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MWSI Project Phase II Conjunctive Use Summary Report - DRAFT August 22, 1995 Well efficiency and aquifer transmissivity declined as a result of the injection/pumping cycles, Additional sand in the pumped water was also observed, Halepaska and Associates (JHA) reported that the reduction in the well's characteristics and larger-than-expected head buildup during injection was attributable to temperature differences between injected and native waters which results in viscosity differences between the two waters. "These thermal gradients create a differential in viscosity, which makes the mixing of these waters a slower process" (Halepaska and Associates, 1989), By the end of 1989 the Willows study had concluded that the injection of treated surface water into the Arapahoe aquifer was feasible, The study recommended that issues related to head buildup during injection and water quality should be examined in additional detail. The cooperative agreement for the Denver Basin Aquifer Recharge Demonstration Project was signed in December, 1990. Denver Water supplied the treated surface water for injection through Conduit Ill. Because Well A-5 was pumping some sand, it was not considered a viable candidate for further injection and recovery testing. The project decided to continue its work with Willows Well A-6A. To monitor aquifer hydraulics and water quality, Arapahoe aquifer monitoring Well M-I was installed 110 feet from Well A-6A in 1991. Injection and recovery work first started at Well A-6A in February of 1992, Injection is performed in the winter when the well is not needed for water supply to the Willows District. After injection and recovery cycles were performed on this well hydraulic analyses indicated a decline in the well's specific capacity. In the summer of 1993, the well's specific capacity had declined to 42 percent of its original value. In the fall of 1993, remediation work was performed on the well in an effort to restore the well to its original specific capacity, Potential causes for the reduction in specific capacity, well screen or gravel pack clogging, and aquifer clogging were thought to be iron bacteria (biofouling), chemical precipitation of iron and manganese, a build up of suspended solids, dissolved gases in the injectant water which may come out of solution, entrained air and the temperature of the injected water. A well remediation program was prepared to improve the well. The well rehabilitation was performed from October to December, 1993. The well was chlorinated, swabbed, bailed and treated with liquid carbon dioxide, A subsequent pumping test showed improvement to the well but did not result in the expected increase in specific capacity. The results of the remediation work strengthened JHA's belief that the reduction in specific capacity was related to the low temperature of the injected water when compared to the background aquifer temperature, The pre-injection aquifer water temperature was 15.3 degrees centigrade. The temperature of the injected water generally ranged from more than three to less than 15 degrees centigrade. Halepaska and Associate's experience with A-6A demonstrates to them that water temperature and viscosity differences can cause significant reductions in the hydraulic characteristics in an injection-recovery well. Because of the stratified and heterogeneous nature of the Arapahoe aquifer sandstone, more water is injected into the sandstone layers with the better hydraulic properties. JHA reports that cold water injected into these more permeable layers reduces the conductive nature of the aquifer. Data and analyses concerning the 10