How many Connecticut spills reached groundwater?

Of roughly 47,800 spills reported to the Connecticut Department of Energy and Environmental Protection (CT DEEP) between 2015 and 2025, about 195, approximately 0.4%, were recorded as reaching groundwater. Around 78% were surface releases on the ground, and the remainder reached inside buildings, containment, surface water, groundwater, or air. The single largest category was small vehicle and equipment fluids (motor oil, antifreeze, hydraulic oil), followed by fuel oil and gasoline.

If only 0.4% reached groundwater, is my well water safe?

Not necessarily, because the two questions are different. '0.4% reached groundwater' describes spill events at the time they were reported, and one-off spills are usually not the main threat to a private well. The larger, longer-term risk is a persistent nearby source (a former dry cleaner leaching solvents, or a leaking underground tank) that releases slowly over years. This statistic is a reason not to panic about spills, not a reason to skip testing your well.

What counts as a spill 'reaching groundwater'?

CT DEEP records the medium a spill reached (ground surface, inside a building, surface water, containment, or groundwater) based on conditions observed when the release was reported. A gasoline spill that soaks into soil above the water table may be logged as a ground-surface release even if some fraction later migrates downward. So 'reached groundwater' is a conservative, point-in-time field, not a long-term fate study. That's one reason proximity to a documented source still warrants a private-well test.

Which Connecticut towns had the most spills that reached groundwater?

The counts are small everywhere. The towns with the most spills recorded reaching groundwater over 2015 to 2025 were Danbury (7), Hartford (6), and Southington, Groton, Bloomfield, and Westport (5 each), a handful of events each, over a decade. Because the numbers are so low, these should be read as 'documented instances,' not as a ranking of well-water safety.

CT Spills That Reached Groundwater: The 0.4%

Connecticut logged about 47,800 spills from 2015 to 2025. Roughly 78% were surface releases on the ground, and only 195, about 0.4%, were recorded reaching groundwater. That's reassuring about spills. It is not a clean bill of health for any well.

● Key Takeaways

Of ~47,800 spills reported to CT DEEP (2015 to 2025), about 195 (0.4%) were recorded reaching groundwater; ~78% were surface releases that are typically cleaned up. The largest categories were everyday vehicle and equipment fluids (~48%) and fuel or gasoline (~37%), not exotic chemicals. That's a real reason not to panic about spills. But "recorded reaching groundwater" is a point-in-time field. The bigger long-term risk to a private well isn't a one-off spill. It's a persistent source nearby: an old dry cleaner, or a leaking tank. If you're on a private well, that's the reason to test.

The Short Answer: About 195 Out of 47,800

Connecticut requires that spills of oil, chemicals, and other regulated materials be reported to the Department of Energy and Environmental Protection (CT DEEP). Those reports pile up fast: roughly 47,800 of them over the decade from 2015 to 2025, an average of more than a dozen a day.

When you sort those reports by the environmental medium the release reached, the picture is not what most people expect:

Where the spill was recorded reaching	Share of reports
Ground surface (soil, pavement, roadway)	~78%
Inside a building / containment	~9%
Surface water (streams, ponds, Long Island Sound)	~3%
Groundwater	~0.4% (about 195 reports)
Air / other / mixed	remainder

The headline is the last row. Across a decade and nearly forty-eight thousand reports, about 195 spills were recorded as reaching groundwater, the layer that private wells draw from. That is a genuinely reassuring number, and it deserves to be said plainly: the typical Connecticut spill is a small, surface-level release that gets cleaned up.

What Actually Spills in Connecticut

The substances reinforce the point. These are not, for the most part, industrial poisons. Classified from the reported substance, the mix is dominated by everyday fluids:

Vehicle and equipment fluids (~48%): motor oil, antifreeze, hydraulic oil, transformer oil, transmission fluid. The single biggest bucket, mostly small quantities from crashes, equipment failures, and leaks.
Fuel and gasoline (~37%): home heating oil (#2 fuel oil is the most common single substance), gasoline, and diesel.
Everything else (~15%): sewage releases, cleaning chemicals and acids, and a small tail that includes metals like mercury and materials like asbestos.

Even among the substances most able to move through soil (gasoline, diesel, fuel oil), the number recorded reaching groundwater is small. Of more than 8,000 home-heating-oil (#2 fuel oil) spills, 29 were recorded reaching groundwater. Of ~4,500 gasoline spills, about 20.

Why "0.4%" Is Reassuring but Not the Whole Story

Here's the caveat, and it matters. "Recorded reaching groundwater" is a point-in-time field. It reflects what responders observed when the spill was reported, not a long-term study of where every molecule ended up. A gasoline release that soaks into soil above the water table can be logged as a ground-surface spill even if a fraction later migrates down. So treat 0.4% as a floor, not a guarantee.

More importantly, one-off spills are usually not the main threat to a private well. The releases that actually put contaminants into groundwater over time tend to be persistent sources: a dry cleaner that used the solvent PCE for decades, or an underground tank that leaked slowly for years before anyone noticed. Those don't show up as a single dramatic "spill." They show up as a plume. We looked hard at whether Connecticut's spill counts predict what's actually measured in groundwater, and the answer is clarifying. The number of spills in a town does not predict what's in its water, but the presence of certain long-standing sources does.

So the reassurance is real and the caution is real, and they're about different things:

Don't panic about spills. The vast majority are small surface releases that were cleaned up. A spill on a road three towns over is not in your water.
Do test your private well, not because of one spill, but because Connecticut does not test private wells for you, and a persistent nearby source is the risk a single statistic can't rule out.

Reading this inside ChatGPT or Claude?

This page can tell you the general science, but not what is actually in spills and groundwater near a Connecticut address. That depends on your exact address. You can get your specific answer two ways:

Inside the chat: ask your assistant to “check my tap water with CheckYourTap”. Our connector returns your ZIP code’s measured contaminant levels, including the derived dog and cat safe levels, and, only if you ask it to, can email you the report or arrange a specialist callback.
On the web: open CheckYourTap.com and enter your ZIP code for a free 30-second report.

Which Towns, and Why the Numbers Are So Small

Because only ~195 spills were recorded reaching groundwater statewide, no town has more than a handful. Over the full decade, the most were Danbury (7), Hartford (6), and Southington, Groton, Bloomfield, and Westport (5 each). These are counts of documented instances over ten years, not a safety ranking. A town with six recorded groundwater-reaching spills is not meaningfully "worse" than one with four. Larger and more industrial towns also simply generate and report more spills of every kind, which is why raw counts track activity more than water quality.

If you want to understand a specific place, the useful question isn't "how many spills were reported here." It's "what persistent sources are documented nearby, and am I on a well that could draw from that groundwater?"

What To Do With This

If you're on public water, a nearby spill is almost never a drinking-water concern: your utility treats and monitors the supply. If you're on a private well, use this the right way: don't lose sleep over the spill map, but do test your well, because no one does it for you. Test for the basics (bacteria, nitrate, arsenic), and if you're near a documented long-standing source like a former dry cleaner or a known leaking tank, add the matching contaminants (solvents, or fuel-related VOCs) to the list.

Keep Reading

Sources: Connecticut Department of Energy and Environmental Protection (CT DEEP) spill reporting records, 2015 to 2025 (analyzed by CheckYourTap); U.S. EPA, Private Drinking Water Wells; Connecticut Department of Public Health, Drinking Water Section. Spill medium and substance categories are as reported to CT DEEP at the time of each release; "reached groundwater" is a point-in-time field, not a long-term fate determination.

How Many of Connecticut's 48,000 Reported Spills Actually Reached Groundwater?