Sampling of Ground Water Monitoring Wells - Part 2
Mixing of contaminated with uncontaminated waters in both the subsurface and in a monitoring well can occur when purging and sampling are improperly performed. This problem increases with longer well screens, higher pump rates, bailing and when variable stratigraphy exists across the screened interval.
High pump rates also can yield false plume locations and higher-than-actual contaminant concentrations, especially when combined with the above long-screen scenarios. Figures 1 and 2 illustrate that wells with long screens can provide incorrect, low contaminant values but a potential overestimation of plume thickness. Figure 3 shows the potential for high pump rates to pull contaminated water into a zone where there previously was none. The figure depicts this occurrence via high-permeability sand, but fractured rock also could be very susceptible to such errors. This scenario results in overestimation of the natural extent of the plume as well as spreading of the contamination into uncontaminated zones. Although horizontally induced migration is depicted, field data has shown that vertically induced migration through the aquifer also is possible.
Figure 4 depicts such a sampling scheme. In this nested well scheme, there are separate sampling points, with no sandpacks, and the aquifer has been allowed to collapse around the sampling point. These can be installed with small diameter augers or with the use of drive-point technologies. These types of monitoring points probably are best sampled through low-flow purging and sampling techniques, with the monitoring of purging parameters such as turbidity, Eh and DO. It also is possible to sample such monitoring wells using passive, or unpurged sampling, depending on the natural ground water flow to maintain purged conditions. ND