DNAPL source depletion: linking architecture and flux response.

We study the migration of sorbing tracers through crystalline rock by combining relatively simple transport measures with particle tracking in a discrete fracture network. The rock volume is on a 1 ...

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003WR002655

Stochastic simulation of radionuclide migration in discretely fractured rock near the Äspö Hard Rock Laboratory

[1] This work presents a stream tube–based analytical approach to evaluate reduction in groundwater contaminant flux resulting from partial mass reduction in a nonaqueous phase liquid (NAPL) source zone. The reduction in contaminant flux, Rj, discharged from the source zone is a remediation performance metric that has a direct effect on the fundamental drivers of remediation: protection of human health and the environment. Closed form expressions are provided for analyzing remediation performance under conditions of joint spatial variability of both groundwater flow and NAPL content. The performance measures derived here are expressed in terms of measurable parameters. Spatial variability is described within a Lagrangian framework where aquifer hydrodynamic heterogeneities are characterized using nonreactive travel time distributions, while NAPL spatial distribution heterogeneity can be similarly described using reactive travel time distributions. The combined statistics of these distributions are used to evaluate the relationship between reduction in contaminant mass, Rm, and Rj. A portion of the contaminant mass in the source zone is assumed to be removed via in situ flushing remediation, with the initial and final conditions defined as steady state natural gradient groundwater flow through the contaminant source zone. The combined effects of aquifer and NAPL heterogeneities are shown to be captured in a single parameter, reactive travel time variability, which was determined to be the most important factor controlling the relationship between Rm and Rj. It is shown that as heterogeneity in aquifer properties and NAPL spatial distribution increases, less mass reduction is required to achieve a given flux reduction, although the overall source longevity increases. When rate-limited dissolution is important, the efficiency of remediation, in terms of both mass and flux reduction, is reduced. However, at many field sites the combined effects of field-scale heterogeneities and site aging will result in favorable relationships between mass reduction and flux reduction.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004WR003825

Groundwater contaminant flux reduction resulting from nonaqueous phase liquid mass reduction

[1] Particle tracking through stochastically generated networks of discrete fractures provides an alternative to the conventional advection-dispersion description of transport in fractured rock. However, discrete fracture network simulations are computationally intensive and usually limited to small scales. An approach for direct upscaling of discrete fracture simulations is described. Trajectories for nonreacting tracer particles are first extracted from relatively small discrete fracture network simulations. Tracer-rock interaction is represented by also calculating a cumulative reactivity parameter along each trajectory. The residence time/reactivity information is then used in a Monte Carlo simulation to construct artificial particle trajectories of any length, thereby achieving the upscaling objective. In its simplest form the procedure has the form of a random walk evolving in a two-dimensional space. Tests using site-specific and generic networks show that it is necessary to modify the random walk to produce sequential correlation along the trajectories. We achieve this by using a discrete state Markov process to direct the random walk. The procedure is computationally efficient, easily implemented, and compares well with the network simulations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004WR003682

Upscaling discrete fracture network simulations: An alternative to continuum transport models

Relationships between in-stream dissolved solute concentrations (C) and discharge (Q) are useful indicators of catchment-scale processes. We combine a synthesis of observational records with a parsimonious stochastic modeling approach to test how C-Q relationships arise from spatial heterogeneity in catchment solute sources coupled with different timescales of reactions. Our model indicates that the dominant driver of emergent archetypical dilution, enrichment, and constant C-Q patterns was structured heterogeneity of solute sources implemented as correlation of source concentration to travel time. Regardless of the C-Q pattern, with weak correlation between solute-source concentration and travel time, we consistently find lower variability in C than in Q, such that the predominant solute export regime is chemostatic. Consequently, the variance in exported loads is determined primarily by variance of Q. Efforts to improve stream water quality and ecological integrity in intensely managed catchments should lead away from landscape homogenization by introducing structured source heterogeneity.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017GL072630

Emergent archetype patterns of coupled hydrologic and biogeochemical responses in catchments

Contaminant transport in aquifers with spatially variable hydraulic and sorption properties

H. Cheng

Papers

Contaminant transport in aquifers with spatially variable hydraulic and sorption properties