CIV 913 Environmental Assessment and Sustainability Eutrophication Eutrophication of Freshwaters - Harper D Freshwater Ecology - various Limnology - various

Eutrophication Objectives Definition Causes Limnology and Lake Ecology Effects Control Strategy Definition The enrichment of waters by inorganic plant nutrients.

Eutrophication Cause - sources of Nitrogen and Phosphorous. External Municipal and Industrial wastewaters. (Main source of Phosphorous) Land run-off. (Main source of nitrogen) Atmospheric Deposition. Internal Nutrient regeneration from bottom sediments. Groundwater seepage (sub-surface flow) Historical incidence recent (demographic growth - consumerism)

Eutrophication Limnology Lake vs River renewal time years vs days Stratification in Lakes. EPILIMNION THERMOCLINE HYPOLIMNION

Eutrophication Limnology Stratification. Formed by temperature gradient. Most of the heat from light penetration is absorbed in top 1 or 2 metres. Wind gives rise to mixing to form: epilimnion at the top hypolimnion at the bottom a transitional zone, the metalimnion. in which a thermocline exists. Temperature range may be: 20`C to 4`C in temperate lakes. 29`C to 25`C in tropical lakes (but can be equally stable stratification)

Eutrophication Limnology Nutrients in lakes Nitrogen fixation, sediment denitrification Internal Phosphorus Cycling Forms of P bound to Ferric hydroxides bound to Calcite (CaCO3) or hydroxyappetite (Ca5OH(PO4)3 bound to clay released by extreme pH, change in redox (anaerobic)

Eutrophication Limnology Trophic classification of Lakes Ultraoligotrophic Oligotrophic Mesotrophic Eutrophic Hypertrophic Numerical Classification Trophic State Index (TSI) scale 0 - 100 by Secchi depth - 64m= 0; 32m= 10; 16m=20; etc see OECD categories

Eutrophication Limnology OECD Trophic Categories CATEGORY Ultraoligotrophic Oligotrophic Mesotrophic Eutrophic Hypertrophic P 4  10 10 - 30 35 - 100 100 Chl.  1  2.5 2.5 - 8 8 - 25 25 Max Chl.  2.5  8 8 - 25 25 - 75 75 Secchi (m) 12 6 6 - 3 3 - 1.5  1.5 Secchi (min) (m) 6 3 3 - 1.5 1.5 - 0.7  0.7

Eutrophication Productivity Rates of Primary Production in Lakes. Oligotrophic Eutrophic Natural Polluted Mean rates in growing season. (mgC/m2/d) 30-100 300-1000 3000 - 15000 Annual Rates (mgC/m2/d) 7-25 75-250 350-700

Eutrophication Prediction of Water Treatment Plant Problems. UK study in 1960s by Lund to predict effects: Winter maximum PO4 > 5g/l Winter maximum NO3 > 300 g/l Produces Algae > 3000 cells/ml Models in 1960’s by Vollenweider where TP is total phosphorus L is surface loading of P z is depth p is flushing (renewal per year) O is sedimentation rate coefficient of P

Eutrophication Predicting Permissible P Loading Using OECD Formulae. Developed relationships between: Chlorophyll A (annual mean and maximum),[Chl] P inlet concentration [P]i and hydraulic residence time Tw [Chl]mean = [P]i / (1+(Tw)0.5) mg/m3

Eutrophication Effects. Freshwater. Fish diversity reduced. Low/no DO in hypolimnion, hence reduced fauna and flora diversity. Algal blooms and adverse aesthetics. Algal blooms and water treatment difficulties. affects drinking water quality and treatment costs.

Eutrophication Effects Seawater. Algal blooms. Red tides (phaecocystis) and toxins affect coastal fisheries. Corals. Suffocated by algal sedimentation. Macrophytes in shallow coastal waters. Increased biomass (fish).

Eutrophication Adverse Effects of Algae in Water Treatment physical blocking of filters 3000cells/ml detrimental polysaccharides chelate Fe and Al ions (enter treated water) THM production Taste and odour toxins animal infestation in distribution system industrial ion exchange poisoned deposits block valves

Eutrophication Water Quality Objectives for Lakes. Must take account of intended use. Develop a nutrient load control strategy. Using algal biomass as a trophic response indicator: set target for mean algal biomass set target for peak algal biomass Determine phosphorous load to be removed. Control point sources, then diffuse sources.

Eutrophication Typical Controls. Municipal sewage treatment. chemical precipitation biological removal combinations. Pre-reservoirs (>15 day HRT, aerobic) Chemical precipitation in the lake. High flow-through lake. 3 - 5 day HRT. Hypolimnetic aeration. Artificial water circulation. Land use practices. Removing polyphosphates from detergents Flushing Dredging see UWWT Directive

Nutrient Removal - Standards - UWWT Directive (1991): Pop >10,000 N<15mg/l P<2mg/l Pop >100,000 N<10mg/l P<1mg/l or 80% removal of Total P 70 - 80% removal of Total N (The above applies to “sensitive waters”)