Groundwater-derived nutrient fluxes and offshore mixing rates along the New Jersey coast

Groundwater discharge to the coastal ocean is a vital source of nutrients and trace metals that support photosynthetic life. This study uses radium isotopes as tracers to calculate coastal mixing rates during upwelling and steady state conditions and to quantify the groundwater-derived nutrient flux to the ocean along the southern New Jersey coast. Our best estimate for the coastal mixing rate (apparent horizontal eddy diffusivity) was 18 km 2 d −1 (possible range of 11 – 38 km 2 d −1 ) based on 224 Ra. A box model was used to determine that the flux of groundwater discharge to the coastal ocean is 5.4 × 10 7 m 3 d −1 , with a possible range of 2.8 × 10 7 – 1.4 × 10 8 m 3 d −1 . The Ra-derived estimate of submarine groundwater discharge was combined with groundwater nutrient concentrations to determine nutrient fluxes of 5.8 × 10 5 mol d −1 for nitrate + nitrite (NO x ), 4.3 × 10 4 mol d −1 for PO 4 , and 2.5 × 10 4 mol d −1 for NH 4 + . These results indicate that groundwater discharge is a major source of water and nutrient inputs along the New Jersey coast, similar to the fluxes delivered by the Delaware River and exceeding those delivered by the nearby Mullica River by ∼200%. These values serve as baseline estimates for assessing future changes in the magnitude and quality of groundwater discharge driven by human activity and climate change.