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SPATIALLY-BALANCED SURVEY DESIGN FOR GROUNDWATER USING EXISTING WELLS
Citation:
Olsen, A R. SPATIALLY-BALANCED SURVEY DESIGN FOR GROUNDWATER USING EXISTING WELLS. Presented at Joint Statistical Meetings, New York, NY, August 3-7, 2003.
Description:
Many states have a monitoring program to evaluate the water quality of groundwater across the state. These programs rely on existing wells for access to the groundwater, due to the high cost of drilling new wells. Typically, a state maintains a database of all well locations, including limited information on type of well and aquifer. Since wells are drilled mainly by landowners and municipalities, the density of wells tends to be higher near urban areas, resulting in a very uneven spatial distribution of wells. If the objective is to estimate the water quality of wells, then this does not present any particular problem. If the objective is to estimate the quality of underlying groundwater aquifers, then the question is how to design a survey of wells that will adequately sample the aquifer. Using an example set of well locations in a subregion of Florida, we develop an approach that results in a spatially-balanced sample of the aquifer (within limits of geographic coverage of wells). The design is based on the Generalized Random Tessellation Stratified (GRTS) survey design. First, we create an estimated 2-dimensional density surface of wells using a 2-dimensional kernel density estimator. We use the inverse of the well density surface as unequal inclusion probability densities for use in the GRTS survey design. This approach is compared to a simple random sample of wells and two-stage stratified area sample of wells. Currently, only spatial patterns of the sample can be compared.