Bradbury, K. R.; Borchardt, Mark A.; Gotkowitz, Madeline B.; Hunt, R. J. / Assessment of virus presence and potential virus pathways in deep municipal wells
Procedures and methods, pp. 9-14 PDF (2.7 MB)
Procedures and Methods Selection of wells for sampling The Madison Water Utility currently operates 27 deep high-capacity wells completed in bedrock aquifers. The wells draw from a Cambrian-age sandstone aquifer underlying the city (Bradbury and others, 1999). This aquifer lies beneath 30 to 100 feet of glacially- deposited sand and gravel and lake sediment. Most of these high-capacity wells are over 700 feet deep and cased to about 200 feet below the surface. Water enters the wells through open boreholes in the rock below the casing. Although the well casings are supposed to be sealed to the surrounding geologic materials using cement grout, the integrity of these grout seals is often suspect and nearly impossible to test. About one- third of the wells are cased through the Eau Claire shale, a regional aquitard described by Bradbury and others (1999) and thought to provide excellent protection to the underlying sandstone aquifer. The other two-thirds of the wells, most of which are the older wells, are "cross-connecting"; open both above and below the shale or drilled in places where the shale is thin or absent. These wells are more vulnerable to contamination than the deeply cased wells. The funding level for this project prohibited sampling of all 27 Madison wells. In order to understand the scope of the virus problem we decided initially to sample 11 wells and then sample fewer wells in subsequent rounds. Our rationale was to insure that we were working with some virus-positive wells and that we had a variety of well construction and well locations. We chose six wells reported to be multi-aquifer wells (open both above and below the Eau Claire aquitard) and five wells reported to be cased through the aquitard. We sampled surface water from Lakes Mendota, Monona, and Wingra as well as clarified sewage influent at the Madison Metropolitan Sewage District. Samples were also collected for inorganic chemistry and isotope analyses. Following the initial sampling rounds we chose six wells for repeated monthly sampling. Figure 1 shows the spatial distribution of wells, and figures 2 and 3 show the construction of the sampled wells. Figure 2 also shows the typical conceptualization of subsurface hydrostratigraphy in Madison. The complex geologic stratigraphy is simplified to consist of upper glacial materials (till, sand and gravel, or lake sediment) covering a shallow bedrock aquifer composed of sandstone and dolomite. Shale of the Eau Claire Formation forms a regional aquitard and separates the upper bedrock aquifer from a deep bedrock aquifer composed of sandstone. Crystalline PreCambrian rock bounds the bottom of the system. Vertical hydraulic gradients in groundwater beneath the city are downward due to a regional cone of depression beneath the Madison metropolitan area (Bradbury and others, 1999). Figure 2 shows this diagrammatically - the potentiometric surface of the deep sandstone aquifer is lower than the water table in the shallow aquifer. In this situation water and any contaminants in the upper aquifer have the hydraulic potential to move vertically downward and reach the underlying deep aquifer. Wells are typically cased and grouted through the upper geologic units and consist of open holes below the casing.