Demonstrating In-Situ Biodegradation
Natural Attenuation of Groundwater Contaminants: New Paradigms, Technologies, and Applications
Cleaning up the large number of groundwater contamination sites is a significant and complex environmental challenge. The environmental industry is continuously looking for remediation methods that are both effective and cost-efficient. Over the past 10 years there have been amazing, important developments in our understanding of key attenuation processes and technologies for evaluating natural attenuation processes, and a changing institutional perspective on when and where Monitored Natural Attenuation (MNA) may be applied. Despite these advances, restoring groundwater contaminated by anthropogenic sources to allow for unrestricted use continues to be a challenge. Because of a complex mix of physical, chemical, and biological constraints associated with active in-situ cleanup technologies, there has been a long standing focus on understanding natural processes that attenuate groundwater contaminant plumes. We will build upon basic environmental science and environmental engineering principles to discover how to best implement MNA as a viable treatment for groundwater contamination plumes. Additionally we will delve into the history behind and make predictions about the new directions for this technology. Any professional working in the environmental remediation industry will benefit from this in-depth study of MNA. We will use lectures, readings, and computational exercises to enhance our understanding and implementation of MNA. At the completion of this course, students will have updated understanding and practical tools that can be applied to all possible MNA sites.
Better Technologies: Compound Specific Isotopes & Molecular Biological Tools
In this series of lectures, we will focus on several established and emerging methods for demonstrating that biological and other attenuation processes are occurring. We will start with an overview on lines of evidence for biological degradation processes, and then transition to newer molecular methods for identifying and quantifying biodegradation potential for a variety of compounds. Several of this week’s lectures also examine compound-specific isotope analysis, which has been emerging as a key resource for MNA studies. You will learn how stable isotope analyses can provide critical information on whether attenuation is occurring, the rate at which it is occurring, and even the pathway that is responsible.
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
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.