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a gradient estimated from the groundwater elevation located 12 miles from the center of the confined zone,4.)Inflow from the area east of the study with an underflow perimeter width of~14 miles,and 5.) Inflow from the area north of the study area with a perimeter width of--8 miles.' The Bookman-Edmonston"hydrologic inventory analysis"and the Kennedy/Jenks"historical trend analysis" indicate that an operational yield of approximately 9,000 acre-feet per year may be applicable for use by the City of Tracy. This is an increase of 2,300 acre-feet from the operational yield recommended in the Kennedy/Jenks Study. The volumes referenced above should be compared with the Bookman-Edmonston Study which has estimated the volume of groundwater in storage under the Tracy area, and is based upon the following information: With a lower zone thickness of 500 feet, an area of 69,000 acres as used in the "hydrologic inventory analysis," and with a specific yield of 10 percent, there is about 3.45 million acre-feet in storyge in the lower aquifer under the Tracy area. (Specific yield is the ratio of the volume of water that a given mass of saturated soil will yield by gravity, to the volume of that mass). Regarding the CSA-44A well, it is our understanding that there are to be approximately 280 (Castello Estates=84 +Linne Estates=195) homes connected to CSA-44A, in addition to possible Neighborhood Commercial properties(C-N Zoning)and Public Facilities(P-F Zoning). Water use for each home is anticipated to be approximately one Equivalent Consumer Units (ECU)per year, which is equal to one-half acre-feet of water (162,926 gallons). If this water use projection is doubled to incorporate possible additional users,then the CSA-44A well must supply 280 acre-feet (9.12 x 10'gallons) of water per year from a lower groundwater basin under the Tracy area that contains 3.45 million acre-feet of groundwater in storage (1.12 x 1012 gallons). WATER QUALITY A Nitrate Loading Study was required by PHS-EHD for Mr. Olmstead's Castello Estates and part of the Linne Estates subdivision projects. It was determined from this study that significant nitrate impact has occurred in the upper zone aquifers. The nitrate in the underlying groundwater showed increasing,although fluctuating nitrate concentrations over time. Consequently,PHS-EHD and the Regional Water Quality Control Board have required wastewater treatment for these two subdivisions. It has also been determined from the 2000 Water Quality Report for the City of Tracy that the drinking water has a nitrate concentration ranging between 3.6 ppm and 24.3 ppm (as NOD. The lower nitrate concentrations are probably from wells perforated in the lower zone below the Corcoran Clay. The higher concentrations may be from wells that are "cross-zoned," or wells perforated in both the upper and lower zones. It is appears that the Corcoran Clay layer is attenuating nitrate impact to the lower zone aquifers, probably through denitrification. Nitrate impact is also attenuated by the use of conjunctive water. Page -3- Vaffey Ag Research