SU0006056_SSNL - Laserfiche WebLink

Home Browse Search

7ou nding Analysis Since the groundwater was measured at 20.5 feet from sounding the on-site domestic well, adverse mounding potential should be nonexistent and inconsequential. Additionally, since a large percentage of sand was encountered in the 10 ft test boring, this would also, in all likelihood, / prevent the formation of the mound phenomenon under the disposal area. A distance of approximately 20 feet between the soil/effluent interface and the top of the underlying water table should be maintained. For wastewater treatment(destruction of bacteria and viruses) to occur, the distance effluent should travel under unsaturated conditions through the soil environment is generally regarded to be 5 feet. E. SURFACE WATER INFORMATION Storm water management is proposed to be three sets of subsurface infiltration chambers, as illustrated on the Site Plan produced by RBF Consultants. There should be no impact to percolating water from these subsurface structures, and in fact are upgradient to the future wastewater disposal area. Since the groundwater directional flow is north-northwest, percolating rainwater from these structures into the top aquifer mixing layer may create a dilution effect to the downgradient percolating septic system effluent. Groundwater dilution effects can only be truly discerned with complicated groundwater monitoring and modeling. According to a scientific paper authored by Shaw and Turyk (1994), reduction of nitrate concentrations in groundwater can occur primarily through dispersion, or by percolating rainwater recharge. li a* ,ata: Rainfall versus Evapotranspiration NL§3.2. It is generally and scientifically accepted that when precipitation exceeds evapotrans- piraion (E o), deep percolation of surface rainwater may occur. Evapotranspiration refers to water evapora 'on that takes place both at the soil surface and transpiration by plants, and is defined as water loss from a large field of 4-to-7 inch tall, cool-season grass that is not water stressed. The following chart illustrates rainfall and evapotranspiration for the Stockton area, which is the closest data to Manteca available. Precipitation data was obtained from the Soil Survey of San Joaquin County. The conservative "2 years in 10" data was used and not the "average rainfall" data. Evapotranspiration data was obtained from the California Irrigation Management Information System (CIMIS): TABLE 3 PRECIPITATION AND Eto DATA FOR STOCKTON, CA Month 2 Years in 10 will have more Eto Average Potential Deep precipitation than: (in/mo) (in/mo) (in/day) Percolation (in/mo) January 4.42 1.24 0.04 3.18 February 3.20 1 1.96 0.07 1.24 7 Chesney Consulting