Empirical decryption of point source and nonpoint source loading in the context of a total phosphorus TMDL

Abstract

ABSTRACTLehman JT. 2016. Empirical decryption of point source and nonpoint source loading in the context of a total phosphorus TMDL. Lake Reserv Manage. 32:259–269.A graphical and linear regression method was used to distinguish among point sources, nonpoint sources, and in-channel sinks of nitrogen, phosphorus, and other nutrients, relying on the observation that point-source loads and nonpoint-source loads are not correlated. Point-source loads from wastewater outfalls did not change proportionally with streamflow, whereas landscape erosion did. Point sources and sinks could be deduced from changes in the intercept of linear regressions of in-stream loads of nutrients at different points along the stream channel, whereas deviations of the slope from 1.0 revealed nonpoint source loading or diversions. The method demonstrates (1) how total phosphorus and total nitrogen released from a wastewater treatment facility is sequestered within macrophyte beds downstream from the outfall, and (2) how reductions in nonpoint source loading of total phosphorus coincident with a lawn fertilizer ordinance can be quantified. A related application of the method was used to quantify reduced internal loading and increased retention of dissolved phosphorus, total phosphorus, and silica in a polymictic impoundment after it was subjected to artificial mixing to prevent anoxia. The results revealed conceptual flaws in a total maximum daily load model for the watershed and provided information to improve the model. Regulatory restrictions on point-source discharges are ineffective when point-source nutrients are sequestered upstream of a lake that exhibits nuisance algal blooms and when most of the external nutrient loading to the lake of concern derives from runoff and other nonpoint sources across the landscape.