Inflow and infiltration: why rain ends up in the sanitary sewer
Updated October 30, 2026 · 3-minute read · Old House Sewer Lines
Inflow and infiltration is clean rain and groundwater that gets into a sanitary sewer meant to carry only household waste. Inflow arrives through direct hookups such as roof leaders and sump pumps. Infiltration seeps in through cracks and loose joints.
Westchester County treats too much of this clear water as a legal problem, not a nuisance. The county's sewer law, Chapter 824 of its Laws (the Sewer Act), bars groundwater, storm water and excessive inflow and infiltration from the county trunk sewers, according to a county model agreement. Under § 824.72, as that document quotes it, the threshold is anything above 150 gallons per capita per day for the people a municipality's sewers serve. Penalties are in § 824.77.
Where the water gets in
Rye's city code and the Village of Mamaroneck's sewer chapter both spell out inflow at length. The list runs from roof leaders, sump pumps, cellar drains and foundation drains to yard and area drains, catch basins, cooling towers, manhole covers and cross-connections between storm and sanitary lines (Village of Mamaroneck, § 282-13). Most of those sit on private property. The hookups homeowners are most likely to have are covered in leaders and sump pumps tied into sanitary lines.
Infiltration is slower and harder to spot. The US EPA lists leaky joints that let groundwater in among the ordinary causes of sewer trouble (EPA sanitary sewer overflow questions). Its 2004 report to Congress names faulty joints, root intrusion, failed bedding or backfill, material failure in aging laterals and missing or broken cleanout caps as typical lateral defects (EPA 2004, Appendix L). Old clay sewer pipe is one place to look, since its own industry concedes that joints on very old runs may be below today's standard.
What it means for one house
Those two codes also write the goal into their lateral maintenance standards. A private lateral must show no infiltration and no exfiltration, with tight joints and no breaks, openings or voids (Rye Building Department notice). Both check for this when a house changes hands, through a discharge compliance certificate explained in lateral certificates at sale.
In EPA's assessment, digging out and replacing a bad lateral, in whole or in part, is the usual, proven fix for clear water coming from private property. Whether a town will order that work on a particular house is a question for its building department or public works office.
Background
The county program rests on an Order on Consent with the state Department of Environmental Conservation, entered in 2004 and amended in 2008, that requires the county to reduce inflow and infiltration. Under the county's template agreement with towns on the Long Island Sound side, each was to put a point-of-sale law before its board by July 1, 2016, and to complete a Sewer System Evaluation Study and a CMOM (capacity, management, operation and maintenance) program. The copy on file is unsigned, and which towns signed it is not confirmed. It also refers to the federal case brought by Save the Sound, No. 15-CV-6323 in the Southern District of New York.
Mount Vernon shows how the numbers are applied. Since 2010, county policy has required new development there to offset added flow by removing existing leakage: three gallons out for each gallon added at market-rate units, and one for one at affordable units (Mount Vernon CMOM plan). That flow ends up at the county's Yonkers Joint treatment plant. Yonkers has combined sewers in places, built to carry rain and sewage together; the county reported 13 CSO outfalls and 31 overflow events in 2024 (NYSDEC CSO annual report). The county sewer districts item lists which district serves each town.
Video from Metropolitan Council, an outside channel: Metropolitan Council staff explain how rain and groundwater get into sanitary sewers from homes and why that causes basement backups. Nothing loads from YouTube until you press play.
Key facts
- The county Sewer Act (Chapter 824) bans groundwater, storm water and excessive inflow and infiltration from trunk sewers (Westchester County DEF).
- "Excessive" means more than 150 gallons per capita per day, under § 824.72 as the county quotes it.
- A 2004 state Order on Consent, amended in 2008, obliges the county to cut the extra flow.
- Laterals in Rye and Mamaroneck village must show no infiltration or exfiltration (ecode360).
- Mount Vernon developers must remove 3 gallons of leakage per gallon added at market-rate units (CMOM plan).