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Washington water profile

Nitrate in Washington community water systems

Washington is two nitrate stories: the irrigated Columbia Basin in the east, where roughly a fifth of drinking-water wells have exceeded the federal limit, and the glacial-outwash lowlands of the Puget Sound in the west. See where higher public-system results concentrate and what each reporting system measured in the three years from August 9, 2023 to August 9, 2026.

Browse all 2,141 Washington systems →

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

93%
of the state’s 2,313 active community water systems have a current result here
2,141
systems with a current nitrate result
6,347,891
people served by those reporting systems
5.7%
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 Washington's systems fall across the five bands

Most systems report below 3 mg/L

Natural background
0.00–0.99 mg/L as N
1,307 systems · 2,836,082 served
Human impact likely
1.00–2.99 mg/L as N
509 systems · 2,505,740 served
Human impact clear
3.00–4.99 mg/L as N
202 systems · 839,604 served
Health-study range
5.00–9.99 mg/L as N
113 systems · 158,827 served
Above the federal limit
10.00 mg/L and above
10 systems · 7,638 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. Of the 123 systems reporting 5 mg/L or more, 65 stayed at or above 5 across the whole three-year range; the other 58 reached it only on a high sample and otherwise ran lower.

Most recent reading Jul 2026; the typical system was last tested around Dec 2025. 2,098 systems sampled their own water; 43 systems carry a supplier’s result.

The named systems

Every reading here belongs to a utility with a name

Highest recent readings
SystemMost recent resultBandBased onPeople served
Coxville Water Assn #120.60 mg/LJul 2024Above the federal limit5 samples26
Basin City Water Sewer District16.00 mg/LMar 2026a high point — runs lowerAbove the federal limit10 samples1,740
Delta Water Association14.80 mg/LApr 2026Above the federal limit31 samples450
Cascade Foothills11.80 mg/LFeb 2026a high point — runs lowerAbove the federal limit5 samples61
Sundance Improvement Association11.70 mg/LJun 2026Above the federal limit12 samples100
Waterville Town Of10.80 mg/LNov 2025a high point — runs lowerAbove the federal limit22 samples1,140
Grove Terrace Mhp Two Llc10.50 mg/LJun 2026Above the federal limit19 samples220
Mesa Water Department10.50 mg/LSep 2025a high point — runs lowerAbove the federal limit9 samples495
Largest systems
SystemMost recent resultBandBased onPeople served
Tacoma Water Division City Of2.58 mg/LFeb 2026Human impact likely17 samples446,170
Vancouver City Of4.60 mg/LMay 2026a high point — runs lowerHuman impact clear83 samples373,047
Spokane City Of2.34 mg/LJul 2025Human impact likely3 samples343,167
Bellevue City Of0.25 mg/LOct 2025Natural background6 samples321,349
Kent Water Department0.88 mg/LAug 2025Natural background8 samples166,421

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

Higher results split between the irrigated east and the Puget lowlands

Human impact is clear on drinking water served in 28 of Washington’s 39 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.

WA 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.Pierce County — 21 systems at 3.00–4.99 mg/L as N; 8 systems at 5.00 mg/L as N or aboveAdams County — 7 systems at 3.00–4.99 mg/L as N; 5 systems at 5.00 mg/L as N or abovePend Oreille County — 1 system at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveKing County — 5 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveChelan County — 7 systems at 3.00–4.99 mg/L as N; 3 systems at 5.00 mg/L as N or aboveKittitas County — 1 system at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveGarfield County — 0 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveGrant County — 22 systems at 3.00–4.99 mg/L as N; 16 systems at 5.00 mg/L as N or aboveOkanogan County — 4 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveFranklin County — 2 systems at 3.00–4.99 mg/L as N; 11 systems at 5.00 mg/L as N or aboveClark County — 3 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveWalla Walla County — 3 systems at 3.00–4.99 mg/L as N; 7 systems at 5.00 mg/L as N or aboveCowlitz County — 2 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveLewis County — 3 systems at 3.00–4.99 mg/L as N; 0 systems at 5.00 mg/L as N or aboveClallam County — 3 systems at 3.00–4.99 mg/L as N; 2 systems at 5.00 mg/L as N or aboveLincoln County — 3 systems at 3.00–4.99 mg/L as N; 2 systems at 5.00 mg/L as N or aboveMason County — 4 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveStevens County — 0 systems at 3.00–4.99 mg/L as N; 4 systems at 5.00 mg/L as N or aboveWhatcom County — 8 systems at 3.00–4.99 mg/L as N; 9 systems at 5.00 mg/L as N or aboveDouglas County — 4 systems at 3.00–4.99 mg/L as N; 6 systems at 5.00 mg/L as N or aboveSnohomish County — 10 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveThurston County — 26 systems at 3.00–4.99 mg/L as N; 9 systems at 5.00 mg/L as N or aboveYakima County — 10 systems at 3.00–4.99 mg/L as N; 8 systems at 5.00 mg/L as N or aboveBenton County — 7 systems at 3.00–4.99 mg/L as N; 7 systems at 5.00 mg/L as N or aboveIsland County — 18 systems at 3.00–4.99 mg/L as N; 11 systems at 5.00 mg/L as N or aboveSpokane County — 12 systems at 3.00–4.99 mg/L as N; 9 systems at 5.00 mg/L as N or aboveWhitman County — 2 systems at 3.00–4.99 mg/L as N; 1 system at 5.00 mg/L as N or aboveKitsap County — 8 systems at 3.00–4.99 mg/L as N; 0 systems 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

