For protecting drinking water supplies, the locations of areas with reliance on private domestic wells (hereafter referred to as “wells”) and their relationship to contaminant sources need to be determined. A key resource in the U.S. was the 1990 Census where the source of domestic drinking water was a survey question. Two methods are developed to update estimates of the areal density of well use using readily accessible data. The first uses well logs reported to the states and the addition of housing units reported to the Census Bureau at the county, census tract and census block group scales. The second uses housing units reported to the Census and an estimated well use fraction. To limit the scope and because of abundant data, Oklahoma was used for a pilot project. The resulting well density estimates were consistent among spatial scales, and were statistically similar. High rates of well use were identified to the north and east of Oklahoma City, primarily in expanding cities located over a productive aquifer. In contrast, low rates of well use were identified in rural areas without public water systems and in Oklahoma’s second largest city, Tulsa, each attributable to lack of suitable ground water. High densities of well use may be expected in rural areas without public water systems, expanding cities and suburbs, and legacy areas of well usage. The completeness of reported well logs was tested by counts from neighborhoods with known reliance on wells which showed reporting rates of 20% to 98%. Well densities in these neighborhoods were higher than the larger-scale estimates indicating that locally high densities typically exist within analysis units. A Monte Carlo procedure was used to determine that 27% of underground storage tanks that had at least one well within a typical distance of concern of 300 m (1000 ft).
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