ARCHIVED REPORTS_2013_7 - Laserfiche WebLink

Home Browse Search

Based on individual well aquifer recharge tests conducted during the EMP field investigation, hydraulic conductivity values in the uppermost aquifer range from 3.83E-02 to 5.9E-03 ft/day with a geometric mean of 1.19E-02 ft/day. Published values of effective porosity for fractured, clayey sand range from 2 to 25 percent (Fetter, 1994). Using these values, groundwater flow velocities were calculated ranging from 0.0011 foot per day (using minimum values) to 0.086 foot per day (using maximum values), with an average flow velocity of 0.023 foot per day (or about 8.3 feet per year). Historical groundwater equipotential data suggest that groundwater in the deeper aquifer flows to the northeast at a gradient of about 0.004 ft/ft, generally parallel to the regional topographic gradient toward the floor of the Central Valley. Hydraulic properties are not available for the deeper aquifer. 2.4 WELLS WITHIN ONE MILE OF THE SITE A search of groundwater supply wells within one mile of the site indicated that eight wells have been constructed in the vicinity of the CHSL (Figure 4). Of these, four have been abandoned, and the other four are screened in the deep, regional aquifer (GLA, December 2012). 2.5 NATURE AND EXTENT OF GROUNDWATER IMPACT Understanding of the nature and extent of groundwater impacts is based on historical groundwater chemistry data from routine monitoring reports and data obtained from the recent EMP completed for the site (GLA, 2012). 2.5.1 Conceptual Model for the Release Area Groundwater in the release area is encountered perched above or within the Corcoran Clay at depths ranging from 17 to 83 feet below ground surface. The depth and dip of the Corcoran Clay and anecdotal evidence for site workers at the time suggest that its southern limit is likely to have been beneath the footprint of the northern third of the landfill, but was excavated during landfill construction. Based on the difference in first encountered and static water levels, it appears that water in the Corcoran Clay is perched or under partially confined conditions. • Groundwater samples from wells MW-5 and MW-8, which monitor the shallow aquifer, have historically contained trace and low quantifiable concentrations of VOCs. Data from the Engineering Feasibility Study Geo-Lo Page ■ 6 Corral Hollow Sanitary Landfill ASSOCIATES January 25,2013