Subsidence

Subsidence

Subsidence of a residence in Florida (USGS photo)

Tide gauges in the Chesapeake Bay indicate that sea level there is rising at twice the global average. However since tide gauges are a relative measure of sea level height (see Variations in Sea Level), it is impossible to discern a sea level rise from land subsidence. To resolve this ambiguity, we are installing GPS receivers in the Chesapeake Bay region. The GPS receivers will monitor the absolute motion of the crust in this region (see GPS Section of this document).

Land subsidence, in the Chesapeake Bay region, may result from


Sediment Loading

The Chesapeake Bay is a depositional basin that is being filled by sediment

[Hobbs et al, 1990]. During the 100 year period ending in the mid-1950's net deposition in the estuary was approximately 2,915 million metric tons. To get a feel for whether the subsidence caused by this load would be significant, we assume the load was deposited at a constant rate over the last 100 years and we further assume that the sediment was all deposited in an area of 100 kmē. (This area limit is unrealistic, but it will help us bound the upper limit of the subsidence). Using Farrell's Green's functions for an elastic earth, we find that the subsidence would be on the order of 0.6 mm/year which is too small to contribute to the observed rate of sea level rise. We dismiss sedimentation as a source of the subsidence.


Groundwater Extraction

The Maryland Geologic Survey's Department of Hydrology has been monitoring wells in the area surrounding the Chesapeake Bay since the 1940's [Frederick Mack, personal communication]. Annual maps displaying the depth of the water table (see Figure 2 for an example) of the Aquia Aquifer in Southern Maryland, indicate that the surface of the water table has dropped on average by approximately 10 meters [Curtin et al., 1980-1991]. Specific wells display drops of as much as two to five times that amount. Ground subsidence is expected to result from the subsequent compaction of the aquifer. We would expect to see a larger subsidence in the GPS data near areas of maximum water withdrawal as opposed to areas distant from pumping stations.


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