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FAAR-24-99 16 44 From WOODWARD CLYDE ¢ T-T35 P 11/34 Job-462 Fust and foremost,it is necessary to understand the medium to be sampled,the envsrornttnent in which the sampling is to %kc place, and exactly what is to be expected ft=n the sampling event. It is also pertinent to consider the consequences of using any particular type of equipMent or procedure, either rased on the equipment itself,or On unposed standards Current sampling standards are based on antiquated equipment and involve the production of large amounts of additional hazardous waste as a result of the sampling event, in order to obtain a repeatable average sample This produced waste, must then be discharged elsewhere or treated and discharged, generally adding greatly to the contusion of exactly what the regional chemical composition of groundwater is at any one site_ There has been much study completed on the problem of obtaining reyresmtatwe groundwater samples from rapidly recovering ground-water monitoring wells. Scalf, et al reported in a 1981 paper that bacterial activity can dramatically affect VQC analysis results Dissolved VOCs can effervesce in as little as 2 hours after sample acquisition Seanor and Brannaka noted in their 1983 paper that water in ground-water monitoring wells is stagnant, this means that,typically,water standing in a well,is usually considerably above the screensd interval and this water is not in communication with formational water This standing water is subject to different Chemical equilibria and often has different pH,Eh,temperature,and dissolved solids Wilson and Dworkin reported in their 1986 paper that rust and scale from ground-water monitoring wells may interfere with laboratory analyses, as may the presence of colloids and clay platelets Bnrcelona and Helfrich concluded in 1986, that no or unproper pur9mg of a ground-water monitoring well was a greater cause for sampling error than was sampling method,well construction materials,or sampler materials They went so far as to state that improper purging was the dominant error factor noblerms With Sampling Loss of dissolved volatiles by aeration-is one of the greatest errors in the sampling process This loss can be realized at any time during the process of well.purging,bailer sampling,or transfer to the receiving sample container Loss though effervescence created by the relief of depth pressures and through turbulence caused by the sampling bailer or COLAWASA,is also a concern Loss of dissolved volatiles through sample turb�� electric 11 t �d ananon d enc encountered m the he sampling process Clay and silt(some silts are organic)part y Y g 8 chain molecules, making them unavailable for detection using EPA standard analysis methods Clay and silt particles from the surrounding stratigraphic formations migrate, over tithe,through the sand pack and into the well casing where the slowly settle-out_ The art of purging most wells disturbs these settled particles., creatmS highly turbzd samples Gain of dissolved volatiles by sampling through Free Product - is not uncommon m leaking underground storage tank(LUST)investigations This probleat w eo evident and prevalent,that many Sites cannot be sampled until all of the free product has been removed from the groundwater table Laboratory analyses from wells containing free product are generally useless as representative groundwater samples under the influence of free product This is because the bailer must be continuously lowered down through the free product during the purging Process, creating a thick layer of emulsion and an entire well of organic-nch contaminated groundwater If another clean' bailer is used to obtam a sample of this broth, it, too, must be lowered through the emulsion to obtain a sample, thereby coating and contarnmating the bailer and rendering the sample invalid Purging prior to sampling can adversely affect the collected sample due to artificially produced turbidity Wilson and Dworlan went so far as to state that the necessity for such large purge volumes was to thoroughly remove all traces of turbidity Purging in wells with long well screens tends to average the contaminant concentrate netrue entire vertical screen length-13ut unknown hydrogeoiogic effects,which can confound our understanding of contaminant distributions and concentrations, do the greatest dam ge to sample integrity The effects of stratigraphic variation,most generally represented by zones of high natural groundwater flow layered weth zones of lesser permeability, can greatly affect the character of a contaminated aquifer Purging long-screened wells only serves to muddle our undentandmg of the tame nature of the m-ratu conditions. Well Purging-Pugh Flour The universal goal of well purging, according to the literature, is to provide TupTeseretattve samples of formation 0 7k124199 4 22 YM 2 of 1 o