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f <br /> Ms. Kim Schwab -2- <br /> CORRAL HOLLOW SANITARY LANDFILL <br /> GROUNDWATER AND SURFACE WATER MONITORING <br /> The proposed work to investigate the apparent impact to groundwater at MW-4 was concluded <br /> (Appendix 1). We attempted to evaluate the possibility that the nitrate detected at MW-4 was <br /> from an off-site source. Unfortunately,the results of the evaluation are inconclusive because we <br /> were able to determine concentration gradients due to the inconsistency of the shallow aquifer, <br /> and other soil conditions. To further investigate this phenomenon,we have asked Emcon to <br /> measure all nitrogen species (Nitrate,Nitrite,NH4, and TKN)in the shallow aquifer monitoring <br /> wells. By comparing the concentration of these compounds at MW-4 to that of the impacted <br /> well,MW-5,we may see a pattern that allows the determination of the impact source at MW-4. <br /> We continue to be puzzled by the apparent recharge of groundwater at SB-1. We will have the <br /> shallow aquifer wells resurveyed this next quarter to verify that the top-of-casing elevations are <br /> correct. It is unlikely,but possible, that a past error in surveying has caused a miscalculation of <br /> the water elevation at that well. <br /> Two new wells (MW-6 and MW-7),were installed this quarter to monitor the deep aquifer. The <br /> data from two samples at each well are included in this report. Acetone at MW-6 was initially <br /> reported at 18 ppb,but the second sampling indicated 1.0 ppb. While we would like to think that <br /> this decrease indicates remediation by the LFG system,we think it more likely that the acetone <br /> was introduced during installation of the well, and is being removed by sampling. As noted <br /> above, resampling is scheduled for the first week of February. <br /> If you have any questions,please call me at (209) 468-3066. <br /> Sincerely, <br /> W. MICHAEL CARROLL, PE <br /> Senior Civil Engineer <br /> WMC:sc <br /> N:\1Corral]Hollow\Groundwater Mon itoring\Repts2003\4thQtr03letter.doc <br /> Enclosure <br /> c: Robert McClellon,Environmental Health Department <br /> LA 21850 <br />