ARCHIVED REPORTS_1996_2 - Laserfiche WebLink

Home Browse Search

L4 � I L E water -holding capacity and a high shrink -swell. potential. Although not classified as hydric, these soils may contain inclusions of soil types that are classified as hydric (U.S. Soil Conservation Service 1991). Soils within the active landfill have been highly modified and have been reclassified as "dumps", a general category that includes landfills and associated activities. 11 P4 0 -,T#NMB W a. F3 :77ij v DUR kv The project site is located within the Littlejohns Creek watershed. Topographic modifications of the landscape associated with the current landfill and historical agricultural practices have influenced the site's hydrologic characteristics (Environmental Science Associates 1993). The portion of Littlejohns Creek on the project site has a UWzoidal cross section with 2:1 side slopes (Jones & Stokes Associates 1995) (Figure 2-1). The average floodplain width is 46 feet and the average low -flow channel wide is 16.6 feet. The channelbanks are earthen and steep, and a strongly cemented soil hardpan forms the channel bed and lower bank for most of the reach. Surface runoff draining from the existing landfill surfaces has contributed to the fomiation of gullies and bank erosion perpendicular to the banks. Drainage ditches, one near the landfill entry and two parallel to the western property boundary, have piped outfalls that discharge to the creek onsite (Environmental Science Associates 1993). 0 Compared with heavier winter flows, creek flows in summer and fall are greatly reduced (Environmental Science Associates 1993). Figure 2-2 documents the City's observations of water levels during winter 1995; the photographs in Figure 2-3 (taken in spring 1995) represent typical spring flows for the creek. The SJCFCD projects along the south fork of L&kjohns Creek, which includes the north • south branches of the south fork, axe generally maintained at a 30- to 50 -year level of protecti tYe nro�lj siJ 07,7� 1&Ici lo,JxItfig dti-s�,Tq a ,At*)* I TO'S P"T (Z31 The project area is not subject to substantial groundwater recharge, although some infiltration and percolation to the groundwater system is expected along the north branch of the south fork of Uttlejohns Creek. Area groundwater levels have dropped by approximately 10 feet over the last 20 years as a result of agricultural pumping demand and drought. Typically, the water table is at depths averaging 69 feet below the surface but may seasonally fluctuate by from 13 to 23 feet depending on the amount of rainfall. Groundwater in the project area generally flows north. The existing landfill has a groundwater treatment system designed to remove volatile organic compounds. The treated groundwater is discharged to a storm drain and then into the north branch of the south fork of Littlejohns Creek. Wetland Mitigation and Monitoring Plan for the Chapter 2. Existing Conditions Austin Road landfill Expansion. U. S. Arm , y Corps of Engineers File 199400974 Ciry of Stockton 2-2 April J. 1996