Researchers:
Dongxiao Zhang, Los Alamos National Laboratory
Thomas Harter, University of California,
Davis
Mathematical
models of flow and contaminant transport in highly
heterogeneous (variable) sediments, while mathematically
complicated, are greatly simplifying nature's
complexities. Yet they are extremely powerful:
- They
provide scientists with fundamental insight
into the behavior of water flow and contaminant
transport through aquifers with highly variable
geologic-sedimentologic architecture. It turns
out that if we consider water flow or contaminant
transport over distances of several tens of
feet (field scale) or several thousands of feet
(regional scale), its behavior is fundamentally
different from what we observe at the laboratory
scale, where measurements are made on small
samples (an inch or few inches in length). With
mathematical models that account for the sedimentologic
heterogeneity, many of these discrepancies can
be explained.
- These
mathematical models provide a tool to predict
the certainty or uncertainty of predictions
in future groundwater flows and future groundwater
contamination. Because we can never know the
exact geologic properties of an aquifer at each
and all locations, predictions of groundwater
flow and contaminant transport are necessarily
associated with uncertainty. By mathematically
or statistically describing the character or
type of heterogeneity in aquifers or soils,
we can say something about the degree of certainty
with which we can make predictions.
This particular
project specifically investigates water flow and
contaminant transport from the land surface to
the water table, where pores are only partly filled
with water. (Hence the name 'unsaturated' flow
and transport). Depending on whether sediments
are tight or fairly coarse, water content in these
pores can change quickly. Unlike previous models,
the mathematical model that we developed takes
into account that water content can be highly
variable. In our research paper, we describe the
mathematical model and demonstrate how important
it is to include a measure for variable water
content, particularly when evaluating water flow
and contaminant transport in relatively dry soils
(where flow and transport happens only over long
periods of time, that is, months or years). 
References:
Harter,
T., and D. Zhang, Water flow and solute spreading
in heterogeneous soils with spatially variable
water content, Water Resour. Res., 35(2),
415-426, 1999.
Harter,
T., T.C.J. Yeh, Flow in unsaturated random porous
media, nonlinear numerical analysis, and comparison
to analytic stochastic models, Adv. in Water
Resour., 22(3), 257-272, 1998.
Harter,
T., T.C.J. Yeh, 1996, Stochastic analysis of solute
transport in heterogeneous, variably saturated
porous media, Water Resour. Res.,20, 1585-1595
Harter, T., T.C.J. Yeh, 1996, Conditional stochastic
analysis of solute transport in heterogeneous,
variably saturated soils, Water Resour. Res.,
20, 1597 - 1609.
Harter,
T., 1994, Unconditional and Conditional Simulation
of Flow and Transport in Heterogeneous, Variably
Saturated Porous Media, Dissertation, University
of Arizona, 418 p.
Also see:
Stochastic
Analysis of Reactive Transport
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