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S Problems 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 through 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 turbidity - is the next common error encountered in the sampling process. Clay and silt(some silts are organic) particles are electrically charged and may adsorb organic ring or chain molecules, making them unavailable for detection using EPA standard analysis methods Clay and silt particles from the surrounding strabgraphic formations migrate, over time, through the sand pack and into the well casing where the slowly settle-out The act of purging most wells disturbs these settled particles, creating highly turbid samples Gain of dissolved volatiles by sampling through Free Product - is not uncommon in leaking underground storage tank (LUST) investigations This problem is so 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-rich contaminated groundwater If another "clean" bailer is used to obtain a sample of this broth, it, too, must be lowered through the emulsion to obtain a sample, thereby coating and contaminating the bailer and rendering the sample invalid Purging prior to sampling can adversely affect the collected sample due to artificially produced turbidity. Wilson and Dworkin 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 concentration over the entire vertical screen length But unknown hydrogeologic effects, which can confound our understanding of the true contaminant distributions and concentrations, do the greatest damage to sample integrity. The effects of stratigraphic variation, most generally represented by zones of high natural groundwater flow layered with zones of lesser permeability, can greatly affect the character of a contaminated aquifer. Purging long-screened wells only serves to muddle our understanding of the true nature of the in-situ conditions. Well Purging- High Flow The uruversal goal of well purging, according to the literature, is to provide representative samples of formation waters, while at the same time, creating as little disturbance to the groundwater flow regime In all cases found in the literature, the most common reason for purging is to "refresh" the "stagnant' bore water volume. In general, one well bore volume is described as the volume of water standing in the well casing above the screened interval This