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water to wells. However, for water rights determinations, the SEO considers these <br />interbedded sandstone units as a single hydrologic unit, and wells are typically <br />completed through all the sandstone units. <br />While water levels have declined significantly over time, and will continue to do so, the <br />Arapahoe aquifer over much of the Denver Basin still is in a confined condition (i.e., the <br />water in the Arapahoe aquifer is confined under hydrostatic head greater than <br />atmospheric pressure by overlying relatively impermeable strata.) When a well under <br />confined conditions is pumped, the elastic properties of the aquifer control the movement <br />of the water to the well. Under confined conditions, hydrostatic head is reduced due to <br />pumping, which results in an increased aquifer load. This increase in the aquifer load <br />results in a compression of the aquifer that forces some water from it. Additionally, this <br />lowering of the hydrostatic head causes a small expansion, with a subsequent release of <br />water (Todd, 1980). Simply stated, assuming a nonleaky aquifer, producing water under <br />confined conditions results in a lowering of head in the entire confined portion of the <br />aquifer. Likewise, injection of water into a confined aquifer simply increases head in the <br />entire confined portion of the aquifer. <br />Project Activities <br />As described in the Plan of Development (John C. Halepaska & Associates, Inc. [JCHA], <br />August 1990), the technical portion of the demonstration project involved a cyclical series <br />of injection runs and pumping cycles. Water was injected into well A -6A over periods <br />ranging from 7 to 42 days. During the period of injection, a datalogger was used to <br />monitor water level changes in both the injection well (A -6A) and the monitoring well <br />(M -1) as well as to continuously monitor the injection flow rate. <br />Field water quality parameters (pH, Eh, specific conductance, temperature, and dissolved <br />oxygen) were monitored continuously during each injection run with a flow- through cell <br />equipped with separate probes to measure the above parameters. In addition, a <br />chlorimeter monitored residual free and total chlorine concentrations. To supplement the <br />field water quality parameters, an independent certified laboratory analyzed weekly <br />water quality samples from the source water for a select set of water quality parameters. <br />A more complete set of water quality parameters was analyzed quarterly. <br />Periodically, the injection runs were shut down to allow well A -6A to be cleaned through <br />a constant - discharge pump test. These pump cycles between each injection run not only <br />allowed the well to be cleaned to minimize potential clogging but also helped evaluate <br />changes in aquifer hydraulic characteristics and well efficiencies as the injection study <br />proceeded. Typically, each pump cycle would consist of a 4 -hour constant - discharge <br />pump test and a 4 -hour step- discharge pump test to evaluate hydraulic changes in either <br />the formation or the well. <br />Water quality samples were also obtained from the injection well and the monitoring well <br />during each pump cycle. As with the water quality samples obtained from the source <br />water, well water was generally analyzed after each injection run for a select water <br />quality parameter list and quarterly for a more comprehensive list of water quality <br />parameters. <br />M <br />High Plains States Groundwater Demonstration Program <br />