mm TOTI RESOURCES Hamilton County Open File Report 86-40 WRD Compiled by CAROL A. THOMPSON GROUNDWATER RESOURCES OF HAMILTON COUNTY Introduction Approximately 90% of the residents of Hamilton County rely on groundwater as the source of their drinkfng water. It is estimated that the use of groundwater in the county currently approaches 1.0 billion gallons per year. For comparison, this amount would provide each resident with 146 gallons of water a day during the year. Actually, few if any households use this much water, and the rather large annual per capita use reflects the greater water requirements of the county's industries, agribusinesses and municipalities. The users of groundwater in the county draw their supplies from several different geologic sources. Various factors must be considered in determining the availability of groundwater and the adequacy of a supply source: distribution - having water where it is needed accessibility - affects the costs for drilling wells and pumping water yield - relates to the magnitude of the supply that can be sustained quality - determines for what purposes the water can be used In terms of these factors, there are few locations in Hamilton County where the availability of groundwater is not limited to some degree. The most common limitation 1s poor water quality, that is, highly mineralized groundwater. Secondary limitations are generally related to poor distribution, small yields from some sources, and poor accessibility due to the great depths to adequate sources. Occurrence of Groundwater in Hamilton County The occurrence of groundwater is influenced by geology — the position and thickness of the rock units, their ability to store and transmit water, and their physical and chemical make-up. Geologic units that store and transmit water and yield appreciable amounts to wells are called aquifers. The best aquifers are usually composed of unconsolidated sand and gravel, porous sandstone, and porous or fractured limestone and dolostone. Other units composed of materials such as clay and silt, shale, siltstone, and mudstone yield little or no water to wells. These Impermeable units are called aquieludes or aquitards, and commonly separate one aquifer unit from another. In Hamilton County, there are two principal sources from which users obtain water supplies: the loose, unconsolidated materials near the land surface that comprise the surficial aquifer, and several deep-rock aquifers. Between the surficial aquifer and the deep Cambro-Ordovician aquifer are two other water-bearing units, the Mississippi an and the Devonian aquifer systems. However, throughout Hamilton County the water contained in these aquifers is highly mineralized and often of too poor quality for human or livestock use. Figure 1 is a cross section showing the geologic relations of aquifers beneath the county. Each aquifer has Its own set of geologic, hydrologic, and water-quality characteristics which determine the amount and potability (suitability for drinking) of water it will yield. Table 1 lists the geologic and hydrogeologlc characteristics of the aquifer underlying Hamilton County. Surficial Aquifers Unconsolidated deposits at the land surface are comprised of mixtures of clay, silt, sand, gravel, and assorted boulders. The water-yielding potential of surficial deposits is greatest in units composed mostly of sand and/or gravel. Three types of surficial aquifers are used: the alluvial aquifer, the drift aquifer, and the burled channel aquifer. The alluvial aquifers (Fig. 2a) consist mainly of sand and gravel transported and deposited by modern and Pleistocene streams and make up the floodplalns and terraces in major valleys. Alluvial deposits are shallow, generally less than 50-60 feet, and thus may be easily contaminated by the infiltration of surfacewater. The drift aquifer (F1g. 2b) is the thick layer of clay-to boulder-size material (till) deposited over the bedrock by glacial ice which invaded the county several times in the last two million years. The composition of the glacial drift varies considerably, and in many places does not yield much water. There are however, lenses or beds of sand and gravel in the drift, which are thick and widespread*enough to serve as dependable water sources. Usually one or two sand layers can be found in most places that will yield minimum water supplies for domestic wells. The buried channel aquifer (Fig. 3a) consists of stream alluvium of partially filled valleys that existed before the glacial period. The valleys were overridden by the glaciers, and are now buried under the glacial drift. They may or may not coincide with present day alluvial valleys. The distribution, yield, and water-quality characteristics for the surficial aquifers are summarized in Figures 4 and 5 and Table 3. An indication of accessibility can be obtained by comparing the elevations of the top (the land surface) and the bottom (the bedrock surface) of the surficial deposits 1n Figures 6 and 7. The