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total well depth (to 0.1 ft.) be measured the day before, in order to <br />allow for re- settlement of any particulates in the water column. If <br />measurement of total well depth is not made the day before, it should <br />not be measured until after sampling of the well is complete. All <br />measurements must be taken from the established referenced point. <br />Care should be taken to minimize water column disturbance. <br />Check newly constructed wells for the presence of LNAPLs or DNAPLs <br />before the initial sampling round. If none are encountered, <br />subsequent check measurements with an interface probe are usually not <br />needed unless analytical data or field head space information signal <br />a worsening situation. Note: procedures for collection of LNAPL and <br />DNAPL samples are not addressed in this SOP. <br />IV.PURGING AND SAMPLING PROCEDURE <br />SOP #: GW 0001 <br />Region I Low Stress <br />(Low Flow) SOP <br />Revision Number: 2 <br />Date: July 30, 1996 <br />Page 5 of 13 <br />Sampling wells in order of increasing chemical concentrations (known <br />or anticipated) is preferred. <br />1. Install Pump <br />Lower pum , safety cable, tubing and electrical lines slowly (to <br />minimize disturbance) into the well to the midpoint of the zone to be <br />sampled. The Sampling and Analysis Plan should specify the sampling <br />depth, or provide criteria for selection of intake depth for each <br />well (see Section I). If possible keep the pump intake at least two <br />feet above the bottom of the well, to minimize mobilization of <br />particulates present in the bottom of the well. Collection of turbid <br />free water samples may be especially difficult if there is two feet <br />or less of standing water in the well. <br />2. Measure Water Level <br />Before starting pump, measure water level. If recording pressure <br />transducer is used - initialize starting condition. <br />3. Purge Well <br />3a. Initial Low Stress Sampling Event • <br />Start the pump at its lowest speed setting and slowly increase the <br />speed until discharge occurs. Check water level. Adjust pump speed <br />until there is little or no water level drawdown (less than 0.3 <br />feet). If the minimal drawdown that can be achieved exceeds 0.3 feet <br />but remains stable, continue purging until indicator field parameters <br />stabilize. <br />Monitor and record water level and pumping rate every three to five <br />minutes (or as appropriate) during purging. Record any pumping rate <br />adjustments (both time and flow rate). Pumping rates should, as <br />needed, be reduced to the minimum capabilities of the pump (for <br />example, 0.1 - 0.4 1 /min) to ensure stabilization of indicator <br />