Salinity and low temperature effects on the performance of column biochemical reactors for the treatment of acidic and neutral mine drainage.

Passive biochemical reactors (PBRs) represent a promising option for the treatment of mine drainage. In this study, the influence of temperature (22 and 5 °C), salinity (0 and 20 g/L) and hydraulic retention time (HRT) on the efficiency of PBRs for the treatment of acidic and neutral mine drainage (AMD and NMD) was evaluated. To do so, eight 11 L PBRs were set-up and operated with vertically upward flow. Synthetic AMD and NMD, with two salinities (0 and 20 g/L), were tested at ambient temperature (22 ±â€¯0.5 °C) during the first 3 months, then at low temperature (5 ±â€¯1 °C), for 5 additional months. The HRT tested was 0.5 and 1 day, for NMD, and 2.5 and 5 days, for AMD. Results showed a consistent efficiency, above 65%, with higher HRTs (1 vs. 0.5 day for NMD and 5 vs. 2.5 for AMD). At room temperature, metals and sulfate removal was better for non-saline synthetic effluents (>99% vs 95% for Cu, 99% vs >74% for Ni, 90% vs 75% for Fe, and <99% vs <96% for SO42-), after 3 months. At 5 °C, removal efficiency decreased especially for Ni, from 99% to 74%, for both mine drainage qualities. However, sulfate removal was found to be better in saline AMD (<40% vs <10%). The simultaneous effect of low temperature and high salinity further decreased PBR performance. Although higher HRTs entailed better removal efficiency, hydraulic problems such as decreases in permeability of the reactive mixture may still lead to inhibition of long-term PBR efficiency.

Issues and Options in the Delineation of Well Capture Zones under Uncertainty.

The delineation of wellhead protection areas (WHPAs) under uncertainty is still a challenge for heterogeneous porous media. For granular media, one option is to combine particle tracking (PT) with the Monte Carlo approach (PT-MC) to account for geologic uncertainties. Fractured porous media, however, require certain restrictive assumptions under this approach. An alternative for all types of media is the capture probability (CP) approach, which is based on the solution of the standard advection-dispersion equation in a backward mode, making use of the analogy between forward and backward transport processes. Within this context, we review the current controversy about the correct form of the conceptual model for transport, finding that the advection-diffusion model, which represents the diffusive interchange between streamtubes with differing velocities, is more physically realistic than the conventional advection-dispersion model. For mildly to moderately heterogeneous materials, stochastic theories and simulation experiments show that this process converges at the field scale to an effective advection-dispersion process that can be simulated with conventional transport models using appropriate macrodispersivity values. For highly heterogeneous materials, stochastic theories do not yet exist but there is no reason why the process should not converge naturally as well. Macrodispersivities appear to be formation-specific. The advection-dispersion model can be used for capture zone delineation in heterogeneous granular media. For fractured porous systems, hybrid equivalent porous medium and discrete fracture network or CP-based approaches may have potential. In general, capture zones delineated by PT without MC will always be too small and should not be used as a basis for land-use decisions.

Delineating baseflow contribution areas for streams - A model and methods comparison.

This study addresses the delineation of areas that contribute baseflow to a stream reach, also known as stream capture zones. Such areas can be delineated using standard well capture zone delineation methods, with three important differences: (1) natural gradients are smaller compared to those produced by supply wells and are therefore subject to greater numerical errors, (2) stream discharge varies seasonally, and (3) stream discharge varies spatially. This study focuses on model-related uncertainties due to model characteristics, discretization schemes, delineation methods, and particle tracking algorithms. The methodology is applied to the Alder Creek watershed in southwestern Ontario. Four different model codes are compared: HydroGeoSphere, WATFLOW, MODFLOW, and FEFLOW. In addition, two delineation methods are compared: reverse particle tracking and reverse transport, where the latter considers local-scale parameter uncertainty by using a macrodispersion term to produce a capture probability plume. The results from this study indicate that different models can calibrate acceptably well to the same data and produce very similar distributions of hydraulic head, but can produce different capture zones. The stream capture zone is found to be highly sensitive to the particle tracking algorithm. It was also found that particle tracking by itself, if applied to complex systems such as the Alder Creek watershed, would require considerable subjective judgement in the delineation of stream capture zones. Reverse transport is an alternative and more reliable approach that provides probability intervals for the baseflow contribution areas, taking uncertainty into account. The two approaches can be used together to enhance the confidence in the final outcome.

Oxygenated gasoline release in the unsaturated zone, Part 2: Downgradient transport of ethanol and hydrocarbons.

In the event of a gasoline spill containing oxygenated compounds such as ethanol and MTBE, it is important to consider the impacts these compounds might have on subsurface contamination. One of the main concerns commonly associated with ethanol is that it might decrease the biodegradation of aromatic hydrocarbon compounds, leading to an increase in the hydrocarbon dissolved plume lengths. The first part of this study (Part 1) showed that when gasoline containing ethanol infiltrates the unsaturated zone, ethanol is likely to partition to and be retained in the unsaturated zone pore water. In this study (Part 2), a controlled field test is combined with a two-dimensional laboratory test and three-dimensional numerical modelling to investigate how ethanol retention in the unsaturated zone affects the downgradient behaviour of ethanol and aromatic hydrocarbon compounds. Ethanol transport downgradient was extremely limited. The appearance of ethanol in downgradient wells was delayed and the concentrations were lower than would be expected based on equilibrium dissolution. Oscillations in the water table resulted in minor flushing of ethanol, but its effect could still be perceived as an increase in the groundwater concentrations downgradient from the source zone. Ethanol partitioning to the unsaturated zone pore water reduced its mass fraction within the NAPL thus reducing its anticipated impact on the fate of the hydrocarbon compounds. A conceptual numerical simulation indicated that the potential ethanol-induced increase in benzene plume length after 20 years could decrease from 136% to 40% when ethanol retention in the unsaturated zone is considered.

Migration and fate of ethanol-enhanced gasoline in groundwater: a modelling analysis of a field experiment.

Ethanol use as a gasoline additive is increasing, as are the chances of groundwater contamination caused by gasoline releases involving ethanol. To evaluate the impact of ethanol on dissolved hydrocarbon plumes, a field test was performed in which three gasoline residual sources with different ethanol fractions (E0: no ethanol, E10: 10% ethanol and E95: 95% ethanol) were emplaced below the water table. Using the numerical model BIONAPL/3D, the mass discharge rates of benzene, toluene, ethylbenzene, xylenes, trimethylbenzenes and naphthalene were simulated and results compared to those obtained from sampling transects of multilevel samplers. It was shown that ethanol dissolved rapidly and migrated downgradient as a short slug. Mass discharge of the hydrocarbons from the E0 and E10 sources suggested similar first-order hydrocarbon decay rates, indicating that ethanol from E10 had no impact on hydrocarbon degradation. In contrast, the estimated hydrocarbon decay rates were significantly lower when the source was E95. For the E0 and E10 cases, the aquifer did not have enough oxygen to support complete mineralization of the hydrocarbon compounds to the extent suggested by the field-based mass discharge. Introducing a heterogeneous distribution of hydraulic conductivity did little to overcome this discrepancy. A better match between the numerical model and the field data was obtained assuming partial degradation of the hydrocarbons to intermediate compounds. Besides depending on the ethanol concentration, the impact of ethanol on hydrocarbon degradation appears to be highly dependent on the availability of electron acceptors.