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'o::t' :;.,') M N C o usually occur or when sudden thundershowers swell the river. It was thought that the extraction of sa1 ine ground water would need to be suspended during high flows to avoid infiltration of fresh water into the aquifer being pumped. The barrier could be constructed as a buried earth dam by excavating a trench in the alluvial bed down to an impermeable "bottom" and placing a compacted earth "dam" or by installing a relatively thin (5 to 8 feet) curtain of impermeable clay by "slurry trench" construction. The latter case would not require excavating a dry trench because the dense clay and gravel slurry would support the sidewalls of the excavation as it progressed, permitting construction "in the wet." To minimize cost, the barrier would be constructed at a point where the combined river and flood plain were relatively narrow. The sa1 ine ground water extracted from the riverbed would be disposed of in evaporation ponds on relatively even terrain adjacent to the ri ver. Subsurface Exploration To check for the presence of the suggested sa 1 i ne underflow, a row of test wells was drilled along a line across the river and flood plain at a potential barrier site about 4 miles downstream from Riverside, as shown on the Frontispiece Map. The drilling program was designed to obtain data of various types needed for the investigation. The extent of the alluvium through which a barrier would be needed was determined by drilling through the alluvium and locating the under- lying impermeable formation. The hydraulic properties of the alluvium were determi ned by pumpi ng a test well. The sa 1 i nity of the ground water was determined by taking water samples from the wells. Four wells were drilled under contract. The well spacing, depths, and the physical data obtained are shown on Figure 2. It was found that the riverbed consisted of alluvial fill up to 91 feet deep 15