Residual Light Non-aqueous Phase Liquid Research Articles

Simulation of the effect of remediation on EDB and 1,2-DCA plumes at sites contaminated by leaded gasoline

An analytical model is used to simulate the effects of partial source removal and plume remediation on ethylene dibromide (EDB) and 1,2-dichloroethane (1,2-DCA) plumes at contaminated underground storage tank (UST) sites. The risk posed by EDB, 1,2-DCA, and commingled gasoline hydrocarbons varies throughout the plume over time. Dissolution from the light nonaqueous phase liquid (LNAPL) determines the concentration of each contaminant near the source, but biological decay in the plume has a greater influence as distance downgradient from the source increases. For this reason, compounds that exceed regulatory standards near the source may not in downgradient plume zones. At UST sites, partial removal of a residual LNAPL source mass may serve as a stand alone remedial technique if dissolved concentrations in the source zone are within several orders of magnitude of the applicable government or remedial standards. This may be the case with 1,2-DCA; however, EDB is likely to be found at concentrations that are orders of magnitude higher than its low Maximum Contaminant Level (MCL) of 0.05 μg/L (micrograms per liter). For sites with significant EDB contamination, even when plume remediation is combined with source depletion, significant timeframes may be required to mitigate the impact of this compound. Benzene and MTBE are commonly the focus of remedial efforts at UST sites, but simulations presented here suggest that EDB, and to a lesser extent 1,2-DCA, could be the critical contaminants to consider in the remediation design process at many sites.

In situ chemical oxidation of residual LNAPL and dissolved‐phase fuel hydrocarbons and chlorinated alkenes in groundwater using activated persulfate

AbstractA treatablity study (TS) was conducted to evaluate the efficacy of in situ chemical oxidation (ISCO) using activated persulfate, alone and in combination with air sparging (AS), for treating a source area contaminated with residual light nonaqueous‐phase liquid (LNAPL), dissolved‐phase fuel hydrocarbons (HCs), and dissolved‐phase chlorinated alkenes at Edwards Air Force Base (AFB), California. The TS was implemented in two phases. Phase I included injecting a solution of sodium persulfate and sodium hydroxide (NaOH) into groundwater via an existing well where residual LNAPL and dissolved‐phase contaminants were present. Because the results of Phase I indicated a limited distribution of the activated persulfate, Phase II was performed to assess whether AS could enhance the distribution of the sodium persulfate. Each phase was followed by groundwater monitoring and sampling at the injection well and at three monitoring wells, located 20 to 44 feet from the injection well. Results from Phases I and II of the TS indicated that (1) alkaline‐activated persulfate was effective in promoting the dissolution of LNAPL and the degradation of dissolved‐phase contaminants, but only at the injection well; (2) the addition of AS was effective in enhancing the radius of persulfate distribution from less than 20 feet to greater than 44 feet, and (3) persulfate alone (i.e., not in an activated state) was effective in reducing the concentrations of dissolved‐phase fuel HC and chlorinated alkenes. © 2009 Wiley Periodicals, Inc.

In‐situ apparent conductivity measurements and microbial population distribution at a hydrocarbon‐contaminated site

We investigated the bulk electrical conductivity and microbial population distribution in sediments at a site contaminated with light nonaqueous‐phase liquid (LNAPL). The bulk conductivity was measured using in‐situ vertical resistivity probes; the most probable number method was used to characterize the spatial distribution of aerobic heterotrophic and oil‐degrading microbial populations. The purpose of this study was to assess if high conductivity observed at aged LNAPL‐impacted sites may be related to microbial degradation of LNAPL. The results show higher bulk conductivity coincident with LNAPL‐impacted zones, in contrast to geoelectrical models that predict lower conductivity in such zones. The highest bulk conductivity was observed to be associated with zones impacted by residual and free LNAPL. Data from bacteria enumeration from sediments close to the resistivity probes show that oil‐degrading microbes make up a larger percentage (5–55%) of the heterotrophic microbial community at depths coincident with the higher conductivity compared to ∼5% at the uncontaminated location. The coincidence of a higher percentage of oil‐degrading microbial populations in zones of higher bulk conductivity suggests that the higher conductivity in these zones may result from increased fluid conductivity related to microbial degradation of LNAPL, consistent with geochemical studies that suggest that intrinsic biodegradation is occurring at the site. The findings from this study point to the fact that biogeochemical processes accompanying biodegradation of contaminants can potentially alter geoelectrical properties of the subsurface impacted media.

