The Federal Clean Water Act requires states to develop Total Maximum Daily Loads (TMDLs) for lakes and rivers that are not meeting water quality goals. A TMDL is an estimate of the amount of a pollutant that a body of water can receive without impairing vital uses, such as drinking water supply or support of aquatic life. Because excess phosphorus from a variety of sources has impaired water quality in many parts of Lake Champlain, the states of Vermont and New York developed a phosphorus TMDL in 2002.
In 2011, the EPA disapproved the Vermont portion of the TMDL based on two concerns: the TMDL did not provide sufficient assurance that phosphorus reductions from polluted runoff would be achieved, and there was not an adequate margin of safety to account for uncertainty in the original analysis (particularly for four segments of the Lake: Missisquoi Bay, St. Albans Bay, Northeast Arm, and South Lake). EPA collaborated with the Vermont Department of Environmental Conservation and Agency of Agriculture, Food and Markets to establish a new TMDL for Vermont Segments of Lake Champlain in June 2016. The 2002 TMDL remains in place for New York.
The state of Vermont released a Phase I Implementation Plan, which provides a framework for how phosphorus loading limits established in the TMDL will be met. The Phase I plan includes policy commitments, including enhanced regulatory programs, funding and financial incentives, and technical assistance, that address reduction of nonpoint sources of pollution at a basin-wide scale. Phase 2 Tactical Basin Plans identify in more detail the specific measures and management practices to be implemented to achieve limits for each lake segment.
The original Lake Champlain phosphorus TMDL was based on phosphorus loading and in-lake concentration targets defined in a 1993 Water Quality Agreement signed by New York, Vermont, and Québec. Loading targets were updated with those established by the 2016 Vermont TMDL. Targets and actual in-lake concentrations for each segment are updated in the State of the Lake report.