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COI Funded Project: Investigating Submarine Groundwater Discharge to Coastal Waters Using Radium Isotopes

Project Duration: 6/1/98-12/31/99
Key Words: Boston Harbor, groundwater flux, radium geochemistry, modeling, toxic chemicals

Final Report


Background

The importance of coastal groundwater discharge in delivering dissolved nutrients, such as nitrate and phosphate, to coastal waters has often been overlooked, primarily because it is difficult to estimate (Johannes, 1980; Nixon et al., 1986; Simmons, 1992). The problem lies in the fact that the flow of groundwater through coastal marine sediments, called submarine groundwater discharge (SGWD), is difficult to quantify using traditional methods such as seepage meters since the discharge is often patchy and may vary with time. Unlike rivers, submarine fluid discharge bypasses the estuary filter, which is an important mechanism for contaminant removal in many coastal settings (Moore and Shaw, 1998). Even if SGWD rates are modest, dissolved nutrient concentrations in groundwater may be sufficiently high to have a significant impact on the nutrient budgets for receiving waters. Recently, radium has been shown to be a useful chemical indicator of SGWD and, having four isotopes with half-lives ranging from four days to 1600 years, can be used to estimate rates of SGWD on a wide range of time-scales (Moore, 1996; Rama and Moore, 1996).

A key biogeochemical problem associated with coastal groundwater flow on Cape Cod is the introduction of "new" nitrogen entrained by groundwater plumes as they pass through septic tank fields located along the coastline (Valiela et al., 1992; Weiskel and Howes, 1991). As a result, nitrate concentrations may be several orders of magnitude greater than the receiving waters (Valiela et al., 1990, 1992; Andrews et al., 1999). Here, we present a study of SGWD in Waquoit Bay, MA utilizing radium isotopes as tracers of SGWD-derived dissolved inorganic nitrogen (DIN) flux to the estuary. Lastly, we compare these results with productivity estimates from the literature in an attempt to determine if the estuary is a net source of nutrients to coastal waters.

Study Area
Waquoit Bay is an enclosed estuary located on the south shoreline of Cape Cod, MA. Its watershed comprises nearly 65 km2 extending roughly 10 km north from the head of the bay. The bay on average is relatively shallow with a mean depth of 1 m; major freshwater sources include the Quashnet River (to the east) and the Childs River (to the west). In terms of the total freshwater budget, these rivers are a minor component compared with direct groundwater discharge (Cambareri and Eichner, 1998). Surrounded by a population of over 8,000 year-round residents (not including summer residents), groundwater-derived nutrients from private septic systems have led to an increasing number of eutrophication events in this watershed (Valiela et al., 1992)

Eutrophication in Waquoit Bay has been directly linked to sewage-derived nitrogen inputs via direct groundwater discharge to the estuary (Valiela et al., 1990; McClelland et al., 1997). The coarse, unconsolidated sands that characterize this watershed contribute significantly to the rapid transport of nutrients to coastal waters. The most notable ecological impact has been a decline in shellfish population and sea grass coverage. The former is likely due to seasonal the seasonal decline in dissolved oxygen. The latter is caused by secondary effects, namely an increase in epiphytes which intercept light from their growth substrate, eel grass blades (Valiela et al., 1992)

Summary of Results
Due to rapid increases in population, anthropogenic sources of nitrogen have adversely impacted the water quality of coastal ponds on Cape Cod. A major source of "new" nitrogen to these estuaries is groundwater, which intercepts septic tank fields in its flow path to the coastline. Many attempts have been made to quantify this process; however, groundwater discharge is often patchy in nature and therefore difficult to study using traditional techniques such as seepage meters. In Waquoit Bay, MA, we tested an approach based on radium, which is naturally enriched in aquifer fluids and has four isotopes with half-lives ranging from 4 days to 1600 years. Groundwater entering the bay was low in salinity and contained several orders of magnitude greater radium and dissolved inorganic nitrogen (DIN) relative to ambient bay water. Using a mass-balance approach for radium, we calculated a submarine groundwater flux of ~28,000 m3 d-1, which compared well with aquifer recharge rates calculated from rainfall. From the DIN content of the groundwater, we estimated that ~1500 mol N d-1 was directly input to the estuary. However, this nitrogen flux was small in comparison to DIN fluxes from the heavily populated subestuaries. Furthermore, our results suggest that groundwater flux of DIN was assimilated by plant biomass during the summer but may be exported from the embayment to coastal waters during the winter months.

Originally published: January 1, 1998