Storm-Driven Turbidity Spike at a Single-Source Surface Water Plant

The controlling objective is to keep settled water stable so the filters can hold combined filter effluent under 0.3 NTU; everything else is secondary.

1. Jar-test the current raw water within the hour. Storm flows change coagulant demand in hours, not days. Reset alum/polymer to the tested optimum and confirm with a settled-water turbidity check two hours later rather than waiting for the next shift.
2. Increase monitoring frequency: raw turbidity, color, pH, and alkalinity every 2 hours; settled and filtered turbidity every 2–4 hours; particle counts if you have them. The alkalinity drop from 55 to 32 mg/L means the same alum dose now drives pH lower and floc becomes smaller and weaker.
3. Bring all four filters online and throttle to lower the loading rate before turbidity reaches the filters; a lower rate buys run time and protects effluent quality.
4. Start the emergency well to shave surface production. At 2.2 MGD average demand, 0.8 MGD from the well leaves 1.4 MGD on the reservoir. First confirm well chlorine residual and collect a coliform sample; expect manganese near 0.25 mg/L, an aesthetic issue, not a reason to refuse the water.
5. Protect storage and prepare notifications. 1.5 MG is roughly 16 hours of average demand, so do not draw it down unnecessarily. Pre-notify MassDEP and draft public notice language now, not after a turbidity exceedance.

Exceeding the 25 NTU design basis is a warning, not an automatic stop point - conventional plants routinely treat above design for short periods if coagulation is reset correctly. I would stop or curtail when treatment can no longer reliably produce compliant water, specifically:

• Jar tests can no longer produce settled water low enough to keep filters under 0.3 NTU, or filtered turbidity exceeds 0.3 NTU on consecutive readings with no filter-to-waste available.
• Filter run times collapse to the point that backwash water or rate limits prevent maintaining production.
• Raw Cryptosporidium risk rises sharply after the storm (beaver pond and disturbed watershed upstream), especially if the state's monitoring triggers additional treatment requirements the plant cannot meet.
• The intake itself plugs with debris.

The capacity arithmetic limits the option. The well supplies only 0.8 MGD - about 36 percent of average day demand and 25 percent of peak day - and the 1.5 MG of storage is about 16 hours at average demand. A full switch therefore means mandatory conservation, suspension of hydrant flushing and nonessential uses, and possibly an interconnection or hauling, not just opening a valve. I would also weigh manganese and any well-specific water quality issues against the microbial risk of continuing on storm-impacted surface water; microbial risk normally wins.

The timber harvest is the immediate suspect and the easiest to control. I would inspect the silt fence and stormwater controls on the 60-acre cut, document any sediment reaching tributaries, confirm a state-approved forest cutting plan is in place, and report deficiencies to the state forestry and environmental agencies. Sediment delivery from an active cut can persist for months after the rain stops, so this is not a one-time inspection.

The beaver pond matters because beaver are competent hosts for Giardia and Cryptosporidium. A pond 1,500 feet above the intake, flushed by the same storm, is a plausible pathway for protozoa that chlorination alone does not reliably inactivate. That argues for increased protozoan monitoring, considering a deeper or alternate intake withdrawal, and evaluating whether the pond can be managed under the appropriate state wildlife permits.

Finally, I would update the source water assessment and the emergency response plan with what was learned: the actual turbidity response curve, coagulant demand after storm events, the practical production limit of the well, and the timeline for conservation measures. The next storm should be met with a written trigger table rather than ad hoc decisions.