Journal of Environmental Engineering

Controls on vadose-zone gas transport beneath and adjacent to a southern High Plains ephemeral lake (playa) were investigated. Under dry conditions, vertical gas permeability and tortuosity were enhanced by cracks and root tubules in the upper 2.5 m. Below this depth, the horizontal components of both permeability and tortuosity tensors were dominant. Both atmospheric pumping and pneumatic tests were used to determine the gas permeability tensor; whereas gas tracer tests were used to estimate the tortuosity tensor. The field data were elevated in a multilayer numerical model. The results suggest that gas movement can be dominated by both advection and diffusion, with vertical movement dominating in the shallow zone under dry conditions. After a large precipitation event, vertical gas permeability was reduced and large pressure differentials (as high as 1.5 kPa) were produced, creating driving forces for advective gas transport.

The performance of soil vapor extraction systems in the field commonly is less than optimum and is difficult to predict. The factors that cause deteriorating performance are not well understood or quantified. To gain a better understanding, the venting behavior of a single compound (toluene) and a binary mixture (toluene and m-xylene) from a silt loam soil was investigated in a series of multiple column experiments. The contaminants were present initially as non-aqueous-phase liquids. The venting experiments were conducted at three water contents of air-dry, 16 and 22 wt%. The vapor extraction was influenced strongly by the soil's water content. Equilibrium conditions predominated for the air-dry and 16% water content experiments. At a water content of 22%, venting became mass transfer limited. This mass transfer limitation developed early in the venting process with the non-aqueous-phase liquid still present.

Nonequilibrium air-water mass transfer experiments for six volatile organic compounds (VOCs) were conducted using a bench-scale air sparging system. VOCs used were carbon tetrachloride, trichloroethylene, tetrachloroethylene, chloroform, dichloromethane, and toluene. The average particle size of the porous media used ranged from 0.278 to 1.71 mm. The air-water mass transfer coefficients were estimated by fitting the experimental data to a lumped parameter model. The model assumed that the saturated porous media under air sparging conditions consisted of two zones. In the “mass transfer” zone, VOCs were directly impacted by the flow of air in the air channels, while in the “bulk water” zone, VOCs were not directly affected by the air flow in the air channels. The estimated air-water mass transfer coefficients (K_Ga) were found to increase for higher injected air flow rates and for larger mean particle sizes of porous media but were inversely proportional to the Henry's law constant of the VOCs. An empirical correlation was developed by correlating the Sherwood number with the Péclet number, the Henry's law constant, and the mean particle size of porous media. The estimated fraction (F) of the total volume of the porous media directly affected by air channels was between 5 and 20% for fine sand, indicating that a small fraction of the porous media was affected by the advecting air stream.

A series of permeameters (columns) was used to evaluate the effects of the percolation of water and 1,000 μg/mL of zinc chloride solution through a mixture of montmorillonite clay, sand, and lime. The column test results show that the addition of lime changes the chemical and physical properties of the clay. The hydraulic conductivities for the mixture of clay with different percentages of lime at first increases with increasing lime and then decreases with increasing lime. The breakthrough curves indicate that the Zn(II) capture is increased and Zn(II) breakthrough is delayed with increasing lime addition. Lime also enhances the clay/lime mixture's ability to resist puncture by sharp objects. Based on the effects of lime on Zn(II) captured by the clay, an explanation for the interacting effects of lime and Zn(II) capture on changing hydraulic conductivity is suggested. The results of this research demonstrate the potential of using lime-treated clay liners for landfills. Such liners would have lower hydraulic conductivity, better resistance to puncture, and enhanced ability to capture heavy metals.

Soil samples artificially contaminated with 10 different lead compounds to produce 5,000 mg/kg Pb were washed with acid and ethylenediaminetetraacetic acid (EDTA) solutions. For variable pH, the highest washing efficiencies were achieved at pH 2, the lowest value examined. Washing with EDTA enhanced the removal of lead, the removal increasing with an increase in the EDTA:lead molar ratio. High removals (79–106%) of adsorbed lead (as lead nitrate), lead carbonate, basic lead carbonate, lead sulfate, and lead oxide were achieved with both types of washing. Although not washed effectively with acid, significant lead dioxide removal occurred with EDTA wash. The removals of lead sulfide, lead paint, lead dimethyldithiocarbamate, and elemental lead were low (near 0–16%) under all washing conditions. The removal efficiency of the lead is affected by the compound solubility, lead solid dissolution kinetics, and lead sorption into the soil. Results clearly indicate the importance of the form of lead contamination in determining the success of a soil washing operation. Comparison of these results with others suggests that soil washing success and soil lead bioaccessibility are related phenomena.

