Idaho nitrate priority areas
Idaho DEQ has designated 35 nitrate priority areas covering about 1,913,216 acres. Across them, 682 of 4,393 sampled wells (16%) came back above the EPA limit of 10 mg/L.
What these numbers are not
They describe the wells DEQ sampled when drawing each boundary. Being inside an area does not mean a particular well is contaminated, and being outside one does not mean it is clean. Nitrate is colourless and tasteless. The only way to know what is in a well is a laboratory test of that well.
Ranked by share of sampled wells above the EPA limit
| Area | Above limit | Wells sampled | Highest reading | Acres |
|---|---|---|---|---|
| Fort Hall | 58%(10) | 17 | 23.6 mg/L | 17,277 |
| Weiser | 50%(75) | 150 | 60 mg/L | 21,462 |
| Bruneau | 50%(4) | 8 | 92 mg/L | 13,420 |
| Black Cliffs | 50%(14) | 28 | 28.68 mg/L | 1,030 |
| South Fremont County | 46%(6) | 13 | 38 mg/L | 4,964 |
| NE Star | 46%(22) | 47 | 44 mg/L | 3,180 |
| Grand View | 40%(13) | 32 | 110 mg/L | 9,173 |
| Marsing | 40%(14) | 35 | 56 mg/L | 5,994 |
| Mountain Home Afb | 33%(11) | 33 | 27.9 mg/L | 5,983 |
| Mountain Home | 32%(17) | 53 | 40 mg/L | 2,014 |
| Notus | 30%(6) | 20 | 16 mg/L | 4,288 |
| Lapwai Creek | 29%(11) | 37 | 18.8 mg/L | 49,168 |
| Glenns Ferry | 29%(5) | 17 | 73.3 mg/L | 13,398 |
| Parma | 26%(8) | 30 | 16 mg/L | 4,980 |
| Homedale | 25%(10) | 40 | 17.1 mg/L | 8,765 |
| Lindsay Creek | 23%(15) | 65 | 21 mg/L | 26,246 |
| Mink Creek | 23%(8) | 34 | 21 mg/L | 1,576 |
| Clearwater Plateau | 22%(31) | 138 | 52 mg/L | 268,361 |
| Marsh Creek | 20%(81) | 403 | 40 mg/L | 101,345 |
| Lower Payette | 18%(38) | 207 | 61 mg/L | 26,205 |
| Ashton/Drummond | 16%(35) | 209 | 38.3 mg/L | 145,111 |
| SW Jerome County | 16%(5) | 30 | 30 mg/L | 7,901 |
| Bliss | 16%(4) | 24 | 19 mg/L | 6,218 |
| Preston | 15%(13) | 82 | 27.75 mg/L | 94,761 |
| Blackfoot | 13%(3) | 22 | 16 mg/L | 32,620 |
| Malad | 12%(2) | 16 | 11.51 mg/L | 22,379 |
| Ada Canyon | 11%(130) | 1,117 | 38.4 mg/L | 251,883 |
| Grace | 10%(6) | 60 | 42.57 mg/L | 95,693 |
| NE Canyon County | 9%(17) | 176 | 27 mg/L | 18,653 |
| Georgetown_benn | 9%(2) | 22 | 13.3 mg/L | 17,764 |
| N. Pocatello | 8%(2) | 25 | 8.9 mg/L | 5,511 |
| Minidoka | 7%(27) | 347 | 83 mg/L | 145,083 |
| Mud Lake | 5%(5) | 97 | 26 mg/L | 111,709 |
| Emmett North Bench | 5%(2) | 40 | 21 mg/L | 5,414 |
| Twin Falls | 4%(30) | 719 | 41 mg/L | 363,687 |
Idaho Department of Environmental Quality, published through IDWR, retrieved 2026-10-19. "Above limit" is the share of wells DEQ sampled in that area reading over 10 mg/L, with the count in brackets. Sample sizes differ enormously between areas — read the denominator before comparing two rows.
What a priority area designation means
DEQ designates an area when sampling shows nitrate in groundwater at levels worth tracking. The designation is about the aquifer, not about any address inside it, and the boundaries are drawn from the samples the agency happens to hold.
Nitrate in Idaho groundwater is generally associated with land use at the surface — irrigated farmland, dairies, septic systems and fertiliser. That is why the designated areas sit over the Treasure Valley and Magic Valley, which is also where most Idaho acreage changes hands.
Why the spread between areas is so wide
Fort Hall has the highest share above the limit at 58% of 17 wells. Twin Falls is the largest by area at 363,687 acres with 719 wells sampled. Those are different kinds of problem, and a single ranking flattens them.
Sample size is the thing to watch. An area with 8 sampled wells and one bad reading reports 13% above the limit; an area with 1,117 wells reports a far more reliable figure. Every row above shows its denominator for exactly this reason.
What to do if a parcel is inside one
- Test before you buy, and test the well you will actually drink from.
- Use a laboratory certified for drinking water analysis. Nitrate is a standard, cheap test.
- Ask the seller for any previous results, and note the date — a reading from 2012 says little about today.
- If you are buying raw land, ask a driller whether depth changes exposure here. Deeper water is often better protected from surface sources, but that is a question for someone who knows the local geology, not an assumption.
Nitrate is treatable. Reverse osmosis, ion exchange and distillation all address it, and which is appropriate depends on the reading and the household. A water treatment professional and your DEQ regional office are the right people to ask.
Check a specific location
The free lookup shows whether a point falls inside a designated area, alongside the well records nearby.
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