Preferential Flow Modelling In The Vadose Zone
Using MACRO 5.0: Cape Flats Sands And
Mpumalanga Clays Case Studies (South Africa)
Author(s)
N. Z. Jovanovic, K. A. Majola, M. Ginster & S. Adams
Abstract
Understanding fluid flow and solute transport within the vadose (unsaturated)
zone is an essential prerequisite for protection of groundwater from contaminant
sources occurring overland. Preferential flow paths in the vadose zone pose a
significant problem because they are potential avenues for rapid transport of
chemicals from contamination sources to the water table. The objectives of this
study were i) to test the adequacy of a model, namely MACRO 5.0, in simulating
bimodal water flow and generic contaminant transport in the vadose zone, and ii)
to run a sensitivity analysis with MACRO 5.0 in order to determine which input
parameters are the most relevant in describing the effects of preferential water
flow, solute transport and exchange between macropores and matrix. Two case
studies were investigated, the first at a landfill site overlying sandy unconfined
aquifer (Coastal Park, Cape Town), and the second at an industrial site overlying
cracking clayey soil and fractured rocks (Secunda, Mpumalanga Province). For
the Coastal Park site, simulations of soil water content and leaching of a mobile
tracer were compared to monitored soil water contents and chloride
concentrations in groundwater. For the Secunda site, simulations of soil water
content and concentrations of boron and fluoride originating from effluent
irrigation were compared to soil profile measurements. In both cases, the
MACRO 5.0 model predictions agreed well with measurements, appropriate
input calibration data were used. The sensitivity analysis indicated that soil water
properties related to preferential flow (hydraulic conductivity at the boundary
between macropores and matrix, soil water content and tension, and diffusion
pathlength) have influence on simulation results. Similarly, the solute balance is
mostly influenced by degradation rate coefficients (both in solid and liquid
phases), sorption distribution coefficients and solute concentrations.
Keywords: cape flats, MACRO 5.0, preferential flow, Secunda, sensitivity
analysis, vadose zone.
Keywords
cape flats, MACRO 5.0, preferential flow, Secunda, sensitivityanalysis, vadose zone.
Related Book
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