With regard to the total external input of these essential nutrients, SGD represents 29–45 % of dissolved inorganic nitrogen input to Ninigret, Point Judith, and Quonochontaug ponds and as much as 93 % for Winnapaug pond.

On a daily basis, the SGD supply of dissolved inorganic nitrogen and phosphorus is estimated to represent ∼1–6 % of the total amount of these nutrients in surface waters of Ninigret, Point Judith, and Quonochontaug ponds and up to 84 and 17 % for Winnapaug, respectively, which may reflect a greater SGD nutrient supply to this pond because of the proximity of fertilized golf courses. Results range from 80 to279 mmol N m −2 year −1 and 4 to 15 mmol P m −2 year −1 for Ninigret, 48 to 265 mmol N m −2 year −1 and 4 to 23 mmol P m −2 year −1 for Point Judith, 31 to 62 mmol N m −2 year −1 and 1 to 2 mmol P m −2 y −1 for Quonochontaug, and 668 to 1,586 mmol N m −2 year −1 and 29 to 70 mmol P m −2 year −1 for Winnapaug ponds, respectively. Measurements of groundwater-dissolved inorganic nitrogen (nitrate + nitrite + ammonia) and phosphate concentrations were combined with recent, radium-based, submarine groundwater discharge (SGD) fluxes and prior estimates of SGD determined from Darcy’s Law, a hydrologic model, and total recharge to yield corresponding SGD nutrient fluxes to Ninigret, Point Judith, Quonochontaug, and Winnapaug ponds, located in southern Rhode Island.

Ninigret pond hydraulic dredge