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MAR-24-99 16 36 From WOODWARD CLYDE ¢ T-T35 P 03134 .lob-M depending on which contaminant or suite of contaminants are present Num®nick,et al S found that raprosentative samples could be obtained after purging only ono bore volume One Well bore volume, in general,is described as the volume of water standing in the well casing above the screened interval This is because waters in the scr©®ned interval and the Sand pack ara free to interact wan the formation water,while the bore water is cut-off from communis-ation with tho formation water,and is thorofore subtract to ddlcrent physical and geochemical rogimes The literaturo reports that different contaminants have diftmnt physical parameters,and therefore have different purging and sampling method requiroments Dissolvable constituents of gasoline,for example,are much lighter than thoso of 1,1,1-trichloroethylene,and will therefore be distributed in any"stable"system nearer to the surface with the highest concentrations very close to the surface It has been demonstrated(Hall,et al, 1984)that organic chemicals which dissolve with groundwater assume a more or less graded vertical distribution according to thair molecular weight The dissolved 1,1.1-tnchloroothylone constituents would be found much towor in the same system IntiprAW Loewe in the Sam l{ling Plgress Losses due to moring-Relatively Wo work has been published on the vertical diatnbution of contaminant concentrations due to a lack of adequate sampling apparatus.Preliminary field studies, however(Farr,et.af, 1 goo), have demonstrated that m vorticaRy graded concentration distribution will bo established in any hydrogeologic rggtme for any dissolved organic compound due to the physics of temperature and pressure and solution chemistry "Mixing" of that graded diatnbution occurs as a result of well recharge following purging,giving a false indioation of the true nature of the contaminant plume Lasses due to aeration-Loss of dissolved volatiles due to aeration occurs continuously throughout the sampling process tosses are realized during the process of well purging,bailer sampling,and sample transfer tv the meaning samplo container tosses also occur through effervescence created by tho relief of depth pressures and through turbulence caused by a check-valve bailer,the act of purging,or the use of a bladder pump eampfer Lose of dissolved voiatTles through sample turbidity is the next most common error encountered in the sampling process • 2 of 14 3/24199 4 21 PM