Comparative evaluation of contaminant mass flux and groundwater flux measurements in fractured rock using passive flux meters

Abstract

Peer reviewed report on cost effectiveness and reliability of techniques for the characterization of contaminated fractured rock aquifers. Two important characteristics of contaminant transport are groundwater velocity (or flux) and contaminant mass flux. Conventional methods for characterization in fractured rock cannot directly measure groundwater or contaminant mass flux. Therefore, the purpose of this project was to assess the ability of two technologies, the modified standard passive flux meter (MSPFM) and the fractured rock passive flux meter (FRPFM), to measure groundwater and contaminant mass flux in fractured rock. These measurements were compared to more conventional fractured rock characterization methods, including a borehole dilution (BHD) test. The comparative study of these techniques was conducted in a well at the former Naval Air Warfare Center Research site located in West Trenton, New Jersey. This research site, which consists of fractured sedimentary rock, is operated by the U.S. Geological Survey (USGS) as part of the USGS Toxic Substances Hydrology Program. Tests conducted in the well centered on a transmissive fracture identified at 28.7 m (94 ft) below ground surface through previous characterization activities.
Average groundwater flux measurements from the BHD test, MSPFM, and FRPFM were 1.5 cm/day, 2.6 cm/day, and 2.7 cm/day, respectively. Estimates of groundwater flux based on the MSPFM and FRPFM were very similar, but were almost a factor of two higher than the BHD test results. Measurements of groundwater flux vertical distributions were also completed with the MSPFM and FRPFM. However, the spatial patterns of groundwater flux as measured with the MSPFM and FRPFM were not similar. Average trichloroethene mass flux measurements from the BHD test, MSPFM, and FRPFM were 18.8 mg/m2/day, 31.5 mg/m2/day, and 116 mg/m2/day, respectively. Likewise, averagecis-1,2-dichloroethene mass flux measurements from the BHD test, MSPFM, and FRPFM were 14.6 mg/m2/day, 40.7 mg/m2/day, and 68.2 mg/m2/day, respectively. In both cases, the BHD gave the lowest estimates while the FRPFM gave the highest. As with groundwater flux, spatial measurements of contaminant flux based on the MSPFM and FRPFM were not similar. Differences in results between the technologies most likely stem from differences in measurement design and method, but natural variability in conditions during the tests may also be a factor. Moreover, damage to the FRPFM during retrieval may also have been a factor in the results obtained.
The MSPFM, compared to the FRPFM, was easier to implement, and is judged less likely to be damaged during deployment and retrieval. However, because of its design it is also more susceptible to sampling bias during deployment and retrieval. The FRPFM was the most complex method to use compared to the MSPFM and BHD test. The FRPFM was damaged during retrieval in this study, suggesting this technology is more fragile than the BHD test or MSPFM. At present, the best use for the FRPFM would be those applications where high resolution data is needed over short intervals. Further development of the FRPFM technology may result in a more widely applicable measurement method. Comparisons of the spatial distributions of groundwater flux and contaminant mass flux between the MSPFM and FRPFM as measured in this project indicate more research is needed to further assess the accuracy and reliability of the measured spatial distributions. Controlled experiments in which the true distribution is known would be helpful in this regard.