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N ~ N N o o , . Gypsum. an evaporite mineral responsible for the high dissolved-solid~ concentration in the shallow aquifer. is relatively abundant in the near surface soils of the Whitney area. SOlI samples IOdlcate abundances as great as 15 percent of the total sample (Bmme and Prodic, 1991). Much smaller percentage~ of gypsum, generally 1-4 percent, were found at greater depths, The one available sample taken from a reddIsh clay layer Wh Evidence suggests that discharge by evapotranspiration during summer months may be nearly equivalent to the combination of ground-water and surface-water recharge to the detention basin, Nitrate concentrations at well site PG264 (fig. 4) increase to twice their yearly average during late summer, reflecting the high concentrations in the wash 50 ft away. In other words. the wash acts as a sink during most of the year. but it can act as a source of water for the shallow aquifer during late summer. However. the quantity of water supplied by the wash during this time probably is small and localized. Furthennore, streamflow rates along a section of the wash in the Whitney area have been reported to decrease during late summer (Schmidt and Hess, 1980, p. 20), indicating that the wash may be a source of water to the shallow aquifer during late summer. Ground water may discharge from the detention basin in areas farther from the wash (south of the line of section W-Z shown in figure 4). but the quantity is probably small. ,~'. ". The tota1 discharge by evapotranspiration is difficult to quantify. owing to the large seasonal fluctuations resulting from variations in temperature and water consumption by plants. A rough conjectural estimate of the annual discharge by evapotranspiration within the detention basin is 0.5 ff3/s, on the basis of an evapotranspiration rate of 30 in/yr, During tbe summer, evapotranspiration and surface-water flow through the oudet structure probably are the most significant sources of discharge from the detention basin; ground-water discharge is probably minor. Inflow of surface water is at its maximum during summer months and is estimated to be as much as 0.8 ft3/s. However, even during peak evapotranspiration. some surface flow--probably no more than 0.3 rfls--passes through the outlet structures, Hence, the maximum net surface-water recharge that is consumed by evapotranspiration during the summer is about 0.5 ff3/s, If evapotranspiration consumes all the estimated ground-water inflow (0.2-1.0 ft3/s) during summer months, the total evapotranspiration can range from 0.7 to 1.5 fiJ/s, or 40-300 percent greater than the conservative annual rate estimated above for the entire detention-basin area. During winter months, surface-water inflow nearly equals surface-water outflow. Measurements at the inlet structure indicate that inflow is at its yearly minimum of about 0.2 ft3/s during the winter. Measured outflow at the structure along the south side of the dike was 0.3 fiJ/s during the same period. Hence, evapotranspiration during winter months approximately equals 'the quantity of ground water recharging the detention basin, minus the amount of ground water discharging from the detention basin. The rate of evapotranspiration during the winter probably is about 0.1 ft3/s. when temperatures are much cooler and hydrophytes are donnant. Thus. ground-water outflow would range from about 0.1 to 0.9 ft3 Is. Until further work can be done to better quantify evapotranspiration, these estimates represent preliminary approximations. Chemical Quality of Ground Water in the Study Area Water-quality samples were collected at all available well sites in the Whitney area in April and September 1986, and January and June 1987. The water-quality data and the interpretation of the geochemical analyses are presented by Emme and Prudic (1991). This report deals with the dissolved-solids concentrations along section W-Z shown in figure 4. The dissolved-solids concentrations along this line of section are used with the estimated aquifer properties and recharge and discharge estimates to simulate the effect the proposed sluny wall would have on the flow and chemical quality of the aquifer system. Dissolved-Solids Concentrations The average dissolved-solids concentration (based on samples from wells at all sites) of ground water in the shallow alluvial aquifer beneath the detention basin, for the four sampling periods during 1986-87, was 6,600 milli.arams per liter (m2IL)-much high" thSln in A""I:U:~ fl:l1"'th...... tn fJ.._ '"M''' '"-.____"----1 T -