K.U. Mayer

Vadose zone attenuation of organic compounds at a crude oil spill site - interactions between biogeochemical reactions and multicomponent gas transport.

Contaminant attenuation processes in the vadose zone of a crude oil spill site near Bemidji, MN have been simulated with a reactive transport model that includes multicomponent gas transport, solute transport, and the most relevant biogeochemical reactions. Dissolution and volatilization of oil components, their aerobic and anaerobic degradation coupled with sequential electron acceptor consumption, ingress of atmospheric O(2), and the release of CH(4) and CO(2) from the smear zone generated by the floating oil were considered. The focus of the simulations was to assess the dynamics between biodegradation and gas transport processes in the vadose zone, to evaluate the rates and contributions of different electron accepting processes towards vadose zone natural attenuation, and to provide an estimate of the historical mass loss. Concentration distributions of reactive (O(2), CH(4), and CO(2)) and non-reactive (Ar and N(2)) gases served as key constraints for the model calibration. Simulation results confirm that as of 2007, the main degradation pathway can be attributed to methanogenic degradation of organic compounds in the smear zone and the vadose zone resulting in a contaminant plume dominated by high CH(4) concentrations. In accordance with field observations, zones of volatilization and CH(4) generation are correlated to slightly elevated total gas pressures and low partial pressures of N(2) and Ar, while zones of aerobic CH(4) oxidation are characterized by slightly reduced gas pressures and elevated concentrations of N(2) and Ar. Diffusion is the most significant transport mechanism for gases in the vadose zone; however, the simulations also indicate that, despite very small pressure gradients, advection contributes up to 15% towards the net flux of CH(4), and to a more limited extent to O(2) ingress. Model calibration strongly suggests that transfer of biogenically generated gases from the smear zone provides a major control on vadose zone gas distributions and vadose zone carbon balance. Overall, the model was successful in capturing the complex interactions between biogeochemical reactions and multicomponent gas transport processes. However, despite employing a process-based modeling approach, honoring observed parameter ranges, and generally obtaining good agreement between field observations and model simulations, accurate quantification of natural attenuation rates remains difficult. The modeling results are affected by uncertainties regarding gas phase saturations, tortuosities, and the magnitude of CH(4) and CO(2) flux from the smear zone. These findings highlight the need to better delineate gas fluxes at the model boundaries, which will help constrain contaminant degradation rates, and ultimately source zone longevity.

A detailed field-based evaluation of naphthenic acid mobility in groundwater.

An anaerobic plume of process-affected groundwater was characterized in a shallow sand aquifer adjacent to an oil sands tailings impoundment. Based on biological oxygen demand measurements, the reductive capacity of the plume is considered minimal. Major dissolved components associated with the plume include HCO(3), Na, Cl, SO(4), and naphthenic acids (NAs). Quantitative and qualitative NA analyses were performed on groundwater samples to investigate NA fate and transport in the subsurface. Despite subsurface residence times exceeding 20 years, significant attenuation of NAs by biodegradation was not observed based on screening techniques developed at the time of the investigation. Relative to conservative tracers (i.e., Cl), overall NA attenuation in the subsurface is limited, which is consistent with batch sorption and microcosm studies performed by other authors. Insignificant biological oxygen demand and low concentrations of dissolved As (<10 microg L(-1)) in the plume suggest that the potential for secondary trace metal release, specifically As, via reductive dissolution reactions driven by ingress of process-affected water is minimal. It is also possible that readily leachable As is not present in significant quantities within the sediments of the study area. Thus, for similar plumes of process-affected groundwater in shallow sand aquifers which may occur as oil sands mining expands, a reasonable expectation is for NA persistence, but minimal trace metal mobilization.

Biogeochemical processes controlling the mobility of major ions and trace metals in aquitard sediments beneath an oil sand tailing pond: laboratory studies and reactive transport modeling.

Increased production and expansion of the oil sand industry in Alberta are of great benefit to the economy, but they carry major environmental challenges. The volume of fluid fine tailings requiring storage is 840×10(6) m(3) and growing, making it imperative that we better understand the fate and transport of oil sand process-affected water (OSPW) seepage from these facilities. Accordingly, the current study seeks to characterize both a) the potential for major ion and trace element release, and b) the principal biogeochemical processes involved, as tailing pond OSPW infiltrates into, and interacts with, underlying glacial till sediments prior to reaching down gradient aquifers or surface waters. Objectives were addressed through a series of aqueous and solid phase experiments, including radial diffusion cells, an isotope analysis, X-ray diffraction, and sequential extractions. The diffusion cells were also simulated in a reactive transport framework to elucidate key reaction processes. The experiments indicate that the ingress and interaction of OSPW with the glacial till sediment-pore water system will result in: a mitigation of ingressing Na (retardation), displacement and then limited precipitation of exchangeable Ca and Mg (as carbonates), sulfate reduction and subsequent precipitation of the produced sulfides, as well as biodegradation of organic carbon. High concentrations of ingressing Cl (~375 mg L(-1)) and Na (~575 mg L(-1)) (even though the latter is delayed, or retarded) are expected to migrate through the till and into the underlying sand channel. Trace element mobility was influenced by ion exchange, oxidation-reduction, and mineral phase reactions including reductive dissolution of metal oxyhydroxides - in accordance with previous observations within sandy aquifer settings. Furthermore, although several trace elements showed the potential for release (Al, B, Ba, Cd, Mn, Pb, Si, Sr), large-scale mobilization is not supported. Thus, the present results suggest that in addition to the commonly cited naphthenic acids, remediation of OSPW-impacted groundwater will need to address high concentrations of major ions contributing to salinization.

