Environmental Engineering Reference
In-Depth Information
TABLE 6.14. Half-Lives of Insecticides
in Soil
The time,
t
, for the concentration of atrazine to decay
from 19 to 0.003 mg/L is given by Equation (6.44),
where
half-Life
(Months)
Pesticide
c
( )
t
=
c
( )
0
e
k t
−
d
aq
aq
Aldrin
3-8
Chlordane
10-12
0 003 19
.
=
e
−
0 00693
.
t
DDT
∼30
Dieldrin
∼27
which yields
heptachlor
8-10
Lindane
12-20
t
= 1263 days
Source of data
: Kuhnt (1995).
Therefore, it will take approximately 3.5 years (= 1263
days) for the concentration of atrazine to decay to the
drinking water standard, provided that no additional
atrazine is added to the soil.
(6.43)
dc
dt
aq
=
constant
Typically, due to lack of information, it is not possible
to quantify specific degradation rates. The only informa-
tion usually available for many organic chemicals is
their half-life and/or overall persistence of the chemical
in the soil. Using the first-order reaction given by Equa-
tion (6.42), the aqueous concentration (in the soil water)
as a function of time is given by
6.3.3 Best Management Practices
Best management practices are methods and practices
for preventing or reducing nonpoint source pollution to
a level compatible with water-quality goals. When select-
ing BMPs, it is important that the pollutants and the
forms in which they are transported be known.
BMPs can be selected in two ways: (1) to control a
known or suspected type of pollution (e.g., phosphorus
or bacteria) from a particular source (e.g., runoff from
a cornfield or a dairy feedlot); or (2) to prevent pollu-
tion from a category of land use activity (such as agri-
culture row crop farming or containerized nursery
irrigation return flow). The recommended procedure
for selecting BMPs to solve water-quality problems is as
follows:
c
( )
t
=
c
( )
0
e
k t
−
(6.44)
d
aq
aq
where
c
aq
(
t
) and
c
aq
(0) are the aqueous concentrations
at time
t
and zero, respectively, and
k
d
is the overall
degradation coefficient. Using Equation (6.44), the half-
life,
t
0.5
(T), is related to the overall degradation coeffi-
cient,
k
d
(T
−1
), by the relation
ln( . )
0 5
t
= −
(6.45)
0 5
.
k
Step 1.
Identify the water-quality problem: for
example, annual summer algal blooms in a lake.
Step 2.
Identify the pollutants contributing to the
problem and their probable sources: for example,
nutrients from septic systems adjoining the lake
and runoff from a nearby horse pasture.
Step 3.
Determine how each pollutant is delivered to
the water (e.g., soluble nutrients from septic tank
drain fields rise to the surface when the systems
are overloaded and are carried to the lake by over-
land flow during rainstorms and snowmelt).
Step 4.
Set a reasonable water-quality goal for the
water body and determine the level of treatment
needed to meet that goal.
Step 5.
Evaluate feasible BMPs for water-quality
effectiveness, effect on groundwater, economic
feasibility, and suitability of the practice to the site.
d
The half-lives of several pesticides (insecticides) in soils
are given in Table 6.14.
EXAMPLE 6.8
The half-life of atrazine is estimated to be 100 days.
If the pore water concentration of atrazine in a soil
is found to be 19 mg/L, estimate how long it will take
for the concentration of atrazine to decrease to the
drinking-water standard of 0.003 mg/L.
Solution
From the data given,
t
0.5
= 100 days,
c
aq
(0) = 19 mg/L,
and
c
aq
(
t
) = 0.003 mg/L. The overall degradation coeffi-
cient,
k
d
, is given by Equation (6.45) as
ln( . )
0 5
ln( . )
0 5
100
When selecting BMPs to prevent a problem, a more
technology-based approach can be followed. Such an
k
= −
= −
=
0 00693
.
d
−
1
d
t
0 5
.
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