Laserfiche WebLink
<br />{JiJliilo <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />polyphosphate in the injectate water did not sequester iron and prevent its oxidation and <br />subsequent precipitation. Thus, the addition of polyphosphate in the injectate water would <br />not reduce the potential for plugging from iron oxidation during a recharge event. <br /> <br />The computer program PHREEQE was used to model chemical interactions between the <br />injectate and groundwater as they mix and equilibrate. Volume-based mix proportions of <br />groundwater to injectate water were used to simulate the range of expected mixtures ahead, <br />across, and behind the mixing zone. The model results confirmed the iron precipitation <br />potential. However, the model results also indicated the buffering capacity of the native <br />groundwater could control pH increases that cause calcite precipitation. <br /> <br />Column experiments were also used to demonstrate water quality interactions at the mixing <br />front as the injectate moved through the aquifer formation. These experiments used strata <br />material actually recovered from test holes at the recharge well site to represent aquifer <br />conditions. Results showed removal of calcium, magnesium, and manganese, coinciding <br />with an unexpected high pH front at the injectatel groundwater interface. The elevated pH <br />was speculated to indicate precipitation. Increased head loss in the aquifer media was <br />attributed to such precipitate deposits. <br /> <br />General Conclusions <br /> <br />The water use permit issued to the State of South Dakota allowed for recharge for any <br />90-day period from March 1 until July 31, subject to minimum flow requirements in the <br />James River. For example, riverflows during 1992 and 1993 would not have been sufficient <br />to allow the project to conduct recharge. The first recharge event occurred from June 2 <br />through July 29,1994, for a total of 58 days. Approximately 9 million gallons of treated <br />surface water were injected into the aquifer. Recharge operations were suspended several <br />times due to problems at the Huron water treatment plant and a pipeline failure, resulting <br />in 50 days of actual recharge. The injection rates for recharge ranged from 100 to 150 gpm, <br />the rate determined by the requirement to keep the water level approximately 5.0 feet below <br />the ground surface at the injection well, with the rate of recharge gradually decreasing as <br />the recharge event progressed. The second recharge event occurred from June 14, 1995, to <br />September 13, 1995. Approximately 9 million gallons of treated surface water were injected <br />during this period. Due to the high water levels in the aquifer resulting from abnormally <br />high rainfall in 1994 and 1995, the recharge rate was about 75 gpm. The head level was <br />maintained at about 2 feet below the ground surface. One flushing event was conducted <br />during the recharge interval in an attempt to improve the hydraulics of the system-this <br />resulted in only a short-term improvement. <br /> <br />The potentiometric water levels in the confined glacial aquifer were increased above <br />background water levels due to recharge (shown in figure 6). Monitoring of native ground- <br />water and injectate quality indicated the injected water migrated as a bubble about 90 to <br />150 feet in the southerly direction from the injection well, displacing the native <br />groundwater. <br /> <br />Dispersion and mixing occurred at the interface between the native groundwater and the <br />injectate. It appears that ion exchange may have produced removal of calcium and <br /> <br />13 <br />