Previous: Ground Subsidence Up: Chesapeake Bay Bolide Next: Faulting
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.
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.
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.
Previous: Ground Subsidence Up: Chesapeake Bay Bolide Next: Faulting
Back to Chesapeake Bay Bolide home page.