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WORK PLAN <br /> DECOMMISSIONING MONITORING WELLS MW-3,MW-4, and MW-7 <br /> 2130 WEST WASHINGTON STREET <br /> STOCKTON, CALIFORNIA <br /> SIERRA-PACIFIC PROJECT NO. 92-033.04 <br /> L� <br /> 1.0 INTRODUCTION <br /> Union Oil Company of California dba Unocal authorized Sierra-Pacific Groundwater Consultants, <br /> L� Inc. (Sierra-Pacific) to install five ground-water monitoring wells and collect ground-water data at a <br /> former PureGro (currently Westway, Inc.) molasses feed product facility located at 2130 West <br /> Washington Street, Stockton, California (Figures 1 and 2). Background information is provided in <br /> r earlier Sierra-Pacific work plans and reports listed in Section 7.0 of this report. <br /> Sierra-Pacific completed a comprehensive soils investigation at the 2130 West Washington Street, <br /> Stockton California in 1994. Ground-water monitoringhas continued at the site since 1993 and the <br /> , <br /> r frequency of sampling was reduced from quarterly to semi-annually in September 1996. Near-surface <br /> L.i ground water has flowed generally west since 1993. Ground water was flowing west-southwest, with <br /> a gradient of 0.0066, on March 9, 1998. Ground-water levels rise and fall seasonally, and the water <br /> level at monitoring well MW-2 fluctuates more than the other monitoring wells due to ponding and <br /> infiltration of storm water near the southeast comer of the site. <br /> Nitrate concentrations in ground-water samples from wells MW-2 and MW-8 are always higher than <br /> concentrations detected in samples from other wells, and the nitrates are derived from an off-site <br /> (eastern) source. Nitrate-as-nitrogen (NO3-N) concentrations detected at MW-8 have fluctuated from <br /> 13.7 to 60 mg/1. NO3 N concentrations detected at MW-2 have attenuated with time from 47.3 mg/1 <br /> to 20 mg/1 in three years. Previous sampling events have recorded anomalous nitrogen spikes in <br /> ground-water samples from side-gradient well MW-3. The "side-gradient" nitrogen spikes are due <br /> to seasonal shifting of near-surface ground water and up-gradient sources of nitrate. Ground-water <br /> samples collected from the down-gradient monitoring well (MW-7) have consistently had little or no <br /> nitrates and ammonium. <br /> Sierra-Pacific Groundwater Consultants,Inc. 1 <br /> �I <br />