ ⎞
= + = + = +⎜ ⎟( )( )s flushing s sH
J C q v C Hk v C vτ
= + = + = +⎜ ⎟⎝ ⎠
Vollenweider Model10
Eutrophic1
(gP/
m2 yr
)Eutrophic
0.01
0.1J
OligotrophicEmpirically:C = 10 mg/m30.01
1 10 100 1000
H*kflushing (m/yr)
g p Cmeso 10 mg/mCeutro = 20 mg/m3
vs = 10 m/yr
R.A. Vollenweider awarded Tyler Prize for Environmental Achievement, 1986, “The Vollenweider Model has subsequently been adopted as the basis for eutrophication control programs of
http://www.usc.edu/dept/LAS/tylerprize/vollenweider
been adopted as the basis for eutrophication control programs of most countries of the western world.”
Example: What is an allowable l di t i t i P i T h Lloading to maintain P conc. in Torch L.
at 15 mg/m3?10
H = 15m1
P/m
2 yr) Kflushing = 1/yr
0.1J (g
P
0.011 10 100 1000
H*kflushing (m/yr)
Example Problem
Wh t i ll bl l di t T h L k ?What is an allowable loading to Torch Lake?
3 2 2
1515 10 375 0.375sH mg m m mg gJ C v
⎛ ⎞ ⎛ ⎞= + = + = =⎜ ⎟ ⎜ ⎟3 2 21.0s
w m yr yr m yr m yrτ⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠
Loading to TLUAL: Forest – 0.01 kg/ha-yr = 0.001 g/m2yr
Agriculture – 1.7 kg/ha-yr = 0.17 g/m2yrUrban – 1.3 kg/ha-yr = 0.13 g/m2yr
0.02 g/m2yr3.56 g/m2yr2 67 g/m2yrUrban 1.3 kg/ha yr 0.13 g/m yr
Torch Lake Catchment:Lake area ratio = 20:1
2.67 g/m yr
Example 1.
A lake is found to suffer from excessively high nutrient y gconcentrations, and the eutrophic condition of the lake impairs its use for drinking water and contact recreation (swimming), two of this lake’s designated uses As a result of the watertwo of this lake s designated uses. As a result of the water quality impairment, a Total Maximum Daily Load (TMDL) study was conducted and a TMDL plan is now being implemented. As part of this plan, stormwater detention ponds are being constructed for the urban section of the catchment. The primary purpose of the detention ponds is to reduce the p y p p pphosphorus runoff that reaches the lake.
E.g. 1 cont’d
The design goal for one particular stormwater detention pond is to remove (i.e., retain) 85% of the phosphorus inputs to the pond. Phosphorus is retained in the pond through burial of both macrophytes and algae as well as limited harvesting of macrophytesburial of both macrophytes and algae as well as limited harvesting of macrophytes. Phosphorus removal in the pond occurs via a first-order reaction with a settling velocity (v) of 50 m/yr. dPV W QP vPA= − −
In this equation, V is volume (m3), P is phosphorus concentration (mg/m3), t is time, W is phosphorus loading or input (mass/time), Q is flow (volume/time), v is the rate
V W QP vPAdt
constant (expressed as a velocity) for the internal processes that remove phosphorus from the water, and A is the area of the pond. The constraints on the design of the pond are the phosphorus loading to the pond and the water inflow rate. The pond
i d i f 5 k 2 f id i l Th i l d f h hreceives drainage from 5 km2 of a residential area. The unit area load of phosphorus from this urban area is 0.78 kg/ha-yr, and the annual water inflow is 2.44 million cubic meters.
What area and depth of pond are needed to meet the design goal?
Example 1: Simple P modelCatchment inputs River
outflow dCV W QC v ACTotal P
outflow
@ :
sV W QC v ACdtSS
= − −
Settlings
WCQ v A
=+
15%in
C transfer CoefficientC
β = = = ⋅
Water inflow: 2.5x106 m3/yrKnowing the P loading and the water inflow, one can calculate Cin:
P loading: (0 78 kg/ha yr)(5 km2)(106 m2/km2)(103 g/kg)(10-4 ha/m2)P loading: (0.78 kg/ha-yr)(5 km2)(106 m2/km2)(103 g/kg)(10 4 ha/m2)Cin = (P loading)/(water inflow) = (3.9x105 g/yr)/2.5x106 m3/yr) = 0.156 g/m3
Knowing the transfer coefficient and Ci one can calculate C:Knowing the transfer coefficient and Cin, one can calculate C:C = Cin β = (0.156 g/m3) (1-85%) = 2.34x10-2 g/m3
37
3901 67 10
kgW myrA i il i C i
At this point we also know the assimilation capacity:
7
63
1.67 1023.4 10
W myrAssimilation Capacity a xmg kgC yrm mg
−⋅ = = = =
⋅
We can also now calculate the area required for the pond:WC
Q A=
5
1
3 9 10
s
s
Q v AWA QC v
g
+
⎛ ⎞= −⎜ ⎟⎝ ⎠
⎛ ⎞53
6 4 2
3
3.9 1012.5 10 28.3 10
0.0234 50
gxmyrA x x mg myr
m yr
⎛ ⎞⎜ ⎟⎜ ⎟= − ⋅ =⎜ ⎟⎜ ⎟⎝ ⎠
Pond depth can be selected based on other criteria (e.g., expense f i d h b d k d h bl fof excavation, depth to bedrock, depth to water table, safety,
topography, variability in rainfall, etc.). A typical depth would be 0.5-2 m. A depth of 1.5 m is used in the calculations below.
Hydraulic residence time = V/Qo = 2.0 months
p