Analytical solutions for contaminant fate and transport in parallel plate fracture-rock matrix systems with poiseuille flow.

Modeling contaminant transport in fractured-rock matrix systems often approximates the effect of the parabolic flow field in the fractures (i.e., Poiseuille flow) on transport by adding a dispersion term to the uniform flow field. In this study, an analytical solution is derived to model contaminant transport in a parallel-plate fractured-rock matrix that explicitly simulates Poiseuille flow in the fractures, eliminating the need for the dispersion approximation. In addition to simulating Poiseuille flow in the fracture, the contaminant transport model developed here includes: (1) two-dimensional contaminant diffusion in the fractures and matrix, (2) first-order decay in the aqueous phase, and (3) rate-limited sorption onto matrix solids. It should be noted, however, that this model, much like the commonly employed Taylor dispersion approximation, neglects macro dispersion, thereby limiting the model's applicability to systems having wide fracture apertures with extremely high flow velocities (P e > 104). Model equations are analytically solved in the Laplace domain and numerically inverted. In addition, analytical expressions for the zeroth, first, and second spatial moments of the concentration profiles along the fractures are derived for both the new Poiseuille flow model as well as a model that approximates the effect of Poiseuille flow on transport by using a dispersion term. The first and second moment expressions are used to quantify how well the dispersion term approximates the effect of Poiseuille flow. Simulations confirm that the dispersion approximation will be adequate for natural fractures at long times. However, if a modeler is concerned with short-time transport behavior or transport behavior in systems with relatively wide-aperture fractures and high groundwater velocities where macro dispersion can be ignored, such as may be found at engineered geothermal systems and carbon capture and storage sites, there may be significant differences between model simulations that explicitly incorporate Poiseuille flow and those that approximate Poiseuille flow with a dispersion term. The model presented here allows the modeler to analytically quantify these differences, which, depending on the modeling objective, may cause the dispersion approximation to be inadequate. Simulations were also run to examine the effect of adsorption rate on remediation of fractured-rock matrix systems. It was shown that moderate adsorption rate constants could lead to very long remediation times, if remediation success is quantified by achieving low concentrations within the fracture.

Analytical solutions for a soil vapor extraction model that incorporates gas phase dispersion and molecular diffusion.

To greatly simplify their solution, the equations describing radial advective/dispersive transport to an extraction well in a porous medium typically neglect molecular diffusion. While this simplification is appropriate to simulate transport in the saturated zone, it can result in significant errors when modeling gas phase transport in the vadose zone, as might be applied when simulating a soil vapor extraction (SVE) system to remediate vadose zone contamination. A new analytical solution for the equations describing radial gas phase transport of a sorbing contaminant to an extraction well is presented. The equations model advection, dispersion (including both mechanical dispersion and molecular diffusion), and rate-limited mass transfer of dissolved, separate phase, and sorbed contaminants into the gas phase. The model equations are analytically solved by using the Laplace transform with respect to time. The solutions are represented by confluent hypergeometric functions in the Laplace domain. The Laplace domain solutions are then evaluated using a numerical Laplace inversion algorithm. The solutions can be used to simulate the spatial distribution and the temporal evolution of contaminant concentrations during operation of a soil vapor extraction well. Results of model simulations show that the effect of gas phase molecular diffusion upon concentrations at the extraction well is relatively small, although the effect upon the distribution of concentrations in space is significant. This study provides a tool that can be useful in designing SVE remediation strategies, as well as verifying numerical models used to simulate SVE system performance.

Organophosphate vapor detection on gold electrodes using peptide nanotubes.

Peptide nanotubes (PNTs) encapsulating horseradish peroxidase and surface coated with acetylcholinesterase (AChE) were attached to gold screen printed electrodes to construct a novel gas phase organophosphate (OP) biosensor. When the sensor with the AChE enzyme is put in contact with acetylthiocholine (ATCh), the ATCh is hydrolyzed to produce thiocholine, which is then oxidized by horseradish peroxidase (HRP). Direct electron transfer between HRP and electrode is achieved through PNTs. The signal produced by the electron transfer is measured with cyclic voltammetry (CV). The presence of an OP compound inhibits this signal by binding with the AChE enzyme. In this study, gas phase malathion was used as a model OP due to the fact that it displays the identical binding mechanism with acetylcholinesterase (AChE) as its more potent counterparts such as sarin and VX, but has low toxicity, making it more practical and safer to handle. The CV signal was proportionally inhibited by malathion vapor concentrations as low as 12 ppbv. Depending on the method used in their preparation, the electrodes maintained their activity for up to 45 days. This research demonstrates the potential of applying nano-modified biosensors for the detection of low levels of OP vapor, an important development in countering weaponized organophosphate nerve agents and detecting commercially-used OP pesticides.

