James M. Martin-Hayden

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.

Mass balance evaluation of monitoring well purging: Part II. Field tests at a gasoline contamination site

Abstract The mass balance model simulations in Part I indicated that contaminant concentration data from typical ground water monitoring wells are inherently biased by mixing and averaging effects. This paper presents the results of field purging studies conducted at a site of subsurface gasoline contamination to test the mass balance purging model and to verify composite averaging effects. Samples collected from wells with different screen lengths, and from multi-level sampling pipes within both the aquifer and the sand pack of two wells provided data that confirmed mass balance inferences. The concentrations of aromatic contaminants and other water quality parameters in the monitoring wells depended on: (1) the well screen length; (2) the water levels achieved in the well during purging (as well as the volume of water purged); (3) when the well was sampled during water level recovery (as opposed to sampling method); (4) sand pack characteristics; and (5) vertical concentration variations in the ground water. The study also revealed that post-purging well concentrations underestimated ground water contamination by orders of magnitude due to composite averaging. These results indicate that typical monitoring wells provide only qualitative contaminant information, irrespective of how the wells are purged. Consequently, contaminant data collected from typical monitoring wells may only have limited value with respect to delineating, modeling and remediating contaminant plumes. As predicted by the mass balance model simulations in Part I and verified by this field study, water samples must be collected at discrete depths to quantitatively delineate ground water contamination.

Controls of Wellbore Flow Regimes on Pump Effluent Composition

Where well water and formation water are compositionally different or heterogeneous, pump effluent composition will vary due to partial mixing and transport induced by pumping. Investigating influences of purging and sampling methodology on composition variability requires quantification of wellbore flow regimes and mixing. As a basis for this quantification, analytical models simulating Poiseuille flow were developed to calculate flow paths and travel times. Finite element modeling was used to incorporate influences of mixing. Parabolic velocity distributions within the screened interval accelerate with cumulative inflow approaching the pump intake while an annulus of inflowing formation water contracts uniformly to displace an axial cylinder of pre-pumping well water as pumping proceeds. Increased dispersive mixing forms a more diffuse formation water annulus and the contribution of formation water to pump effluent increases more rapidly. Models incorporating viscous flow and diffusion scale mixing show that initially pump effluent is predominantly pre-pumping well water and compositions vary most rapidly. After two screen volumes of pumping, 94% of pump effluent is inflowing formation water. Where the composition of formation water and pre-pumping well water are likely to be similar, pump effluent compositions will not vary significantly and may be collected during early purging or with passive sampling. However, where these compositions are expected to be considerably different or heterogeneous, compositions would be most variable during early pumping, that is, when samples are collected during low-flow sampling. Purging of two screen volumes would be required to stabilize the content and collect a sample consisting of 94% formation water.