Well water in Ada County, Idaho

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Testing strongly recommended

tested well samples in Ada County exceeded the EPA limit for total coliform (0 MPN/100mL), based on public data from USGS Water Quality Portal and Idaho statewide ground water quality monitoring (IDWR EDMS).

17,749 test results across 14 contaminants from at least 1,306 wells, 1947 to 2025; nitrate has the most at 4,324. Data refreshed September 15, 2026.

Compiled by Nick at Well Water Nerd from public records. How these pages are built

An estimated 41,778 people in Ada County get their drinking water from a private well, per USGS 2015 county water use. That is about 10% of residents.

Separately, the USGS national arsenic model estimates that 4% of the 41,778 people on private wells in Ada County draw water above 10 ug/L (the model's range is 3% to 4%), from geology and well depth rather than local tests.

What individual samples found

Each row summarizes every public sample for that contaminant inside Ada County. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.

Contaminant	Above limit	Samples	Detected	Median	Highest	Limit
Total coliform 2010 to 2023	22%	169	22%	0	435	0 MPN/100mL EPA limit
Uranium 1991 to 2025	20%	204	81%	5.3	157	30 ug/L EPA limit
Arsenic 1977 to 2025	15%	1,489	92%	3.6	39	10 ug/L EPA limit
Nitrate 1947 to 2025	10%	4,324	90%	2.5	164	10 mg/L EPA limit
Fluoride 1951 to 2025	2%	1,821	97%	0.4	24	4 mg/L EPA limit
Copper 1981 to 2020	0%	885	50%	0	0.065	1.3 mg/L EPA limit
Lead 1977 to 2012	0%	597	12%	0	6	15 ug/L EPA limit
Nitrite 1991 to 2023	0%	583	5%	0	0.042	1 mg/L EPA limit
E. coli 1990 to 2008	0%	372	0%	0	0	0 MPN/100mL EPA limit
Chloride 1947 to 2025	no limit	2,090	100%	7.47	155	250 mg/L nuisance level
Iron 1951 to 2025	no limit	1,640	49%	0	4,530	300 ug/L nuisance level
Sodium 1947 to 2025	no limit	1,637	100%	31	230	20 mg/L health guideline
Manganese 1959 to 2025	no limit	1,585	44%	0	2,430	300 ug/L health guideline
Radon 1991 to 2001	no limit	353	100%	600	2,800	2,000 pCi/L health guideline

Over time

Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.

Contaminant	before 2000	2000s	2010s	2020s
Nitrate	11%2,892 samples	11%887 samples	8%411 samples	4%134 samples
Fluoride	4%902 samples	<1%547 samples	0%289 samples	0%83 samples
Arsenic	17%620 samples	12%503 samples	15%257 samples	14%109 samples
Copper	0%523 samples	0%142 samples	0%216 samples	n/a
Lead	0%531 samples	0%61 samples	n/a	n/a
Nitrite	0%181 samples	0%195 samples	0%155 samples	0%52 samples

Nitrate, fluoride, arsenic, copper, lead, nitrite have held within a few points between the two most recent periods with enough samples.

Closer than the county

See the sampled wells near your address

The same records, drawn around one point: which wells within a few miles were sampled and what they found, how deep the wells next to you were drilled, and any leaking tank or cleanup site within a mile.

Check my address

What this means

The contaminants most often above their limit here are total coliform (22% of 169 samples), uranium (20% of 204 samples), and arsenic (15% of 1,489 samples). A well next door can differ sharply from yours because bedrock fractures, well depth and plumbing all matter, so these rates describe the area, not your tap.

Chloride is widespread in local groundwater. It is mainly a staining and taste problem, but high manganese also carries a health advisory for infants.

What to test for in Ada County

1
Total coliform
22% of 169 local samples were above the limit.
2
Uranium
20% of 204 local samples were above the limit.
3
Arsenic
15% of 1,489 local samples were above the limit.
4
Nitrate
10% of 4,324 local samples were above the limit.
5
Fluoride
2% of 1,821 local samples were above the limit.
6
Coliform bacteria
Recommended for every private well, every year.
7
Radon
Median local reading is 600 pCi/L.

Test your Ada County well for total coliform, uranium and arsenic

Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.

Order the Tap Score well water test kit, from $209 Already tested? Upload the report

Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.

Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.

Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.

Where to get a well tested in Idaho: certified labs, state programs and prices.

Already have results back? Get your lab report read against the Ada County numbers, $29. Every line explained, ranked by what matters, with the fix for each.

The rock under Ada County

Snake River Plain basin-fill aquifers

Unconsolidated sand and gravel. Wells in sand and gravel are usually shallow and productive, and they recharge quickly from the surface. That makes them the most exposed to nitrate, bacteria and anything spilled nearby, and the least likely to carry arsenic or radon from the rock.

Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the county centroid. A single well can sit in a different unit, especially near a boundary.

Official help in Idaho

Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.

Wells, Idaho DEQ
Drinking Water Testing and Certification, Idaho Department of Health and Welfare

Treatment that fits the local profile

Total coliform

Inspect and repair the well cap and casing, then shock chlorinate and retest; UV disinfection whole-house; Chlorination system

Uranium

Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange

Arsenic

Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange

Nitrate

Point-of-use reverse osmosis; Anion exchange; Distillation

Fluoride

Point-of-use reverse osmosis; Activated alumina; Distillation

Systems we would look at first

Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.

For total coliform

Aquasana Rhino Well Water with UV

whole-house, at the point of entry, about $2,871

Whole-house system built for wells: sediment pre-filter, KDF and carbon, and a UV lamp rated to kill 99.99% of bacteria and viruses. It does not remove arsenic, nitrate, uranium or radon.

For uranium

Under-sink reverse osmosis system

under-sink or countertop, about $300

Look for NSF/ANSI 58 certification listing the contaminant you need removed.

For uranium

Whole-house arsenic and uranium filter

whole-house, at the point of entry, about $2,500

Adsorptive iron-based media; sized to your well yield.

For arsenic

Aquasana SmartFlow reverse osmosis

under-sink or countertop, about $225

Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.

For manganese

Iron and manganese oxidizing filter

whole-house, at the point of entry, about $1,800

Air-injection or greensand media; fixes staining at the source.

Free, no obligation

Get on the list for a free in-home water test in Ada County

No water treatment company serves Ada County through Well Water Nerd yet. We are signing licensed companies on county by county: they test at your tap for free, walk you through the numbers, and quote treatment only if something needs fixing. Leave your details and you are first in line. Nothing is shared until a company joins for Ada County, and you get an email naming it first.

Common questions

Is there arsenic in well water in Ada County?

Arsenic was detected in 92% of 1,489 public well samples in Ada County, and 15% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.

How high is radon in well water in Ada County?

The median radon level across 353 public samples in Ada County was 600 pCi/L, with a high of 2,800 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.

How often should I test a private well in Ada County?

Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.

Counties near Ada County

Closest first. Groundwater does not stop at a line on a map, so the next county over is a fair second opinion.

Want the numbers themselves? Download the Idaho dataset as CSV, every county and town, with the sources named.

Sources

USGS Water Quality Portal: 11,233 samples in Ada County. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
Idaho statewide ground water quality monitoring (IDWR EDMS): 6,516 samples in Ada County. Ground water quality results from the Idaho Department of Water Resources statewide monitoring program and partner agencies (ISDA, IDEQ, INL), EDMS bulk download. Retrieved September 9, 2026.
USGS national arsenic model (Ayotte 2017): estimated share of 41,778 people on private wells in Ada County above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 2026.

Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.