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• reduced stress on the formation (minimal drawdown); sampling,it is recommended that an in-line water quality • less mixing of stagnant casing water with formation measurement device(e.g.,flow-through cell)be used to water; establish the stabilization time for several parameters(e.g., • reduced need for filtration and,therefore, less time pH, specific conductance, redox, dissolved oxygen, turbidity) required for sampling; on a well-specific basis.Data on pumping rate,drawdown, • smaller purging volume which decreases waste and volume required for parameter stabilization can be used disposal costs and sampling time; as a guide for conducting subsequent sampling activities. better sample consistency; reduced artificial sample variability. The following are recommendations to be considered ' before, during and after sampling: Some disadvantages of low-flow purging are: • use !ow-flow rates(<0.5 Vmin),during both purging • higher initial capital costs, and sampling to maintain minimal drawdown in the • greater set-up time in the field, well; • need to transport additional equipment to and from the • maximize tubing wall thickness, minimize tubing site, length; • increased training needs, • place the sampling device intake at the desired • resistance to change on the part of sampling practitio- sampling point; ' ners, • minimize disturbances of the stagnant water column • concern that new data will indicate a change in above the screened interval during water level conditions and trigger an action. measurement and sampling device insertion; • make proper adjustments to stabilize the flow rate as soon as possible; IV. Low-Flow(Minimal Drawdown)Sampling • monitor water quality indicators during purging; Protocols • collect unfiltered samples to estimate contaminant The following ground-water sampling procedure has loading and transport potential in the subsurfacesystem. evolved over many years of experience in ground-water sampling for organic and inorganic compound determinations B. Equipment Calibration and as such summarizes the authors'(and others)experi- ences to date(Barcelona et al., 1984, 1994; Barcelona and Prior to sampling, all sampling device and monitoring Helfrich, 1986; Puls and Barcelona, 1989; Puls et.al. 1990, equipment should be calibrated according to manufacturer's 1992; Puls and Powell, 1992; Puls and Paul, 1995). High- recommendations and the site Quality Assurance Project Plan quality chemical data collection is essential in ground-water monitoring and site characterization. The primary limitations (C�APP)and Field Sampling Plan (FSP). Calibration of pH to the confection of representative ground-water samples should be performed with at least two buffers which bracket the expected range. Dissolved oxygen calibration must be include:mixing of the stagnant casing and fresh screen corrected for local barometric pressure readings and eleva- waters during insertion of the sampling device or ground- water level measurement device; disturbance and tion. resuspension of settled solids at the bottom of the well when C Water Level Measurement and Monitoring using high pumping rates or raising and lowering a pump or bailer;Introduction of atmospheric gases or degassing from It is recommended that a device be used which will the water during sample handling and transfer,or inappropri- least disturb the water surface in the casing. Well depth ate use of vacuum sampling device, etc. should be obtained from the well logs. Measuring to the A. Sampling Recommendations bottom of the well casing will only cause resuspension of settled solids from the formation and require longer purging Water samples should not be taken immediately times for turbidity equilibration. Measure well depth after following well development. Sufficient time should be allowed sampling is completed.The water level measurement should ' for the ground-water flow regime in the vicinity of the monitor- be taken from a permanent reference point which is surveyed ing well to stabilize and to approach chemical equilibrium with relative to ground elevation. the well construction materials. This lag time will depend on site conditions and methods of installation but often exceeds D. Pump Type one week. The use of low-flow(e.g.,0.1-0.5 Llmin)pumps is Well purging is nearly always necessary to obtain suggested for purging and sampling all types of analytes.All samples of water flowing through the geologic formations in pumps have some limitation and these should be investigated the screened interval. Rather than using a general but with respect to application at a particular site. Bailers are arbitrary guideline of purging three casing volumes prior to inappropriate devices for low-flow sampling. 6