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Connecticut water profile

Nitrate in Connecticut community water systems

Connecticut reports few public systems with elevated nitrate — most of the state drinks reservoir water — but the systems that do show up sit in the developed river valleys. See the pattern and what each reporting system measured in the three years from August 9, 2023 to August 9, 2026.

Browse all 61 Connecticut systems →

Search by the utility or system name shown on a water bill. Example: Connecticut Water Company. Private wells are not in this inventory.

13%
of the state’s 475 active community water systems have a current result here
61
systems with a current nitrate result
2,302,273
people served by those reporting systems
1.6%
of reporting systems at or above 5 mg/L
How to read these numbers Each system is placed into a band by its most recent reported result within a rolling three-year window. The bands describe how strongly human activity, read from nitrate, has marked a water source; they are not a safety verdict. Figures describe system reporting, not nitrate measured at an individual tap.

How Connecticut's systems fall across the five bands

Most systems report below 3 mg/L

Natural background
0.00–0.99 mg/L as N
37 systems · 1,016,977 served
Human impact likely
1.00–2.99 mg/L as N
9 systems · 518,405 served
Human impact clear
3.00–4.99 mg/L as N
14 systems · 748,347 served
Health-study range
5.00–9.99 mg/L as N
1 system · 18,544 served
Above the federal limit
10.00 mg/L and above
0 systems · 0 served

Each system appears in one band only, placed by its most recent reported result in the current three-year window. Counts sit beside people served so a small number of larger systems cannot hide behind a row of tiny ones. The one system reporting 5 mg/L or more did not stay there across its whole three-year range — it reached 5 only on a high sample and otherwise ran lower.

The named systems

Every reading here belongs to a utility with a name

Highest recent readings
SystemMost recent resultBandBased onPeople served
Hazardville Water Company9.60 mg/La high point — runs lowerHealth-study rangeCCR summary, unverified18,544
Meriden Water Division4.61 mg/La high point — runs lowerHuman impact clearCCR summary, unverified58,441
Ctwc - Northern Reg-Western System4.47 mg/La high point — runs lowerHuman impact clearCCR summary, unverified101,313
Hazardville - Rye Hill System4.47 mg/La high point — runs lowerHuman impact clearinherited from supplier352
Tolland Water Dept - Torry Road4.47 mg/La high point — runs lowerHuman impact clearinherited from supplier204
Manchester Water Department4.20 mg/La high point — runs lowerHuman impact clearCCR summary, unverified51,198
Ctwc - Northern Reg-Reservoir Heights4.20 mg/La high point — runs lowerHuman impact clearinherited from supplier62
Southington Water Department4.14 mg/LHuman impact clearCCR summary, unverified43,069
Largest systems
SystemMost recent resultBandBased onPeople served
Regional Water Authority3.82 mg/La high point — runs lowerHuman impact clearCCR summary, unverified418,900
Metropolitan District Commission0.05 mg/LNatural backgroundCCR summary, unverified390,887
Aquarion-Eastern Fairfield County2.08 mg/LHuman impact likelyCCR summary, unverified351,756
Aquarion-Stamford0.50 mg/LNatural backgroundCCR summary, unverified119,214
Waterbury Water Department0.03 mg/LNatural backgroundCCR summary, unverified107,271

Each result shows its date and what it rests on, so a lone sample is never taken for a settled measurement. A single high sample is not a complete account of what a household drinks.

Where higher nitrate concentrates

A small valley signal

Human impact is clear on drinking water served in four of Connecticut’s 8 counties. Each is shaded by how many of its systems reported 3.00 mg/L as N or above. Shading counts systems, not severity — a county with many small utilities can outrank one holding a single larger system that reads higher. The table beside the map splits the two bands so that difference stays visible.

