RIVERS AND STREAMS - Water Quality Engineering in Natural Systems - page 124

of violations occur under low-flow conditions, then
point sources are the likely sources of these viola-
tions, while if the majority of violations occur
under high-flow conditions, then nonpoint sources
are the likely culprits. If violations are distributed
across all flow conditions, then both point and
nonpoint sources are likely responsible. In the
example shown in Figure 4.18, it is apparent that
the majority of water-quality violations occur
under high-flow conditions, and so contaminant
loads from nonpoint sources should be targeted
for reduction in order to remediate the quality of
the stream.

Solution

For each pair of concentration and flow measurements,
c i (CFU/dL) and Q i (m 3 /s), respectively, the correspond-
ing load, L i (CFU/d), can be calculated using the follow-
ing relationship

8.64 10 8

(4.166)

L Qc
i

= × ×

i i

Applying Equation (4.166) to the given measure-
ments yields the load duration relationship shown in
Table 4.11.
These data are plotted on the given TMDL LDC
(from Example 4.24), and the results are shown in
Figure 4.19. Based on the results shown in Figure 4.19,
it is apparent that almost all of the TMDL exceedances
occur under dry conditions, which indicates that point
sources are the likely causes of these water-quality
violations.

In some cases, supplementary analyses could be
useful in further isolating the source of impairment.
For example, for elevated levels of indicator bacteria,
bacterial source tracking using the genotypes of
bacteria found in the stream might be particularly
useful.

EXAMPLE 4.25

Several synoptic measurements of FC concentration
and flow rate are taken in the stream that is described
in Example 4.24; these measurements are as shown in
Table 4.10.
If the FC water-quality standard for the stream is
200 CFU/dL, determine whether point sources or non-
point sources are the likely causes of the water-quality
violations.

Estimation of Required Load Reductions. Comparison
of measured contaminant loads with the LDC, as
described in the previous section and illustrated in
Figure 4.18, leads to the identification of one or more
flow conditions where violations of the applicable water-
quality standard occur with an unacceptable frequency.
The required load reductions can then be determined
separately for each flow condition. The important

TABLE 4.10. Synoptic Measurements of Fecal-Coliform Concentration and Flow Rate
Conc
(CFU/dL)

Flow
(m 3 /s)

Exceed
(%)

Conc
(CFU/dL)

Flow
(m 3 /s)

Exceed
(%)

Conc
(CFU/dL)

Flow
(m 3 /s)

Exceed
(%)

190 33.15 0.5 129 3.67 31.5 255 1.08 72.1

160 26.42 1.3 149 3.26 34.2 296 0.98 75.8

140 21.16 2.8 188 2.95 37.4 380 0.92 78.3

180 19.53 3.6 85 2.62 40.6 276 0.82 81.4

166 17.84 4.4 98 2.44 42.8 248 0.74 84.7

124 16.47 5.2 164 2.35 44.3 221 0.66 87.9

143 14.37 6.7 174 2.18 46.7 200 0.61 90.8

192 13.42 7.5 169 2.07 48.2 198 0.60 91.3

70 12.32 8.6 117 1.93 50.5 221 0.58 92.7

172 11.79 9.2 154 1.79 52.7 210 0.57 93.1

155 10.79 10.4 138 1.71 54.3 205 0.54 94.5

126 8.63 13.6 193 1.59 56.6 228 0.53 95.3

181 7.11 16.8 185 1.52 58.4 205 0.49 96.7

195 6.21 19.7 221 1.44 60.4 199 0.46 97.8

56 5.47 22.4 232 1.32 63.8 201 0.44 98.2

171 4.82 25.3 201 1.24 66.7 193 0.40 99.1

148 4.09 28.9 342 1.16 69.2 - - -