FIELD DOCUMENTS AND WORK PLANS 1989

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Gary A. Reents -5- 24 February 1989 The Phase IV report also states that a potential downward vertical gradient . exists in the vicinity of the Marley facility, but the implications of this potential gradient upon plume migration is not discussed. A hydraulic head difference of 1 .94 feet was measured ( in December 1988) in a clustered intermediate and deep well in the North Yard (MW-213 and MW-361) . The RCRA Comprehensive Ground Water Monitoring Evaluation (CME) Report, dated 21 December 1988, notes that potential downward gradients have been measured at the facility since 1985 and that the hydraulic head differences for July and September 1988, were 2.2 and 3.71 feet respectively. The CME Report further suggests that the Stockton Diverting Canal is acting as a seasonal source of recharge, since the vertical and horizontal gradients, at the facility, are greatest during the times of the year when the water levels in the Canal are the highest. However, the Phase IV report states that water levels in the Canal were basically equivalent in December 1988 and September 1988. The hydraulic head differences in the North Yard, dropped 1 .8 feet between these dates. H&A provides no explanation for the changes in the hydraulic heads between zones, nor any discussion/evaluation of the hydraulic relationship between the Stockton Diverting Canal and the site' s ground water regime. In addition, H&A notes that water levels have dropped 2 to 3 feet from September to December 1988, and 13 to 17 feet since July 1987. However, it is unclear if water levels have decreased equally in all three zones, and if the September through December decrease is permanent or seasonal . H&A should examine the water level data collected to date, and determine if seasonal trends exist in the ground water regime either as a result of recharge from the Stockton Diverting Canal and/or recharge from surface water infiltration from the facility. Specifically, the following questions should be addressed: a. What is the cause of this ground water ridge? Ground water recharge from the North Yard and/or the Stockton Drainage Canal are two potential causes. b. Is the ground water ridge a permanent or ephemeral feature of the underlying ground water regime? c. What effect does this ridge have on the movement of contaminants to the north and/or northeast? d. Is there sufficient areal coverage in the shallow and intermediate zones, north of the North Yard, to ensure that a plume has not migrated north- ward? The shallow zone, a fairly tight subunit; has five monitoring wells along north and east boundaries of the North Yard (all showing less than 0.05 mg/l of chromium) , however the intermediate zone (identified as the most transmissive zone underlying the site) has only three (3) monitoring wells along these boundaries ( all containing less than 0.01 mg/l of chromium) .