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~ C~~ "oJ N c:..; C <. The second source of recharge is surface water that enters the detention basin at the north side of the dike from the Monson Road floodway, and treated effluent in the flood plain adjacent to the wash (fig, 3), The quantity of surface-water inflow to the study area probably is similar in volume to ground-water inflow on a yearly average. but varies more widely from season to season than ground-water inflow. Floodway flow and discharge of treated effluent are significantly greater during summer months when water use is aI its peak. Estimated inflow to the detention basin ranges from 0.2 to 0.8 ft3ts, and has a yearly average of about 0.5 ft3ts (inflows can be much higher during stonn events [D. Art Tuma. D,S, Bureau of Reclamation, oral commun" 1987]). Surface-water inflow provides water for vigorous hydrophyte growth in the eastern part of the detention basin (fig. 3). Additional inflow to the detention basin along Tropicana floodway is substantiall,.h\lt because the flood way is on the downgradient side of the detention basin and all the inflow leaves thfough an outflow structure farther to the east (fig. 3), its significance as a source of recharge to the study area is minimal. However, the Tropicana flood way does act as a sink for shallow ground water (Jess than 5 ft below land surface) within the detention basin. Discharge from the Detention Basin To estimate discharge from the detention basin, time-dependent physical processes affecting discharge need to be identified. These processes are important in developing an accurate conceptual model that will be used to simulate potential hydrogeologic effects of the proposed slurry wall. The quantity of discharge from the detention basin varies greatly from season to season, primarily owing to evapotranspiration--the main process by which water leaves the study area. Other sources of discharge include ground-water outflow beneath the dike structure and surface-water flow through outlet structures in the south side of the dike. Additional drainage of the aquifer occurs along the Tropicana floodway (fig. 3) when ground-water levels are above the excavated level of the channel. 'lbe total quantity of ground-water outflow is the most important source of discharge to estimate, because ground water is responsible for transporting appreciable dissolved-solids loads to the wash. Evapotranspiration greatly influences the quantity of ground water that leaves the detention basin, Water levels generally are from 2 to 6 ft below land surface in the western pari of the area. and range from o to 4 ft in the eastern part of the area. These shallow water levels are ideal for pbreatophyte and hydrophYle communities to thrive. Hydrophytes occupy the eastern part of the area (fig, 3) where inflow from the Monson Road floodway and flood plain provides enough water to inundate the area with several inches of water during periods of low evapotranspiration; surface water often flows through the detention basin in small stream channels occupying topographically low areas. Upgradient from the inlet structure, in a topographically higher area. pbreatophytes dominate, Evapotranspiration is greatest during summer months, when plants are thriving and temperatures are high (fig. 7). The lowest recorded water levels at most wells coincide with this period of high evapotranspiration. From September to early March. when much of the vegetation is donnant, water levels rise, reaching their peak between January and March. Figures 8 and 9 show the seasonal water-level fluctuations at selected well sites in areas dominated by hydrophytes and phreatophytes. Maximum yearly changes in water levels due to evapotranspiration range from 3.5 ft in phreatophyte-dominated areas to 2.0 ft in hydrophyte-dominated areas, The smaller net change in the hydrophyte areas probably is due to the influx of surface water that buffers the water-level response caused by evapotranspiration, -19-