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SOP-009 Standard Operating Procedure Low-Flow Groundwater Quality and Sample Collection Page 4 of 9 Revision No. 1 April 2019 Sharpe Army Depot Ideally, there should be only a slight and stable drawdown of the water column after pumping begins. In some cases, it will be necessary for the well to drawdown a considerable distance (10 ft or more in extreme cases) to maintain a minimal usable pumping rate for sampling (0.2-0.3 L/min; maximum of 0.5 liters per minute). Excessive pump rates and drawdown can result in increased turbidity, or aeration of the sample if the screen is exposed. Stable drawdown is an essential condition of the Low-Flow method. If the stable drawdown condition cannot be met, then one of the other methods should be employed. The target for maximum well drawdown should be 0.3 feet. The water level should be checked and recorded often along with the flow rate to ensure minimal drawdown. Any well that should go “dry” during the course of sampling will be noted in the field notebook as well as on the appropriate field monitoring data sheet. After recharge time is allowed purging may be reattempted at a slower rate. For additional information on Low-Flow sampling see EPA/540/S-95/504, April 1994, "EPA Groundwater Issue Low-Flow Minimal Drawdown Groundwater Sampling Procedures". For additional guidance on low-flow minimal drawdown see USEPA Region 1 EQA SOP-GW 001, Region 1 Low-Stress (Low-Flow) SOP Revision #3, Date, July 30, 1996, Revised January 19, 2010. 4.5 Groundwater Quality Parameter Collection Parameters are measured from the wells by low-flow dedicated or non-dedicated pumping with a water quality multi-meter. Begin well documentation by filling out the top half of the water sampling log as needed. The water quality multi-meter will be connected to a flow through cell. Parameter measurements are recorded on the sample log as follows: • Time of measurement • Pump intake depth • Flow rate (millimeters per minute [mL/min]) • Cumulative volume (liters) • Temperature: (°C) • Conductivity: (mS/cm) • DO: (mg/L) • pH • ORP: (mV) • Turbidity (NTU) • Additional comments, if any Each successive measurement will be recorded within 3-5 minutes. At least three measurements will be taken but there must be three successive readings stabilized according to the requirements below: • pH ± 0.1 units • Electrical conductivity: ± 3 percent (microsiemens per cm) • ORP: ± 10 millivolts