Geoscience Reference
In-Depth Information
sterilized media represent biologically mediated degradation. Figure
16.41
illustrates the dynamics of atrazine concentration in natural (A, C) and sterilized
(B, D) groundwater (A, B) and in groundwater with aquifer sand (C, D), over
120 days. Atrazine concentration does not show any changes during the 120-day
period, in either natural or sterilized waters; there is no chemical or biological
degradation. The degradation rate of atrazine in the natural groundwater-aquifer
sand mixture is greater than that obtained for the sterilized matrix, proving the
occurrence of a biologically mediated process.
Pang et al. (
2005
) assumed that atrazine degradation follows a first-order decay
law,
c ¼ c
o
exp
ð
kt
Þ
ð
16
:
7
Þ
where c is the atrazine concentration at time t (mg/L), c
o
is the initial atrazine
concentration (mg/L), k is the degradation rate coefficient (d
-1
), and t is time (d).
The calculated chemical and biological degradation rate, k, and the half-life, t
1/2
(d), of atrazine and other pesticides in the groundwater and sand mixtures are
given in Table
16.10
. In this particular case, biologically mediated degradation of
atrazine is about 8 % greater than chemical degradation, and this result is reflected
in the half-life of the compound.
The degradation of atrazine in saturated aquifer materials was studied under
static and recirculating laboratory conditions by Schwab et al. (
2006
). Aquifer and
soil samples were collected from south central Kansas (Hutchinson) and from the
floodplain of the Kansas River (Topeka and Ashland). In general, groundwater
used in the experiments was collected from the same location and depth as the
aquifer solid material. The majority of the aerobic microorganisms from the
aquifers were bacteria and actinomycetes, although the population of actinomy-
cetes was 100-fold less than in soils. No information on anaerobic microorganism
activities in the aquifers is presented. The contribution of biodegradation to
atrazine transformation was determined by comparing natural and sterilized
(autoclaved) variables. The experiments were performed temperatures of 15 and
25 C. The degradation patterns of atrazine incubated in natural and sterilized
aquifers are presented in Fig.
16.42
.
Atrazine degradation was different for the sterilized and natural aquifer treat-
ments after the first 80-100 days, at which point atrazine disappearance continued
in natural aquifers but almost ceased in the sterilized system. This behavior may be
explained by a joint chemical and biological effect in the first 100-day period.
After 100 days of incubation, chemical processes cease and atrazine is transformed
only through a minor biologically mediated process. Circulation of the water
through aquifer material resulted in the highest disappearance of atrazine. Under
recirculating conditions, no difference between atrazine degradation in sterilized
and nonsterilized aquifers was observed, indicating that chemical degradation is
the predominant degradation pathway. It is likely that the recirculating pathway
extends contact with active sites of the aquifer solid phase, favoring continued
