Remember Groundwater Flow

Abstract

Groundwater flow is governed by groundwater hydraulics and is key to understanding the subsurface migration of groundwater, dissolved contaminants, and emplaced injection fluids. Yet groundwater flow appears to be largely forgotten or ignored in many subsurface injection-based remedial studies. Over the past decade, numerous studies of subsurface injection technologies (in situ oxidation, enhanced biodegradation, etc.) have been presented at conferences and published as peer-reviewed papers. These studies discuss the effectiveness of these remediation technologies and present chemical and biological data to support their results. However, rarely do these studies consider the impact of groundwater flow on data interpretation and their results. Often the studies do not even provide basic information on groundwater seepage velocities. I recently read a professional paper, an in situ oxidation (ISCO) study, that prompted me to write this editorial. I will use this unnamed study as an example, but I could equally use one of numerous other studies as an example. This ISCO study showed reductions in contaminant concentrations with some rebound and, in many respects, was well designed and well documented. The groundwater seepage velocity was reported to be about 46 ft/year. Three ISCO injection events were completed, during a period of about 9 months, at the source and at other injection locations extending about 30 ft downgradient from the source. The monitoring network consisted of three transects of discrete monitoring wells located 15, 45, and 75 ft downgradient from the source injection location. Evaluations of the effectiveness of the injections were completed based on a groundwater sampling event 3 months after the last injection. A final groundwater sampling event, which occurred 11 months after the last injection event, was used to evaluate rebound. Do you see what troubles me here? Consider this ISCO study and assume idealized horizontal groundwater flow while disregarding aquifer heterogeneity that complicates groundwater flow. Based on the distances of the monitoring transects (15, 45, and 75 ft downgradient) and the groundwater seepage velocity (46 ft/year), it would take approximately 119 (almost 4 months), 357, and 595 days for the injected ISCO fluid from the source injection to flow past the three monitoring transects, respectively. Yet this ISCO study evaluated the results and effectiveness of the ISCO injections based on groundwater samples collected 3 months after the last injection, before the injected fluid had migrated past the closest monitoring well transect. During the final sampling event, 11 months postinjection, the injected fluid would be expected to be found around two of the three well transects under these idealized groundwater flow conditions. The real world is not homogeneous and isotropic. Heterogeneity can result in varying migration rates for injected fluid and can affect the composition of well samples. Injected fluids in higher permeable zones will flow past downgradient monitoring wells faster than injected fluids in lower permeable zones. Groundwater samples reflect a combination of groundwater from the different permeability zones intersected by the well. If a groundwater sample is collected from a well that intersects both a higher permeable zone, where the original contaminated groundwater has displaced the injected fluid, and a lower permeable zone, which still has the noncontaminated injected fluid, the resulting groundwater sample would reflect a mixture of groundwater from these two zones. If, for example, 70% of the sample water comes from the higher permeable zone and 30% of the sample water comes from the lower permeable zone, the resulting groundwater sample would indicate a contaminant concentration 30% lower than the original groundwater contaminant concentration. This could be interpreted to suggest that the postinjection groundwater concentrations had declined by 30%. Groundwater flow needs to be understood and specifically addressed during studies of injection-based in situ remediation because groundwater flow is key to understanding the subsurface migration of groundwater and emplaced injection fluids. The characterization and heterogeneity of groundwater flow need to be integral to the design, data collection, and data interpretation of injection-based remedial studies if we wish to truly understand the effectiveness of our in situ injection remedial technologies. Therefore, for the design, implementation, interpretation, and presentation of studies involving injection based in situ remedial technologies, remember groundwater flow. Guy Swenson, BA, MS, Marcellus, NY; [email protected]