Data Summary

Total Maximum Daily Load (TMDL) - Lakes

2022 - Pennsylvania Department of Environmental Protection

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The Clean Water Act Section 303(d) establishes the Total Maximum Daily Load (TMDL) program. The purpose of the TMDL program is to identify sources of pollution and allocate pollutant loads in places where water quality goals are not being achieved. This layer shows the list of waters for which technology-based or other required pollution controls are not stringent enough to meet water quality standards. The TMDLs themselves specify a pollutant budget that must be achieved to meet state water quality standards and allocates pollutant loads among pollution sources in a watershed, e.g., point and nonpoint sources. This layer represents lakes with TMDL associated with them. Lakes have characteristics that differentiate TMDLs from other waters. Lakes are not free-flowing like streams, but are contained waters that trap pollutants for long periods. Most lake impairments are from high nutrient or sediment loads. Wherever possible, lake TMDLs are developed with the information in the lake study reports that were sponsored by local watershed groups or other local interests. Target acceptable pollutant loads are set at the level of a watershed largely unaffected by human induced impacts. Load allocations are given to the pollutant sources using the same methods as nonpoint source TMDLs. Other indicators of water quality are also considered in the evaluation of a lake. One indicator is the Trophic Status Index (TSI), which refers to the degree of nutrient enrichment in the lake. Nutrient enrichment causes growths of algae that consume oxygen and interfere with the health of the aquatic organisms in the lake. The TSI is affected by factors such as lake volume, water residence time and nutrient loads to the lake. After target loads are set, the TSI is evaluated under reduced nutrient load conditions to assure that the pollutant reductions will bring the TSI into an acceptable range. Implementation of lake TMDLs is best accomplished though local participation. This layer is based on the High Resolution National Hydrography Dataset (NHD).