[1] Large-scale groundwater pumping or deep fluid injection in a multilayered subsurface system may generate pressure perturbation not only in the target formation(s), but also in over-and underlying units. Hydraulic communication in the vertical direction may occur via diffuse leakage through aquitards and/or via focused leakage through leaky wells. Existing analytical solutions for pressure perturbation and fluid flow in such systems consider either diffuse leakage or focused leakage, but never in combination with each other. In this study, we developed generalized analytical solutions that account for the combined effect of diffuse and focused leakage. The new solutions solve for pressure changes in a system of N aquifers with alternating leaky aquitards in response to fluid injection/extraction with any number, N I , of injection/pumping (active) wells, and passive leakage/recharge in any number, N L , of leaky wells. The equations of horizontal groundwater flow in the aquifers are coupled by the vertical flow equations in the aquitards and by the flow continuity equations in the leaky wells. The solution methodology, described in detail in this paper, involves transforming the transient flow equations into the Laplace domain; decoupling the resulting ordinary differential equations (ODEs) coupled by diffuse leakage via eigenvalue analysis; solving a system of N L Â N linear algebraic equations for the unknown rates of flow through leakage wells; and superposing the solution of pressure buildup/drawdown in aquifers and aquitards resulting from flow in the N I active and N L leaky wells. Verification of the new methodology was achieved by comparison with existing analytical solutions for diffuse leakage and for focused leakage, and against a numerical solution for combined diffuse and focused leakage. Application to an eight-aquifer system with leaky aquitards and one leaky well demonstrates the usefulness and efficiency of the approach, and illustrates the pressure behavior over a spectrum of leakage scenarios and parameters.Citation: Cihan, A., Q. Zhou, and J. T. Birkholzer (2011), Analytical solutions for pressure perturbation and fluid leakage through aquitards and wells in multilayered-aquifer systems, Water Resour. Res., 47, W10504,

Large-scale pressure buildup in response to carbon dioxide (CO 2 ) injection in the subsurface may limit the dynamic storage capacity of suitable formations, because elevated pressure can impact caprock integrity, induce reactivation of critically stressed faults, drive CO 2 and/or brine through conductive features into shallow groundwater resources, or may affect existing subsurface activities such as oil and gas production. It has been suggested that pressure management involving the extraction of native fluids from storage formations can be used to control subsurface pressure increases caused by CO 2 injection and storage, thereby limiting the possibility of unwanted effects. In this study, we introduce the concept of "impact-driven pressure management (IDPM)", which involves optimization of fluid extraction to meet local (not global) performance criteria (i.e., the goal is to limit pressure increases primarily where environmental impact is a concern). We evaluate the feasibility of IDPM for a hypothetical CO 2 storage operation in an idealized multi-formation system containing a critically stressed fault zone. Using a newly developed analytical solution, we assess alternative fluid extraction schemes and test whether a predefined performance criterion can be achieved, in this case the maximum allowable pressure near the fault zone. Alternative strategies for well placement are evaluated, comparing near-injection arrays of extraction wells with near-impact arrays. Extraction options include active extraction wells and (passive) pressure relief wells, as well as combinations of both, with and without reinjection into the subsurface. Our results suggest that strategic well placement and optimization of extraction may allow for a significant reduction in the brine extraction volumes. Additional work is required in the future to test the general concept of IDPM for more complex and realistic CO 2 storage scenarios.Page 2 IntroductionFor geologic carbon sequestration (GCS) to have a positive effect on reducing or at least stabilizing atmospheric carbon levels, the anticipated volume of CO 2 that would need to be injected in the subsurface is very large (e.g., Zhou and Birkholzer, 2011). One single coal-fired power plant alone may emit as much as 5-10 million tons of CO 2 per year. Unless storage is conducted in depleted oil or gas reservoirs, where fluids have been previously extracted as a result of production, the pore space in suitable storage formations is already filled with saline water. The CO 2 volume injected into saline formations then needs to be accommodated by expansion of reservoir pore space and compression of fluid in response to pressure buildup and, if reservoir boundaries are open, by pressure-driven migration of native brines into neighboring formations. Large and lasting pressure perturbation in the subsurface is an expected feature of GCS operations (e.g., Nicot, 2008;, and careful monitoring and management of pressure increases is generally considered of great importance to the saf...

Bare soil evaporation is a key process for water exchange between the land and the atmosphere and an important component of the water balance. However, there is no agreement on the best modeling methodology to determine evaporation under different atmospheric boundary conditions. Also, there is a lack of directly measured soil evaporation data for model validation to compare these methods to establish the validity of their mathematical formulations. Thus, a need exists to systematically compare evaporation estimates using existing methods to experimental observations. The goal of this work is to test different conceptual and mathematical formulations that are used to estimate evaporation from bare soils to critically investigate various formulations and surface boundary conditions. Such a comparison required the development of a numerical model that has the ability to incorporate these boundary conditions. For this model, we modified a previously developed theory that allows nonequilibrium liquid/gas phase change with gas phase vapor diffusion to better account for dry soil conditions. Precision data under well‐controlled transient heat and wind boundary conditions were generated, and results from numerical simulations were compared with experimental data. Results demonstrate that the approaches based on different boundary conditions varied in their ability to capture different stages of evaporation. All approaches have benefits and limitations, and no one approach can be deemed most appropriate for every scenario. Comparisons of different formulations of the surface boundary condition validate the need for further research on heat and vapor transport processes in soil for better modeling accuracy.

Concentrated suspensions of polymer-modified Fe(0) nanoparticles (NZVI) are injected into heterogeneous porous media for groundwater remediation. This study evaluated the effect of porous media heterogeneity and the dispersion properties including particle concentration, Fe(0) content, and adsorbed polymer mass and layer thickness which are expected to affect the delivery and emplacement of NZVI in heterogeneous porous media in a two-dimensional (2-D) cell. Heterogeneity in hydraulic conductivity had a significant impact on the deposition of NZVI. Polymer modified NZVI followed preferential flow paths and deposited in the regions where fluid shear is insufficient to prevent NZVI agglomeration and deposition. NZVI transported in heterogeneous porous media better at low particle concentration (0.3 g/L) than at high particle concentrations (3 and 6 g/L) due to greater particle agglomeration at high concentration. High Fe(0) content decreased transport during injection due to agglomeration promoted by magnetic attraction. NZVI with a flat adsorbed polymeric layer (thickness ∼30 nm) could not be transported effectively due to pore clogging and deposition near the inlet, while NZVI with a more extended adsorbed layer thickness (i.e., ∼70 nm) were mobile in porous media. This study indicates the importance of characterizing porous media heterogeneity and NZVI dispersion properties as part of the design of a robust delivery strategy for NZVI in the subsurface.