SR0082543_SSNL - Laserfiche WebLink

Home Browse Search

Revised November 4, 2020 BGG Job No. G164.01 development area, soil denitrification factor, rainfall, rainfall nitrate concentration, waste effluent quantity, and waste effluent nitrate concentration from onsite septic systems. Mass Transport Soil Properties Field percolation test results indicate that the surface soils are moderately permeable.The site soils will permit some infiltration of water through the surface soils.The longer the effluent remains near the surface, the more biologic treatment may occur. If permeable soils exist to the depth of the ground water aquifer, it is possible that any remaining surface contaminants may be transported to the saturated zone of the upper aquifer. Nitrate Sources Septic systems and dairies have been identified to be the most significant sources of nitrate impact to groundwater. Orchard fertilization in sandy soils is also a significant source of nitrate contamination in shallow or sandy aquifers in San Joaquin County. Rainfall Contribution An estimate for rainfall quantity and quality was required for this nitrate loading analysis. The H&F method utilizes the"average recharge rate of rainfall"for calculating potential nitrate impacts to the aquifer. Some portion of total rainfall will dissipate by evapotranspiration,the sum of evaporation and plant transpiration, hence reducing the amount of rainfall percolation into the aquifer and percolating effluent, especially in summer months. Rainfall recharge (deep percolation of rain) is a significant factor in the nitrate loading estimate; however,data on rainfall recharge rates is not widely available. For H&F's study conducted in the Chico area,a recharge rate was calculated to be 75 percent of the average rainfall. Since the Chico area is similar geologically to San Joaquin County region, an estimated recharge rainfall for this study was calculated based upon 75 percent of annual rainfall for the project area. Rainfall data for the IHO site area was reviewed from many sources, including data from the California Irrigation Management Information System(CIMIS)Staten Island Station#242 and the U.S. Department of Commerce National Oceanic and Atmospheric Administration(NOAA). The CIMIS Staten Island Station,located approximately 12.5-miles west-northwest of the site,reports the average monthly evapotranspiration(ETo)to be 4.88 inches and the NOAA Stockton station reports the total annual rainfall from 1948 through 2019 to be 13.73 inches. Assuming 75 percent of this rainwater percolates to the aquifer, a value of 10.3 inches of average recharge of rainfall is calculated. H&F estimates that the nitrate concentration of rainfall is approximately 1 mg/L-N. Quantity and Quality of Domestic Wastewater The IHO nursery operates five days per week.A maximum of 50 employees per day is anticipated. The proposed nursery improvements will include the addition of two new ADA restrooms,to contain two toilets and two lavatories. Additionally,the existing nursery facility is estimated to contain three restrooms, with a total of three toilets, one urinal, and four lavatories. The estimated sewage flow rates for various uses are available from several references. The uses presented below were guided by the San Joaquin County On-Site Wastewater Treatment Systems Standards(2017)and US EPA(1980). A use of 15 gallons per day for each employee is typical for BAEZ GEOTECHNICAL GROUP Soil Suitability Study and Nitrate Loading Study-I Hsin Orchids Page 6