SITE HISTORY - Laserfiche WebLink

Home Browse Search

3-2 Regional Groundwater Occurrence and Flow The aquifers occurring in the unconsolidated rocks may be divided into two water-bearing zones: (1) an upper water-bearing zone and (2) a lower water-bearing zone. The upper water-bearing zone, containing confined, semiconfined, and unconfined water , lies within the ;Quaternary age alluvial deposits and the upper section of the Tulare - Formation. In the absence of near--surface confining strata, the top of the upper zone is the water table. Where near-surface confining strata exist, the top of the upper zone is at the base of. these strata, and a separate, perched shallow water-bearing zone. containing unconfined water occurs above these strata. The lower ��water-bearing zone contains confined water in the lower section of °t'he Tulare Formation. The . base of freshwater is considered to be at the base of the Tulare Formation. Groundwater occurring in °the underlying consolidated rock is generally saline. The location of ' the nearest municipal well is shown on Figure 1-1. The static water level in this well is approximately 45 feet Lelow .ground surface . This measurement was obtained from the City of Tracy and is included in this report with their permission. It is unknown " at what time of year the measurement was made. Itis assumed that, this static water level represents the water table surface of an aquifer in the unconsolidated deposits. Recharge to the regional groundwater system occurs by percolation of rainfall and applied irrigation water , by percolation from canals and streams, and by ' subsurface inflow from the west. Regional groundwater movement is generally to the northeast towards the San Joaquin River : The City of Tracy well identified on Figure 1-1 is upgradient from the ARCO station being investigated. Agricultural and municipalipumpage may affect the flow direction 'iocally. Site Geology Geology underlying; the ARCO service station was determined from the borehole logs of" the four monitoring' wells installed at the site. These logs are in Appendix D. The borehole logs of wells ATR-1, ATR-3, . and ATR-4 were used to construct cross section A-A' shown on Figure 3-1. Cross section B-B' was constructed from the borehole logs of wells ATR-2 and ATR-3 :as shown on Figure 3-2. Locations of the sections are shown on 'Figute 2-1. Immediately underlying the station is a ; series of sand, silt, and clay. There appears to be a relatively continuous layer of silt from ground surface to approximately 20 feet below ground surface. BROWN AND CALDWELL CONSULTING ENGINEERS 5