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