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1 STANDARD OPERATING PROCEDURE FOR LOW-FLOW/ I' MINIMAL DRAWDOWN GROUNDWATER SAMPLE COLLECTION "Micro-purge Method" ' INTRODUCTION '. Groundwater sample collection can be a source of variability through differences in sample personnel, sampling procedures, equipment, and ambient variability in subsurface and environmental conditions. Data quality necessitates that the personnel collecting groundwater samples are trained and aware of proper sample collection procedures. The purpose of this standard is to provide a method that, l) minimizes the negative impact of purging on groundwater chemistry, and 2) minimizes the volume of water disposed. The SOP was developed by the Superfund/RCRA Groundwater Forum and follows ASTM Procedure D6771-02. SCOPE AND APPLICATION The primary objective of this method is to minimize the disturbance to water within the monitoring well prior to and during sampling. Disturbing or churning of the water caused by dropping or plunging tools down the well will stir up sediment and colloidal material, and will modify the chemistry by introducing oxygen, thereby yielding spurious analytical results. Prior to conducting field work, information on the construction of the well and well development should be obtained, e.g., well construction diagram,borehole diameter, casing diameter, casing material, screen interval, well depth, etc. These data will assist in designing the sampling program. Suitable monitor wells are those M that have a screen or an open interval with a length of ten feet or less. The analytes may be volatile, semi- volatile or gain compounds, pesticides, PCBs, metals and other inorganic compounds. The pump intake should be placed at or near the known source of the contamination within the screened interval. If the source of contamination is not known, the pump intake should be placed midway in the screened interval. It is critical to place the pump intake at the same depth for each successive sampling event; this depth should be recorded on the sample log for referral during subsequent sampling events. This requirement argues for the use of dedicated sampling devices (such as dedicated tubing or a dedicated down-hole pump) whenever possible. ' The pump intake should not be placed near the bottom of the screened interval to avoid disturbing any sediment that may have settled at the bottom of the well. Neither should well depth measurements be taken until after sampling is completed, for the same reason. Water quality indicator parameters and water levels are measured periodically during purging. Recommended water quality parameters are the following: electrical conductance (eH), dissolved oxygen (DO), turbidity, oxidation-reduction potential (ORP), pH,and temperature(T). All these parameters may not always appear to be appropriate; however, values should be recorded because they can be used as a double check for future oxidation conditions, intrinsic bio-attenuation, and fate and transport issues. In addition, depth to water must be measured during purging. Stabilization of the water quality indicator parameters is the criterion for actual water sampling to begin. If stabilization is not occurring and the procedure has been followed, then sample collection can take place one three(minimum)to six(maximum)casing volumes have been removed. EQUIPMENT o Depth-to-water measuring device and interface probe. • Sampling pump (if not dedicated)— Submersible or bladder pumps with adjustable controls are preferred. Peristaltic pumps can be used when depth to pump intake is less than 27 feet below ground surface (bgs).. 1