Effect of the initial soil water saturation on the behaviour of a mixed LNAPL and heavy metal contaminated glaciofluvial deposit

SummarySoil contamination by mixtures of petroleum hydrocarbons and heavy metals is common in urban and industrial localities. Interactions between these contaminants have an impact on the mobility and the management of contamination. We have characterized the modifications to the transport of heavy metals (Cd, Cu, Pb, Zn) in soil induced by residual light non‐aqueous phase liquid (LNAPL) for two conditions of trapping. Experiments on the elution of tracers and heavy metals in columns of soil were performed with a glaciofluvial material as the soil. Tracer experiments were modelled with the mobile–immobile (MIM) system of partial differential equations. The experiments were designed to compare water flow and metal transport in LNAPL‐contaminated soil with a control set. Residual LNAPL was trapped in water‐saturated and dry soil to ensure preferential wettability of soil surfaces, namely either water‐wet or LNAPL‐wet.In water‐wet soil columns, LNAPL decreased water flow by two orders of magnitude and increased the fraction of immobile water. Solute residence times (SRTs) suggested that heavy metals resided mainly in mobile water where the reaction time was sufficient to reach steady‐state retention. The SRTs also indicated that a fraction of the heavy metal flux diffused to the immobile water where its retention was limited by diffusion. Retention of heavy metals was significantly greater than in the control columns. In LNAPL‐wet soil columns, the obstruction of small pores and surface coating by residual LNAPL significantly decreased the attenuation capacity of the soil by decreasing the diffusion of heavy metals to immobile water and surface sites. Evidently, the individual behaviour of heavy metals can be significantly modified by non‐miscible organic contaminants. These modifications can have important implications for risk evaluation, contamination management and in situ remediation of soil that is contaminated by mixtures of petroleum hydrocarbons and heavy metals.

Surfactant Effects on Residual Water and Oil Saturations in Porous Media

A series of soil column tests was performed to determine surfactant effects on residual water and oil saturations in porous media. In particular, these tests focused on the impact of submicellar surfactant solutions and the potential application of these low concentration systems to light nonaqueous phase liquid (LNAPL) contamination in the vadose zone. One set of tests involved surfactant flushing in soil‐filled columns followed by drainage to residual water saturation and LNAPL injection to determine the subsequent residual LNAPL saturation. Another set of tests involved surfactant application to a soil‐filled column already holding residual LNAPL saturation to promote the release of the previously trapped LNAPL. Test results showed surfactant systems could reduce both residual water and oil saturations by up to 50%. In addition, submicellar surfactant systems were equally effective as supramicellar solutions in reducing residual water saturations and potentially more effective at reducing residual oil saturations. Submicellar surfactant applications to a medium‐grained (0.85–0.425 mm) soil containing residual LNAPL saturations were effective at releasing up to 50% of the previously trapped residual LNAPL. These applications were less successful in a fine‐grained soil as full drainage of water and LNAPL was unachievable due to high capillary pressures. Overall, observations suggest low concentration surfactant solutions may have the ability to release significant amounts of previously trapped LNAPL in the vadose zone, potentially increasing free‐product recovery rates and lowering LNAPL saturations to levels more favorable for biodegradation. The decrease in overall saturations (both water and oil) in a contaminated vadose zone could also present an increase in air permeability, thus enhancing other vadose zone treatment technologies such as bioventing or soil vapor extraction.

Surfactant Effects on Residual Water and Oil Saturations in Porous Media

A series of soil column tests was performed to determine surfactant effects on residual water and oil saturations in porous media. In particular, these tests focused on the impact of submicellar surfactant solutions and the potential application of these low concentration systems to light nonaqueous phase liquid (LNAPL) contamination in the vadose zone. One set of tests involved surfactant flushing in soil-filled columns followed by drainage to residual water saturation and LNAPL injection to determine the subsequent residual LNAPL saturation. Another set of tests involved surfactant application to a soil-filled column already holding residual LNAPL saturation to promote the release of the previously trapped LNAPL. Test results showed surfactant systems could reduce both residual water and oil saturations by up to 50%. In addition, submicellar surfactant systems were equally effective as supramicellar solutions in reducing residual water saturations and potentially more effective at reducing residual oil saturations. Submicellar surfactant applications to a medium-grained (0.85–0.425 mm) soil containing residual LNAPL saturations were effective at releasing up to 50% of the previously trapped residual LNAPL. These applications were less successful in a fine-grained soil as full drainage of water and LNAPL was unachievable due to high capillary pressures. Overall, observations suggest low concentration surfactant solutions may have the ability to release significant amounts of previously trapped LNAPL in the vadose zone, potentially increasing free-product recovery rates and lowering LNAPL saturations to levels more favorable for biodegradation. The decrease in overall saturations (both water and oil) in a contaminated vadose zone could also present an increase in air permeability, thus enhancing other vadose zone treatment technologies such as bioventing or soil vapor extraction.

