DEVELOPMENT OF A TMDL FOR NUTRIENTS AND SEDIMENT FOR STANDING BEAR LAKE

The Nebraska Department of Environmental Quality sponsored the development of a Total Maximum Daily Load (TMDL) for Standing Bear Lake (Omaha, Nebraska), for use as a training tool and template for developing additional TMDLs. This paper describes the development of proposed TMDLs to address impairment by organic enrichment/low dissolved oxygen and sediment, and highlights some of the challenges associated with selecting an appropriate TMDL endpoint and developing a TMDL with little site-specific data.Standing Bear Lake, a 135-acre flood control reservoir, was listed on the 1998 Nebraska Section 303(d) List of Impaired Waters due to organic enrichment and low dissolved oxygen, and was also listed on the 2002 Nebraska 303(d) List of Impaired Waters due to excess sedimentation. As such, it was targeted as a priority water for TMDL development. Because it was among the first in the state, the Nebraska Department of Environmental Quality (NDEQ) is using this TMDL as a template for the approximately 97 waterbodies in the State needing 116 TMDLs.One of the critical components of this TMDL was the selection of numeric endpoints, the targets used to evaluate the attainment of acceptable water quality. Selection of a numeric endpoint can be confounded when numeric standards do not exist for the constituent of concern, and/or there is not a directly defined relationship between the loading constituent and the water quality constituent of concern. Both of these confounding factors will typically be in place for nutrient and sediment TMDLs, and require selection of surrogate parameters designed to protect the beneficial uses. Rigorous assessment of the relationships between nutrient loading, algal growth, and resulting dissolved oxygen depletion would have required a much larger quantity of data than was available for Standing Bear Lake and is likely available for other Nebraska lakes. A Trophic Status Index (TSI) utilizing phosphorus concentration, Secchi depth, and chlorophyll-a was selected as a surrogate parameter to represent organic enrichment/oxygen depletion, based on NDEQ's existing procedures and management objectives for the lake. For sedimentation, capacity loss was the primary concern. In the absence of a water quality standard for sedimentation or capacity loss, NDEQ set a target of 0.75% loss/year.Pollutant sources consist solely of nonpoint sources. While undeveloped and cropland areas had historically comprised the majority of the land uses, conditions were changing rapidly, with residential and other development on the rise. Pollutant loads were estimated based on limited observed data, as well as through the application of a watershed model. The Generalized Watershed Loading Function (GWLF) model was selected as the appropriate watershed model, based on consideration of management objectives, available data, personnel resources, and budget/schedule constraints. Application to existing conditions determined that construction and cropland were the primary sources of phosphorus and solids loads.The BATHTUB reservoir model was applied to define the relationship between pollutant loading and resulting water quality in Standing Bear Lake. BATHTUB provided the best balance between management objectives and available data. The results showed a somewhat surprising result: as land uses shift in the future from construction and cropland (relatively high pollutantgenerating uses) to residential (relatively lower generating use), the water quality objectives are expected to be achieved. Thus, the only control required was use of best management practices for construction activities during future development in the watershed.