ARCHIVED REPORTS_1981 - Laserfiche WebLink

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Hydrologic Conditions/Water Quality Surface Water The proposed site is located in the uppermost reaches of the South Paddy Creek watershed. Drainage on-site flows across the property from east to west into a small tributary to South Paddy Creek. This flows :into South Paddy Creek, then to Bear Creek, then to Mosher Slough between Bishop Tract and Atlas Tract northwest of Stockton. Mosher Slough flows to Disappointment Slough of the Sacramento - San Joaquin Delta System. The approximate watershed size in- cluding the project site is 850 acres. A small portion of runoff from the south end of the site could drain southerly to Mosher Creek; however, this appears to be negligible. Surface water on-site is intermittent and originates from two sources. Most is storm runoff; 70 percent of which occurs between December and March. During the irrigation season, however, it is not unusual for irrigation discharges from higher fields to flow in the tributary to South Paddy Creek. It is estimated that a storm with a one chance in a hundred probability of occurrence, would produce approximately 250 cubic feet per second of run- off from the site. Existing runoff at the site flows largely as sheet flow north to northwesterly. The U.S. Army Corps of Engineers has indicated that the site is not sub- ject to flooding by the Calaveras River or Mosher Creek. The Corps of Engineers also indicated that flooding in the study area was not studied in detail because of the small size of the South Paddy Creek watershed. However, San Joaquin County 's flood management maps show the site not within a one chance in a hundred flood plain. r,rntindwatar The proposed landfill site lies within the San Joaquin Basin. An on-site field exploration to collect groundwater and hydrogeological data was made by J. H. Kleinfelder and Associates. Data relating to the hydrogeological setting was obtained from 16 geologic bore holes. As part of this on-site drilling pro- gram, one boring situated in the extreme southwest property corner was drilled to ascertain the location of groundwater and perched water or lack of it. The geologic formation which underlie this region influence the occur- rence, movement, and quality of groundwater. The oldest significant fresh water bearing formation on the east side of the County is generally considered to be the Mehrten formation. Many of the sands of the Mehrten formation are highly permeable and furnish water of good quality to deep wells. The hydro- logic characteristics of the Laguna formation which overlies the Mehrten for- mation are variable. Bodies of perches water are common in its outcrop area. Despite the thinness of the Quaternary all-uvial sediments, a number of wells obtain large yields from the sand and gravel stringers. However, most of these wells often penetrate the underlying laguna and in many cases, the Mehrten formation. Most water used in the eastern portion of the County is