You are here:
Sources of Sampling Bias in Long-Screened Well
Citation:
ACREE, S. D. Sources of Sampling Bias in Long-Screened Well. Presented at The USEPA Ground Water Forum/Technical Support Project Meeting, San Diego, CA, January 26 - 29, 2009.
Impact/Purpose:
To observe the results obtained from ground-water sampling in long-screened wells which are often influenced by physical factors such as geologic heterogeneity and vertical hydraulic gradients.
Description:
Results obtained from ground-water sampling in long-screened wells are often influenced by physical factors such as geologic heterogeneity and vertical hydraulic gradients. These factors often serve to bias results and increase uncertainty in the representativeness of the samples. Geologic heterogeneity introduces heterogeneity in the hydraulic conductivity structure within the aquifer adjacent to the well screen. Under the influence of pumping during sampling, water moves to the well from each aquifer interval in direct proportion to the hydraulic conductivity of that interval compared to the remainder of the screened zone. This leads to sample dilution from intervals with the highest hydraulic conductivity which increases the complexity in evaluating the sampling results. In situations where vertical hydraulic gradients exist within the screened zone, a vertical flow field may exist within the well under ambient conditions. This flow field can dominate well hydraulics during sampling and result in samples representative of only a tiny fraction of the well screen. In general, these sources of bias can be expected to increase in significance with increasing screen length.