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HomeMy WebLinkAboutNC0021890_Instream Assessment_19820323DIVISION 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