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Description
Water level fluctuations, water resources, and the general hydrogeology of Ballena Valley were evaluated using hydrographs, geophysical surveys, water chemistry, a water budget, and change-in-storage relationships. Seasonal water level fluctuations in fractured-rock wells range from 400 feet in the central east sub-basin to less than 10 feet in other areas. The large fluctuations correspond to water level declines of 15 feet annually, and are associated with a deep and extensive summer cone of depression that has been linked to pumping from three large irrigation wells. Water level declines of 1 to 5 feet annually are attributed to a natural, drought-induced decline or cumulative pumping effects. High Na, K, Mg, and Cl, and moderate SO42- ion concentrations correspond to the cone of depression, indicating that pumping, and perhaps irrigation, are contributing factors. Sulfate concentrations ranging from 100 to 263 ppm in 4 wells, coupled with elevated cation concentrations, are attributed to sulfide oxidation. The extent of the cone of depression and the associated high ion concentrations indicate that fractures feeding the east sub-basin wells are hydraulically continuous, although local discontinuities apparently exist. Geophysical survey interpretation suggests that some fractures are clay-filled. A water resource analysis was approached by estimating recharge using the soil moisture balance method. Over a 35 year period, estimated recharge occurred in 9 of 23 years prior to 1977 and 11 of 12 years after 1977, corresponding to dry and wet periods, respectively. Hydrograph responses to rainfall and recharge suggest that recharge occurs more frequently overall than the soil moisture balance predicts. Proposed mechanisms include direct recharge through surface fractures and concentrated recharge in stream channels. Estimated recharge and measured rainfall were used as independent inflow components to estimate specific yield from aquifer water budget equations. Linear regression analyses produced specific yields ranging from 0.01 to 0.035, which are thought to represent the residuum, or a combination of the residuum and fractured-rock. Hydrographs and cascading water indicate that the water table remains in the residuum, which drains into and recharges the fractured-rock aquifer. The water budget regression equations were shown to be a satisfactory means of predicting long-term water level trends and evaluating safe yield. Predicted hydrographs generated from the water budget equations for a 35 year period show that basin safe yield was greatly exceeded in the 1980’s. Safe yield was estimated to be a maximum of 664 acre-ft./yr. (rainfall as inflow component) and a minimum of 358 acre-ft./yr. (estimated recharge as inflow component), based on eventual recovery of water levels during wet periods. To meet the safe yields would require 26% to 60% reductions from irrigation pumping in the 1980’s, estimated as 801 acre-ft./yr. These values represent order of magnitude safe yield estimates, and are considered maximums as they allow a reduced groundwater decline to occur during extended dry periods. Current irrigation pumping needs to be reduced and more evenly distributed throughout the year in order to meet safe yield requirements, which include reducing the long-term decline as well as the superimposed extreme seasonal fluctuations.
