Saltwater Intrusion in Coastal Wells in Fractured Bedrock (04/10/08)
Localized saltwater intrusion into coastal fractured bedrock aquifers has already been documented through a number of coastal town-wide studies the author has conducted over the past 30 years. With the expected rise in sea level due to global warming more wells will be at risk. Given the difficulty, energy, and expense required to remove salt from water, it is important for land use planners to know which geologic factors increase the chances of saltwater intrusion. Using the USGS density-dependent groundwater model SEAWAT2000 in several environments, the author will describe geologic conditions that enhance the chances of saltwater intrusion. The extent of inland saltwater intrusion is inversely proportional to the distance to the groundwater divide and to the precipitation recharge rate between the ocean and the divide. It is also inversely related to the thickness of sediments covering the intertidal and subtidal area but directly related to the vertical permeability of these sediments. The greater the ratio of the distance to the groundwater divide divided by the width of an estuarine channel, the less the chance of saltwater intrusion. Fracture anisotropy and single high-transmissivity fracture orientations also affect the likelihood of saltwater capture. Other evaluations are in progress and will be reported at the conference. One could use these factors to develop guidelines for allowable coastal residential development densities where the water supply relies on drilled wells.

Click here to download the slides of Robert G. Gerber's presentation at the Maine Water Conference at the Augusta Civic Center on March 19, 2008.