Mass balance evaluation of monitoring well purging: Part I. Theoretical models and implications for representative sampling

Abstract Mass balance models were developed to examine how monitoring well purging influences the collection of representative ground water samples. Modeling indicates that monitoring wells may provide only qualitative information on the absolute and relative abundances of solutes in ground water. Solute concentrations in a purged well may vary by over an order of magnitude depending on well construction, purging procedure, vertical concentration distributions of solutes in the ground water and the hydrogeological properties of the aquifer. Further, these dependencies limit setting a priori criteria for purging, using simple field measurements for monitoring purging completeness, and extrapolating the results of empirical purging studies. Water samples obtained from typical monitoring wells using standard purging procedures may understimate ground water contamination by orders of magnitude. Mass balance effects that occur during purging can complicate the interpretation of physical, chemical and biological conditions and processes occuring within an aquifer. Recognising that solute concentrations are likely to vary in three dimensions, the quantitative assessment of ground water requires sampling at discrete depths within an aquifer.