August's rain surplus tests a sewer system already behind schedule

The early-summer drought is finished.¹ Severe drought covered as much as all of Kings County from mid-June through early August, and by the week of August 25 to 31 the county had none of any drought category, not even the abnormally dry tier.¹ The sharp reversal is supported by an August running at roughly double normal rainfall: the Central Park gauge has logged 8.5 inches this month against a normal of 4.08 inches for these calendar days, a surplus of 4.4 inches.²

Where that water went matters as much as how much of it fell. Much of Brooklyn and Queens drains through a combined sewer system, a single pipe carrying street runoff, household sewage and industrial wastewater to one treatment plant. The design includes a relief valve. When incoming flow exceeds what the pipe and the plant can handle, the excess discharges untreated through permitted outfalls directly into the receiving water. That threshold is crossed by rainfall intensity rather than by monthly totals: an inch spread across a day mostly gets treated, an inch in an hour does not. Storm-by-storm rainfall figures for August aren't in hand, so how much of the 4.4-inch surplus arrived in bursts is an open question, but a month running more than double normal in a place where most of the ground is paved is the shape of month that tests the relief valve.

The infrastructure on the receiving end of that test is already behind. NYCDEP's 26th Ward Water Pollution Control Plant, which discharges to Jamaica Bay about five miles from here, is in significant non-compliance with the EPA for missing a deadline in its compliance schedule by more than 90 days. The city's municipal storm sewer permit is separately in non-compliance for late progress reporting on its own schedule. Neither record documents an overflow event this month; what they document is that two pieces of the stormwater system are formally overdue. Jamaica Bay's western and eastern segments, meanwhile, are already on the state's impaired waters list and still awaiting a cleanup plan.³

NYC H2O teaches local water ecology across the city, and is a good organization to learn from for anyone who wants to understand the health of these marine ecosystems — and where the rain on their block ends up.