Direct Simulation of Groundwater Age

Abstract

Abstract. A new method is proposed to simulate groundwater age directly, by use of an advection-dispersion transport equation with a distributed zero-order source of unit (1) strength, corresponding to the rate of aging. The dependent variable in the governing equation is the mean age, a mass-weighted average age. The governing equation is derived from residence-time-distribution concepts for the case of steady flow. For the more general case of transient flow, a transient governing equation for age is derived from mass-conservation principles applied to conceptual "age mass." The age mass is the product of the water mass and its age, and age mass is assumed to be conserved during mixing. Boundary conditions include zero age mass flux across all noflow and inflow boundaries and no age mass dispersive flux across outflow boundaries. For transient-flow conditions, the initial distribution of age must be known. The solution of the governing transport equation yields the spatial distribution of the mean groundwater age and includes diffusion, dispersion, mixing, and exchange processes that typically are considered only through tracer-specific solute transport simulation. Traditional methods have relied on advective transport to predict point values of groundwater travel time and age. The proposed method retains the simplicity and tracer-independence of advection-only models, but incorporates the effects of dispersion and mixing on volume-averaged age. Example simulations of age in two idealized regional aquifer systems, one homogeneous and the other layered, demonstrate the agreement between the proposed method and traditional particle-tracking approaches and illustrate use of the proposed method to determine the effects of diffusion, dispersion, and mixing on groundwater age.

Groundwater flow and the 4He distribution in the Great Artesian Basin of Australia

“…The numerical model also calculates the distribution of groundwater residence time (or groundwater age) from the predicted rates of advection and hydrodynamic dispersion (but not molecular diffusion), as described by Goode [1996] and Fritzel [1996]. The calculation does not account for the possibility of groundwater from basement rocks mixing into basin strata.…”

Residence time, mineralization processes and groundwater origin within a carbonate coastal aquifer with a thick unsaturated zone

“…However a sample taken at a given location is often a mixture of waters that have been transported via various flow paths. Thus this residence time should be mostly interpreted as a weighted mean of idealized residence times (Bethke and Johnson, 2008;Goode, 1996;Jodar et al, 2014;Suckow, 2014;Torgersen et al, 2013;Turnadge and Smerdon, 2014). The major advantage of CFCs and SF 6 resides in the possibility to discriminate different water bodies considering various mixing models (piston flow, binary or exponential), providing information on aquifer functioning (Cook et al, 1995;Jodar et al, 2014;Kashiwaya et al, 2014;Zuber et al, 2005).…”

Constitution of a catchment virtual observatory for sharing flow and transport models outputs

“…We propose that the comparison of travel time from catchments from differing biomes could lead to a more systematic understanding of catchment behaviour and general theory. Because biogeochemistry is tightly linked to residence time, a better understanding of the travel time distributions will improve the assessment of nutrient removal capacity (Pinay et al, 2015), climate change impacts on stream chemistry (Abbott et al, 2015;Goode, 1996;McGuire et al, 2005) and exposure time (Ginn, 1999;Frei et al, this issue;Oldham et al, 2013).…”