Application Note: Low-Flow Sampling Of Water Quality Parameters Used In Determining Groundwater Stability

In April 1996, the U.S. EPA Office of Research and Development, Office of Solid Waste and Emergency Response, developed and published a document entitled Low-Flow (Minimal Drawdown) Ground-Water Sampling Procedures (Publication EPA540/S-95/504). Since then, the use of low-flow sampling in ground water has increasingly been used to support site assessment and remedial performance monitoring objectives. There are many documents on this subject and this particular document isn't intended to be all-inclusive. For the purposes of this paper, the focus will be primarily on the water quality parameters used to indicate stable well situations.

The document states that "the most common ground water purging and sampling methodology is to purge wells using bailers or high speed pumps to remove 3 to 5 casing volumes followed by sample collection." Adverse impacts can occur through this method affecting sample quality by increasing levels of turbidity. An overestimation of certain analytes — namely metals or hydrophobic organic compounds — may affect results with this method through the inclusion of otherwise immobile artifactual particles. Filtration of these turbid particles has proved undesirable in rectifying the turbidity problem and may, in fact, bias the results of contaminant concentration on the low side by potentially removing mobile (contaminant-associated) particles. These problems can often be mitigated by using low-flow purging and sampling to reduce sampling-induced turbidity.