APPCOR11234

• Permit No: C1981039
• IBM Index Class Name: Application Correspondence
• Doc Date: 1/4/1984
• Doc Name: RESPONSES TO PERMIT REVIEW LETTER
• Media Type: D
• Archive: No

Jtl tiIi UIM1tEN r-('u.\~1~RUL'rliC llN lljtlES, CENTRAL P);N NSYLVANIA j
Figure 26 is a graph showing the suspended-sediment concentration o(the
water in the east pond versus the number of days since the last storm that
alfected the pond. The graph is (orthe period from September 19 to October
18, 1973. In this particular case, the sediment concentration just alter the
storm was 1,800 mg/Land 10 days after the storm it was 800 mg/ L. Because
storms seem to run in cycles-several during one 2-week period, then none
for 2 weeks or so-it may be desirable to increase the efficiency of offstream
ponds, possibly by adding a coagulant to increase the settling velocity of the
suspended matter.
The effectiveness of the offstream ponds was computed on the basis of
their trap efficiency and the ability,to reduce turbidity. The ponds were
operational fur 39 precipitation events which ranged from 0.05 to 2.35 in. (1
to 60 mm) of water. In figures 27 and 28, the percent reduction in sediment
load and mean turhidity is based on what the sediment load and mean
turbidity would have been in the stream if the sediment-control ponds had
not been installed. The reductions were calculated on the basis of the
sediment load and turbidity measured from basin 3, where no offstream
ponds were used.
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SEPTEMBER 1973 OCTOBER 1973
F7m xe 26.--Suspended-sediment Concentration of water stored m the earl offsvcam pond,
b;tsin 2A, September 19 to October IB, 1973.
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P:ILOtiIUN ANU SE UIM IiN I'-('UNI'Ii UI, MIiASlilt liti 4t
Figure 27 shows the percent the offstream ponds in basin 2A reduced the
sediment load versus the amount of precipitation for 32 storms. Dt)t~) fur
seven storms that only produced sm:lll runoff events while the ponds were
operational are not included, For storms that produced 1,25 in. (]2 mntl u(
precipitation or less, the median reduction in thesediment load was about 70
percent, and for storms that produced over 1.25 in. (32 mm) ofprecipitation,
the median reduction was about IS percent, The efficiency of the ponds
decreases as storm size in(reases bec;IUSe detention time is decreased by the
increased runoff. The median reduction forallstorms shown in figure 27 wits
about 60 percent.
Mean turbidity of streamflow was calculated over a 5-day period
including the day of the storm and the following four days. Figure 28 shows
the percent increase or decrease in the mean 5-day turbidity of the stream
draining basin 2A. From (figure 28 it can be seen that the median decreltse in
mean turbidity was about 60 percent.
Although the offstream ponds were relatively effective, the effectiveness
could be increased in several ways. The water in the ponds could be treated
PRECIPITATION, IN MILLIMETERS
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PRECIPITATION, IN INCHES
Rruar 27. -Red action in the storm-ru null eu.pendeJ-sethmeut load In un ha+w 24
due lu the ullst ¢am pnnJ.. (1cloher 2. 1971. to AuEoV 1, 147J
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