20160216 Ver 1_WQ Monitor Rept-1 July 2018_20180717

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ENVIRONMENTAL CONSULTING

Environmental Field Studies ■ Wetland & Stream Delineation ■ 404-401 Permits ■ Mitigation Plans & Banking
18 July 2018

Ms. Chonticha McDaniel
NC Division of Water Resources
401 Wetlands & Buffer Permitting
1617 Mail Service Center
Raleigh, NC 27699-1617

Subject: Hope Mills Dam Water Quality Monitoring, May to July 2018
USACE Action ID # SAW-2010-01336 DWR Project # 16-0216

Dear Ms. McDaniel,

Hope Mills Lake in Cumberland County was re-filled on 29 January 2018, nearly eight years after the
lake was drained in 2010 due to structural failure of the spillway. The repaired dam employs a
concrete labyrinth spillway with two labyrinth cycles built at 104-ft crest elevation for passing low to
moderate flows, and three additional cycles built at 105-ft crest elevation for passing higher flows.
Outflow from the lake cascades over the spillway onto a concrete slab 18 feet below the low-flow crest
(elev = 86 ft), then drops another 8 ft over two 4-ft tall steps into Little Rockfish Creek below the dam.
Normal baseflow elevation in the creek is 79 to 80 ft, so the second step is partially submerged most of
the time.

NC-DWR expressed concern during the Section 404-401 permit review that water released from the
lake may be excessively warm or low in dissolved oxygen (D.O.) during the summer and fall season,
and may adversely affect aquatic life. The Section 404 Permit issued by the US Army Corps of Engineers
(ACE) and Section 401 Certification issued by NC Division of Water Resources (DWR) in August 2016 for
the dam repair project included several Special Conditions for compliance, including a water quality
monitoring plan to assess the impacts of the lake on temperature and dissolved oxygen concentration
in Little Rockfish Creek below the dam. This report presents the first two months of monitoring data,
from May 7 to July 16, 2018.

METHODS: In accordance with the water quality monitoring plan approved by DWR in December
2017, Mogensen Mitigation Inc (MMI) with assistance from Hope Mills Public Works staff installed an
Onset Hobo dissolved oxygen logger (model U-26) and a pair of pressure loggers (model U-20L) just
downstream of the spillway on May 7, 2018. The D.O. logger and one pressure logger were mounted
inside a 3-inch diameter perforated PVC pipe strapped vertically to an I-beam in the middle of the
channel, 180 feet downstream of the spillway steps. The logger sensors are positioned a few inches
above the stream bed to minimize interference from sediment. The second pressure sensor (for
ambient air pressure) is mounted on a nearby tree outside the stream channel. To assess whether any
observed low D.O. events are short-term depressions or continuous multi-day events, the sampling
interval on each logger was set for 4 hours.
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Logger data is periodically downloaded on site using an Onset Hobo Waterproof Shuttle (model U-
DTW-1) and processed with Onset’s HoboWare-Pro software. Monitoring will occur from May through
September 2018. If no significant water quality violations occur during the first year, then further
monitoring in subsequent years may not be necessary, pending DWR review and approval. If low D.O.
appears to be a significant recurring problem, monitoring will be continued next year and the Town
will consult with DWR regarding dam operation strategies to improve downstream conditions.

DATA SUMMARY & CONCLUSIONS: MAY - JULY 2018

From May 7 (loggers deployed) until July 1, dissolved oxygen in Little Rockfish Creek below Hope Mills
Dam generally remained in the 7.0 to 8.5 mg/L range. From July 1 to July 10 it was generally in the 5.0
to 7.0 mg/L range, and from July 10 to July 14 it was generally in the 2.0 to 4.5 mg/L range. The NC
Water Quality Standard for aquatic life in Class C non-trout streams is 5.0 mg/L daily average
(minimum) and 4.0 mg/L instantaneous minimum (15A NCAC 02B.0211). This standard was not met
for four consecutive days, based on the data collected. From July 15 to July 16 (final two days before
the loggers were downloaded) the dissolved oxygen concentration was back up in the 4.8 to 6.1 mg/L
range, with daily averages greater than 5.0 mg/L. A graph of the D.O. data is provided below, and the
full data set in an Excel file is provided by email.

Given the large quantity of decaying organic matter (BOD) in a newly filled lake, summer oxygen
depletion is not surprising. The dissolved oxygen drop in the creek began one day after a high flow
event (July 8 to 9), and may have been caused by oxygen-depleted bottom water (hypolimnion)
pushed to the surface by high inflow into the lake. This effect may subside over time as the bulk of
the initial BOD load is oxidized, provided that annual BOD loading to the lake is not excessive. Another
possible explanation is that a clump of algae, leaves, or other material may have temporarily lodged on
the U-26 sensor, causing localized oxygen depletion at the sensor that is not representative of stream
conditions. No obvious blockage was apparent during the July 16 data download, but the sensor did
have some loosely attached slime that we rinsed off. The next data download will occur in late August
or early September. If you have any questions, feel free to contact me.

Sincerely,

Gerald Pottern
Mogensen Mitigation, Inc
919-556-8845 [e-mail]

CC:
Don Sisko, Hope Mills Public Works, Deputy Director
Emily Greer, US-ACE Wilmington Regulatory Field Office

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Onset Hobo Dissolved Oxygen data logger U-26, data from May 7 to July 16, 2018.

The complete Excel data file containing stream depth, temperature, and dissolved oxygen data was
submitted by email.