PEDRO ALVAREZ: Contaminant concentrations can decrease at a site due to abiotic processes, such as dilution and volatilization. And it is important to discern the importance of biodegradation to support decisions to accept or reject monitored natural attenuation as a remediation strategy. One strong supporting line of evidence that biodegradation is occurring satisfactorily is to demonstrate significant loss of contaminants at the field scale. This involves acquiring extensive data over long spatial and temporal scales. As a simple first step, you can use graphs to visualize whether plume contours are shrinking over time, as illustrated here on the left, and whether contaminant concentrations at all, if not most, monitoring wells are also decreasing. Of course, this requires installing a monitoring well network that fully encompasses the plume, and ideally that it also allows monitoring along the flow pass, as recommended by this figure, even though we need to recognize that the flow direction may change from one season to another. But upgrading wells are also important to include to allow comparison to background groundwater quality. Well transects perpendicular to the flow paths and screened at multiple levels can also be very useful to determine how the flux of a pollutant is changing as the plume migrates. Flux here is defined as the mass flow rate per a unit's cross-sectional area along the flow path. And a decrease in flux from one transect to the next is strong evidence of biodegradation. Although multiple well transects can be costly to install, they provide useful information to also determine the first-order decay coefficient, lambda, that is directly associated with biodegradation. And this coefficient, which can be used in fate and transport models, is obtained as the slope of the regression line between the natural logarithm of the flux versus the travel time. The travel time here can be given as the distance divided by the groundwater flow velocity. Another direct quantitative method for demonstrating natural attenuation is to calculate the total mass of dissolved contaminants by integrating and interpolating beta from a dense monitoring well network. This method is particularly useful for non-steady plumes and can facilitate demonstrating that a plume is disappearing, rather than expanding. This method allows you to directly calculate, also, the decay coefficient lambda, unless the plume is at steady state. This does not work for steady-state plumes, because the mass of dissolved contaminants would be relatively constant. And converging lines of evidence can be provided by the geochemical footprint of biodegradation, such as consumption of electron acceptors for oxidative processes, as illustrated in this time-course pattern for a fuel spill in the left-hand panel. Here, we can see the sequential depletion of electron acceptors in the light blue line, in order of their oxidation potential. We start with oxygen, which is being preferentially utilized; then nitrate; and then you also see, eventually, the experience of reduced products, such as ferrous iron and, eventually, methane. And this results, of course, in zones of different electron-accepting conditions, with stronger reducing anaerobic conditions, such as methanogenesis near the source zone, where the demand for electron acceptor is highest. Separately, if you have chlorinated solvents instead of a benzene, in this case, you want to see the release increase as the concentration of chloride as a sign of reductive dechlorination, as well as the appearance of known dechlorinated byproducts. In situ microcosms can also be used to demonstrate in situ biodegradation. These devices are introduced down a well and spiked with a contaminant that is introduced and released, along with a conservative tracer, so that we can monitor the removal or the appearance of known metabolites. The removal data is then compared to that of a conservative tracer, such as bromide, to account for potential dilution since the bottom of the microcosm is open. A similar approach is to inject into the aquifer a finite amount of pollutant along with a conservative tracer, allow it to remain in the system for sufficient time to undergo some biodegradation, and then pull it back-- extract it and quantify the amount of mass that has been removed relative to the tracer. The difference can be attributed to biodegradation. And finally, there are a couple of emerging biodegradation forensic approaches that hold significant promise for widespread use. These topics will be covered in the next two lectures. To wrap it up, the key points of this lecture is that converging lines of evidence are usually needed to demonstrate that in situ biodegradation is a significant attenuation process. Strong evidence here can be provided by showing graphically that plume contours are shrinking or that the contaminant flux is decreasing along the flow path alone, or that the dissolved mass of contaminants is decreasing. Geochemical indicators such as stoichiometric consumption of electron acceptors and accumulation of their reduced byproducts, or metabolites, are also useful-- along with microcosm study as supporting evidence.
