Consistency of Modeled River Tracer Test Breakthrough Curve Moments With
Data.
Abstract
Precise modeling of surface water quality requires transport theory
consistent with observed data. Efforts to achieve model fits to river
tracer test breakthrough curves (BTCs) have led to different approaches
to the one-dimensional modeling of river corridor transport at the
macroscale. The memory function model form practically generalizes most
current approaches including the transient storage (TSM), multirate mass
transfer, decoupled continuous time random walk, and time-fractional
advection dispersion models. An alternative formulation of the memory
function approach is the phase exposure-dependent exchange (PhEDEx)
model, which is the TSM model that includes residence-time dependence of
the rate coefficient for mass exchange from the hyporheic zone to the
river. Using temporal moment analysis and numerical simulations, we
explore the general consistency of the PhEDEx model with observed BTC
data. With this approach the moments of the memory function can be
estimated without requiring any prior information about the memory
function form. We also examine means of estimating river/transport
process parameters using the results of temporal moments analysis. The
results show the failure of the PhEDEx model, and therefore the memory
function approach, in capturing in particular the observed constancy of
the BTC coefficient of skewness (CSK). The temporal moments equations
can be applied as a useful tool to estimate some river/tracer test
parameters such as the ratio of HZ cross-sectional area to the main flow
cross-sectional area, which are time/cost consuming to measure. The
applications of the analyses are demonstrated with some case studies.