Case Study of Impact of Total Maximum Daily Load Allocations on Nitrate Leaching

Abstract

An analysis of the impact of total maximum daily load (TMDL) allocations on the potential leaching of nitrates to groundwater in the Muddy Creek/Dry River watershed of Virginia is presented. The Muddy Creek/Dry River watershed has experienced nitrate impairments to both surface water and groundwater. Using the Nitrate Leaching and Economic Analysis Package, this work estimates the change in the potential mass of nitrate leaching into the subsurface, given a range of feasible load allocations developed in the nitrate TMDL of this watershed. Modeling analysis suggests that TMDL allocations would reduce the leaching mass and concentrations by 8.5% on average in the Muddy Creek watershed. Furthermore, while feasible load allocations performed similarly with respect to surface water reductions, the selected TMDL allocation scenario provides only half the leaching reduction of other feasible allocation scenarios. While expanding the scope of this TMDL to also protect the subsurface water resources would have been a reasonable and prudent management approach, the expanded complexity was impossible within the short completion schedule allowed for TMDLs.