Groundwater and Nitrogen Modeling to Prioritize Management Strategies for Suffolk County’s Estuaries

Nitrogen loading to groundwater from surface activities has caused degradation of aquifer and surface water quality. In Suffolk County, New York, more than 70% of the county relies on on-site wastewater disposal systems such as septic systems and cesspools. Nitrogen loading from these systems impact both ground and surface waters. Due to the appearance of harmful algal blooms, the loss of sea grass as well as increasing nitrate concentrations in water supply wells of the sole source aquifer, evaluating nitrogen loading within the County is a priority.

Utilizing groundwater models allows for a better understanding of the impacts of historic, existing and potential future land uses on aquifer nitrogen concentrations and ultimate discharge to water supply wells and estuaries. Groundwater models were utilized to better understand nitrogen loading from various land uses within watersheds as well as the impact of potential nitrogen reduction strategies such as the use of innovative alternative onsite wastewater treatment (I/A OWTS), clustered/decentralized treatment, or traditional sewering. Nitrogen loading models have been coupled with groundwater models to simulate the fate and transport of parcel-specific nitrogen loading throughout the County.

Suffolk County has launched a Subwatersheds Wastewater Plan in which baseflow contributing areas (subwatersheds) were simulated for more than 180 surface water bodies. For each subwatershed, time of travel (from the water table to surface discharge) zones are quantified and nitrogen loading from surface activities including wastewater, fertilizer application and atmospheric deposition are calculated. The nitrogen loads were simulated as parcel based point sources using fate and transport groundwater models which ultimately provided discharge loads to the subwatersheds and more than 700 community water supply wells. Simulated nitrogen loads to surface waters were compiled with quantitative and qualitative data and utilized in a decision support tool to rank and prioritize the subwatersheds for future management strategies.