Water Quality Monitoring Plan_revised_137236.docx

Mill Creek Fish Ladder Water Quality Monitoring Plan U.S. Army Corps of Engineers Walla Walla District Aquatics ID# 137236

November 5, 2024

This water quality monitoring plan was prepared to comply with certain Clean Water Act requirements necessitated by the Washington Department of Ecology (Ecology). Ecology will review and approve this plan prior to implementation. Ecology will also review and approve all substantive changes to the plan prior to implementation.

The U.S. Army Corps of Engineers (USACE), Walla Walla District, proposes to construct a new fish ladder at the Mill Creek Diversion Dam on the federally-owned portion of Mill Creek in Walla Walla County, Washington. Construction activities will be conducted in 2025 and 2026, spanning at least two in-water work seasons. All of the in-water work will be conducted between June 15 and October 31 of both years. After placement of the fish ladder cofferdam, which will isolate the work area, work on the ladder would continue outside of the in-water work window. Sections of the channel will first be dewatered so that most of the work can be performed in dry conditions. Water quality monitoring will be conducted during all in-water work. The work would include construction of a new (replacement) stilling basin access ramp on the left bank, just downstream of the Diversion Dam. This ramp would consist of 475 cubic yards of fill material, some of which would be placed below the ordinary high-water line.

Water quality monitoring will be performed by an independent water quality monitoring service, hired by the contractor. The monitoring service must be regularly engaged in the business of water quality sampling and testing and must have a minimum of 3 years of experience in water quality monitoring for the parameters specified using equipment and methods that meet the requirements listed below.

The contractor will be required to prepare a water quality monitoring plan (plan) for USACE approval prior to beginning in-water work. The plan will contain the following information.

1. Name, address, and phone number of company performing the water quality monitoring.

2. Resume for each employee including a detailed description of their qualifications to perform water quality monitoring.

3. List of all equipment and materials to be used.

4. Manufacturer's specifications for each piece of equipment to be used.

5. Calibration history for each piece of equipment.

Water quality monitoring shall be performed for the following parameters:

1. pH

2. turbidity

3. dissolved oxygen

4. temperature

The contractor shall provide multiparameter probes (sondes), as required, with access to a minimum of two spares, meeting the following requirements:

1. A pH sensor accurate to ± 0.2 pH units with a resolution of 0.01 units.

2. A turbidimeter shall be resistant to shock and weather and shall be reliable and accurate under adverse field conditions. The instrument shall use a near-infrared light source with a wavelength between 860 nm and 880 nm. Meter accuracy shall be ±5% of the reading or ±2 NTU (whichever is greater), with a range from 0-1,000 NTU, and a repeatability of ±1% of the reading.

3. A dissolved oxygen sensor accurate to ± 0.2 mg/L.

4. A temperature sensor accurate to ± 0.10 ºC.

5. Internal memory sufficient to store 10 days of sensor readings at 5-minute intervals.

6. Weighted sensor guard sufficient to hold the sonde and cable vertical and protect the sensors from direct contact with objects larger than 3/4-inch diameter.

The contractor shall perform water quality monitoring for the specified parameters throughout the in-water work. Sampling will be conducted at the following locations.

1. 50 to 100 feet upstream from the work area (to obtain background data)

2. approximately 200 feet downstream of the work area (for compliance data) Comment by Tice, Benjamin J CIV USARMY CENWW (USA): This was changed to 200 feet to be consistent with the WQC for flows between 10 and 100 cfs.

During all in-water work, all water quality parameters shall be monitored at 15-minute intervals. Monitoring shall be done during the entire in-water work window (June 15 through October 31) and during elevated turbidity conditions (5 NTU above background) after the work window is passed. This may require continuous monitoring past the end of the normal workday into the evening or next day. Data sondes are to be downloaded daily after completion of the day's work.

The contractor will provide USACE with monthly data summaries and prepare a water quality monitoring report at the conclusion of the action and provide it to USACE.