Grant leads this layer with 16 reporting systems, followed by Franklin, Island, Spokane. Higher results split between the irrigated east and the Puget lowlands. Coverage note: the map places 98% of Washington’s reporting systems. All 43 of the rest buy their water and carry their supplier’s result, which was measured at the supplier’s source — shading their own county would put the reading in the wrong place. They are counted in the figures above.

Counties with the most systems at or above 3 mg/L as N
County3–4.99≥5Total ≥3
Grant221638
Thurston26935
Island181129
Pierce21829
Spokane12921

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. Coverage note: the map places 98% of Washington’s reporting systems. All 43 of the rest buy their water and carry their supplier’s result, which was measured at the supplier’s source — shading their own county would put the reading in the wrong place. They are counted in the figures above.

Find a specific Washington system

What the statewide pattern says

Two Washingtons for nitrate

Washington's higher nitrate splits by geology: fractured-basalt and basin-fill aquifers under irrigation in the Columbia Basin, and permeable glacial outwash under mixed farm and suburban land around Puget Sound.

East of the Cascades, the Columbia Basin is the documented core. The aquifers — layered flood basalt with sediment interbeds — are highly vulnerable: permeable basalt interflow zones and the irrigation-recharge layer move nitrate readily, regionally into deep supply wells. USGS found nitrate above the federal limit in about 20 percent of Columbia Plateau wells (Rupert and others, 2014), and in the Adams–Franklin–Grant Groundwater Management Area, about 20 percent of drinking-water wells have exceeded the limit in a region that draws more than 80 percent of its drinking water from groundwater (USGS Washington Water Science Center).

The source work there is unusually direct: in the GWMA, the strongest correlates of nitrate above 3 mg/L were fertilizer application and well depth, and irrigated agricultural areas carried the highest predicted risk. Trend analysis found concentrations locally elevated but roughly stable to declining in the early 2000s (Frans and Helsel, 2005). The supported source reading in the east is agricultural — irrigated-cropland fertilizer. The Lower Yakima Valley carries a parallel designation, and the Spokane Valley–Rathdrum Prairie aquifer adds a very permeable glacial-outwash setting in the far east.

West of the Cascades the mechanism differs. The Puget Sound lowlands sit on permeable glacial outwash, and USGS susceptibility modeling there tied elevated nitrate to fertilizer use and septic-system density over unconfined outwash (Tesoriero and Voss, 1997) — matching the map's western footprint in Thurston, Pierce and Island counties. The supported western reading is mixed fertilizer and septic on permeable outwash. Neither reading certifies a source at an 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 Washington pattern does—and does not—say

Where does nitrate concentrate in Washington community water systems?

Grant leads this layer with 16 reporting systems, followed by Franklin, Island, Spokane. Higher results split between the irrigated east and the Puget lowlands.

Why does Washington have two different nitrate patterns?

Two geologies. East of the Cascades, layered basalt and basin-fill under heavy irrigation move fertilizer nitrate deep into supply aquifers. West of them, permeable glacial outwash around Puget Sound carries fertilizer and septic nitrogen to shallow wells. Same recipe — nitrogen plus a fast aquifer — different ingredients.

Can this page say the source is agriculture?

In the Columbia Basin, at the zone level, yes — irrigated-cropland fertilizer is the documented dominant signal. In the Puget lowlands the reading is mixed fertilizer and septic. Neither certifies a source at an individual system.

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: 2,141 Washington 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. Rupert MG, Hunt CD, Skinner KD, Frans LM, Mahler BJ (2014). Groundwater quality in the Columbia Plateau and Snake River Plain basin-fill and basaltic-rock aquifers. USGS Circular 1359 — nitrate above the federal limit in about 20% of Columbia Plateau wells.
  5. Frans LM, Helsel DR (2005). Evaluating regional trends in ground-water nitrate concentrations of the Columbia Basin Ground Water Management Area, Washington. USGS Scientific Investigations Report 2005-5078.
  6. USGS Washington Water Science Center. Columbia Basin GWMA — ~20% of drinking-water wells over the limit; more than 80% of the region’s drinking water from groundwater.
  7. Tesoriero AJ, Voss FD (1997). Predicting the probability of elevated nitrate concentrations in the Puget Sound Basin — implications for aquifer susceptibility and vulnerability. Ground Water 35(6):1029–1039.
  8. Washington State Department of Ecology (2016). Washington Nitrate Prioritization Project — statewide inventory of nitrate-vulnerable groundwater areas.