Heavy metal transport in soil contaminated by residual light non-aqueous phase liquids (LNAPLs)

This paper presents an experimental study on mixed soil contamination, more specifically on heavy metal behaviour in soil contaminated by residual non-aqueous phase liquids (NAPLs). Remediation of mixed contaminated sites is a complex technical goal because of the presence of physically and chemically different contaminants and potential interactions between them. Commonly encountered contaminants in mixed contaminated soils include light and dense organic liquids (LNAPLs, DNAPLs) and heavy metals. This study investigated interactions between three residual LNAPLs and three heavy metals (Cd, Cu, Pb) in a carbonated soil. The objectives of the study were to (i) establish the presence of interactive processes in the behaviour of the contaminants, with a focus on the influence of residual LNAPL on heavy metal transport and retention; and (ii) determine the nature of these interactions. Results showed that the LNAPL having the highest residual saturation enhanced heavy metal mobility and decreased heavy metal retention by the soil. On the other hand, the geochemical distribution of heavy metals was not significantly modified by chemical interactions with the residual LNAPLs. Specific modifications of Pb and Cu geochemical distributions rather appeared to be the result of modifications of soil hydrodynamics by residual LNAPL.Key words: contaminated soil, mixed contamination, heavy metals, LNAPL, preferential flow, contaminant mobility.

Geophysical Discovery of a New LNAPL Plume at the Former Wurtsmith AFB, Oscoda, Michigan

AbstractAlight nonaqueous phase liquid (LNAPL) ground water contaminant plume has been discovered by purely geophysical means at the former Wurtsmith Air Force Base (AFB) near Oscoda, Michigan. It is located near another plume called FT‐02, which is a well‐studied area undergoing natural bioremediation. The plume was discovered by ground penetrating radar (GPR) profiling while extending a long line from FT‐02 to establish background variability around that plume. The new plume was apparent because of a high‐conductivity “shadow’ or GPR reflection attenuation observed below the conductive zone at the top of the aquifer, identical to the pattern observed at the FT‐02 plume. Further GPR surveys were conducted by students of a Western Michigan University geophysics field course to outline the proximal part of the plume. The GPR survey was supplemented by an electromagnetic induction (EM) survey which showed a group of four cables crossing the area. Finally, a magnetometer survey was conducted to search for any buried steel objects which might have been missed by the EM survey. The results of the three geophysical surveys were then used by students of a University of Michigan field course to guide subsurface soil and fluid sampling, which verified the presence of residual LNAPL product and ground water with conductivities 2.5 to 3.3 times above background. The plume source is in the vicinity of a vaulted underground storage tank (UST) formerly used for the collection of waste solvents and fuels for subsequent use in the fire training exercises at FT‐02. This newly discovered LNAPL plume, along with other “mature’ plumes, fits the electrical model which predicts conductive ground water below the decomposing but electrically resistive LNAPLs. Finally, this is a fine example of the cooperative use of a dedicated research site for training by students of two different universities.

Application of gravity drainage to the recovery of residual LNAPL in homogeneous and lensed sand packs

Residual LNAPL (light non-aqueous-phase liquid) trapped as immobile, isolated blobs under the water table often constitutes a large fraction of the total amount of LNAPL in the subsurface. When the water table is lowered using pumping wells, these blobs are remobilized on contract with the layer of free LNAPL that lies on top of the water table. In the dewatered zone above the LNAPL layer, the LNAPL phase is kept continuous by LNAPL films which spread between water coating the pore walls and air occupying the centre of the pores. The LNAPL films can flow under the action of gravity forces, resulting in the accumulation of LNAPL on top of the water table where it can be recovered. Extremely small residual LNAPL saturations in the dewatered zone can be eventually achieved by gravity drainage. However, gravity drainage is a slow process due to the low conductivity of LNAPL films. Two experiments have been carried out in a two-dimensional laboratory model packed with sand. In the first experiment, the time scale involved in the recovery of residual Soltrol oil 100® by gravity drainage in a homogeneous sand pack was investigated. In the second experiment, a heterogeneous (lensed) sand pack has been used to study: (1) the influence of different-permeability lenses on the spatial distribution of LNAPL leaked at the top of the sand pack; (2) the effects of water table fluctuations; and (3) the recovery of residual LNAPL by gravity drainage in a lensed porous medium. Special attention has been given to the case where the lenses have smaller permeability than the surrounding medium.

LNAPL Retention in Sandy Soil

AbstractThe vertical distribution of free and residual aviation gasoline, a light nonaqueous phase liquid (LNAPL), through the capillary fringe of a uniform, medium sand is measured and used to calibrate an existing model. The total saturation profile is governed by a classical moisture retention characteristic, with the matric pressure head scaled by LNAPL/air surface tension in the presence of free LNAPL and water/air surface tension in the absence of free LNAPL. The residual LNAPL is described by an empirical equation reflecting hysteretical trapping due to a fluctuating water table. The existing theory is calibrated with new and published field data at an aviation gasoline spill site in Traverse City, Michigan.