Comparing Natural Source Zone Depletion Pathways at a Fuel Release Site

Mackay, Douglas M.; Hathaway, Emily; Sieyes, Nicholas de; Zhang, Han; Rasa, Ehsan; Paradis, Charles; Schmidt, Radomir; Peng, Juan; Buscheck, Timothy E.; Sihota, Natasha

doi:10.1111/gwmr.12278

Cited by 12 publications

(12 citation statements)

References 18 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Lundegard and Johnson () estimated nominal vadose zone HC losses more than one order of magnitude greater than nominal saturated zone HC losses for the Guadalupe oil field composed of diluent contaminant within an eolian sand aquifer (not accounting for iron precipitation to soils). MacKay et al () also estimated nominal vadose zone losses as dominant (94% of the total based on CO 2 effluxes). To the extent that CO 2 effluxes are an indicator of vadose zone losses and groundwater samples are an indicator of saturated zone losses, the narrower range between the two NSZD components for the present study may be related to the active groundwater extraction in a relatively permeable aquifer and high upgradient sulfate concentrations.…”

Section: Resultsmentioning

confidence: 99%

Influence of Ambient Temperature, Precipitation, and Groundwater Level on Natural Source Zone Depletion Rates at a Large Semiarid LNAPL Site

McAlexander¹,

Sihota²

2019

Groundwater Monitoring Rem

Self Cite

View full text Add to dashboard Cite

Natural source zone depletion (NSZD) is increasingly being integrated into management strategies for petroleum release sites. Measurements of NSZD rates can be used to evaluate naturally occurring hydrocarbon (HC) mass losses, and provide a baseline for evaluating engineered recovery systems. Here, nominal saturated and unsaturated zone HC losses were quantified by groundwater sampling and ground surface CO2 effluxes approximately monthly over a 1‐year period. In addition, subsurface gas profiles and temperature, precipitation, and groundwater elevation were evaluated to elucidate dominant environmental factors controlling NSZD rates. Results showed that NSZD rates estimated by surface CO2 effluxes were, on average, more than a factor of 3 greater than those estimated by uptake of electron acceptors (primarily sulfate) in extracted groundwater. This may indicate that vadose zone mass loss mechanisms (e.g., volatilization and subsequent biodegradation) were dominant in this system, but possible transfer of gases from the saturated zone to the vadose zone confounds this interpretation. Results for this semiarid site revealed that increasing NSZD rates tended to occur with increasing ambient monthly precipitation and temperature when accounting for time lags associated with subsurface transport. However, groundwater elevation was not found to be significantly related to NSZD rates. This result is counter to an expectation that increased smear zone exposure increases HC mass losses, and suggests that the pump‐and‐treat system did not greatly influence total NSZD rates directly through smear zone flushing or indirectly by lowering the regional water table.

show abstract

Section: Resultsmentioning

confidence: 99%

Influence of Ambient Temperature, Precipitation, and Groundwater Level on Natural Source Zone Depletion Rates at a Large Semiarid LNAPL Site

McAlexander¹,

Sihota²

2019

Groundwater Monitoring Rem

Self Cite

View full text Add to dashboard Cite

show abstract

“…Often, the demand for sulfate imposed by dissolved hydrocarbons in the release zone exceeds the amount supplied by the flow of natural sulfate‐laden groundwater into the release zone; consequently groundwater flowing through a portion of the release zones is depleted of sulfate. For example, sulfate reduction accounted for 73 to 95% of the total attenuation of PHCs in groundwater at the site described in this paper (Mackay et al ). Dissolved sulfate concentrations were also found to be depleted within and downgradient of the release zone.…”

Section: Introductionmentioning

confidence: 82%

“…Well depths were selected by prior consultants to explore vertical profiles of PHC contamination in the total depth interval monitored within the single hydrogeologic unit. Site operations ceased in 1993 and much of the originally released LNAPL is now submerged and, as discussed by Mackay et al (, ), most may have been depleted by diffusion into the relatively low permeability media surrounding the thin interconnected more permeable lenses. Prior to installation of PFBs and HPIs, LNAPL had not been detected in any monitoring wells since March 2010.…”

Section: Introductionmentioning

confidence: 98%

“…The method relies on the natural groundwater flow to distribute sulfate from the PFBs to the PHC‐impacted portions of the subsurface. The study site had been evaluated in detail by Mackay et al (, ) and was known to have relatively high‐background sulfate, a PHC release zone in which sulfate was depleted, and benzene persisting above the cleanup goal. Microcosm studies described in Appendix S1 confirmed the site's microbial community can reduce benzene if supplied additional sulfate.…”

Section: Introductionmentioning

confidence: 99%

“…Mackay et al (, ) discuss in detail the background and characteristics of the site, a former service station that closed in 1993, so only a brief summary is presented here. Groundwater velocity at this silty site is very low and estimated to be approximately 0.57 cm/d.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Enhancing Microbial Sulfate Reduction of Hydrocarbons in Groundwater Using Permeable Filled Borings

