From TMDLs to PRBs Using Groundwater Models to Evaluate Nitrogen Management Strategies

Nitrogen loading to groundwater from surface activities has caused degradation of aquifer and surface water quality. Major research programs are in place to evaluate and manage nitrogen loading to regional estuaries including Chesapeake Bay. In Suffolk County, New York, an ambitious Comprehensive Plan was published in 2015 that addresses groundwater and surface water quality impairments resulting from nitrogen loading.

Groundwater models are being utilized to better understand the impact of nitrogen reduction strategies such as sanitary sewering. As part of a total maximum daily load (TMDL) for nitrogen for the Forge River Estuary, nitrogen loading models have been coupled with groundwater models to simulate the fate and transport of parcel-specific nitrogen loading. Where development opportunities exist within Suffolk County, groundwater modeling analyses were completed to evaluate the impact of housing density on shallow water supply wells. Utilizing groundwater models allows for a better understanding of the impact of historic land uses on aquifer nitrogen concentrations and ultimate discharge to receptors (supply wells or surface water).

Permeable Reactive Barriers (PRBs) are designed to intercept and remediate groundwater contaminant plumes. PRBs have generally proven to be an effective and sustainable technology, with recent applications demonstrating their effectiveness at reducing elevated concentrations of nitrate in groundwater. Groundwater models are being used to help design PRBs in Suffolk County by evaluating the zone of capture and anticipated nitrate load reduction.

In both of these applications, and in most groundwater-based analyses, the slow movement of groundwater and the highly dynamic nature of aquifer withdrawals for water supply result in very long travel times and indirect pathways from the source(s) of the nitrogen loading to discharge points. For this reason, it is important municipalities and regulators understand the value groundwater models provide in making major infrastructure decisions.