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Description
Groundwater pumping is the sole source of water supply in the Joshua Tree groundwater sub-basin in the southwestern part of the Mojave Desert; miles from Los Angeles, California. As a result of groundwater pumping for public supply in excess of recharge, water levels in the area have declined more than 50 feet since the 1950’s. The Joshua Basin Water District (JBWD) is concerned about the long-term sustainability of the aquifer and is contemplating managed aquifer recharge using water imported from northern California. The United States Geological Survey (USGS) has completed test drilling and instrumentation in the 400 foot thick unsaturated zone at a proposed site near the community of Joshua Trees. Drill cuttings were collected at 1-foot intervals using the Overburden Drilling Exploration method (ODEX). The method uses air rather than water as a drilling fluid. Extraction and analysis of chloride nitrate, and other soluble anions from selected cuttings of unsaturated alluvium in the Mojave Desert, near Joshua Tree, California was done to determine the accumulation of the soluble salts at the proposed recharge site. The drill cuttings were oven-dried, sieved sediment of different depths, hydrated with water and shaken overnight to extract the soluble salts. The resulting extract was filtered through a 0.45 micrometer pore-sized filter prior to analysis by ion chromatography for soluble anions. The mass of chloride and nitrate in the unsaturated zone is used to estimate the time since recharge and the long-term downward water flux of the arid area. Data from this work were compared with previous data to estimate precision and accuracy of the technique. The previous data of the studied area show that it has been 10,000 to 12,000 years since recharge. The data from this study has showed similar results in that the last time the area saw recharge was 8,000 to 9,000 years.
