NC0021890_Instream Assessment_19810623

DIVISION OF ENVIRONMENTAL M&NAGEMENT
March 23, 1982
MEMORANDUM
T0. Dick Peace, Regional Supervisor
Mooresville Regional Office
FROM: Forrest Westall, dead 7114 `' AV 1,,oz
Technical Services Branch
SUBJECT: In -Stream Assessment
Granite Falls WWTP
Granite Falls, Caldwell County
Per your request, a 'Level u" analysis has been complete" for the
Town of Granite Falls WWTP discharge into Gunpowder Creek. This is a
"C" class stream. At the point of discharge, the creek has a drainage
area of 36 m12 yielding; a 7Q10 of 7.2 efs.
There is a small dam located approximately u.4 miles below the
discharge point on Gunpowder Creek. The plant actually discharges into
the backwaters of the lake created by the dam. The velocity in the lake
was estimated to be 0.009 f.p.s. Beyond the daps, Gunpowder Creek becomes
the backwaters of Lake Hickory (Class A -II and B); the lake is located
3.4 miles beyond the dam.
The town of Granite Falls previously applied for a Special Order
by Consent (see memo from Forrest Westall dated June 23, 1981). At that
time, the town applied for an increase of 15,000 gpd; the F.MC approved
this increase in wasteflow. Presuntly, the town is requesting another
increase of 21,900 gpd domestic waste. In order to assess the impact of
both increases, the model was run assuming an addition of 36,900 gpd.
Given present treatment plant conditions, a "Level B" analysis
indicates that at summer 7Q10 conditions a DO sag of 6.8 mg/l is expected
to occur just above the dam (0.4 miles below the discharges). With the
requested addition of 36,900 gpd, the DO Rag is predicted to drop by
only 0.1 mg/l; the location of the sag will remain the same. Once over
the dam, the water quality of Gunpowder Creek is expected to return to
background quality.

Dick peace
March 23, 1982
- page two -
Since additional wasteflow is domestic and adequate disinfection
is being applied, minimal impact should occur to A -II waters which are
located 3.8 miles below the discharge. No evidence exists to suggest
that toxics are a problem below the Granite Falls outfall. The Granite
Falls WWTP is discharging an effluent low in fecal coliform; assuming
treatment plant efficiency remains the same, the proposed increase should
not affect the use of Lake Hickory as a "B" class water.
Anal sis
Self -monitoring data average:: from May through September, 1981
indicates that the Granite Falls effluent has the following relevant
characteristics:
Flow - 0.232 b1GD
BOD ® 33.9 mg/1
N11 N - 4.43 mg/l
D.F . m 2.44 rig/l
ODultimate - BOD5+4(NH3-N)-51.6 mg/l
Using the above parameters, the model was run at 7Q10 summer conditions
for the present flow as well as the proposed expansion of 39,600 gpd.
The same treatment efficiency was ass ed in both cases.
The results of the model indicate that a D.Q. minimum of 6.8 mg/l is
expected 0.4 miles below the discharge giver: present plant flow. The
addition of 0.0369 MCD is predicted to lower the D.O. sag to 6.7 mg/l;
however, the location of the sag will not be affected. With or without
the requested addition, the water quality of Gunpowder Creek is expected
to return to background conditions with 0.6 miles of the outfall.
Self -monitoring data tend to support the above predictions (Table A).
Self --monitoring reports analyzed from February, 1981, through January,
1982, indicate that the lowest monthly D.O. average downstream of the
discharge was 7.3 mg/1; the lowest D.O. value recorded during this time
was 6.0 mg/l. However, the downstream monitoring station is located
only 500 ft. below the discharge. The model predicts the DO sag to occur
0.4 miles below the outfall. Thus, the actual DO minimums may be
somewhat lower than self -monitoring reports indicate.
Since the requested additions will be domestic, the additional
wasteflow should have: minimal impact upon the drinking water supplies of
Lake Hickory. self -monitoring data, analyzed over the past year, indicate
that fecal coliform numbers below the outfall stayed below 200/100 ml
during the summer months (Table A). Assuming the Granite Falls WWTP

Dick Peace
March 23, 1982
-- page three -
maintains its present disinfection capabilities, the reque3ted addition
should not have adverse impact upon Lake Hickory as "B" class water.
If you have any further questions, please call.
FRW:cs
Attachments
cc: Arthur ouberry

Table A
Granite Falls WWTP
Self -Monitoring Data
Fecal Coliform
DO Upstream DO Downstream Fecal Coliform Fecal Coliform Downstream
Month Year (mg/1) mg/l Effluent Upstream
0/100 ml) (#/100 ml) 0/100 ml)
Jan.
82
12.2
12. 1
< 4
80
< 4
Dec.
81
12.3
12.1
< 4
10
< 4
Nov.
81
10.3
10.1
60
10
195
Oct.
81
9.0
8.7
70
100
130
Sept.
81
7.5
7.0
70
100
130
Aug.
81
7.5
7.4
<2
230
80
July
81
7.3
7.2
<10
320
110
June
81
7.9
7.6
< 4
320
48
May
81
7.5
7.3
220
280
110
April
81
8.1
7.9
40
200
10
March
81
9.4
9.1
540
< 50
<50
February
81
12.0
10.9
1000
385
800