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Percolating Recharge Waters Background One of the most determinant factors in the Hantzsche and Finnemore equation is the rate of recharge from percolating waters, which mix with the downward-percolating effluent generated by on-site septic systems. In the original Hantzsche and Finnemore paper, only "average recharge rate of rainfall" was considered. For the Chico area, Hantzsche and Finnemore present an average rainfall of 22.5 inches per year and an estimated recharge of 16.8 inches per year; the recharge value is 75 percent of the average rainfall. The source of the recharge value is not cited in the paper. Since the 1992 publication of the Hantzsche and Finnemore paper, numerous consultants have attempted to apply the method to calculate nitrate loading rates of projects and developments. Most notably, the City of Chico has continued to use the method to calculate desired urban density (County of Butte, 2000). In a 1996 technical memorandum published to support Chico's urban density requirements, the consulting firm Dames and Moore has discussed that "recharge ... is comprised of two major components: applied water ... and precipitation and streams." Dames and Moore discuss three methods of calculating the rate of recharge from percolating waters, two of which are computer models (SWQCB and HELP) that consider measured water levels, soil types, irrigation, stream flow, and other factors, and the third of which is a monthly consideration of precipitation versus evapotranspiration. The report concludes that estimates using precipitation versus evapotranspiration "appear to be too low" when compared to those values calculated by the two computer models. Dames and Moore eventually determine to use a slight modification of the result of the computer modeling. This method of evaluating ground- water recharge, which includes recharge from a variety of sources, was found to be more applicable to the City of Chico's needs than the simplified rainfall-only method. The value Dames and Moore eventually select (18 inches per year), although derived by the modeling methods discussed rather than from rainfall alone, is equivalent 83 percent of the total rainfall (21.52 inches per year). Methods in use in San Joaquin County Within San Joaquin County, four methods have been identified to derive a value for the rate of recharge from percolating waters: • 100 Percent of Rainfall: This method involves identifying rainfall levels from the closest data station to the project site, and assuming that 100 percent of rainfall deep-percolates to the first saturated zone. This method likely over-estimates the rate of recharge from percolating waters. LOGE 1903 Page 9