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Ms. Kim Schwab -2- CORRAL HOLLOW SANITARY LANDFILL GROUNDWATER AND SURFACE WATER MONITORING The proposed work to investigate the apparent impact to groundwater at MW-4 was concluded (Appendix I). We attempted to evaluate the possibility that the nitrate detected at MW-4 was from an off-site source. Unfortunately, the results of the evaluation are inconclusive because we were able to determine concentration gradients due to the inconsistency of the shallow aquifer, and other soil conditions. To further investigate this phenomenon,we have asked Emcon to measure all nitrogen species (Nitrate,Nitrite,NH4, and TKN)in the shallow aquifer monitoring wells. By comparing the concentration of these compounds at MW-4 to that of the impacted well,MW-5,we may see a pattern that allows the determination of the impact source at MW-4. We continue to be puzzled by the apparent recharge of groundwater at SB-1. We will have the shallow aquifer wells resurveyed this next quarter to verify that the top-of-casing elevations are correct. It is unlikely,but possible,that a past error in surveying has caused a miscalculation of the water elevation at that well. Two new wells (MW-6 and MW-7),were installed this quarter to monitor the deep aquifer. The data from two samples at each well are included in this report. Acetone at MW-6 was initially reported at 18 ppb,but the second sampling indicated 1.0 ppb. While we would like to think that this decrease indicates remediation by the LFG system,we think it more likely that the acetone was introduced during installation of the well, and is being removed by sampling. As noted above,resampling is scheduled for the first week of February. If you have any questions,please call me at(209) 468-3066. Sincerely, W. MICHAEL CARROLL,PE Senior Civil Engineer WMC:sc NAI Con-all Hollow\Groundwater Monitoring\Repts2003\4thQtrO3letter.doc Enclosure c: Robert McClellon,Environmental Health Department LA 21850