The importance of conceptual models in the reactive transport simulation of oxygen ingress in sparsely fractured crystalline rock

Redox evolution in sparsely fractured crystalline rocks is a key, and largely unresolved, issue when assessing the geochemical suitability of deep geological repositories for nuclear waste. Redox zonation created by the influx of oxygenated waters has previously been simulated using reactive transport models that have incorporated a variety of processes, resulting in predictions for the depth of oxygen penetration that may vary greatly. An assessment and direct comparison of the various underlying conceptual models are therefore needed. In this work a reactive transport model that considers multiple processes in an integrated manner is used to investigate the ingress of oxygen for both single fracture and fracture zone scenarios. It is shown that the depth of dissolved oxygen migration is greatly influenced by the a priori assumptions that are made in the conceptual models. For example, the ability of oxygen to access and react with minerals in the rock matrix may be of paramount importance for single fracture conceptual models. For fracture zone systems, the abundance and reactivity of minerals within the fractures and thin matrix slabs between the fractures appear to provide key controls on O(2) attenuation. The findings point to the need for improved understanding of the coupling between the key transport-reaction feedbacks to determine which conceptual models are most suitable and to provide guidance for which parameters should be targeted in field and laboratory investigations.

Reactive Transport Modeling in Variably Saturated Media with MIN3P: Basic Model Formulation and Model Enhancements

Abstract: MIN3P was developed as a general purpose multicomponent reactive transport code for variably saturated media. The basic version of the code includes Richard’s equation for the solution of variably-saturated flow, and solves mass balance equations for advective-diffusive solute transport and diffusive gas transport. Biogeochemical reactions are described by a partial equilibrium approach, using equilibrium-based law-of-mass-action relationships for fast reactions, and a generalized kinetic framework for reactions that are relatively slow in comparison to the transport time scale. MIN3P has been used to support multiple field and laboratory investigations involving the fate of inorganic and organic substances and has served as a platform for additional code development: MIN3P-Bubble, an enhanced version to simulate gas generation and exsolution in the saturated zone, as well as gas entrapment and release due to water table fluctuations; MIN3P-Dusty, a version of the code that includes gas advection and multicomponent gas diffusion based on the Dusty Gas Model (DGM); and MIN3P-Soil, a version that includes plant-soil interactions. The capabilities of the basic code and the follow-up developments are demonstrated by simulating the oxidation of pyrite in mine waste, associated metal release, and subsequent attenuation processes; the interactions between the formation of “excess air” and biogeochemical reactions in the vadose zone and below the water table; the evolution of vadose zone gas composition and transport processes at a petroleum hydrocarbon spill site undergoing natural attenuation; and the effect of plant-soil interactions on mineral weathering and secondary mineral formation in soils and surficial sediments.

Multi-year CO2 efflux measurements for assessing natural source zone depletion at a large hydrocarbon-impacted site

The changing landscape of fuel consumption related, in part, to increased engine efficiency and the inexpensive supply of natural gas, has led to the closure of multiple refineries. As the operational lifetime of many refineries exceeds 100 years, historical releases of oil and refined products is common. To evaluate remediation and rehabilitation options, there is a need to understand the rate and distribution of natural hydrocarbon mass losses across these large properties. Here, surficial CO2 flux measurements were used to evaluate naturally occurring hydrocarbon mass losses at a large-scale former refinery that has been closed since 1982. Natural source zone depletion (NSZD) rates over a five-year period (2012-2016) were derived from surficial CO2 efflux measurements on a high-resolution grid (N > 80). Results demonstrate substantial variations of mass loss rates across the site. Average site-wide mass loss rates ranged from 1.1-5.4 g TPH m-2 d-1 as C10H22 with a multi-year average of 4.0 g TPH m-2 d-1 as decane (C10H22), consistent with observations at other sites. Statistical analysis demonstrated that the same average mass loss rates would have been obtained with fewer measurement locations (N = 20-30). Comparing NSZD rates to site metadata show CO2 fluxes to be a reasonably good proxy for zones of subsurface hydrocarbon contamination - particularly with respect to vadose zone impacts. It is hypothesized that the observed decline of NSZD rates over the study period is related to rise of groundwater levels, leading to increased submergence of the smear zone. Overall, mass loss rates calculated from CO2 fluxes show NSZD can result in substantial contaminant removal, which may rival that obtained from engineered remediation, under some conditions.

Evaluation of single- and dual-porosity models for reproducing the release of external and internal tracers from heterogeneous waste-rock piles

Accurate predictions of solute release from waste-rock piles (WRPs) are paramount for decision making in mining-related environmental processes. Tracers provide information that can be used to estimate effective transport parameters and understand mechanisms controlling the hydraulic and geochemical behavior of WRPs. It is shown that internal tracers (i.e. initially present) together with external (i.e. applied) tracers provide complementary and quantitative information to identify transport mechanisms. The analysis focuses on two experimental WRPs, Piles 4 and Pile 5 at the Antamina Mine site (Peru), where both an internal chloride tracer and externally applied bromide tracer were monitored in discharge over three years. The results suggest that external tracers provide insight into transport associated with relatively fast flow regions that are activated during higher-rate recharge events. In contrast, internal tracers provide insight into mechanisms controlling solutes release from lower-permeability zones within the piles. Rate-limited diffusive processes, which can be mimicked by nonlocal mass-transfer models, affect both internal and external tracers. The sensitivity of the mass-transfer parameters to heterogeneity is higher for external tracers than for internal tracers, as indicated by the different mean residence times characterizing the flow paths associated with each tracer. The joint use of internal and external tracers provides a more comprehensive understanding of the transport mechanisms in WRPs. In particular, the tracer tests support the notion that a multi-porosity conceptualization of WRPs is more adequate for capturing key mechanisms than a dual-porosity conceptualization.