20160216 Ver 1_Year 2 Monitoring Data_20190709

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MOGENSEN MITIGATION INC.
ENVIRONMENTAL CONSULTING

Environmental Field Studies ■ Wetland & Stream Delineation ■ 404-401 Permits ■ Mitigation Plans & Banking
03 July 2019

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, April to July 2019
USACE Action ID # SAW-2010-01336 DWR Project # 16-0216

Dear Ms. McDaniel,

This report summarizes the first 10 weeks of dissolved oxygen and temperature monitoring data for
Year-2 (Apr 18 to July 01, 2019) in Little Rockfish Creek below Hope Mills Dam.

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 s tep 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.

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
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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.

Logger data is periodically downloaded using an Onset Hobo Waterproof Shuttle (model U-DTW-1) and
processed with Onset’s HoboWare-Pro software. DWR requested monitoring from May through
September. Year-1 monitoring (2018) found two periods of low D.O. during mid-July and early
September, when D.O. concentrations were below the NC Water Quality standard for aquatic life in
Class C non-trout streams (5.0 mg/L daily average minimum and 4.0 mg/L instantaneous minimum
(15A NCAC 02B.0211). In response to the findings of the 2018 report, DWQ requested that monitoring
continue in 2019. The loggers were removed in November 2018 and re-deployed with a new D.O.
sensor cap in April 2019.

DATA SUMMARY & CONCLUSIONS: From Apr 18 (loggers deployed) until July 1, the dissolved oxygen
(DO) concentration in Little Rockfish Creek below Hope Mills Dam remained between 9.6 and 6.6 mg/L,
and the maximum water temperature recorded was 83 F. There were no D.O. measurements below
the NC Water Quality standard for aquatic life .

For most of this period, the bottom drain gate was partially open to lower the lake level about 6 feet
for construction of a kayak launch and bulkhead in the swimming area on the western shore of the
lake. At elevation 98 feet, the lake volume was roughly one-third of its full pool volume at elevation
104 feet. The hydraulic residence time was thus reduced by a factor of 3x, and the potential for
thermal stratification further reduced by flow release from the bottom gate rather than over the
spillway. Construction of lakeshore amenities was completed at the end of June and the lake is
expected to refill during July.

A graph of dissolved oxygen and temperature data is provided below. A spreadsheet with dissolved
oxygen, temperature, and stream depth data was submitted concurrently by email. A final water
quality monitoring report for 2019 summarizing data from April through September will be submitted
in October.

Sincerely,

Gerald Pottern
Mogensen Mitigation, Inc
MMI-Raleigh Office
104 East Chestnut Ave
Wake Forest, NC 27587
-----------------------------
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 recorded from April 18 to July 01, 2019.
Black line = Dissolved Oxygen; Blue line = Stream Temperature.