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The observed nitrate concentrations in the upper and lower zones may theoretically be7inequilibrium. This theory is based upon the possibility that the influx of nitrate may be roug equivalent to what is termed"disposal"-or the removal of the underlying groundwater due to flow, pumping,etc. However,due to the highly unpredictable nature of nitrate and the immense number of variable involved, this theory would be extremely difficult to prove. OVERDRAFT As previously referenced,PHS-EHD has expressed concerns of overdrafting of the upper zone which may adversely affect the shallow domestic wells that serve individual houses in and around the Valpico Section. The CSA-44A well cannot cause overdraft of the upper zone,since this well will be screened below the Corcoran Clay layer in the lower zone. As referenced on Page 3.6 and 3.8 of the Kennedy/Jenks report,"The City(of Tracy)pumped a maximum of 5,600 AF of water from the underlying groundwater basin in the 1970's without adversely affecting lower-or upper-zone water levels over the long term." "Seasonal water level fluctuations are less than 10 feet in most cases" and"Long-term upper zone water levels do not appear to exhibit significant trends up or down." Potential overdrafting of the upper zone in the Valpico Section may be attributable to insufficient irrigation recharge caused by the removal of the orchards currently on the land designated for the two subdivisions. Both the Kennedy/Jenks and Bookman-Edmonston Reports suggest the primary recharge to the upper zone is through irrigation recharge. It has been estimated that for the Linne Estates proj ect exclusively,the current agricultural water use over this acreage is approximately three times the proposed domestic water use. The recharge volume to the upper zone is extremely complex and is based upon several factors. Percolation of irrigation water that travels below the root zone is termed effective water and is water that exceeds crop use requirements. At present,the water table for the upper zone is approximately 50 feet bgs(60 feet MSL)near the center of the Valpico Section. The attached E-Log indicates the Corcoran Clay layer to begin approximately 280 feet bgs(-155 feet MSL),and as referenced in the Kennedy/Jenks report, most of the upper zone is saturated. An estimate of the volume of potential irrigation recharge that will be lost to houses is a follows: From the Bookman-Edmonston report (Page 6-6), the following assumptions regarding irrigation applications and operations for the purpose of hydrologic inventory were made:Irrigation efficiency is -65 percent. Of the 35 percent of the irrigation applied which was not consumptively used,it was assumed that about 20 percent would be surface runoff and the remaining 15 percent would recharge the upper zone. Therefore, the entire acreage to be served by the CSA-44A well is approximately 200 acres with 190 acres currently in agricultural production. Approximately 40 inches (3.3 ft)of water is used per growing season over this acreage: 3.3 ft x 190 acres = 627 acre-feet x 15 percent recharge =94 acre-feet of water that will be lost as recharge water. If only the shallow domestic wells that are downgradient from the lost irrigation recharge acreage are evaluated, then this is approximately one-half of the Valpico Section acreage of 320 acres. If the top of the upper zone aquifer contains a porosity of 35 percent,then roughly the water volume contained in three feet of this material over a one acre area is equivalent to one acre foot volume of water. One foot of the aquifer material=35 percent porosity x 320 acres= 112 acre- feet of water. Therefore, 94 acre-feet of lost recharge water is roughly equivalent to the loss of one foot of water table elevation over the north half(320 acres) of the Valpico Section acreage. Page -4- Naffey Ag Research