IMPACTS TO GROUNDWATER QUALITY FROM A LARGE DIRECT SURFACE RECHARGE FACILITY

The Granite Reef Underground Storage Project (GRUSP)is Arizona's largest artificial recharge facility.It is an integral and essential component of the water resources management system of the Phoenix metropolitan area.GRUSP is used by the local municipalities for their short and long term water storage needs to efficiently manage the very limited water resources of this semi-arid area.It is an in-channel water spreading recharge facility with 100 hectares of infiltration surface.In excess of 750 million cubic meters of water have been stored in the overdrafted alluvial aquifer since the facility's completion in 1994.Water for potable use is pumped in the vicinity of GRUSP by several municipalities.And, although the quality of the water from the three water sources used for recharge exceeds that of the local groundwater, there were concerns that possible changes in its chemistry could result in some degree of degradation.A study was carried out to determine if there had been any impacts to the groundwater quality as a result of the recharge operation.The large baseline data base and the results of monitoring the quality of the inflow water and the groundwater in more than 50 monitor wells during seven years of operation was used.Also used were the lithologic information of the vadose zone and the aquifer from borehole drilling.Changes in the chemical composition of the groundwater with time of operation and distance from the facility were examined.These changes were correlated with temporal changes of the chemistry of the recharge water.Other factors considered were reactions in the 'clogging layer',with the rock matrix of the vadose zone and the aquifer,residence time,surface/groundwater blending and mechanisms in the zone of mixing.Results indicate that there is a small increase of total dissolved solids and total organic carbon and a decrease in fluoride,arsenic and nitrate at the site.In the seven years of the recharge operation the groundwater type has remained the same in the vicinity and distant from the facility.