Agriculture Reference
In-Depth Information
For a homogeneous soil, the leached fraction,
F
, according to the Convection Dispersion
Equation is given by
RL
F
exp
;
R
1
K
(6)
s
v
if dispersion is ignored, where R is the retardation factor and
K
s
is the sorption coefficient
for linear sorption. In this solution, we recognize the soil physical and atmospheric forcing
controlled properties (
, v
), and the contaminant specific properties (
K
s
, and degradation
rate parameter ). Hence, a leaching vulnerability assessment system for the applicability of
de-icing chemicals could involve a water flow and tracer leaching investigation, followed by
a contaminant specific leaching investigation. In real soil systems, the various parameters of
equation (6) vary in space and time, and this may have a large effect on the leached fraction.
Therefore, Van der Zee & Boesten (1991) simulated leaching for the heterogeneous case. It
appears that if there is a weak spot in the soil where leaching is relatively large and fast,
then this one spot can contaminate a large volume of water up to the level of the water
quality standard even if elsewhere no leaching of contaminant occurs. For moderate
variability of soil properties, the leaching of contaminant in such a heterogeneous
environment still behaves as 'first order degradation and transport', as described by
equation (5), but the constants in this equation should be appropriately averaged. How this
averaging should be done, focused on the leached fraction, is explained by Van der Zee &
Boesten (1991).
Aimed at the problem of contamination with de-icing chemicals, it appears feasible to
approach surface runoff also from a partly stochastic approach. Therefore, some first
indication of ways to deal with irregular soil surfaces in the case of overland flow are
presented using the framework of Appels et al. (2011). Using these approaches, it appears to
be possible to give a stochastic analysis of the in situ aquifer bioremediation of degradable
contaminants such as Propylene Glycol. Besides illustrating this aspect, many problems and
unresolved questions are outstanding. For instance, mostly soil and groundwater
formations are assumed to conform to Gaussian spatial structures, but in reality, more
complicated structures that are distinctly non-Gaussian, may be more realistic.
5.2 Remediation in heterogeneous soils
Because transport of nonlinear reacting contaminants in spatially variable soils is a very
active field of research, many problems are unresolved, yet managers need to make
decisions, for instance concerning choice of remediation technique. For this reason, it is
appropriate to indicate in what respect management decisions need to take into account this
complexity. For instance, the applicability of the still recommended (e.g. by USA, EPA) air
sparging as a methodology to purify groundwater that is contaminated with organic
biodegradable or volatile contaminants can be judged well on the basis of the physics of this
technique.
In air sparging, air is injected below the phreatic groundwater level, with the purpose to
strip the aquifer from volatile contaminants, but also to increase the oxygen content in those
strata as certain organic contaminants require it to become degraded. Much effort has been
devoted to experimentally investigate how air will flow through the aquifer (upwards, as air
is much less dense than water): in a narrow, vertical funnel or in a gradually upward
broadening cone. The latter occurs if the aquifer material is sufficiently fine textured sand,
