ARCHIVED REPORTS_XR0012657 - Laserfiche WebLink

Home Browse Search

9 December 1991 (GeoAudit CV 4ZF5-1.26) Page 2 of. 7 DEPTH TO GROUNDWATER According to the Fall, 1990, "Map of Depth to Groundwater", preparr:d by San Joaquin County's Department of Public Works, Flood Control Section, groundwater underlies the site at a depth of 30--35 ft (approximately 20--25 ft below sea level) . Drilling at the 1320 W. Weber site, however, reveals that first groundwater is considerably shallower than this. According to the Fhase 11 report of WFF Environmental, wells dr 'led in 1989 and 1991 at that site reportedly encountered first groundwater in "Sand 1" at depths of between 13 and 19 ft. below grade. After well completion, groundwater subsequently rose a few feet above the base of "Clay A" and stabilized at 10-12 ft below grade, suggesting that "Sand A" is a semi.-confined aquifer and that slightly artesian conditions exist. Second groundwater stabilized in "Sand 2" approximately 3 ft below the piezometric surface of first groundwater in "Sand 1" in March, 1990. This indicates that "Sand 2" is a confined aquifer and that "Clay B" hydraulically separates "Sand 1" and "Sand 211. GROUNDWATER GRADIENT The site is located on the western edge of a moderately large groundwater depression that occupies the east-central portion of San Joaquin County. According to the SJCDPW, this depression has enlarged and deepened in the late 1980's and early 1990's, and the present maximum depth to groundwater in the depression is in excess of -70 ft below sea level. The depression is probably due to withdrawal in excess of inflow from the groundwater recharge area in the Sierra Nevada foothills to the east. Hence, regional groundwater flow is toward the center of this depression. Local flow in the vicinity of the site should therefore be in a generally eastward direction. However, the local gradient may be somewhat altered by the Stockton deep-water ship channel, which is located approximately 200 ft north of the site. The'water surface in the channel is approximately at sea level, roughly 10 ft above the depth of groundwater in the two on-site monitoring wells. Theoretically, the local flow could therefore be somewhat more to the south, out of the channel. This is apparently the case, because the groundwater gradient map prepared by WHF Environmental for 1320 W. Weber indicates a southeast gradient of approximately 0.45 ft per 100 ft. PREVIOUS SITE INVEITIGATIONS Thetwo underground tanks were removed by Jim Thorpe-OiT Co in a April, 1991. Neither tank was in use at that time, and neither will .be-replaced. if