Representative Sampling of Ground Water from Short‐Screened Boreholes

Publisher Website
Google Scholar
Abstract
Boreholes terminated with a short (1–3 m) length of screen are often used in ground‐water quality monitoring, although to obtain representative samples, pumping is often required to overcome the effects of stagnant casing storage. Methods for estimating pumping time prior to sampling from a pumped discharge, presented graphically and in summary equations, show that for most requirements, pumping time (ta) to purge well storage (where mixing of casing storage and ground water takes place within the casing) or to overcome casing storage effects (where little mixing occurs within the casing) is given by image Here V is the volume of well storage, Q is the average pumping rate, and m is given by image where Ct, Cg, and Cc are concentrations of some substance in pumpage, and ground water, and the initial concentration in casing storage, respectively. In boreholes which have been developed and which are regularly pumped and where a relative sampling error of 2.5% is considered to be acceptable, then m has a value of 0.1. Pumping time then equates to the time necessary to pump 2.3 volumes of well storage from the borehole.Laboratory and field evaluations using boreholes in transmissive and low permeability formations generally validate the theoretical approach. The theory can be used as a basis for estimating and optimizing pumping times for monitoring programs where ground‐water samples are recovered from short‐screened boreholes. Additionally, the theory allows the determination of an optimum pumping rate.

This publication has 5 references indexed in Scilit: