Publication Summary

The annual mass of dissolved nitrate transported to the southern portion of Puget Sound by direct groundwater discharge was estimated to support the South Puget Sound Nutrient Model Study modeling effort. Data from the region south of the Tacoma Narrows was evaluated to delineate the area from which groundwater flows directly to Puget Sound. This "area of contribution" was subdivided into 19 individual recharge/discharge zones.

The annual mass of dissolved nitrate transported to the southern portion of Puget Sound by direct groundwater discharge was estimated to support the South Puget Sound Nutrient Model Study modeling effort. Data from the region south of the Tacoma Narrows was evaluated to delineate the area from which groundwater flows directly to Puget Sound. This "area of contribution" was subdivided into 19 individual recharge/discharge zones.

An estimated range of annual groundwater discharge volume was developed for each of the zones using a water budget approach. An annual groundwater discharge volume was additionally developed for underflow from the Nisqually River alluvial aquifer. Groundwater quality data for nitrate was compiled, and a statistically representative nitrate concentration was assigned for each zone. Estimates of discharge volume and nitrate concentration were then integrated to determine annual loading to the South Sound.

The loading calculations predict an annual mass of nitrate discharging to Puget Sound via groundwater transport between approximately 160,000-190,000 kg nitrate/yr. These values are based on available data, and were calculated using a number of simplifying assumptions regarding the behavior of nitrate in the subsurface, and the movement of groundwater in a complex hydrogeologic setting. The use of public water supply well data to estimate groundwater quality in the study area remains the greatest source of uncertainty in the estimates. Due to concerns that the Washington State Department of Health data under-predicts the nitrate concentration in groundwater, the load estimates may best define the lower bound of current conditions. Predictions of annual load were also developed to highlight the consequences of rising nitrate concentrations with time.