Freshwater and Saltwater Interactions in Coastal Groundwater Systems May Provide Clues to Chemicals Entering Coastal Waters

Scientists have recently recognized an imbalance in the flow of salty groundwater into the coastal ocean: considerable saltwater discharge into the ocean has been observed, but little or no return flow has been seen. Now it appears that the timing of the discharge may be key to the health of our coastal waters.

New measurements and models suggest that seasonal changes in the water table may provide clues to how water is exchanged and why the largest discharge occurs during the summer, when the coastal ocean may be most vulnerable to the dissolved chemicals in the groundwater because biological activity is at its highest and river inflow at its lowest.

Fresh and salty groundwater flows into coastal waters as submarine groundwater discharge and is an important source of nutrients, contaminants and trace elements to the coastal ocean. Recent research has revealed that a large portion of submarine groundwater discharge is saline water. Although this water was once ocean water, the mechanism controlling its flow into and out of the sediments has not been previously determined. Using seepage meters and geochemical tracers, scientists have directly measured and inferred groundwater flow from land to sea. But they have not previously been able to observe the opposite, large-scale flow or intrusion of seawater into coastal aquifers to balance this exchange.

In a paper published August 25, 2005 in Nature, scientists from the Massachusetts Institute of Technology (MIT) and Woods Hole Oceanographic Institution (WHOI) made both direct and indirect measurements of flows back and forth at Waquoit Bay, Massachusetts at various seasons of the year and compared those results with a general model of a coastal groundwater system. Their findings reveal a lag in the inflows and outflows related to seasonal changes in the water table.

Study co-author Ann Mulligan of the WHOI Marine Policy Center says seawater is drawn into aquifers as the freshwater-saltwater interface or boundary moves landward during winter. The water discharges back into coastal waters as the boundary moves seaward in summer. Since summer is typically 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 of several months between precipitation, groundwater recharge, and associated impacts on saltwater flowing into or out of the aquifer.

“We looked at several mechanisms other than seasonal exchange that could drive 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 a well-defined relatively small area, and could not account for the large discharge we observed during summer in Waquoit Bay.“

The study was conducted at the Waquoit Bay National Estuarine Research Reserve in Falmouth, Massachusetts and supported by the National Science Foundation.

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

“The impact on coastal chemistry could be enormous,” Mulligan says. “Along the U.S. east coast the greatest saltwater discharge may occur in summer, when biological activity is at its highest and river inflow at its lowest. The input of nutrients at certain times of the year may be key to the health of our coastal waters.”