SU0003307 - Laserfiche WebLink

Home Browse Search

Existing groundwater quality data from the groundwater monitoring program indicates that the upper aquifer in the vicinity of the plant is divided into two distinct units. The shallow groundwater sampled by the plant monitoring wells is high in nitrates and dissolved solids. A hydraulic barrier which restricts the vertical movement of groundwater separates the groundwater in the upper stratum from the groundwater in the Laguna formation. The domestic well monitoring program samples groundwater at a depth of approximately 180 ft. During the past 4 years, nitrate concentration for the domestic wells has averaged 13 mg/l while specific conductance has averaged 420 umhos/cm. This evidence suggests that the layer of clay between the 50 and 150 ft level acts as a barrier to prevent mixing of water from the shallow aquifer with the stratum penetrated by the domestic wells. The lateral extent of this clay barrier is unknown. Edward Steffani of the Stockton East Water District maintains that there is some degree of hydraulic continuity between the shallow surface groundwater and the aquifer in the Laguna formation (4) . Locally, however, the clay layer appears to act as a barrier to vertical movement of groundwater. Figure 5 shows the site specific aquifer system. Table 2* WATER QUALITY PARAMETERS FOR DOMESTIC WELL MONITORING PROGRAM Nitrate Conductivity Year (mq/1 ) (umhos/cm) 1985 7.9 353 w � jsir �j��d, u ly 1986 6 326 1987 15.5 367 1988 8.8 479 * Average of four samples It should be noted that well four at 6,383 East Arata yielded water of relatively poor quality. It appears that this well draws water from the 6�� shallow groundwater and it was not used in the evaluation of data for Table 2. Groundwater Gradients The San Joaquin County Flood Control District has prepared equal elevation water contour maps biannually for the past decade. Included in the appendix are the water contour maps for 1985 and 1987. It is evident that the QAP site is directly overlying the low point of the pumping depression northeast of Stockton. For this reason the direction of the gradient may shift quite often. Site specific groundwater gradients estimated on the basis of AMSL elevations in three monitoring wells indicates that flow is generally to the east or northeast of the site. Correspondence between gradients shown on the county equal elevation contour maps and monitoring well data is quite good. 1-7 RP033-L