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Geologic Division ---- Coastal and Marine Geology ---- Woods Hole Field Center

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Disruption of Coastal Aquifers

aquifer system This figure illustrates the hydrogeological framework thought to be typical of southeastern Virginia. In the cross section, the ground-water aquifers (blue) alternate with confining beds (gray). The aquifers are mainly sands, which contain water-filled pore spaces between the sand grains. The pore spaces are connected, which allows the water to flow slowly through the aquifers. The confining beds are mainly clay beds, which have only very fine pores, and these are poorly interconnected, which greatly retards or prevents the flow of water. Before we knew about the Chesapeake Bay crater, this framework of alternating aquifers and confining units was applied to models of groundwater flow and water-quality assessments in the lower Chesapeake Bay region. Now we know that this framework does not apply to the lower bay.

contamination of local aquifers This illustration of the aquifer system at the crater (aquifers in blue, confining units in gray) shows the difference. The first thing we notice is that the orderly stack of aquifers seen outside the crater is not present inside the crater. All the pre-impact aquifers have been truncated at the crater outer rim. Moreover, in the center of the crater, all the original aquifers have been completely excavated. In place of these stacked aquifers, we now have a single huge reservoir with a volume of 4,000 km3. That's enough to cover all of Virginia and Maryland with a breccia layer 30 m thick.

But the most startling part is that this huge new reservoir does not contain fresh water like the aquifers it replaced. It is filled with briny water that is 1.5 times saltier than normal sea water. This water is too salty to drink or to use in industry. Geohydrologists have known of this salty water for decades, but only now are we beginning to grasp the true nature of its origin and distribution.

chlorinity in Brightseat - Upper Potomac Aquifer Here is a published map whose contours represent lines of equal chloride content, which is a substitute measure of salinity. Note how the contours curve around the rim of the crater. The chlorinity gradient increases significantly at the crater rim.

The presence of this hypersaline aquifer has some practical implications for ground-water management in the lower bay region. For example, we need to know how deeply buried the breccia is in order to avoid drilling into it inadvertently and contaminating the overlying fresh-water aquifers. Its presence also limits the availability of fresh water. If you live in Newport News, where the breccia is thin, and if the upper aquifers should become depleted, you might be able to reach additional fresh-water aquifers below the breccia. But if you live on the Eastern Shore, over the deepest part of the crater, only the aquifers above the breccia are available for fresh water. So we need to be especially conservative of ground-water use there.

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