Mackay's Research Home Page
Mackay's group addresses many aspects of groundwater contamination and decontamination, often utilizing highly detailed field studies of existing groundwater contamination and/or highly controlled field experiments. Both are conducted with resolution approaching that of laboratory studies but at scales and often including processes impossible to represent in laboratory studies. Contaminants investigated to date include:
- petroleum hydrocarbons (e.g. gasoline, ethanol-blended gasolines [gasohol], oil, crude oil)
- fuel oxygenates (e.g. in reformulated gasolines)
- chlorinated solvents
- complex mixtures
From 1999 through 2009, both types of field research were conducted at a site at Vandenberg Air Force Base (VAFB) addressing: 1) in situ aerobic biodegradation of methyl tert-butyl ether (MTBE) and tert butyl alcohol (TBA), 2) methods to engineer in situ aerobic biotreatment of MTBE and/or TBA plumes, 3) impact of ethanol on natural attenuation (anaerobic biodegradation) of model components of gasoline (benzene, toluene, and o-xylene), 4) impact of ethanol on anaerobic biotransformation of MTBE to TBA), 5) natural biodegradation of TBA under sulfate-reducing conditions, 6) role of diffusion in controlling the flushing of contaminants from aquifers bounded with low permeability aquitards, 7) methods for estimating the total mass flux (mass discharge) of contaminants carried by groundwater flow through aquifers, and 8) methods for assessing bioremediation performance (including bio-traps, molecular methods applied to water or sediment samples, and compound-specific isotope analyses).
In 2009-2010, we applied molecular methods to characterize the microbial populations in an aquifer at a former refinery site. Our specific goals were to determine if the monitoring tools can confirm the presence and activity of microbes capable of benzene degradation under sulfate-reducing conditions.
In 2011 we initiated new field studies at petroleum-impacted
sites with sulfate-dominated aquifers to explore the feasibility of
enhancing the rate of in situ biodegradation of contaminants. We have
examined various types of monitoring data and concluded that there are
limitations to in situ degradation that remain elusive, though we
suspect a role for polar intermediates of hydrocarbon degradation. We
have estimated that the mass discharge of biodegradation products (expressed as grams carbon per day) is much greater through the vadose zone (CO2 and CH4) than is through the saturated zone (CO2, CH4 and polar intermediates).
We are continuing our research on petroleum-impacted sites in
sulfate-dominated settings, but have also initiated new projects. We
have evaluated a fuel-oxygenate-impacted setting (TBA and MTBE) to
determine feasibility of stimulating in situ biodegradation by
recirculating groundwater amended with oxygen. Laboratory studies of
oxygen demand and microcosm studies seeking evidence of oxygenate
degraders suggest the method would be feasible provided hydrogeologic
(flow) issues are addressed.
Recently we secured approvals for use of a field site near UC
Davis for research on natural attenuation of ethanol and/or
ethanol-blended fuels, focusing initially on the fate of methane
generated by fuel degradation in situ. We have nearly completed a
controlled field release of methane and a tracer, with 3D monitoring in
the vadose zone and 2D monitoring of surface efflux of the released
species, along with CO2 and other gases. The experiment has confirmed
ability of native methanotrophs to oxidize methane and prevent or
greatly reduce its efflux to the atmosphere, provided there is
sufficient soil moisture. We have received approval for two new
controlled field releases, one of E10 gasoline and the other of E85
gasoline. Both will be initiated in 2015, and will involve very slow
continuous release of the ethanol-blended gasoline, with detailed
monitoring as used in the methane experiment just finished. Finally, we
are collaborating on a research project on remediation of hydrocarbon
impacted soils which is headed jointly by Arizona State University and
the University of Waterloo, and involves researchers from those
institutions as well as several others.
Additional topics that Mackay's groups have addressed via field research in the past include: 1) transport, sorption and degradation of various organic contaminants in sandy aquifers, 2) factors limiting pump & treat remediation of contaminated groundwater, 3) comparison of pulsed and continuous pumping for groundwater remediation by pump & treat technologies, 4) pesticide transport and sorption in groundwater, 5) remediation of groundwater via various configurations of permeable reactive biobarriers (e.g., aerobic PRBs using diffusive oxygen releasers or solid oxygen sources; anaerobic PRBs using diffusive hydrogen release or zero valent iron), 6) flushing of a high-concentration, multi-component organic/inorganic contaminant plume from a granular aquifer overlying a fractured aquitard, 7) vulnerability of Mexico City's aquifer to contamination.
Douglas M. Mackay