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Ms.-Carol,Oz February_24, 1999 - _ : :Page two:. ._ Although would cover-the previous PHC~detection in groundwater " sampled from.this depth at boringwould not:provide evidence for any potential'containinants at the same depth.at the other two bonrig'lo.cations. As. such, the cost of;a fourth well:is .not justified and we'.are recommN. ending against installing a fourth well-'and instead recommending modifying_the screened.interval at each of.the three proposed_well locationsfrom 1 O.to 35 feet bgs. In sum, during the referenced-conference call we collective) agreed that this Would be a preferrzd-o ion for,the-following.reasoris:a- - -� • Screening from 10.to 35-feet provides a completion across each',of the coarse`r-grained_units identified beneath the site;(see Figure . of.lnVestigation _ _Re rt).-It a.lsoprdVides screens across�frorri all.of the sampled locations-at- which PHCswere'det�cted;in elthersoils or groundwater during the initial . -- --in w.estigati.on: Lowe.n. the op of the screened .interval from 5 feet to-10 feet bgs meets the g _spirit of PHSIEHD's rationaleao have-screen across;from"tlie likely maximum water table. Historical:water levels(1992.71.998)'from irmonitoring;weils at . - other locations on the Port show-that1998 was--a:-maxirnum-year�for water _ leve s at the site This maximum corresponds to several=above.average • '- e = -rainfall years.; which culminated in=last year'sE1.Nino events.e 1n light of this -year's-La Nina events,'however,-it-is therefor unl.ikely_that water,levels will rise substantially:above the'.1998 level:at.this.'location'(6pproximate1 10 feet bgs.). _ ' The10=foof top of.screen also makes sense'because the re:is-no detectable or likely contamination above that level The bottom elevation of the tanks �ia,+wo � .md.. - L. ''• l.., 'i 1,or F 1 F ,I t �f ¢, rernoveci rron=itnis sne.was at.i i feet=Ylzf- -;Gr, ;contamination would be present above.hat level:. Additionally., sot samples" obtained from 10 feet bgs at'all three locations contained-no detectable'ieveI's of P.HU M :• Screening.abovEi.the 1046ot level could introduce contaminants into soils that F _previously were.not contaminated If the`groundwater beneath the site is ur.der.confined conditions-(fine7grained;-potentially confining soils are present beneath the site starting-at 9 to 13.ffeet bgs), contaminated groundwater in. deeper zones could potentially migrate-upwards,through the casting and into- - - the surroundingJormations.-- Starting the screenat the 10-foot level would limit this potential. :. r .