The anaerobic biodegradation of tetrachloroethene (PCE) is a reasonably well understood process. Specific organisms capable of using PCE as an electron acceptor for growth require the addition of an electron donor to remove PCE from contaminated ground waters. However, competition from other anaerobic microorganisms for added electron donor will influence the rate and completeness of PCE degradation. The approach developed here allows for the explicit modeling of PCE and byproduct biodegradation as a function of electron donor and byproduct concentrations, and the microbiological ecology of the system. The approach is general and can be easily modified for ready use with in situ ground-water models or ex situ reactor models. Simulations conducted with models developed from this approach show the sensitivity of PCE biodegradation to input parameter values, in particular initial biomass concentrations. Additionally, the dechlorination rate will be strongly influenced by the microbial ecology of the system. Finally, comparison with experimental acclimation results indicates that existing kinetic constants may not be generally applicable. Better techniques for measuring the biomass of specific organism groups in mixed systems are required.

The effects of solution chemistry, surface shear, and composition of natural organic matter (NOM) were investigated for their impact on accumulation of foulant material at the surface of charged polymeric nanofiltration membranes. The source of NOM was the Suwannee River. A bench-scale, batch recycle system was used with 20 hollow fiber, nanofiltration membranes. Membrane flux decline and foulant accumulation increased at low pH and high ionic strength as a result of neutralization of charge, electric double layer compression, and the apparent shift in conformation of charged NOM macromolecules. The rate of NOM accumulation decreased with operating time, suggestive of an eventual steady state between adsorption and desorption. The effect of NOM composition on membrane fouling could not be discerned by a standard technique to isolate hydrophobic and hydrophilic NOM fractions, quite possibly because of the fractionation methodology's failure to recover a small but important fouling fraction or because of NOM interactions that are lost when individual fractions are separately tested. However, a greater percentage of the hydrophilic than hydrophobic fraction permeated the membrane, in agreement with prior observations by others. Increasing the cross flow velocity from 85 to 255 cm/s reduced the extent of flux decline, presumably due to hydrodynamic disruption of cake layer formation.

Adsorption equilibrium data in lumped water quality indices obtained for diluted solutions of a peat water and effluents from a wastewater treatment were analyzed using the ideal adsorbed solution theory with the Freundlich equation. The use of the same value for Freundlich exponent 1/n in the ideal adsorbed solution theory resulted in a straightforward and less computationally demanding expression to describe an overall batch adsorption isotherm. A fictive component approach, assuming a logarithmic normal distribution of Freundlich K and a nonadsorbable fraction, adequately described and predicted the overall isotherms for different initial concentrations of diluted solutions presented as a lumped water quality index. The parameters in the distributed fictive component approach were independently sensitive to each portion of an isotherm curvature. Preferential adsorption of the smaller molecular weight (MW) components was observed by a size-exclusion high-performance liquid chromatography. Two-dimensional distribution analysis of organics in terms of Freundlich K and MW revealed a weak tendency of the adsorptive strength to decrease with the increase of the MW.

The Allen Park Clay Mine Landfill, owned by Ford, produces a leachate that occasionally contains cyanide at levels marginally below the discharge limit. The form of the cyanide in the leachate was found to be iron-cyanide complexes that resist oxidation by a conventional treatment method, alkaline oxidation. Furthermore, the leachate also was found to contain a relatively large amount of organics (>1,000 mg/L of chemical oxygen demand), which would exert additional demand for oxidizing agents (e.g., chlorine). A study was performed to determine what treatment technology could be employed in the event treatment becomes necessary because of potential changes in the leachate characteristics and/or discharge limits. In this study, among several chemical oxidation methods, ultraviolet (UV) irradiation with or without ozone was investigated as a treatment option. The following are the primary findings: (1) UV irradiation alone was effective for removing the iron-cyanide complex in both the leachate and the clean water; (2) the demand for UV or ozone by chemical oxygen demand was relatively low for this leachate; (3) ozone alone was not effective for removing the iron-cyanide complex; and (4) UV irradiation alone and UV irradiation with ozone resulted in the same removal for total cyanide in clean-water experiments, but the UV irradiation alone left some free cyanide whereas the UV irradiation with ozone did not.

