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Rodney Estrada _ =� E November 27, 2000 I Page 3 Limitations to the Predicting ATWate-Nitrogen Impacts:Hantzsche and Finnemorel found that over a long period o£time,water quality in the upper saturated ground-water zone is closely approximated by the quality of percolating recharge waters. They have found that nitrate loading is a good planning tool and provides useful information to planners and local officials but with some real limitations. In Hantzscbe and Fennemore's published article in Ground Water they mention that"...tire equation ignores dispersion, lateral flow, and mixing with ground water flow from up-gradient areas. 'these processes would generally contribute to additional reduction of mtrate-nitrogen concentrations in ground water to the extent that the nitrate-nitrogen concentration of ground-water flow from upgradient areas is lower. The Hantzsche and Fennemore equation(used in the subject report)provides a conservative(worst case)first approximation of ground-water nitrate-nitrogen concentration resulting from the combined effect of on-site sewage disposal systems and precipitation. This [the equation and method]is for estimation of long-term effect(i.e.,over years) on ground-water quality,and is not intended for prediction of seasonal changes." 1 The equation is simplistic but provides a means to measure potential nitrate impacts of a .� development. A nitrate loading study can probably be performed using a number of different assumptions and come out with similar results with i:sd arge being the most searszhve. Thie method estimates the iritrate r concentration of the combined rain and effluent over a long period before further mixing with ground water occurs. One hundred percent mixing"below the site"in most cases probably does not occur and is really not important. What is important is the estimation of the combined total concentration of nitrate sources from a site. This is the value of the method. . Clay Layers and Mixing: The recharge from rain may or may not make it's way down to deep ground water below the site. Obviously,the size of the site has a bearing on this process: The recharging rain water probably does mix and mingle with septic effluent and other sources such as`over irrigation" and dilutes into."pockets"of percolating water,perched water,and soil moisture depending on soil/geologic structure. As time goes on,all the sources of recharge water such as rain, septic effluent, over-irrigation, stream recharge,and other sources probably coalesce,mix and then percolate downward where impermeable layers are absent- The downward migration is probably first controlled by surface soils and the near surface depositional structure at the site. Later,percolation is controlled by deeper gently dipping westward geologic layers of the local area. The greater amount of time,the greater degree of mingling and mixing of percolating recharge water occurs. At some point the recharge water either reaches ground water or remains perched until displace by other percolating water. The natural flow of ground water system of the valley is to flow to the San Francisco Bay Delta which is estimated to take possibly 100's of years. At this time,the mixing of ground water systems is probably occurring. Some of this flow is now intercepted by pumping wells of large valley Cities like Stockton. Regarding item no.6:I have misplaced the-originalfield sheet and cannot find it: -Mr. Estrada witnessed the percolation test and signed the original form. After the field work was complete,I tabulated the final percolation rates from all the field forms and wrote them on a note pad. Those values were then drafted on to Plate No. 1. The original field sheet with the inspectors signature was lost sometime after my initial tabulation. I believe that the percolation rate shown on Plate No. 1 is accurate and represents the correct percolation rate for Parcel No. 14. i Hantzshe,N.N.,Finn emore,E.J.1991,Predicating Ground-WaterNitrate NitmgenImpacts_ Ground Water, Vol. 30,No.4. r ti x w r 1fsQG