USING GPS TO QUANTIFY THREE DIMENSIONAL AQUIFER DEFORMATION AND STORAGE IN THE VIRGIN RIVER VALLEY, NEVADA

Quantifying aquifer storage is important to characterize aquifer response and optimize aquifer pumping in large well fields located in thick sedimentary basins like those in the arid southwestern America. The majority of this water is released from storage because of aquifer-system compression. Historically this compression is assumed to occur only in the vertical direction. However, aquifer mechanics and field studies show that strain is three-dimensional and the amount of water released from storage by horizontal strain can be significant. The development of empirically-based analytical techniques that allow for accurate quantification of storage and an assessment of the strain components at various radii from the pumping well are needed.

The horizontal movement of solids in the aquifer matrix propagates radially outward from the pumping well forming a zone of compressional strain. Surrounding this zone is an area of no strain, then, a more distant zone of extensional strain. This strain signature has been confirmed in field data and in mathematical models based on mass balance and the Darcy-Gersevanov Law. The size of displacement is known to be a function of the compressibility of the matrix and head change across a unit of interest. Significant quantities of water can occur from horizontal strain within even coarse-grained aquifer units because the radial pressure gradient can occur over distances orders of magnitude greater than those of the vertical direction. Low cost methods for quantifying these three-dimensional strains are necessary if accurate storage properties are to be determined.

In the Virgin River Valley, near Mesquite, NV, a large-scale aquifer test is being conducted on a new municipal well. The well is located in a Tert. and Quat. alluvial fill in the Basin and Range province. Both the horizontal and vertical strains are being monitored with 10 GPS stations—6 located within 2 km of the well, and 4 monitoring any far field (up to 8 km from the well) deformation that may be occurring. The receivers will take measurements of their absolute position throughout the entire 30-day aquifer test. The deformation data retrieved from the GPS stations will be used to develop analytical tools that allow for more accurate determination of storage that can be used to manage future groundwater removals in a growing area.