M. Flury et al., ANALYTICAL SOLUTION FOR SOLUTE TRANSPORT WITH DEPTH-DEPENDENT TRANSFORMATION OR SORPTION COEFFICIENTS, Water resources research, 34(11), 1998, pp. 2931-2937
Reaction processes in soils, such as degradation and sorption, are oft
en strongly depth dependent. Microbial activity and organic matter, wh
ich play an important role in reaction processes, are mostly confined
to the top few decimeters of the soil. The purpose of this paper is to
analyze the effects of space-dependent reaction coefficients on trans
port of solutes through soil. We derive an analytical solution of the
advection-dispersion equation with depth-dependent transformation or s
orption coefficients. The solution is obtained in Laplace space and is
inverted numerically. Breakthrough curves and concentration profiles
of degrading and sorbing chemicals are presented to illustrate the inf
luence of spatially variable reactions. It is demonstrated that the co
ncentration profiles for depth-dependent reactions vary between the tw
o extreme cases of no and constant reactions, depending on how fast th
e reactions cease with increasing depth. The results emphasize the rel
evance of depth-dependent transformation and sorption to transport of
chemicals in soils. It is further shown that depth-dependent sorption
may cause a bimodal depth profile, even under steady state water flow
regimes.