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Soil Suitability / Nitrate Loading Study Irerracon Thor Property m Stockton, CA July 19, 2018 ® Terracon Project No. NA187029 Hantzsche and Finnemore found that over a long period of time, water quality in the upper saturated groundwater zone closely approximates the quality of percolating recharge waters. They believe that their method is a good planning tool and provides useful information to planners and local officials but with some real limitations. In their article published in Ground Water, Hantzsche and Finnemore mention that "...the equation ignores dispersion, lateral flow, and mixing with groundwater flow from up-gradient areas. These processes would generally contribute to additional reduction of nitrate-nitrogen concentrations in groundwater to the extent that the nitrate-nitrogen concentration of groundwater flow from up-gradient areas is lower." The Hantzsche and Finnemore approach is "...a conservative (worst case)first approximation of groundwater nitrate-nitrogen concentration resulting from the combined effect of on-site sewage disposal systems and precipitation. This (the equation and approach) is for estimation of long- term effect (i.e., over years)on groundwater quality, and is not intended for prediction of seasonal changes" (Hantzsche and Finnemore, 1992). The method takes into account the development area, soil denitrification factor, rainfall volume, rainfall nitrate concentration, waste effluent quantity, and waste effluent nitrate concentration. The formula, assumptions and calculations used for the analysis are shown in Appendix F. The units of nitrate used in the mass balance equation are nitrate-N, whose maximum drinking water contaminant level (MCL) is 10 mg/L. 2.1 Chemical and Physical Properties of Soil 2.1.1 Chemical and Physical Soil Properties For a discussion of on-site soil properties, refer to Section 1.5. 2.1.2 Mass Transport Soil Properties The percolation test has revealed that site clay soils will allow for slow infiltration at 25 feet bgs. The longer the effluent remains near the surface, the more treatment is enhanced due to natural processes within the soil. Based on the slow permeability and high water absorption capacity of the clay soils, effluent is anticipated to remain within the clay soil column much longer when compared to more permeable sands, silts, and loamy soils. 2.2 Groundwater Analysis 2.2.1 Regional Groundwater A discussion of regional groundwater, including depth to groundwater and flow gradient is included in Section 1.4.1. No significant seasonal trends were evident from the sources consulted. Responsive ■ Resourceful ■ Reliable 12