Piecewise linear interpolation (PLI) is used frequently in environmental studies to estimate missing data. However, to evaluate the reliability of these estimates, the variances of these interpolated values must be quantified. We propose a procedure to quantify this PLI variance, which involves establishing a semivariogram with coefficients that are calibrated using a cross-validation technique. Estimated values are written as a linear combination of neighboring data points and the variance is calculated with the help of the variogram. Such interpolated values are unaffected by the variance quantification procedure. We then use the PLI model to calculate the variance of a yearly nutrient load under the assumption that only the nutrient concentrations contained missing values. When these results were compared with those from an arithmetic mean, a flow-weighted mean, and a linear regression model, the PLI model was found to be comparable with the other three models in terms of variance. Selection of an appropriate model depends on the characteristics of the data set. Knowing the variance of estimated loads can help regulatory agencies make better decisions to determine whether water quality in the environment is in compliance with established standards or criteria.

Vegetative storm water controls include grassed swales and vegetated filter strips. The effectiveness of these controls for removing pollutants found in runoff from highways and other urban areas has not been demonstrated to the satisfaction of regulatory agencies; therefore, these technologies have been limited to applications as pretreatment devices for other structural runoff controls. Many highways in Texas and other areas discharge storm water to grassy medians and shoulder areas, which act as both filter strips and grassed swales. The runoff flows overland down the sides of the median as in a filter strip and then parallel to the highway as in a grassed swale. This study investigated the capability of vegetated highway medians for treating storm water runoff in the Austin, Tex. area. Two medians on major highways were monitored to document pollutant removal efficiencies. The medians were designed solely for storm water conveyance and differed in slope and vegetation type. In addition, the highways adjacent to the medians had different average daily traffic counts and contributing drainage areas. Removal efficiencies at the two sites were remarkably similar despite these differences and were comparable with those observed in structural controls such as sedimentation/filtration systems (i.e., greater than 85% removal of suspended solids). The majority of pollutant removal occurred on the sides of the median, indicating the effectiveness of vegetated filter strips. Consequently, the effect of the length of the median (measured parallel to the highway) on the amount of removal was minimal. These data indicate that medians with side slopes of less than 12% and a length of at least 8 m (measured from the pavement edge to the center of the median) are effective in reducing storm water loads from highways.

A simple isotherm model has been developed for the type-5 equilibrium adsorption systems by considering the effects of both primary and secondary adsorptive sites. The model was applied to interpret experimental data describing water vapor adsorption capacities of four activated carbon adsorbents for relative vapor pressures from 0.0 to 0.95 and temperatures between 288 and 308 K. Good agreement was observed between the model and experimental results. The isotherm parameters were found to either correlate well to the Brunauer, Emmett, and Teller specific surface area or have limited effect on describing the adsorption onto adsorbents made from the same precursor but activated with different degrees of burnout. The limiting pore volumes of the adsorbents obtained from the model based on water vapor adsorption compared favorably with those measured using volatile organic compounds.

One-dimensional advective-dispersive-reactive models are often used to simulate the performance of vertical subsurface barriers. However, a review of the literature reveals a lack of consistency in the treatment of boundary conditions (BCs), particularly for field applications. Commonly used simplified BCs are shown to result in nonconservative predictions of contaminant flux when applied to the low flow scenarios expected in the field. Consequently, the constant concentration entrance BC and zero concentration exit BC are recommended as the starting point for the design of slurry walls. Generalized “mixing zone” conditions are also proposed and evaluated for modeling both field installations and laboratory columns. For field scenarios, the exit mixing zone provides flexibility in adjusting between the limiting cases of zero concentration and semiinfinite BCs. For laboratory columns, mixing zones may be used to represent the influence of porous materials placed at the column ends to promote lateral distribution of the permeating fluid.