Environmental Engineering Reference
In-Depth Information
Instantaneous loads in streams are determined from
contemporaneous measurements of
Q
and
c
and are
widely used (at least in the United States) to assess the
instream quality of the water (using
c
), the hydrologic
state of the stream (using
Q
), and the contaminant load
relative to the allowable load for the existing hydrologic
state (using
Qc
). When the measured instantaneous
load (
Qc
) exceeds the allowable load for the observed
flow conditions, then reductions in contaminant source
loading to the stream must be achieved under those flow
conditions in order for the stream to meet its applicable
water-quality standards. In this type of application, the
maximum allowable load is called the
total maximum
daily load
(TMDL).
In contrast to the instantaneous loads that are used
in TMDL analyses, long-term estimates of constituent
loads are sometimes required to determine the total
amount of a constituent that is removed from a water-
shed by streamflow, or the total amount of a constituent
that is entering a downstream waterbody from upstream
sources. These applications commonly involve consider-
ation of suspended solids as the constituent, with total
suspended solids load sometimes being related to the
amount of soil erosion in the watershed, or, in other
applications, the suspended solids load entering a reser-
voir is of concern since these solids will eventually settle
out of the water column and reduce the storage volume
in the reservoir.
v
H
(0.9846)
0.3
s
k
=
f
=
3.0
= 0.09846
d
−
1
b3
p
k
=
k
+
k
+
k
=
0.5704 0.1263 0.09846
+
+
=
0.795
d
−
1
b
b1
b2
b3
Therefore, the first-order decay coefficient of the
indicator bacteria is
k
b
= 0.795 d
−1
. Since the decay
process is first order, Equation (4.149) integrates to
yield
x
V
[
]
=
c
=
c
exp
−
k t
c
exp
−
k
0
b
0
b
where
c
0
is the initial concentration and
x
is the distance
from the source. If
x
90
is the distance for 90% attenua-
tion, then
x
V
90
0.1
0
c
=
c
exp
−
k
0
b
x
90
0.1
=
exp
−
(0.795)
1780
which yields
x
90
= 5160 m. Therefore, the distance
required for 90% attenuation of the indicator bacteria
is 5.16 km.
4.7 CONTAMINANT LOADS
4.7.1 Total Maximum Daily Loads
Contaminant loads are widely used bases for regulating
and managing the water quality in streams. The con-
taminant load in a stream is defined as the amount of
contaminant transported across a given cross section of
the stream in a given interval of time. Contaminant
loads can be categorized as either instantaneous loads
or long-term loads, where instantaneous loads measure
the amount of contaminant transported at a given
instant of time, and long-term loads measure the amount
of contaminant transported over longer durations of
time, within which flow conditions in the stream might
change such as over seasonal and annual time scales.
The instantaneous load,
L
(
t
) (MT
−1
), at any time
t
(T)
is related to the flow rate,
Q
(
t
) (L
3
T
−1
), and concentra-
tion,
c
(
t
) (ML
−3
), by
Water-quality standards typically specify the maximum
allowable concentration,
c
std
(ML
−3
), of a contaminant in
a stream and, for any given flow rate,
Q
(MT
−3
), the
TMDL is defined as
Qc
std
. Since the relative contribu-
tions of various contaminant sources within the water-
shed to contaminant levels in a stream are closely
related to the flow conditions in the stream (e.g., high
low and low low), it is convenient to relate the TMDL
(=
Qc
std
) to the flow condition. The functional relation-
ship between the TMDL and flow condition is called the
load duration curve
(LDC), and this curve forms the
basis for allocating contaminant loading from sources
within the watershed so as to ensure that the water-
quality standards in the receiving stream are met.
Streams in which the applicable water-quality standards
are not being met are called
impaired streams
.
L t Q t c t
( )
=
( ) ( )
,
(4.156)
4.7.1.1 Derivation of the Load Duration Curve.
The
LDC expresses the relationship between the contami-
nant load required to meet a water-quality standard
(i.e., the TMDL) and the corresponding flow condition.
Full appreciation of the meaning and application of the
LDC is best acquired from an understanding of how this
and the long-term contaminant load,
L
T
(M), over a time
T
(T) between times
t
1
(T) and
t
2
(T) is given by
t
t
2
2
∫
∫
L
=
L t dt
( )
=
Q t c t dt
( ) ( )
(4.157)
T
t
t
1
1
Search WWH ::
Custom Search