A computational assessment of representative sampling of soil gas using existing groundwater monitoring wells screened across the water table

Self Cite

This column reviews the general features of PHT3D Version 2, a reactive multicomponent transport model that couples the geochemical modeling software PHREEQC-2 (Parkhurst and Appelo 1999) with three-dimensional groundwater flow and transport simulators MODFLOW-2000 and MT3DMS (Zheng and Wang 1999). The original version of PHT3D was developed by Henning Prommer and Version 2 by Henning Prommer and Vincent Post (Prommer and Post 2010). More detailed information about PHT3D is available at the website http://www.pht3d.org. The review was conducted separately by two reviewers. This column is presented in two parts.

Section: Factors Affecting Recovery Endpointssupporting

confidence: 66%

“…Other single-phase approaches such as SVE may be considered as subcategories of MPE. This would mean that the same modeling approach is applicable in characterizing bulk and composition endpoints for such single-phase approaches (Sookhak Lari et al 2017).…”

Section: Discussionmentioning

confidence: 99%

LNAPL Recovery Endpoints: Lessons Learnt Through Modeling, Experiments, and Field Trials

Lari¹,

Rayner²,

Davis³

et al. 2020

Self Cite

“…Additional depths within the screened interval can be sampled, if needed, to determine the depth of the aerobic/anaerobic (A/A) boundary. Both high (EPA method) and low flow sampling methods were evaluated independently by Sookhak Lari et al (2017) at a field site in Australia. They concluded that the low flow sampling method for monitoring wells provided: “reasonably representative samples of vadose zone gas concentrations”.…”

Section: Methodsmentioning

confidence: 99%

Rapid Field Approach to Evaluating Natural Source Zone Depletion for a Range of Light Non‐Aqueous Phase Liquid Sites

Ririe¹,

Sweeney²

2022

More rapid cost‐effective approaches are required to enable field evaluation of natural source zone depletion (NSZD) at petroleum LNAPL sites. Long and costly cleanups have continued as NSZD has not become widely acceptable as a remedial option. We have developed a low cost rapid field approach (RFA) and expanded the site conceptual models (SCMs) to include NSZD. Results from two sites are presented that illustrate the benefit of the RFA in supplementing existing data and updating SCMs. At present, most NSZD guidelines follow a single generic SCM (aerobic oxidation of methane) as the primary contributor to NSZD. Aerobic oxidation of LNAPL is also possible, and can be documented by subsurface measurements of soil‐gas composition and/or heat generated from biodegradation. The RFA was developed so that sites could be easily separated into those with and without a methane plume overlying LNAPL. This distinction is important as it defines the relative rates of oxygen transport into the soil, and methane generation and release from saturated zone anaerobic biodegradation. For the RFA, evidence for direct LNAPL oxidation is absence of methane (<0.1%‐v) in the unsaturated zone above LNAPL. This measurement can be readily made in the field using existing monitoring wells. The absence of a methane plume above LNAPL would indicate that NSZD is a result of direct biodegradation of LNAPL. This alternative SCM needs to be considered and evaluated prior to implementing more costly data collection or other remediation or site management options.

“…The important assumption implicit in the low purge volume method is that the gas composition in the well bore will be essentially the same as the soil gas in the adjacent soil and sand pack. As shown by SookhakLari et al (), vapor extracted from a well at low flow rates can have a similar composition as the in situ gas at the base of the well. The effectiveness of the low purge volume method for obtaining a vapor sample from near the source can be evaluated in the field on the basis of the O 2 concentration monitored in the field.…”

Section: Introductionmentioning

confidence: 77%

“…This can provide a more rapid and cost‐effective approach compared with installation of deep gas sampling probes at a site. The small purge volume method for sampling monitoring wells is advisable for sampling those sites with low permeability soils (McAlary et al ) and also should be considered, on a site‐specific basis, as an important new approach for evaluating NSZD potential at LNAPL sites using the gradient method (Johnson et al ).We note too that a computational assessment of low‐ and high‐volume gas sampling from existing long‐ and short‐screened wells has been undertaken recently (SookhakLari et al ). It is consistent with the findings presented here.…”

Section: Discussionmentioning

confidence: 99%

Small Purge Method to Sample Vapor from Groundwater Monitoring Wells Screened Across the Water Table

Sweeney¹,

Ririe²

2017

Groundwater monitoring wells are present at most hydrocarbon release sites that are being assessed for cleanup. If screened across the vadose zone, these wells provide an opportunity to collect vapor samples that can be used in the evaluation of vapor movement and biodegradation processes occurring at such sites. This paper presents a low purge volume method (modified after that developed by the U.S. EPA) for sampling vapor from monitoring wells that is easy to implement and can provide an assessment of the soil gas total petroleum hydrocarbon (TPH) and O 2 concentrations at the base of the vadose zone. As a result, the small purge method allows for sampling of vapor from monitoring wells to support petroleum vapor intrusion (PVI) risk assessment. The small purge volume method was field tested at the Hal's service station site in Green River, Utah. This site is well-known for numerous soil gas measurements containing high O 2 and high TPH vapor concentrations in the same samples which is inconsistent with well-accepted biodegradation models for the vapor pathway. Using the low purge volume method, monitoring wells were sampled over, upgradient, and downgradient of the light nonaqueous phase liquid (LNAPL) footprint. Results from our testing at Hal's show that vapor from monitoring wells over LNAPL contained very low O 2 and high TPH concentrations. In contrast, vapor from monitoring wells not over LNAPL contained high O 2 and low TPH concentrations. The results of this study show that a low purge volume method is consistent with biodegradation models especially for sampling at sites where low permeability soils exist in and around a LNAPL source zone.