CT counties shaded by number of community water systems reporting nitrate at or above 3.00 mg/L as NCounties are shaded by how many community water systems reported nitrate at or above 3.00 mg/L as N in the three years from August 9, 2023 to August 9, 2026. The same counts appear in the table below this map.New Haven County — 5 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveFairfield County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveTolland County — 3 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveHartford County — 5 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or above
none1–23–56–1112–2526+Number of drinking water systems reporting ≥3.00 mg/L NO₃–N per county as of August 9, 2026

Hartford leads this layer with 1 reporting system. A small valley signal.

Counties with the most systems at or above 3 mg/L as N
County3–4.99≥5Total ≥3
Hartford516
New Haven505
Tolland303
Fairfield101

Counts use each system's most recent reported result in the current three-year window. A system appears in only one of the two bands.

Find a specific Connecticut system

What the statewide pattern says

Reservoirs for the cities, wells for the rest

Connecticut's modest nitrate signal sits in its stratified-drift river valleys — the setting its small groundwater systems and its large private-well population share.

Connecticut's big cities drink from upland reservoirs; that accounts for part of the small inventory here — about 60 public systems reporting groundwater nitrate, the smallest in this series — and the reach of the source records accounts for the rest. The mid-band systems that appear — Hartford, New Haven and Tolland counties — sit on stratified-drift valley aquifers under dense development.

The regional science sets the baseline: Connecticut lies inside the northern glacial aquifer system, whose private-well nitrate median runs about 0.11 mg/L — among the lowest measured nationally — but whose rare high wells concentrate where oxidizing shallow sand meets nitrogen sources (USGS SIR 2010-5100). USGS estimated high nitrate in about 4 percent of the glacial system's area (Erickson and others, 2019).

Connecticut has no county-level nitrate study this page can cite, so the reading stops at the setting: valley-fill sand and gravel under suburbs and legacy septic, with the state's several hundred thousand private bedrock and valley wells carrying the untested share.

The supported source reading is mixed suburban nitrogen on stratified-drift valley aquifers — not a certified source at any individual system (Nolan and Hitt, 2006).

The general science — how nitrate moves, why some ground is vulnerable, and why the legacy lingers — is laid out with its sources on Where nitrate comes from.

Questions this page answers

What the Connecticut pattern does—and does not—say

Where does nitrate concentrate in Connecticut community water systems?

Hartford leads this layer with 1 reporting system. A small valley signal.

Why does Connecticut have so few systems on this map?

Two things at once: the large cities draw from upland reservoirs, and this series holds current results for only 61 of Connecticut's 475 community systems. The ones that appear sit on stratified-drift valley aquifers under dense development.

Should a Connecticut private well be tested for nitrate?

Yes, especially on valley-fill sand and gravel or near legacy septic. The glacial system's average is low, but its rare high wells sit exactly in that oxidizing shallow setting — and Connecticut wells can also carry arsenic and uranium, arguing for a broad certified-lab panel.

Does a high public-system result mean a nearby private well is high?

No. It is a local sentinel, not a well-specific result. It shows that nitrogen and a transport pathway have reached groundwater in the area; a certified-lab test is still the only way to know a specific private well.

Sources and provenance

What this page rests on

  1. NitrateSmart inventory: 61 Connecticut community water systems with a current nitrate result in the three years from August 9, 2023 to August 9, 2026. Figures count people served by reporting systems, not measured tap exposure.
  2. Nolan BT, Hitt KJ (2006). Vulnerability of Shallow Groundwater and Drinking-Water Wells to Nitrate in the United States. Environmental Science & Technology 40(24):7834–7840.
  3. Dubrovsky NM, and others (2010). Nutrients in the Nation's Streams and Groundwater, 1992–2004. USGS Circular 1350.
  4. USGS (2010). Relations That Affect the Probability and Prediction of Nitrate Contamination in the Glacial Aquifer System. SIR 2010-5100 — glacial-system private-well median 0.11 mg/L as N.
  5. Erickson ML, and others (2019). Drinking water quality in the glacial aquifer system, northern USA. Science of the Total Environment — high nitrate in ~4% of the glacial system area.