Geoscience Reference
In-Depth Information
spatial variability of contaminant concentration, which affects contaminant
redistribution with depth following leaching. If a sorbed contaminant is not of
uniform concentration across all soil-size ranges but is higher in the fine sediment
fraction, the deposition of this soil fraction controls contaminant redistribution in
the subsurface.
The extent of transport of dissolved contaminants in overland runoff is con-
trolled by the topography and morphology of the land (also affected by anthro-
pogenic activity), the depth of chemical incorporation into soil, and the time
between rainfall initiation and surface runoff commencement. In addition to these
factors, transport of adsorbed contaminants on suspended particles is affected by
rainfall intensity, which favors soil erosion.
In general, soluble and nonreactive contaminants are found mainly in dissolved
form in runoff water. For example, a large percentage (up to 90 %) of the most
soluble herbicides present in the soil layer may be partitioned in overland flowing
water. A substantial portion of dissolved nitrogen (8-80 %) and phosphorus
(7-30 %) also may be transported in runoff water (Menzel et al.
1978
; Hubbard
et al.
1982
; McDowell et al.
1984
). Experiments comparing concentrations of
soluble bromide in overland runoff water, as a function of soil type, infiltration
rate, and contaminant incorporation depth, under similar ''rainfall'' conditions,
ranged from 0.1 to 14 % (Ahuja
1982
; Ahuja and Lehman
1983
). Bromide was
released to runoff from depths up to 2.0 cm, and concentrations decreased expo-
nentially below the thin crust layer. In treatments where infiltration was impeded
due to the presence of a deeper clay layer, the amount of bromide in the runoff
water was greater than in treatments with a high infiltration rate. The authors
suggested that the mechanism of chemical transfer from below the soil surface to
runoff is through a turbulent diffusion process caused by raindrop impact. Based on
field experiment results, Walton et al. (
2000
) stated that the type of soil and its
hydraulic and structural properties are the main factors that determine the amount
of bromide transported in runoff water. These results are in agreement with
Wallach et al. (
1997
), who quantify overland flow and total runoff volume by
considering a spatially variable infiltration rate along the slope. The effect of
spatial variability of infiltration on the uncertainty in predicting runoff from sites
with distinct topographic characteristics was discussed by Michaelides and Wilson
(
2007
). They found that increasing the range of spatial correlation of infiltration
rates leads to increased uncertainty in modeled runoff.
A reactive contaminant may be adsorbed on the soil surface prior to rainfall
then, following rainfall that causes erosion, the soil is transported by runoff water
in the form of suspended particles redistributed on the land surface. In general, the
settling velocity distribution during runoff indicates that the finer particles are
resettled initially (Proffitt et al.
1991
), although the details of the settling process
are affected by different environmental factors, such as soil type and rainfall rate.
