THE IMPLICATIONS OF SUBMARINE GROUNDWATER DISCHARGE ON RESOURCE MANAGEMENT DECISIONS

Coastal landfills and hazardous waste sites pose a potential environmental threat to surface water bodies through the exchange of groundwater-borne contaminants. Many of these sites are located adjacent to harbors, bays, estuaries, wetlands, and other coastal environments. The management of these sites often depends upon an understanding of the spatial extent and magnitude of the groundwater discharge entering into the surface water body. Recent results show that the Trident probe provides rapid spatial assessment of both groundwater exchange parameters (temperature contrast and conductivity) and contaminant concentrations. Recent results from the UltraSeep show that groundwater exchange at coastal sites can be an important process in the transport and fate of dissolved contaminants that emanate from terrestrial waste sites. The ability to collect a continuous seepage record is critical to understanding the dynamics of the exchange process, especially in areas with strong tidal influence. In addition, the flow sensing capability allows water samples to be collected in proportion to the seepage rate, enabling the direct quantification of the chemical loading associated with the groundwater discharge. Measurements have been made from over 50 sites in the United States over the last ten years. Hundreds of individual measurements have been made a various hydrogeological settings including coastal bays and estuaries as well as inland rivers, lakes, ponds, wetlands and ditches. Groundwater exchange rates as high as 400 cm/d have been measured. Additionally recharge rates as high as -40 cm/d have been recorded. Tidal influences, losing and gaining stream segments, recharge events, sediment types, ebullition and other natural effects on seepage rates have been recorded. Many interesting features about the nature and characteristics of groundwater seepage have been revealed from this vast and detailed set of measurements. Glimpses at some important process in contaminant mobility at the interface between surface water and sediment have shown the importance of understanding the dynamics is this transition zone.