Chemicals Entering Coastal Waters: Freshwater And Saltwater Interactions In Coastal Groundwater

Fresh and saltygroundwater flows into coastal waters as submarine groundwaterdischarge and is an important source of nutrients, contaminants andtrace elements to the coastal ocean. Recent research has revealed thata large portion of submarine groundwater discharge is saline water.Although this water was once ocean water, the mechanism controlling itsflow into and out of the sediments has not been previously determined.Using seepage meters and geochemical tracers, scientists have directlymeasured and inferred groundwater flow from land to sea. But they havenot previously been able to observe the opposite, large-scale flow orintrusion of seawater into coastal aquifers to balance this exchange.

Ina paper published August 25, 2005 in Nature, scientists from theMassachusetts Institute of Technology (MIT) and Woods HoleOceanographic Institution (WHOI) made both direct and indirectmeasurements of flows back and forth at Waquoit Bay, Massachusetts atvarious seasons of the year and compared those results with a generalmodel of a coastal groundwater system. Their findings reveal a lag inthe inflows and outflows related to seasonal changes in the water table.

Studyco-author Ann Mulligan of the WHOI Marine Policy Center says seawateris drawn into aquifers as the freshwater-saltwater interface orboundary moves landward during winter. The water discharges back intocoastal waters as the boundary moves seaward in summer. Since summer istypically associated with higher temperatures and evaporation,saltwater should intrude inland rather than discharge at the coast.However, the numerical model reveals that there may be a time lag ofseveral months between precipitation, groundwater recharge, andassociated impacts on saltwater flowing into or out of the aquifer.

“Welooked at several mechanisms other than seasonal exchange that coulddrive saltwater circulation, including tides, wave run-up on the beach,and entrainment or trapping of saltwater into fresh,” Mulligan said. “But each of these flows balanced over a tidal cycle and occurs in awell-defined relatively small area, and could not account for the largedischarge we observed during summer in Waquoit Bay.“

The studywas conducted at the Waquoit Bay National Estuarine Research Reserve inFalmouth, Massachusetts and supported by the National ScienceFoundation.

The authors say the global extent of seasonalexchange of freshwater and saltwater is unknown but could be animportant factor in transporting nutrients and contaminants trapped insediments into coastal waters. Because the chemistry of coastal watersis affected, it is important to understand the link between theseasonal hydrologic cycle on land and the saline groundwater system incoastal aquifers. Now that a major driving mechanism of saline waterflow has been determined, important follow-up studies will look at thechemical content of the inflowing and outflowing water over a yearlycycle. Most previous studies have looked at chemical loading fromgroundwater over short time-periods, but this study shows that a majorprocess is occurring on a yearly cycle.

“The impact on coastalchemistry could be enormous,” Mulligan says. “Along the U.S. east coastthe greatest saltwater discharge may occur in summer, when biologicalactivity is at its highest and river inflow at its lowest. The input ofnutrients at certain times of the year may be key to the health of ourcoastal waters.”