Passive sampling of groundwater monitoring wells without purging: multilevel well chemistry and tracer disappearance

Abstract

It is essential that the sampling techniques utilized in groundwater monitoring provide data that accurately depicts the water quality of the sampled aquifer in the vicinity of the well. Due to the large amount of monitoring activity currently underway in the U.S.A. it is also important that the techniques be efficient. It would be desirable to minimize the requirements of sampling time, equipment and quantity of contaminated waters pumped to the surface, without loss of data integrity. If representative samples could be acquired without purging the wells, increased sampling efficiency could potentially be achieved. Purging of multiple borehole volumes is largely routine, based on studies that show changes in the water chemistry as it stands in the casing and is subjected to atmospheric exposure at the top of the column. However, little data are available depicting water chemistry in the screened intervals of wells at equilibrium flow conditions, i.e. with little or no disturbance to the natural flow regime or disruption of the overlying casing waters. This study examines the differences in water chemistry between the casing and screened interval volumes of four wells at a field site, then compares the results to purged values for the same wells. Tracer experiments, utilizing both colloidal particles and dissolved species as tracers, are presented to illustrate differences in natural flushing between the screened and cased intervals. The data from the tracer removal were then utilized to estimate groundwater flow velocities in the vicinities of the boreholes. The results indicate that dedicated low-flow rate samplers set in the screened intervals are appropriate for the 6.1- and 7.6-m wells at this site and could be used without purging of the casing volume. The possibility exists that such passive sampling techniques would be useful at many of the sites currently undergoing routine monitoring.