Milford's plant blows past its new phosphorus limit during drought recovery

On August 8, Milford's wastewater treatment plant, which discharges to the Souhegan River, reported a phosphorus violation of 652 percent over its permitted limit, alongside a repeat copper exceedance the same day. The copper problem is not new; it echoes a violation from late March that was already the facility's second consecutive one. What makes the phosphorus number especially striking is how young that limit is: it comes from a general permit that only took effect this past February, so a blowout of this size arrived within the permit's first six months. EPA currently lists the plant as one of only three facilities in significant non-compliance across this watershed.

The timing sharpens the concern. This stretch of the Souhegan is already on the state's impaired-waters list for nutrients, and a pollution reduction plan for phosphorus loading here exists precisely because this has been a recognized problem before. The violation also landed as Hillsborough County was climbing out of a severe drought, with over 80 percent of the county still in moderate-or-worse drought conditions in late July, easing to roughly a fifth of that by mid-August.

Low summer flows matter here because they mean less river volume to dilute whatever the plant releases. Excess phosphorus in slow, warm water is exactly the kind of load that feeds algal growth and can drive down dissolved oxygen for the fish and mussels downstream. Flow and oxygen readings at the outfall itself were not available, but the conditions on the river this summer were set up for exactly that kind of stress.