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Abstract: The potential for using pairs of so-called horizontal flow treatment wells (HFTWs) to effect in situ capture and treatment of contaminated groundwater has recently been demonstrated. To apply this new technology, design engineers need to be able to simulate the relatively complex groundwater flow patterns that result from HFTW operation. In this work, a three-dimensional analytical solution for steady flow in a homogeneous, anisotropic, contaminated aquifer is developed to efficiently calculate the interflow of water circulating between a pair of HFTWs and map the spatial extent of contaminated groundwater flowing from upgradient that is captured. The solution is constructed by superposing the solutions for the flow fields resulting from operation of partially penetrating wells. The solution is used to investigate the flow resulting from operation of an HFTW well pair and to quantify how aquifer anisotropy, well placement, and pumping rate impact capture zone width and interflow. The analytical modeling method presented here provides a fast and accurate technique for representing the flow field resulting from operation of HFTW systems, and represents a tool that can be useful in designing in situ groundwater contamination treatment systems. [Copyright &y& Elsevier]

• Analytical model for contaminant fate and transport in fracture-rock matrix. • Incorporating advection induced by variably velocity of Poiseuille flow in fracture. • Insights into the difference of Taylor dispersion and Poiseuille flow models. 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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]