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capturing at both the shallow and deep plumes Apparent piezometric gradients within the plumes are <br /> everywhere directed toward an extraction well It is not clear that the current high pumping rates or all <br /> of the well locations are necessary For example, wells E-5 and E-1 may not be needed given their <br /> locations relative to wells E-9 and E-11, respectively The latter wells have capture zones that <br /> adequately cover those parts of the plume The USACE remediation system evaluation recommends a <br /> more elaborate optimization study <br /> 4 13 Subsurface Performance and Response— Contaminant Levels Concentrations of <br /> contaminants in the extracted water prior to treatment have declined from near 50 pg/L after startup to <br /> near 25 µg/L more recently Contaminant levels in all extraction wells have declined to some degree <br /> since start-up of the system or since installation Since the system has been in operation, the <br /> concentrations of contaminants in the ground water near the leading edge of the shallow plume have <br /> been substantially reduced (e g , 36 pg/L to 4 6 µg/L in Monitoring Well B-19) In fact, extraction well <br /> 12, the most downgradient extraction well, has been shut down due to the drop in concentration in the <br /> extracted water from 3 8 µg/L to non-detectable levels (1 0 µg/L reporting limit) While this reduction <br /> would indicate that the remediation is achieving its goals, this trend is not site wide Contaminant <br /> concentrations in monitoring wells elsewhere have generally not declined or in a few cases may have <br /> actually risen slightly Clearly, the remediation of the plume to current goals may involve pumping over <br /> decades, or perhaps longer, if it can be remediated to current goals at all This is not unusual for ground <br /> water extraction projects <br /> 4 14 Effectiveness of the System to Protect Human Health and the Environment The system is <br /> currently protective of human health and the environment There is currently no human or ecological <br /> exposure to contaminated ground water or soil at this site Although there is no treatment of the offgas <br /> from the air strippers, this discharge to the atmosphere is relatively small and the distance to the <br /> installation boundary is relatively large and it seems unlikely that the discharge poses a risk to the <br /> nearby population <br /> 4 1 5 Component Performance <br /> 4 15 1 Wells The extraction and infection wells have performed well, although periodic <br /> rehabilitation is necessary to maintain performance On a weekly basis, pumping water levels in the <br /> extraction wells are measured by hand and the specific capacities of the wells are computed If the <br /> specific capacity declines 30 to 50% from a baseline value, the well is identified for rehabilitation <br /> Rehabilitation of the extraction wells consists of swabbing with a wire brush Approximately 9 of the 16 <br /> extraction wells have been cleaned at least once A recent video of extraction well E-1 indicated serious <br /> clogging of the screen by both inorganic scaling and biomass The infection wells were cleaned with a <br /> sulfamic acid solution and swabbing in 1995 following significant inorganic scaling Scaling has <br /> apparently been controlled since then by the use of the SHMY Wells are also rehabilitates by bailing if <br /> 50% of the screen is occluded by sediment Wellhead vaults appear in excellent shape <br /> 4 1 5 2 Well Pumps Extraction well pumps have been a significant problem The power <br /> supply is plagued by voltage irregularities that often cause the well pump controllers to shut the pumps <br /> down (see Sec 4 1 7 1 Power Outages) The electric motors driving the submersible pumps have a <br /> history of failure after approximately two years The failures appear to be a short in the motor windings <br /> The manufacturer guarantees these motors for two years, but according to the operator they seem to fail <br /> 06/07/99 <br /> Page 6 of 14 <br />