Eutrophication TMDLs


Eutrophic conditions, indicative of excessive algae production, are the predominant cause of impairment in Kansas lakes. When an overabundance of nutrients enter the lake, algal growth increases significantly. Numerous negative effects occur ranging from taste and odor problems in drinking water to objectionable algal growth interfering with recreational activities.

The purpose of the Eutrophication TMDLs is to reduce the amount of nutrients entering the lake so that the designated uses of the lake are supported. For primary contact recreation (i.e., swimming and domestic water supply), the 12 ug/L chlorophyll a target is implemented. The 20 ug/L concentration of chlorophyll a is used for secondary contact recreation (i.e., fishing). Developed by the KDHE Lake Monitoring Program, these endpoints were determined through the use of visual assessment, water quality, and water clarity data. The Carlson Trophic State Index provides the basis which ties chlorophyll a levels (serving as a metric of algal abundance) with total phosphorus which tends to fuel algal productivity.

Before nutrient load reductions are calculated, the nutrient that is causing the impairment needs to be determined. Algal production can be effected by numerous factors in its environment including but not limited to phosphorus and nitrogen concentrations, carbohydrate availability, oxygen, carbon dioxide, and light. For TMDL development, the focus is placed on the limiting factor (phosphorus, nitrogen, a combination of phosphorus and nitrogen, or light). Light tends to be the limiting factor in lakes with excessive silt and suspended organic matter. The most accurate way to determine the limiting factor for algal growth is to do a bioassay that tests the growth response of the algae to the nutrient(s) and light. Because it is not feasible to bioassay each lake in a large monitoring network, calculating the limiting factor with data over the period of record is the commonly used method.

Light, phosphorus, and nitrogen limitations are determined through graphing the deviations of the trophic state variables. For all Eutrophication TMDLs, a total phosphorus reduction is required to meet the chlorophyll a endpoint. If nitrogen is indicated to be more important to algal growth, a nitrogen concentration endpoint would be put in place as well. If a light limitation is indicated and the lake is classified as being Argillotrophic, then a siltation TMDL is written.

The Army Corps of Engineers' BATHTUB/CNET spreadsheet model is used to calculate the total phosphorus load to the lake. Information that is input into the model comes from mapping, water quality monitoring data, and precipitation, evaporation, runoff, and atmospheric phosphorus data. Therefore, the in-lake total phosphorus is used to back-calculate mean phosphorus loads expected from the watershed. The loading capacity within the lake is determined from modeling results relating the necessary phosphorus concentrations corresponding to 12 or 20 ug/L chlorophyll a. Typical total phosphorus concentrations range between 40 and 50 ug/L. A ten percent margin of safety is used for all lake TMDLs. The remaining 90% is divided between nonpoint source pollutants and point source pollutants if applicable.

The point source contribution is derived from monitoring data from the waste treatment plants and other point source pollution contributors. When effluent discharge data is not available, the following concentrations are used to calculate the wasteload allocations for waste treatment plant lagoons and municipal mechanical plants:

Facility Type Total Phosphorus Total Nitrogen
Waste Treatment Plant Lagoon 2.0 mg/L 7.0 mg/L
Mechanical Plant: Trickling Filter 3.5 mg/L 20.0 mg/L
Mechanical Plant: Activated Sludge only fully nitrify 3.5 mg/L 25.0 mg/L
Mechanical Plant: Activated Sludge fully nitrify and de-nitrify 3.5 mg/L 10.0 mg/L

The waste load allocations of industrial facilities are calculated based on baseline design standards for that specific type of facility. If design standards for total phosphorus and total nitrogen are not in place, then best professional judgment is used.

Prior to 2002, for nitrogen limited lakes, total nitrogen loading was determined through modeling based on generic areal loading coefficients. Regional coefficients for different parts of Kansas were developed by the KDHE Lake Monitoring Program. The amount of different types to land use (cropland, grassland, urban, woodland, and water) and regional coefficients were input into the model. The total nitrogen load was calculated. As with the phosphorus loading, a ten percent margin of safety was used.

