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010 NEIL O.ANI'--tSON&ASSOCIATES,INC. GEOTECHNICAL•ENVIRONMENTAL•GROUNDWATER•INSPECTIONS•TESTING•LABORATORY SERVICES SACRAMENTO OFFICE CORPORATE OFFICE 4229 Northgate Boulevard,Suite 322 Houston Lane CHICO (530)691-6304 Sacramento,California 95834 Lodi,California 95240 sTOCKrON (209)472-1091 (916)929-9267 (209)367-3701 RANCHO CORDOVA (916)631-4455 FAX:(916)929-9269 FAX:(209)333-8303 May 21, 2001 Our Job No. LG00-312 Mr.Greg Potts P.O. Box 1358 Manteca,CA 95336 Subject: Addendum Soil Suitability/Nitrate Loading Study 2 Acre, South San Joaquin Educational Center Manteca,CA Dear Mr. Potts, Per your request,we are providing some recommendations that address San Joaquin County PHS- EHD concerns regarding the proposed development.Mr.Mike Huggins,Supervising Env.Health Specialist, after reviewing the Soil Suitability/Nitrate Loading Study, indicated that the percolation rate exceeds the County's acceptable rate(water empties within the specified 10-minute test interval in the percolation test). He is concerned that the use of the existing septic system coupled with the sandy soils and shallow ground water may cause additional impacts of the shallow ground water aquifer which already is affected with nitrate levels near or exceeding the maximum drinking water standard for nitrate(level is 45 mg/L). Our study accounted for the sandy soils by assigning a denitrification factor of 15%and reducing the facilities use from 36 students and 8 staff(6 hours per day)to 25 students and 8 staff for 6 hours per day. The results of the study found that if this was implemented,that the proposed use would not cause a build up of nitrates in the shallow aquifer(over the long term). Mr.Huggins indicated that because the percolation rate exceeds the County's maximum rate,that he wants his concerns addressed in an addendum. After several discussions with Mr.Huggins and you,it appears that the existing quarterly monitoring of nitrate of the drinking water from the onsite well could be coupled with some monitoring of shallow ground water near the septic system to attempt to determine if nitrate build up is occurring. This approach was suggested by Mr.Huggins. It would be necessary to construct a shallow/small diameter monitoring well next to or within the leach field. Quarterly monitoring could be performed by the same service company (same type test,collect samples on same monitoring visit)for some cost savings. We could also provide you with monitoring services for the septic system. The monitoring would need tocontinue during the student body growth period of the school. You should retain us or another qualified consultant to review the data at least yearly to make recommendations for the monitoring program. We will provide you in a separate letter an estimate for the construction of the monitoring well.If you have any questions,please do not hesitate to call. � R D GFO Very truly yours, J. wF��O� NEIL O.ANDERSON&ASSOC.,INC. it O No. 2151 y • CERTIFIED ENGINEERING • )J�Welc�hM.Sc� Senior Geologist 4 f, OLOGIST �a\Q CEG-2151,CHG-653 DJW:djw �FCALIF� CC: Mike Huggins