Buscheck¹,

Mackay²,

Paradis³

et al. 2019

Groundwater Monitoring Rem

Self Cite

View full text Add to dashboard Cite

At a service station closed in 1993, groundwater contained benzene that persisted above the cleanup goal of 1 mg/L in zones depleted of background sulfate. The benzene and other petroleum hydrocarbons (PHCs) were present as much as 36 feet (11 m) below the water table and therefore remediation of a thick saturated zone interval was required. Microcosms using site sediments demonstrated that anaerobic benzene biodegradation occurred only if sulfate was added, suggesting sulfate addition as a remediation approach. Twenty‐four boreholes (9.1″ diameter and 56′ deep) were drilled around four monitoring wells, in which benzene concentrations exceeded 1 mg/L. The boreholes were backfilled with a mixture of gravel and 15,000 pounds of gypsum (which releases sulfate as it dissolves) to create “Permeable Filled Borings” (PFBs). Concurrently, nine high pressure injections (HPIs) of gypsum slurry were conducted in other site locations (312 pounds of gypsum total). PFBs were expected to release sulfate for up to 20 years, whereas HPIs were expected to produce a short‐lived plume of sulfate. Concentrations of benzene and sulfate in groundwater were monitored over a 3‐year period in six monitoring wells. In two wells near PFBs, benzene concentrations dropped below the cleanup goal by two to three orders of magnitude; in one well, sulfate concentrations exceeded 500 mg/L for the most recent 18 months. Benzene concentrations in two other PFB monitoring wells declined by a factor of 2 to 4, but remained above 1 mg/L, presumably due to high‐dissolved PHC concentrations and possibly greater residual PHC mass in adjacent sediments, and therefore greater sulfate demand. However, hydrogen and sulfur isotopic enrichment in benzene and sulfate, respectively, confirmed biodegradation of benzene and stimulation of sulfate‐reducing conditions. Thus, it is hypothesized that the PHC mass in adjacent sediments will decline over time, as will dissolved PHC concentrations, and eventually benzene concentrations will decrease below the cleanup goal. Benzene in two HPI monitoring wells was below the cleanup goal for all but one sampling event before HPIs were conducted and remained below the cleanup goal after HPIs; there was no stimulation of sulfate‐reducing conditions. It is concluded that sulfate released from PFBs contributed to declining benzene concentrations.

show abstract

Feasibility of greenhouse gas emissions offsets for natural source zone depletion of petroleum hydrocarbons

McAlexander¹

2019

Remediation Journal

View full text Add to dashboard Cite

Hydrocarbon biodegradation is an important process for remediating petroleum hydrocarbons and managing large sites. However, this biodegradation results in what are essentially unavoidable CO 2 emissions to the atmosphere. A feasibility assessment was conducted to quantitatively consider reuse options for petroleum brownfields that would offset contaminant respiration emissions rates in the 2 to 10 micromoles CO 2 per meters squared per second ( mol CO 2 m −2 s −1 ) typically observed. Under a wide range of solar resource scenarios, placement of solar panels over only a fraction (no more than 35%) of the site footprint is estimated as necessary to achieve an emissions offset. Similarly, placement of one 30-meter tall wind turbine of moderate rating (approximately 30 to 50 kW) is sufficient to provide an offset for a nominal 1,000 square meters site. For spreading of spent calcium-rich construction materials, under even a high emissions scenario, the required footprint for the offset is less than the site footprint. While these approaches appear feasible, revegetation as forestland is estimated as sufficient only at contaminant respiration rates up to 2 mol CO 2 m −2 s −1 . Revegetation as rangeland and cropland, which sequesters CO 2 mainly in soil organic carbon, is estimated as requiring more than the site footprint under many contaminant respiration rates. Revegetation as a wetland fares slightly better from a carbon storage perspective, but it also has the potential for N 2 O and CH 4 emissions that may largely undo the benefit from sequestration in soil organic matter. Overall, the results indicate several methods that are viable for achieving emissions offsets and a quantitation method that can be honed with site-specific input parameters as appropriate.

show abstract

Comparing Natural Source Zone Depletion Pathways at a Fuel Release Site

Cited by 12 publications

References 18 publications

Influence of Ambient Temperature, Precipitation, and Groundwater Level on Natural Source Zone Depletion Rates at a Large Semiarid LNAPL Site

Influence of Ambient Temperature, Precipitation, and Groundwater Level on Natural Source Zone Depletion Rates at a Large Semiarid LNAPL Site

Enhancing Microbial Sulfate Reduction of Hydrocarbons in Groundwater Using Permeable Filled Borings

Feasibility of greenhouse gas emissions offsets for natural source zone depletion of petroleum hydrocarbons

Contact Info

Product

Resources

About