Since 2002, a new model has been used to determine the current, in-lake nitrogen concentration and to calculate how much of a nutrient reduction is need to meet water quality standards. The model, which was developed by Ed Carney of the KDHE Lake Monitoring Program, uses an average chlorophyll a concentration to estimate the nitrogen concentration of a lake or wetland. Ed ran a regression on the 2000 and 2001 lake data and 1997 to 2000 wetland data. The following total nitrogen to chlorophyll a model will be used in all lake and wetland TMDLs requiring a total nitrogen endpoint:

Log TN = 0.4738(Log Chl) + 2.2807


Dissolved Oxygen, pH, and
Aquatic Plants TMDLs

There are a number of lakes on the 303(d) list with impairments cited for eutrophication, dissolved oxygen, pH, and aquatic plants. In such cases, the impairments are bundled together because all of these impairments are linked to elevated nutrient levels. The TMDLs are developed based on the belief that nutrient level decreases would induce lower algal productivity with corresponding reductions in incidents of depleted dissolved oxygen and elevated pH. Reduced nutrient availability also limits uptake by aquatic plants and tempers their growth. In all these bundled TMDLs, the desired endpoint is to reduce the average summer chlorophyll a concentrations so that the designated uses are achieved. The implementation measures would work toward reducing the limiting nutrient(s).

Dissolved oxygen problems arise within the water column of lakes through the decomposition of organic matter. Excessive algal growth in the water creates dissolved oxygen problems in three ways. First, the obvious crash of the algal bloom places dead and decomposing organic matter within the water column, exerting an oxygen demand as the decomposition process ensues. The second, more subtle impact on oxygen is the shading effect the near-surface plankton have on algae at lower depths. Effectively blocking sunlight from reaching those lower depths shifts the biological process of the deeper algae from oxygen production to oxygen uptake through respiration. Finally, the growth stemming from primary productivity is driven during daylight hours by the presence of sunlight, but the resulting biomass reverts to oxygen demanding respiration during the night. Reductions in nutrients, particularly phosphorus, should result in diminished algal growth and primary productivity, thereby lowering the amount of organic matter which the lake water must assimilate through the decomposition process, using its oxygen reserves dissolved within the water column.

Levels of pH typically rise above 8.5 under vigorous photosynthesis. Photosynthesis drives the biological system by converting carbon dioxide and water through sunlight into sugar and oxygen. An additional end-product from the photosynthesis process are hydroxyl ions, stripped of hydrogen atoms in the production of glucose. Therefore, not only is carbon dioxide taken up from the water column, where it tends to form carbonic acid with disassociated hydrogen ions, but the addition of the hydroxyl ions in combination with bicarbonate ions in the water column raises pH levels. Explosive primary productivity driven by photosynthesis and results in pH rises above the desired 8.5 level. Therefore, more moderate productivity, in terms of rate and biomass volume, induced by lower available nutrients should yield more temperate rises of pH and maintain conditions within the 6.5-8.5 level expressed as water quality standards.

Although they may be a nuisance, aquatic plants are less of an impact to the designated uses of a lake than algal blooms are. The aquatic plant community provides shoreline protection and habitat for fishes and other aquatic life. Lakes are considered impaired for recreation only if aquatic plants cover greater than 70% of the lake surface. The growth of aquatic plants can be reduced to acceptable levels (30 to 40% cover) if the nutrient level is reduced. Generally, total phosphorus levels less than 50 ppb tended to maintain healthy plant communities where macrophyte restoration was the goal. Greater total phosphorus levels tended to allow nuisance species to re-invade, or plants to succumb to algal blooms shading them out.


Bibliography


  • Carlson, Robert E. 1991. Expanding the Trophic State Concept to Identify Non-Nutrient Limited Lakes and Reservoirs. Enhancing the States' Lake Management Programs, p. 59-71.


  • Carney, C. E. 1999. Lake and Wetland Monitoring Program Report. Kansas Department of Health and Environment.


  • Carney, C. Edward 1999, Requested information on the two TMDL "review themes" you received from EPA which relate to lakes [Memorandum] 5 Aug. 1999


  • Liscek, Bonnie C. 2001, Reference for Determining Limitation/Co-Limitation of Nutrients [Memorandum] 18 Jun. 2001


  • Liscek, Bonnie C. 2002, Total Nitrogen Modeling[Memorandum] 12 Apr. 2002


  • North American Lake Management Society. 2001. Managing Lakes and Reservoirs. p. 160-163.


  • Stiles, Thomas C. 1999, Rationale and Reference to Selected TMDL Issues [Memorandum] 6 Aug. 1999


  • Stiles, Thomas C. 2000, Relationship between Total Phosphorus, Dissolved Oxygen, pH, and Aquatic Plants [Letter] 10 Aug. 2000


  • Tate, Michael B., Mueldener, Karl W., Geisler, Rodney R., and Dillingham, Edward W. 2002. Wastewater Stabilization Lagoons - Are They Still an Option?. Kansas Department of Health and Environment.


  • Wetzel, R. G. 1983. Limnology, Second Edition. Saunders College Publishing, New York.