Gary A. Robbins

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.

Field examination of ground water quality as an indicator of microbiological activity at gasoline contaminated sites.

Various portable electrodes and an on-line colorimetric test kit were used in the field to examine ground water quality as an indicator of natural bioremediation across two sites in Connecticut having subsurface gasoline contamination. The parameters examined included dissolved oxygen, dissolved carbon dioxide, direct redox potential (Eh), nitrate, ammonia and pH. These parameters permitted delineating regions of aerobic and anaerobic microbiological activity. Variations in these parameters over an eighteen month period along with gas chromatographic analyses of certain gasoline components in the ground water indicated that in-situ bioremediation was effective at containing the petroleum contamination at both sites. It was found that a new on-line colorimetric test kit for the determination of oxygen was more accurate than a commonly used dissolved oxygen electrode.

Use of sequential purging with the static headspace method to quantify gasoline contamination

The static headspace method used with portable gas chromatographs has become an important means of field screening environmental samples for gasoline contamination. A major limitation in using this method is the simultaneous detection (coelution) of other volatile gasoline constituents with those of interest. This is particularly problematic in the quantitation of methyl-t-butyl ether (MTBE), benzene and toluene. A sequential purging technique was used with static headspace analysis to remove coeluting compounds and to improve the accuracy in the quantitation of MTBE and aromatic constituents. Aqueous constituent concentrations determined using sequential purging were generally within 20% of those determined from laboratory purge and trap/gas chromatography (EPA method SW 846-602) analyses. Without sequential purging, constituent concentrations determined using the static headspace method were found to be 2 to over 10 times that of laboratory analyses. Further, in very contaminated samples, sequential purging permitted quantitation of constituents which were not resolvable in the initial headspace analysis due to coelution.