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Ms. Mary Meays Page 5 <br /> February-25, 1993 <br /> TPH-D, TPH-G or BTEX are.detected in the soils. If floating petroleum <br /> products were situated in this' confined aquifer, -the screened intervals in. <br /> these wells .would allow for the product to enter the well and rise.lto the top of . <br /> -the potentiometric surface where.they would be detected. during Quarterly <br /> Monitoring events. <br /> .Even:if-.floating product was-on top^of`the potentiometric surface and we <br /> could-not see it with our screened intervals,:'we would be able.to see.a <br /> dissolved phase in the water; we have not seen any signs'of-dissolved <br /> petroleum products in MW-3A and -MW-4A: Thus, it is not necessary to <br /> have.screens located all the way to.the-top of the.potentiometric surface. <br /> Because the data indicate.confine&a'q zifer conditions, ERM and Catellus <br /> respect€ally.informed PHS/EHD that::=we were-unwilling 'to_install monitoring <br /> wells by screening them to the tope-of.the pofention etric-.surface.- We are <br /> concerned about the potential°.for introducing petroleum compounds to <br /> uncontaminated soils. <br /> -MW-2A-.was installed-in..a perched .water zone that is situated within the <br /> - gravel-filled tank excavation. _The tank was removed-from silty soils and the <br /> excavation-was backfilled withgravel. The gravel is not covered with any <br /> impermeable surface such as.asphalt;:-concrete or-plastic: Thus, rainwater <br /> -subsequently filled the bottom,portion of the-excavation.. ''The perched water <br /> was initially encountered in the-lower portions of the gravel during drilling <br /> of MW-2A. As-drilling progressed: into the.silts which are located beneath the <br /> gravel,'the.groundwater level.'dropped..by-approximately one foot._ The drop <br /> in'water-level indicated that the pooled.water was filling-the void:created by <br /> drilling:at the bottom of the tank excavation. Drilling was.stopped to avoid <br /> mixing'tl a perched water which potentially contained.petroleum <br /> II constituents with lower water.-bearing zones. .Upon cessation of drilling;the <br />!� water ,level remained at the lower level:_ The boring was.completed as a: <br /> monitoring well screened entirely within the silty soils. During subsequent <br /> monitoring activities, the water'level;drops:during.purging-.arid:does not <br /> immediately-recharge as it does-in the other wells. Furthermore, in contrast, <br /> to'the behavior-of all other monitoring_Wells at :the site, the:water levels in <br /> this well do not appear to'rise or fall with.tides.- Finally, the water levels at <br /> MW-2A do riot match the trends established by'the`other wells, and appear to <br /> rise during periods of rainfall: <br /> Based-on the information_obtained during our investigation activities, we <br /> have determined that 30 feet.-is the maximum lateral extent of detectable <br /> TPH-G, TPH-D and-BTEX in'the,groundwater to the east and south:of the tank <br /> .excavation. Catellus and ERM,agree that the extent of these constituents to <br /> = the north has not yet-been determined. <br /> t <br />