Setting

Hays Kansas, located in the central High Plains (Fig. 2), has been the focus of an ongoing effort to determine Well Head Protection Areas (WHPAs) by EPA's R.S. Kerr Environmental Research Center (RSKERC) [Kraemer and Burden, 1994]. The Hays site has an exceptional number of boreholes, estimated at around 2000 wells in a 65 km$^2$ area (Paul Montoia, Hays Wellfield Manager, pers. commun. 1996), 600 of which have readily-available lithologic logs. Only limited hydrologic testing has been carried out at Hays, requiring hydrologic models to be lithology-based, and limiting the applicability of more quantitative approaches (e.g. kriging of hydraulic parameters).

Figure 2: Location of study area: the vicinity of Hays, Kansas (black rectangle).

Hays is located in west-central Kansas (Fig. 3) along a large creek (Big Creek), Interstate 70, and the east-west trunk of the Union Pacific railroad. Big Creek has eroded a moderate valley (averaging 50 m deep, 5-10 km in width, Fig. 4) into underlying impermeable shales (Kc). The Big Creek watershed extends about 140 km west-northwest of Hays.

Figure 3: Hays (red dot) lies in Ellis County, which occupies an eroded bedrock area of the High Plains. Shaded relief map (lit from the WNW) after http://gisdasc.kgs.ukans.edu/kanview/slope/ks_slope.html. Select image to view full-sized version.

Figure 4: Location of City of Hays within Ellis County. Red lines are township boundaries, black E-W line is I-70 freeway, blue lines are major streams, including Big Creek along the southern Hays city limit. Shaded relief map (lit from the WNW) after http://gisdasc.kgs.ukans.edu/kanview/slope/html/Ellis.html.

During the Neogene, Big Creek began filling the valley with silt, sand and gravel [Latta, 1948]. Hays itself is built on Quaternary terrace deposits (Qt), primarily composed of moderately permeable silts, sands, and clays (cross-section Fig. 5, surficial geology, Fig. 6). Concentrated at the base of these deposits and toward the valley center are lenses of sand and gravel, comprising the primary drinking water aquifer for Hays (historically referred to as ``alluvium'', Qal [Latta, 1948]). The lithologies at Hays can therefore be broken into three classifications: shale (the bedrock, Kc), alluvium (sand-gravel channel facies deposited by Big Creek, Qal) and terrace (Big Creek overbank deposit facies, Qt). The concentration of alluvium at the base of the Quaternary deposits (Fig. 5) allows the system to be described by three variable-thickness hydrostratigraphic units: a basal very low permeability Cretaceous shale layer, overlain by a highly permeable sand-gravel layer, which is overlain by moderately permeable terrace deposits.

Figure 5: Geologic cross-section, oriented N-S along Vine St. (black line in Figure 4, central Hays), after Latta (1948). Qal is concentrated at base of Quaternary section and toward the center of the valley, but dispersed Qal bodies are present throughout the section. Blue line is approximate water table.

Lithologic logs are available from three types of wells at Hays. The city of Hays has drilled a number of water-supply wells (Fig. 6), at present 13 are considered active, although some wells are pumped only during peak demand periods. A large number of private water-supply wells are present within the Hays city limits (Fig. 7), and a summary of the lithologic logs for these wells is available [Bermudez, 1986]. During early evaluation of water supplies at Hays an additional series of test wells were drilled (Fig. 7), although apparently no hydraulic testing took place [Latta, 1948].

Figure 6: Local surface features Hays area. Axes show UTM coordinates. Hays city limit shown by solid line, wells used for city water supply shown by circles, geologic contact between impermeable Carlisle Shale (Kc) and water-bearing terrace deposits (Qt) shown by red line (after [Latta, 1948]). Principal ground-water/surface-water interaction occurs via Big Creek (blue line). Select figure to view full-sized version.

Figure 7: Location of wells used for stratigraphic information, Hays area. Approximately 40 distant wells not shown. Number of wells: Latta = 80 [Latta, 1948], ``other'' = 170, Perez = 366 [Bermudez, 1986], City = 16, for a total of 632. Axes show UTM coordinates. Select figure to view full-sized version.