William R. Wise

Analysis of constructed treatment wetland hydraulics with the transient storage model OTIS

Abstract The model one-dimensional transport with inflow and storage (OTIS) was calibrated to results from tracer experiments conducted in the Orlando Easterly Wetland in order to quantify short-circuiting of the treatment volume and temporary storage of tracer in isolated, low-flow regions. OTIS was found to fit experimental data very well for both steady and unsteady flow conditions. The model calibrations indicate the presence of three different hydraulic ‘zones’ of the wetland. The first zone is the actively flowing main channel; the second, a temporary storage zone where water and constituents are exchanged with the main flow channel; and the third, completely isolated, ‘dead’ water. The uncertainty of storage zone parameters was determined to be low due to the experimental Damkohler number that quantifies intra-zone mass transfer processes. The appropriateness of the use of a one-dimensional model for treatment wetlands is also addressed.

Controlled release, blind test of DNAPL remediation by ethanol flushing

A dense nonaqueous phase liquid (DNAPL) source zone was established within a sheet-pile isolated cell through a controlled release of perchloroethylene (PCE) to evaluate DNAPL remediation by in-situ cosolvent flushing. Ethanol was used as the cosolvent, and the main remedial mechanism was enhanced dissolution based on the phase behavior of the water-ethanol-PCE system. Based on the knowledge of the actual PCE volume introduced into the cell, it was estimated that 83 L of PCE were present at the start of the test. Over a 40-day period, 64% of the PCE was removed by flushing the cell with an alcohol solution of approximately 70% ethanol and 30% water. High removal efficiencies at the end of the test indicated that more PCE could have been removed had it been possible to continue the demonstration. The ethanol solution extracted from the cell was recycled during the test using activated carbon and air stripping treatment. Both of these treatment processes were successful in removing PCE for recycling purposes, with minimal impact on the ethanol content in the treated fluids. Results from pre- and post-flushing partitioning tracer tests overestimated the treatment performance. However, both of these tracer tests missed significant amounts of the PCE present, likely due to inaccessibility of the PCE. The tracer results suggest that some PCE was inaccessible to the ethanol solution which led to the inefficient PCE removal rates observed. The flux-averaged aqueous PCE concentrations measured in the post-flushing tracer test were reduced by a factor of 3 to 4 in the extraction wells that showed the highest PCE removal compared to those concentrations in the pre-flushing tracer test.

A comparison of modeling approaches for steady-state unconfined flow

Abstract The Dupuit-Forchheimer, the fully saturated flow, and the variably saturated flow models, are compared for problems involving steady-state, unconfined flow through porous media. The variably saturated flow model is the most comprehensive of the three and requires more parameters. The performances of the three models are compared for different soil properties, problem dimensions, and flow geometries. There are certain types of problems where the simpler models may yield satisfactory results. For soils with large pores and/or broad pore-size-density functions, the variably saturated flow model solutions to steady-state problems approach those of the fully flow model, owing to the manner in which the soil-water retention curve and relative permeability function, respectively, affect the variably saturated flow model solutions. For problems of significant size, the fully saturated flow model may be sufficient, as the effects of the vadose zone are relatively diminished. For problems with radial symmetry (e.g. steady flow to a well), the fully saturated flow model performs well because the variably saturated flow model is relatively insensitive to the parameters describing the soil properties, as the amount of vadose zone flow, compared with the total flow, is relatively insignificant in such problems.

A wetland–aquifer interaction test

An understanding of the hydraulic connectivity between an isolated wetland and its underlying groundwater is required to help assess the ecological impact that changes in the groundwater level may induce. Literature values for the hydraulic conductivity of peat vary up to ten orders of magnitude, indicating the absolute necessity of obtaining site-specific information. Horizontal and vertical variability in peat layers makes the process of extrapolating point-based measurements to predict system-level behavior difficult. By inducing or augmenting a flow up from the underlying aquifer into the wetland through a rapid lowering of wetland water level, the system-level hydraulic connectivity of a wetland to the groundwater may be directly measured. At a study site, a small, seasonally flooded depression mash wetland in Florida, the method and subsequent analysis yielded a value for the hydraulic resistance of the organic layer of 6 days, indicating a significant connection between the wetland and the aquifer.

NAPL characterization via partitioning tracer tests: quantifying effects of partitioning nonlinearities

The inherent nonlinearity in the equilibrium interphase distribution of a partitioning tracer between a NAPL and water affects the manner in which the partitioning tracer moves throughout a region of the subsurface contaminated with the NAPL. A semi-analytic solution is developed to describe this behavior. Analysis of a breakthrough curve of the partitioning tracer in such a system using a linear partitioning based model results in systematic overestimation of the NAPL present in the region swept during the test.

Non-aqueous phase liquid characterization via partitioning tracer tests: a modified Langmuir relation to describe partitioning nonlinearities

The equilibrium interphase behaviors of alcohol partitioning tracers between NAPL and water are inherently nonlinear in nature. This nonlinearity may be described using an unfavorable form of the Langmuir partitioning relation with a good degree of agreement up to tracer mole fraction values of 0.3. The presence of co-tracers tends to move the equilibrium partitioning closer to Raoults law. It may be possible to manipulate tracer tests systems to minimize the effects of nonlinearities through the use of co-tracers.

Thermodynamic analysis of effluent tailing observed during dissolution of binary oil mixtures

Tailing phenomena observed during the dissolution of binary oil mixtures are manifestations of non-equilibrium effects and are not induced by non-ideal partitioning. This conclusion follows a thermodynamically-based analysis of the dissolution process in a chromatographic framework. A criterion, both components of binary oil mixtures having enhanced or unaffected solubilities relative to Raoults law, serves as an indicator for systems in which non-linear-partitioning induced tailing is precluded; tailing must be the result of non-equilibrium processes. This condition is the rule, since chemical dissimilarities tend to increase activity coefficients in mixtures.