thickness of the glacial drift or the depth of buried channels, is determined by subtracting the elevations at selected locations. Water levels in the surficial aquifers are difficult to analyze, because water rises to different levels in wells drilled into alluvial, buried-channel, and drift aquifers. The water table in the shallow drift aquifer generally slopes from high land areas toward the streams, and, changes noticeably throughout the year in response to recharge from precipation. Water levels in the alluvial aquifer fluctuate somewhat 1n the same way as those in the shallow drift aquifer; however, the main influence on the alluvial aquifer is the stage (level) of the associated streams. Water levels will be high during periods of high stream stage and low during low-stage periods. Deeper drift and buried channel aquifers are under confined (artesian) conditions and are generally unaffected by local recharge-discharge relationships. Water levels in the drift aquifers commonly are from 10-50 feet below the land surface, and those in the buried-channel aquifers have been reported to be as low as 50 feet below the land surface. The water levels 1n alluvial wells are from 4 to 20 feet below the flood plain surface. Rock Aquifers Below the drift and other surficial materials 1s a thick sequence of layered rocks, formed from deposits of shallow seas that alternately covered the state during the last 600 million years. The geologic map (Figure 8) shows the geologic units which form the surface of this rock sequence. The aquifers (F1g. 3b) are the water-bearing rock units, mostly limestones and sandstones. Rocks of Pennsylvanian and Mississippian age lie below the glacial drift in Hamilton County. The Pennsylvanian rocks are mostly shales and siltstones with minor sandstones and limestones. Because shales dominate, the Pennsylvanian sequence acts as an aquielude and only locally can water be produced from the permeable members. The water yields are generally low. Water quality data extrapolated from nearby counties indicate that the water is highly mineralized with high concentrations of dissolved solids, sulfate and sodium. The Mississippian Aquifer is the most frequently utilized groundwater source in Hamilton County, and consists of a series of limestones and dolostones. Yields range from 10-75 gpm. The Devonian aquifer 1s used locally by rural residents. The main water-producing units in the Devonian are a series of limestones and dolostones. The Cambro-Ordovician aquifer is the major deep aquifer in the county, and includes the St. Peter Sandstone, the Prairie du Chien Dolomite, and the Jordan Sandstone, the latter being the major water producer. The maps 1n Figure 12 refer to the Jordan aquifer, the lower two units of the Cambro-Ordovician aquifer. The St. Peter sandstone, being highly friable, is generally cased-out in the deep wells. The relative accessibility of groundwater 1n rock aquifers depends on the depth to the aquifer. The deeper a well must be, the greater the cost for well construction and pumping. The depths to, and thicknesses of, units at specific sites, will vary somewhat because of irregularities In the elevation of the land surface, and in the elevation of the underlying rock units. Estimates of depths and thicknesses can be made by comparing Figure 6 with the maps of aquifer elevations in Figures 10, 11, and 12. The range in depth below land surface to the top of the county's principal bedrock aquifers 1s given for each township in Figure 9. A second factor affecting groundwater accessiblility is the level to which the water will rise in a well (the static water level). Throughout the county, water in the rock aquifers is under artesian pressure, and rises in wells once the aquifer is penetrated. This can reduce the cost of pumping. Average static water levels for Hamilton County wells are shown in Figures 12, 13 and 14. Average rates of yield and water quality characteristics for each of the aquifers are summarized in the maps in figures 10, 11, 12 and Table 4. Figure 14 shows typical wells in Hamilton County, which are indexed in Figure 13. Table 1 GEOLOGIC AND HYDROGEOLOGIC UNITS IN HAMILTON COUNTY Vator-Bearlng Geologic Ate Rock Unit Description Thickness Range Hydrogeologlc Unit Characteristics Alluvlua Sand, gravel, tilt and clay Fair to large yields (JS-SO goal Quaternary Glacial Drift Predominantly till contain M-*m Surficial Aquifer low yields ing scattered Irregular (less than 10 gpa) oodles of sand and gravel Burled Channel Sand, gravel, silt and clay Snail to large yields 0eposIts Pennsylvanian Cherokee Gp. Shale, clay, tlttstone, 0-145 Aqul elude Low yields from llaestone sandstone and llaestone and sandstone St. Louis Fa. llaestone and doloalte, ?rm-3sn cherty, oolitic Burlington Fin. Mississippian Gllnore City Fb. Mississippian Fair to low yields Aquifer Hampton Fn. North Hill Gp. Shales inn-ISO Aqulclude Vellow Spring Gp. Does not yield water devonian Line Creek Fa. Doloalte Cedar Valley Fa.