SITE INFORMATION AND CORRESPONDENCE

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-2- 0/LDC STAFF REPORT detectable in most of the wells above concentrations of 5 µg/l with the exception of MW-5. MW-5 has had nickel concentrations ranging from 7 to 12 µg/1 within the last year. Concentrations of chromium in the upgradient well , MW-4, have remained at or below detection limits (as low as 1 µg/1) since it was first analyzed in 1987. Wells MW-1 and MW-2 are downgradient from the impoundment. The concentration of chromium in MW-1 was 80 µg/1 when the well was first analyzed in December 1985. The levels of chromium have fluctuated since that time. However, chromium concentrations have generally MW grand periodicallythe inrMWa2ysilncemMarchum nofm1989t Level (MCL) of 50 µg/ The maximum concentrations of total chromium reported in MW-1 and MW-2 were 130 and 81 µg/1 , respectively. A Ground Water Monitoring and Reporting Program, Order No. 90-804, was issued for the facility in June 1990. Analysis for hexavalent chromium was added to the monitoring program in October 1990. The primary MCL for chromium applies to total chromium in drinking water and does not distinguish between trivalent and hexavalent chromium. Hexavalent chromium is of greater concern because it is more mobile and toxic than trivalent chromium. Sample results have shown that up to 100 percent of the total chromium in the samples is hexavalent. Wells for domestic purposes are within one-half mile immediately downgradient of the 0/LDC ground water plume and could potentially be impacted. Staff sampled a single domestic well within one-half mile downgradient of the facility in February 1991 . The depth of the well is unknown. Lab analysis results showed the sample contained total chromium at 1 A911 . The discharger has made several requests to discontinue ground water monitoring and properly abandon the monitoring wells. In October 1991, the discharger notified the Board that they had instructed their consultant to discontinue ground water monitoring indefinitely. However, the wells were sampled in December 1991. The discharger submitted the results in a quarterly monitoring report in January 1992, as required by Order No. 90-804. Staff has contended that there are wells downgradient from the former surface impoundment that have chromium concentrations above the MCL and the extent of the plume has not been defined. There is no way of knowing how far the plume extends off the property or what the concentrations of chromium are further downgradient with the existing monitoring system. Therefore, staff recommends the adoption of the proposed Cleanup and Abatement Order requiring 1) more wells and 2) corrective action.