Using GPS to Quantify Three Dimensional Storage and Aquifer Deformation in the Virgin River Valley, NV
Warner, Sandra McCarthy
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The horizontal component of land subsidence is typically assumed to be negligible, although recently, theoretical simulations have shown that horizontal strain is significant. A field based investigation in Mesquite, NV, was undertaken from May to July, 2003, for the purpose of evaluating the significance of horizontal strain during an aquifer test. The hydraulic heads in the aquifer were monitored within a meter of the municipal pumping well used for the aquifer test and also at a distance of approximately 1,470 meters from the pumping well. Aquifer deformation in the horizontal and vertical directions were measured using GPS for the first 22 days of pumping in 10 different locations at radial distances from the well varying from 100 meters to 2500 meters. From 22 to 60 days of the aquifer test, the number of GPS stations monitoring deformation was reduced to five. Radial displacement was measured at all monitoring stations during the aquifer test, indicating that the aquifer is moving closer to the pumping well. The greatest magnitude displacement measured 140 m from the well was approximately 10 mm at the land surface. A zone of radial compression occurred between the pumping well and the first monitoring station 140 m away from the pumping well. Vertical displacement was measured in decreasing magnitude with increasing distance from the well. Because GPS is not as precise a tool in the vertical direction as it is in the horizontal, the vertical signal of displacement is not as accurate. Numerical simulations using the BIOT and IBS codes were performed to reproduce the aquifer test and land deformation. The model included six layers representing three hydrogeologic units: a bottom aquifer (four layers) in which pumping occurred , a top aquifer (one layer) in which the monitoring well was screened, and a semi-confining bed (one layer) between the two aquifers that represented an equivalent thickness of interbeds and clays layers. The Biot code was used to simulate radial and vertical movements in an axisymmetrical simulation, while the IBS code was used to simulate only vertical displacement but also provided for the simulation of elastic and inelastic storage and compression. The vertical distribution of radial displacement was simulated using the BIOT code. At the onset of pumping, the greatest radial displacement occurred in the bottom aquifer in which pumping occurred. At a distance of 2,000 m from the well, the radial displacement was uniform over all depths indicating that the differences in hydraulic diffusivity are not as important a factor at distance from the well. The change in porosity that occurred as a result of horizontal strain was greatest in the bottom aquifer. Using the strain calculated directly from the GPS measurements at the land surface, vertical strain comprised almost 99% of the volume strain at the land surface. However, when the strain was simulated over the entire aquifer system, the radial and hoop strain contributed more than vertical strain in the bottom aquifer at a radius of 100 m from the pumping well at the onset of pumping until the aquifer reached near equilibrium, at which time vertical strain again dominated.
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