Impact of DNAPL contact on the structure of smectitic clay materials.

Smectitic clays have a flexible structure that may be impacted by contact with dense nonaqueous phase liquids (DNAPLs) present at hazardous waste sites. Measurements of the basal spacing of air-dry clays contacted with pure chlorinated solvents and chlorinated DNAPL wastes showed that the intraparticle spacing is similar to that in air. Basal spacings of water-saturated clays contacted with pure chlorinated solvents are similar to those in contact with water, even after extended equilibration times (300 d). In contrast, contact with chlorinated DNAPL wastes reduced the basal spacing of water-saturated sodium smectites in a relatively short time frame, resulting in cracks that were as large as 1mm in aperture. The penetration of these wastes into the intraparticle spacing of clay and the resultant cracking may contribute to the accumulation of chlorinated compounds in clay layers observed in the field and the extended remediation times associated with this mass storage.

Validation of two innovative methods to measure contaminant mass flux in groundwater.

The ability to quantify the mass flux of a groundwater contaminant that is leaching from a source area is critical to enable us to: (1) evaluate the risk posed by the contamination source and prioritize cleanup, (2) evaluate the effectiveness of source remediation technologies or natural attenuation processes, and (3) quantify a source term for use in models that may be applied to predict maximum contaminant concentrations in downstream wells. Recently, a number of new methods have been developed and subsequently applied to measure contaminant mass flux in groundwater in the field. However, none of these methods has been validated at larger than the laboratory-scale through a comparison of measured mass flux and a known flux that has been introduced into flowing groundwater. A couple of innovative flux measurement methods, the tandem circulation well (TCW) and modified integral pumping test (MIPT) methods, have recently been proposed. The TCW method can measure mass flux integrated over a large subsurface volume without extracting water. The TCW method may be implemented using two different techniques. One technique, the multi-dipole technique, is relatively simple and inexpensive, only requiring measurement of heads, while the second technique requires conducting a tracer test. The MIPT method is an easily implemented method of obtaining volume-integrated flux measurements. In the current study, flux measurements obtained using these two methods are compared with known mass fluxes in a three-dimensional, artificial aquifer. Experiments in the artificial aquifer show that the TCW multi-dipole and tracer test techniques accurately estimated flux, within 2% and 16%, respectively; although the good results obtained using the multi-dipole technique may be fortuitous. The MIPT method was not as accurate as the TCW method, underestimating flux by as much as 70%. MIPT method inaccuracies may be due to the fact that the method assumptions (two-dimensional steady groundwater flow to fully-screened wells) were not well-approximated. While fluxes measured using the MIPT method were consistently underestimated, the method's simplicity and applicability to the field may compensate for the inaccuracies that were observed in this artificial aquifer test.

An assembly model for simulation of large-scale ground water flow and transport.

When managing large-scale ground water contamination problems, it is often necessary to model flow and transport using finely discretized domains--for instance (1) to simulate flow and transport near a contamination source area or in the area where a remediation technology is being implemented; (2) to account for small-scale heterogeneities; (3) to represent ground water-surface water interactions; or (4) some combination of these scenarios. A model with a large domain and fine-grid resolution will need extensive computing resources. In this work, a domain decomposition-based assembly model implemented in a parallel computing environment is developed, which will allow efficient simulation of large-scale ground water flow and transport problems using domain-wide grid refinement. The method employs common ground water flow (MODFLOW) and transport (RT3D) simulators, enabling the solution of almost all commonly encountered ground water flow and transport problems. The basic approach partitions a large model domain into any number of subdomains. Parallel processors are used to solve the model equations within each subdomain. Schwarz iteration is applied to match the flow solution at the subdomain boundaries. For the transport model, an extended numerical array is implemented to permit the exchange of dispersive and advective flux information across subdomain boundaries. The model is verified using a conventional single-domain model. Model simulations demonstrate that the proposed model operated in a parallel computing environment can result in considerable savings in computer run times (between 50% and 80%) compared with conventional modeling approaches and may be used to simulate grid discretizations that were formerly intractable.

Use of tandem circulation wells to measure hydraulic conductivity without groundwater extraction.

