SU0002167 SSNL - Laserfiche WebLink

Home Browse Search

B. SOIL PHYSICAL CHARACTERISTICS AND ANALYTICAL TEST RESULTS The project site consists of level terrain with a slight declination toward the northeast as illustrated on the attached USGS map. As referenced in the Soil Survey of San Joaquin County, the indigenous soils consist of Stomar Clay Loam. The soils in the area of the leachfield are extremely clayey and tight. On April 26, 1999, a Case backhoe was used to observe soil strata delineation, obtain soil samples and excavate down to the water table for static water table measurements. The attached log of boring illustrates the subsurface soil characteristics. The surface soil (0-2') is composed of dark brown, high plasticity clay, often referred to as a "gumbo" clay. From three to 11 feet, exists a light brown silty clay stratum. Both of these strata possess slow permeability. The groundwater or zone of saturation was encountered at 10 feet below existing grade. A hydrometer test was performed on a composited soil sample taken from 12 inches to 48 inches below existing grade. This test was done under the American Society of Testing and Materials (ASTM) D 422: Standard Test Method for Particle-Size Analysis of Soils. Test results indicate the soils from this depth possess a clay content of 42%. This assigns the soil a clay identification under the United States Department of Agriculture (USDA) classification system. To further characterize the soil, an additional test was done, referenced as ASTM D4318: Standard Test Method for Liquid Limit Plastic Limit and Plasticity Index of Soils. Test results indicate the soil has a liquid limit (LL) of 48 and a plastic limit(PL) of 18. The Plasticity Index (PI) =LL - PL; therefore, the PI of the soil is 30. This gives the soil a Unified Soil Classification System (USCS) designation as a "lean" clay (CL). In addition to the physical tests conducted on the soil samples, chemical analyses of soil composited from the 12 to 48 inch depths was done to quantify several constituents. The important parameters for nitrate loading assessment are the organic matter,pH, cation exchange capacity (CEC), and nitrate-nitrogen(NO3-N) content. As noted,the organic matter and nitrate- nitrogen in the soil is low and indicates a potentially small influence on additional nitrate- nitrogen loading in the future. The pH of the soil is alkaline which may slightly inhibit ammonium (NH4) formation and stability. The CEC measures the ability of the soil to theoretically trap and hold ammonium molecules. A CEC of 23.5 as measured, indicates there is potential for the retention of ammonium molecules for microbial uptake. C. PERCOLATION TEST RESULTS Perc tests were conducted on February 15 & 16, 1999 by Mr. Curtis. The tests were done under U.S.E.P.A. and San Joaquin County Environmental Health guidelines near the existing leachfield. Test results indicate a failed perc rate of 120 minutes/inch (min/in), which is twice the minimum acceptable rate of 60 min/in. This perc rate is indicative of the high clay content native soils and demonstrates that the installed conventional leachfield cannot manage the current effluent flows as referenced above. 2 Vaffey Aq Wfsearck