89 research outputs found
Reply to comment by J.-P. Renaud et al. on “An assessment of the tracer-based approach to quantifying groundwater contributions to streamflow”
This is the published version. Copyright Wiley [Commercial Publisher
Hydrologic response of catchments to precipitation: Quantification of mechanical carriers and origins of water
This is the published version. Copyright American Geophysical Union[1] Precipitation-induced overland and groundwater flow and mixing processes are quantified to analyze the temporal (event and pre-event water) and spatial (groundwater discharge and overland runoff) origins of water entering a stream. Using a distributed-parameter control volume finite-element simulator that can simultaneously solve the fully coupled partial differential equations describing 2-D Manning and 3-D Darcian flow and advective-dispersive transport, mechanical flow (driven by hydraulic potential) and tracer-based hydrograph separation (driven by dispersive mixing as well as mechanical flow) are simulated in response to precipitation events in two cross sections oriented parallel and perpendicular to a stream. The results indicate that as precipitation becomes more intense, the subsurface mechanical flow contributions tend to become less significant relative to the total pre-event stream discharge. Hydrodynamic mixing can play an important role in enhancing pre-event tracer signals in the stream. This implies that temporally tagged chemical signals introduced into surface-subsurface flow systems from precipitation may not be strong enough to detect the changes in the subsurface flow system. It is concluded that diffusive/dispersive mixing, capillary fringe groundwater ridging, and macropore flow can influence the temporal sources of water in the stream, but any sole mechanism may not fully explain the strong pre-event water discharge. Further investigations of the influence of heterogeneity, residence time, geomorphology, and root zone processes are required to confirm the conclusions of this study
An assessment of the tracer-based approach to quantifying groundwater contributions to streamflow
This is the published version. Copyright American Geophysical Union[1] The use of conservative geochemical and isotopic tracers along with mass balance equations to determine the pre-event groundwater contributions to streamflow during a rainfall event is widely used for hydrograph separation; however, aspects related to the influence of surface and subsurface mixing processes on the estimates of the pre-event contribution remain poorly understood. Moreover, the lack of a precise definition of “pre-event” versus “event” contributions on the one hand and “old” versus “new” water components on the other hand has seemingly led to confusion within the hydrologic community about the role of Darcian-based groundwater flow during a storm event. In this work, a fully integrated surface and subsurface flow and solute transport model is used to analyze flow system dynamics during a storm event, concomitantly with advective-dispersive tracer transport, and to investigate the role of hydrodynamic mixing processes on the estimates of the pre-event component. A number of numerical experiments are presented, including an analysis of a controlled rainfall-runoff experiment, that compare the computed Darcian-based groundwater fluxes contributing to streamflow during a rainfall event with estimates of these contributions based on a tracer-based separation. It is shown that hydrodynamic mixing processes can dramatically influence estimates of the pre-event water contribution estimated by a tracer-based separation. Specifically, it is demonstrated that the actual amount of bulk flowing groundwater contributing to streamflow may be much smaller than the quantity indirectly estimated from a separation based on tracer mass balances, even if the mixing processes are weak
Use of groundwater lifetime expectancy for the performance assessment of a deep geologic waste repository: 1. Theory, illustrations, and implications
Long-term solutions for the disposal of toxic wastes usually involve
isolation of the wastes in a deep subsurface geologic environment. In the case
of spent nuclear fuel, if radionuclide leakage occurs from the engineered
barrier, the geological medium represents the ultimate barrier that is relied
upon to ensure safety. Consequently, an evaluation of radionuclide travel times
from a repository to the biosphere is critically important in a performance
assessment analysis. In this study, we develop a travel time framework based on
the concept of groundwater lifetime expectancy as a safety indicator. Lifetime
expectancy characterizes the time that radionuclides will spend in the
subsurface after their release from the repository and prior to discharging
into the biosphere. The probability density function of lifetime expectancy is
computed throughout the host rock by solving the backward-in-time solute
transport adjoint equation subject to a properly posed set of boundary
conditions. It can then be used to define optimal repository locations. The
risk associated with selected sites can be evaluated by simulating an
appropriate contaminant release history. The utility of the method is
illustrated by means of analytical and numerical examples, which focus on the
effect of fracture networks on the uncertainty of evaluated lifetime
expectancy.Comment: 11 pages, 8 figures; Water Resources Research, Vol. 44, 200
Use of groundwater lifetime expectancy for the performance assessment of a deep geologic radioactive waste repository:2. Application to a Canadian Shield environment
Cornaton et al. [2007] introduced the concept of lifetime expectancy as a
performance measure of the safety of subsurface repositories, based upon the
travel time for contaminants released at a certain point in the subsurface to
reach the biosphere or compliance area. The methodologies are applied to a
hypothetical but realistic Canadian Shield crystalline rock environment, which
is considered to be one of the most geologically stable areas on Earth. In an
approximately 10\times10\times1.5 km3 hypothetical study area, up to 1000 major
and intermediate fracture zones are generated from surface lineament analyses
and subsurface surveys. In the study area, mean and probability density of
lifetime expectancy are analyzed with realistic geologic and hydrologic shield
settings in order to demonstrate the applicability of the theory and the
numerical model for optimally locating a deep subsurface repository for the
safe storage of spent nuclear fuel. The results demonstrate that, in general,
groundwater lifetime expectancy increases with depth and it is greatest inside
major matrix blocks. Various sources and aspects of uncertainty are considered,
specifically geometric and hydraulic parameters of permeable fracture zones.
Sensitivity analyses indicate that the existence and location of permeable
fracture zones and the relationship between fracture zone permeability and
depth from ground surface are the most significant factors for lifetime
expectancy distribution in such a crystalline rock environment. As a
consequence, it is successfully demonstrated that the concept of lifetime
expectancy can be applied to siting and performance assessment studies for deep
geologic repositories in crystalline fractured rock settings.Comment: 14 pages, 14 figures; Water Resources Research, Vol. 44, 200
Review of Inverse Laplace Transform Algorithms for Laplace-Space Numerical Approaches
A boundary element method (BEM) simulation is used to compare the efficiency
of numerical inverse Laplace transform strategies, considering general
requirements of Laplace-space numerical approaches. The two-dimensional BEM
solution is used to solve the Laplace-transformed diffusion equation, producing
a time-domain solution after a numerical Laplace transform inversion. Motivated
by the needs of numerical methods posed in Laplace-transformed space, we
compare five inverse Laplace transform algorithms and discuss implementation
techniques to minimize the number of Laplace-space function evaluations. We
investigate the ability to calculate a sequence of time domain values using the
fewest Laplace-space model evaluations. We find Fourier-series based inversion
algorithms work for common time behaviors, are the most robust with respect to
free parameters, and allow for straightforward image function evaluation re-use
across at least a log cycle of time
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