Conventional methods to measure the hydraulic conductivity of an aquifer on a relatively large scale (10-100 m) require extraction of significant quantities of groundwater. This can be expensive, and otherwise problematic, when investigating a contaminated aquifer. In this study, innovative approaches that make use of tandem circulation wells to measure hydraulic conductivity are proposed. These approaches measure conductivity on a relatively large scale, but do not require extraction of groundwater. Two basic approaches for using circulation wells to measure hydraulic conductivity are presented; one approach is based upon the dipole-flow test method, while the other approach relies on a tracer test to measure the flow of water between two recirculating wells. The approaches are tested in a relatively homogeneous and isotropic artificial aquifer, where the conductivities measured by both approaches are compared to each other and to the previously measured hydraulic conductivity of the aquifer. It was shown that both approaches have the potential to accurately measure horizontal and vertical hydraulic conductivity for a relatively large subsurface volume without the need to pump groundwater to the surface. Future work is recommended to evaluate the ability of these tandem circulation wells to accurately measure hydraulic conductivity when anisotropy and heterogeneity are greater than in the artificial aquifer used for these studies.

Field evaluation of in situ source reduction of trichloroethylene in groundwater using bioenhanced in-well vapor stripping.

Two technologies in combination, cometabolic bioremediation and in-well vapor stripping, were applied to reduce trichloroethylene (TCE) concentrations in groundwater at a contaminant source area without the need to pump contaminated groundwater to the surface for treatment. The vapor-stripping well reduced source TCE concentrations (as high as 6-9 mg/L) by over 95%. Effluent from the well then flowed to two bioremediation wells, where additional reductions of approximately 60% were achieved. TCE removal was extensively monitored (for research and not regulatory purposes) using an automated system that collected samples about every 45 min at 55 locations over an area of approximately 50 x 60 m2. During 4.5 months of system operation, total TCE mass removal was 8.1 kg, 7.1 kg of which resulted from in-well vapor stripping and 1.0 kg from biotreatment. The system reduced the average TCE concentration of about 3000 microg/L in the source-zone groundwater to about 250 microg/L in water leaving the treatment zone, effecting greater than 92% TCE removal. A 6 month rebound study after system operation ceased found TCE concentrations then increased significantly in the treatment zone due to diffusion from the fractured rock below and perhaps other processes, with mass increases of about 1.5 kg in the lower aquifer and 0.3 kg in the upper aquifer.

Dissolved organic matter effects on the performance of a barrier to polycyclic aromatic hydrocarbon transport by groundwater.

In order to contain the movement of organic contaminants in groundwater, a subsurface sorption barrier consisting of sand or clay minerals coated with a cationic surfactant has been proposed. The effectiveness of such a sorption barrier might be affected by the presence of dissolved organic matter (DOM) in the groundwater. To study the impact of DOM on barrier performance, a series of batch experiments were performed by measuring naphthalene and phenanthrene sorption onto sand coated with cetylpyridinium chloride (CPC) and bentonite coated with hexadecyltrimethylammonium bromide (HDTMA) in the presence of various concentrations of DOM. The overall soil-water distribution coefficient (K*) of naphthalene and phenanthrene onto CPC-coated sand decreased with increasing DOM concentration, whereas the K* of the compounds onto HDTMA-coated bentonite slightly increased with increasing DOM concentration. To describe the overall distribution of polycyclic aromatic hydrocarbons (PAHs) in the systems, a competitive multiphase sorption (CMS) model was developed and compared with an overall mechanistic sorption (OMS) model. The modeling studies showed that while the OMS model did not explain the CPC-coated sand experimental results, a model that included competitive sorption between DOM and PAH did. The experimental results and the modeling study indicated that there was no apparent competition between DOM and PAH in the HDTMA-coated bentonite system, and indicated that in groundwater systems with high DOM, a barrier using HDTMA-coated bentonite might be more effective.

Combined effect of natural organic matter and surfactants on the apparent solubility of polycyclic aromatic hydrocarbons.

Both natural organic matter (NOM) and surfactants are known to enhance the apparent aqueous solubility of hydrophobic organic contaminants (HOCs) in aqueous systems. In this study, the combined effect of NOM and surfactants on enhancing the solubility of HOCs was investigated, since both may occur and affect the fate and transport of HOCs in natural aqueous environments. Experimental results indicated that the apparent solubility of naphthalene, phenanthrene, and pyrene in NOM and anionic surfactant solution was lower than their solubility in NOM solution alone. However, the apparent solubility of an HOC in NOM and nonionic surfactant solution is almost the same as the sum of the HOC's solubility in NOM solution plus its solubility in nonionic surfactant solution. The observation that apparent aqueous solubility of HOCs in NOM and anionic surfactant solution is decreased is probably due to the fact that the cations that are released when the anionic surfactant dissociates may form ion pairs with acidic or phenolic groups associated with the NOM. This serves to increase the size of hydration of these groups, thereby decreasing the effective size of the nonpolar moieties associated with the NOM, and thus decreasing hydrophobic partitioning of the HOCs into the NOM. The results presented here will help us to understand the effect of NOM and surfactants on the fate and transport of HOCs in aquatic systems.