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<br />WATER <br /> <br />The water resources of the Big Blue Basin consist of <br />variable surface water supplies and ground water supplies of <br />considerable magnitude. Interest by basin residents in <br />control and conservation of the ground water supply is <br />increasing. <br /> <br />Ground Water <br /> <br />Ground water is the primary source of irrigation water for the nearly half-million acres <br />of land being irrigated in the basin. The best sources of ground water are the Quaternary age <br />deposits of sand and gravel having high permeability. The bedrock materials underlying the <br />aquifers are mostly impermeable in nature. Some bedrock in the southeastern part of the basin <br />is somewhat permeable but yields water' too highly mineralized for most uses. The geology of <br />the basin reflects the xinfluence of glaciation periods. The glaciation extended only into the <br />eastern parts of the basin but the advance and retreat of four major glaciations are reflected in <br />the underlying deposits throughout the basin. The most permeable deposits in the basin are <br />generally the sand and gravel layers of the Illinoian age (the third major glaciation). The <br />thickness of these layers is variable and ranges from a few feet to about 200 feet. Gravel <br />layers of other glaciation periods also occur but normally are separated by glacial drift or clay <br />lenses. These sand and gravel layers occasionally occur in a continuous strata, resulting in an <br />aquifer several hundred feet thick. <br /> <br />The thickness of the water bearing material has a bearing on the yield of water from <br />the aquifer, however, the porosity and transmissibility primarily determine the actual arilOunt <br />of water available. <br /> <br />Porosity refers to the void space in the water bearing material and is usually expressed <br />as a percentage of the total volume. Effective porosity, or specific yield, is the part of this <br />water filled void space which can be removed by gravity. Much of the water contained in an <br />aquifer is bound by surface tension to the, surrounding material and is thus unavailable. The <br />degree of this retention establishes the specific yield. <br /> <br />Transmissibility is a measure of a material's ability to transmit water. The coefficient of <br />transmissibility refers to the amount of water in gallons per day passed through a one-foot <br />vertical strip of aquifer when the slope of the water table is 45 degrees. Transmissibility is <br />sometimes expressed in units of gallons per day per mile of aquifer under a gradient of one <br />foot per mile. Figure II shows areas having transmissibility rates of 20,000 to 200,000 gallons <br />per day per mile. These quantities can be roughly converted to gallons per minute yield of <br />wells by dividing by 100, assuming that the static water level is 100 feet above the ,base of the <br />permeable section and the drawdown while pumping is about 30 feet. <br /> <br />25 <br />