Laserfiche WebLink
Water Quality <br />Water quality studies by USGS at the Rillito Recharge Project provide the best characteri- <br />zation of storm runoff from ephemeral stream systems in the Tucson basin. The studies <br />showed that urban runoff carries pollutants, such as heavy metals and organochlorine <br />pesticides, into surface waters. The vast majority of the pollutants are contained in the <br />sediments rather than dissolved in the water. Trace element concentrations in stream <br />sediments are similar to soils elsewhere in the Western United States. Based on review of <br />the existing data, USGS plans to incorporate the water quality station at Alamo Wash <br />into the National Water Quality Assessment Program to represent water quality of an <br />urbanizing area. <br />The Rillito Recharge Project also assessed groundwater quality from 14 wells in the <br />vicinity of the proposed recharge project. With the cooperation of the city of Tucson, two <br />sets of piezometers were constructed at the project site. Analysis of samples from <br />piezometers located near the channel before and after large floods showed that even <br />recently recharged water met drinking water standards. Low concentrations of <br />organochlorine pesticides and priority pollutants were detected in surface water samples <br />which contributed to recharge, but none were detected in groundwater samples. <br />Concentrations of dissolved organic carbon were higher in surface water than <br />groundwater, perhaps due to adsorption or microbial degradation. <br />Recharge Techniques and Approaches <br />The U.S. Geological Survey previously estimated that approximately 70 percent of the <br />average annual inflow to Tucson's major watercourses infiltrates (Burkham, 1970). <br />Davidson (1973) estimated that 90 percent to 100 percent of the infiltrating water is <br />recharged. <br />To improve efficiency of the natural system, stormwater recharge efforts must focus on <br />capturing flows that would otherwise leave the area of interest. Where damsites are <br />lacking, this may be accomplished by diverting the peak flows of large, infrequent storms <br />from channelized streams using structures such as side weirs and inflatable dams. Flows <br />would be directed into sedimentation basins and large off - channel storage and recharge <br />areas. Costs would be high due to the construction and land area involved. Maintenance <br />costs would be higher than for CAP or effluent recharge projects due to the greater <br />suspended sediment loads carried by stormwater. <br />Alternatively, areas with high groundwater tables may be pumped to increase the <br />subsurface storage available during natural recharge events. This would diminish the <br />availability of water to sustain mature riparian trees if pumping occurs during the <br />growing season. The approach has already been used along Tanque Verde Creek, <br />although the objective was to meet summer pumping demands, not to increase <br />subsurface storage. Public concern regarding impacts to riparian trees (documented in <br />Stromberg et al., 1992) provoked City Council to restrict pumping in the area. <br />In cooperation with the ADWR and Tucson Water Department, recharge techniques were <br />evaluated, and various methods and their advantages and disadvantages were outlined. <br />The Department of Water Resources distributed over 200 copies of the evaluation to <br />IV <br />High Plains States Groundwater Demonstration Program <br />