SU0006024 SSNL - Laserfiche WebLink

Home Browse Search

D. GROUNDWATER INFORMATION Groundwater depths and elevations throughout San Joaquin County are illustrated on maps published by the County Flood Control and Water Conservation District. Due to the high degree of variability in the elevations in the Tracy area, groundwater lines of equal depth and equal elevation are not drawn for this area. The last map denoting lines of equal elevation for the Tracy area was published in 1992 and show the general groundwater flow to be in a northeasterly direction, toward the San Joaquin River. This has been regarded as the generalized groundwater directional flow, primarily based on hi&hly accurate groundwater studies t the Tracy Defense Depot. tiz Jl lie NL-S — ,'ak 555 C SS § 2. The Monitoring Report regarding th(Yon-site hydrocarbon contamination,produced by \Amenfrin Geological Services, Inc. 6£Sonorii, California, indicates the groundwater directional flow to e in a northwest direction. This flow direction is contrary to the generally recognized northeast flow direction, and may be attributable to a number of reasons: 1.) There may be a change in underlying geology, 2.) The on-site well may be influencing flow direction(This is somewhat unlikely, and is highly dependent upon numerous factors.), or 3.) The wells were surveyed, or the monitoring well water levels were measured inaccurately(Again, unlikely.) NLS2.1 SSS§4.1 The lines of equal elevation shown i I�Monitorin Re suggest the ground ter gradient to be (44.40 ft -44.10 ft)_ 106.67 ft=0.0 281 ft/ft. Velocity of the fir` a groundwater under the project site can be estimated by the following formula: o� K(h -I x s Vequiter= L h 7.5 p Where: K=Hydraulic conductivity of aquifer formation in gals/day/ft, =500 gals/day/ftz (est.based on a silty clay formation as observed in the 15 ft test boring.) h,= 44.40ft,h,=44.10ft L= 106.67 ft p=porosity of aquifer= 35%(est.) VN.i, =0.53 ft/day or 195 ft/yr !moi The groundwater contamination has subsequent) emediated. In response to the request of EHD, the Monitoring Report/Casr Request dated March 24, 2004, and the Closure Letter from EHD is found i endix . The Case Closure Summary says the highest groundwater depth below ground surface (bgs)was 8 ft and the lowest was 12 ft. All three of the soil/groundwater borings referenced above, determined the static water table depth to be approximately 11 ft bgs. After this water level measurement, the test borings were closed in with bentonite pellets, as witnessed by David Van Dyne of EHD. A rise in the water table elevation may be primarily attributed to the cropland surrounding the property to the south and east. Irrigation recharge can have a significant influence on the underlying and surrounding water table. The critical issue regarding the groundwater elevation is: 1.)There must be sufficient distance between the soil/effluent interface and the highest anticipated depth to groundwater for sufficient treatment of effluent to occur, which is generally accepted to be five feet, and 2.) The water table cannot encroach upon the bottom of the septic tanks. This may cause buoyancy of the tanks. 6 Chesney Consulting