Consultant schedules tracer study at Bass Lake clearwell to measure baffling factor and CT

Guy (presenter) told State Water Resources Control Board staff and local plant operators that a modified step‑dose tracer study will be used to measure the baffling factor and the T10 contact time for the Bass Lake clearwell and that the field test is planned for next week.

Guy said the study will use a modified step‑dose tracer method that he described as faster than full step‑dose or slug‑dose tests and that the result will be combined with the tank’s hydraulic residence time to compute a daily T10 value used for plant CT calculations. “This is what we call water age distribution curve,” Guy said when introducing the method, and he explained that EPA practice uses the T10 (the time by which 10% of the exiting water has an age equal to or less than that value) as the basis for ensuring disinfectant exposure.

The presentation laid out specific, test‑day procedures for Bass Lake: the team will operate the plant at a low flow of 300 gallons per minute, hold the clearwell level at 19 feet, and run the tracer test for one hour and 52 minutes. Based on the tank geometry and that level, Guy calculated the clearwell volume as 62,700 gallons and a hydraulic residence time of 209 minutes; multiplying the tracer‑study baffling factor by that residence time yields the T10 used in CT calculations.

Field logistics and sampling frequency were described in detail. Guy said outlet samples will be collected at three‑minute intervals for the first 42 minutes (intended to resolve baffling factors in the 0.0–0.2 range) and at two‑minute intervals for the remainder of the run (to resolve baffling factors in the 0.2–0.5 range). The team will prepare 50 numbered outlet bottles and a set of inlet bottles (bottles 51–60) for the inlet/bucket averages. A bucket will be used at the outlet to produce an averaged sample; Guy instructed staff to rinse bottles, record exact times when intervals are missed, and keep backup timers.

Operational details included tracer dosing and pump timing. Guy said he left 1.5 liters of the sodium‑fluoride tracer at the site; operators will mix that concentrate into roughly 2.5 gallons of water (resulting in about 2.9 gallons of solution) and feed it with a peristaltic metering pump rated at 24 gallons per day (about 63 mL per minute). At the planned 300 gpm flow the calculated dose from that pump setting would be about 1.78 milligrams per liter. Guy emphasized verifying the pump rate and the time lag between the pump and the injection point before starting the clock for the test.

Sampling and safety notes included disabling automatic plant features that could interrupt the run (such as automatic chemical feed restart) and running the plant for 30 minutes beforehand to reach hydraulic steady state. Ophelia (staff member) agreed to assist with laboratory instruments and analysis; Guy said he will collect the bottles on his return trip and perform the analysis if needed. Sean (water system staff) confirmed availability windows and operations support; one participant noted staff availability on Tuesday, Wednesday and Thursday.

Guy identified several site‑specific concerns discovered during his preview: the clearwell is an almost 70,000‑gallon bolted steel tank with three baffles; inlet/outlet piping and bolted joints might allow short‑circuiting under the baffles; and the planned temporary gravity‑to‑waste routing required a sampling workaround because a high‑service pump sample tap would be on a dead leg when the system flows to waste. A proposed solution was to route waste via a nearby hydrant so the pump‑station sample tap can be used and staff can collect samples into a bucket.

Next steps agreed in the meeting: Bruce and Ophelia will coordinate with plant staff to schedule the site test for a mutually acceptable day next week (Guy said he would be in the area Monday and available thereafter), operators will set gate valves and tank level (19 feet) before the team arrives, staff will prepare bottles and buckets and verify pump/line transit delays, and Guy will bring standards and collect samples after the run. Guy warned that “there’s always a hiccup” but that the team can usually resolve issues on site.

The discussion covered only test planning and procedures; no formal board action or regulatory decision was taken during the meeting.