Embayment scale assessment of submarine groundwater discharge nutrient loading and associated land use

• Submarine groundwater discharge was measured by surface water 222Radon method.
• Porewater nutrient concentrations used to estimate total nutrient flux to an embayment.
• Total nitrate flux from submarine groundwater similar to point source sewage input.
• Modeled nitrate flux with aquifer land use accounts for 79% of data variance.
• Percent of high density development is the best predictor of nitrate flux.

A shoreline survey of porewater nutrient concentrations and 222Rn surface water activities was performed in Port Jefferson Harbor, NY, an embayment of Long Island Sound. Submarine groundwater discharge (SGD) was calculated for individual shoreline segments; shallow porewater nutrient concentrations were applied to calculated groundwater flux values in order to spatially depict the nutrient flux to overlying harbor water. Nitrate was the dominant form of inorganic nitrogen as porewater concentrations of ammonium were negligible. Land use analysis showed that nitrate to phosphate ratios (N:P) in SGD were positively correlated with medium to high development of inland aquifer watersheds. A multivariate regression model was developed which describes 79% of the variability in shoreline nitrate discharge. Three modes of nutrient transport were observed; 1) High SGD rates coupled with nutrient rich groundwater derived from high density development, 2) moderate SGD rates coupled with low nutrient groundwater from low density development/forested watersheds and 3) negligible SGD rates associated with recirculated seawater adjacent to low density development areas. Transport mode 1 dominated total SGD nitrate inputs primarily due to steep hydraulic gradient, a characteristic of tunnel valleys in glacial deposits, which highlights the critical role hydrogeology plays in groundwater nitrogen loading to surface water. Total nitrate inputs to the harbor from shoreline SGD alone are 980 mol d−1, similar to average daily sewage treatment plant nitrogen inputs of 870 mol d−1. The techniques used in this study represent an effective methodology for calculating SGD derived nutrient loads where a surface water nutrient source is present.

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