Well water in Clark County, Idaho
In Clark County, 2% of 119 tested well samples exceeded the EPA limit for arsenic (10 ug/L), based on public data from USGS Water Quality Portal and Idaho statewide ground water quality monitoring (IDWR EDMS).
1,306 test results across 13 contaminants from at least 55 wells, 1957 to 2022; nitrate has the most at 243. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 330 people in Clark County get their drinking water from a private well, per USGS 2015 county water use. That is about 38% of residents.
Separately, the USGS national arsenic model estimates that 2% of the 330 people on private wells in Clark County draw water above 10 ug/L (the model's range is 1% to 2%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Clark 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 |
|---|---|---|---|---|---|---|
| Arsenic 1979 to 2022 | 119 | 89% | 1.8 | 13.2 | 10 ug/L EPA limit | |
| Fluoride 1957 to 2022 | 147 | 95% | 0.266 | 7.38 | 4 mg/L EPA limit | |
| Nitrate 1957 to 2022 | 243 | 95% | 0.74 | 14.4 | 10 mg/L EPA limit | |
| Copper 1981 to 2018 | 86 | 31% | 0 | 0.012 | 1.3 mg/L EPA limit | |
| Lead 1979 to 2012 | 76 | 26% | 0 | 4.74 | 15 ug/L EPA limit | |
| Nitrite 1991 to 2011 | 15 | 7% | 0 | 0.02 | 1 mg/L EPA limit | |
| E. coli 1990 to 2008 | 15 | 0% | 0 | 0 | 0 MPN/100mL EPA limit | |
| Uranium 1957 to 2022 | 10 | 50% | 0.15 | 3.7 | 30 ug/L EPA limit | |
| Chloride 1957 to 2022 | no limit | 149 | 100% | 6.44 | 330 | 250 mg/L nuisance level |
| Sodium 1957 to 2022 | no limit | 148 | 100% | 10 | 72 | 20 mg/L health guideline |
| Iron 1957 to 2022 | no limit | 146 | 35% | 0 | 1,500 | 300 ug/L nuisance level |
| Manganese 1957 to 2022 | no limit | 136 | 32% | 0 | 209 | 300 ug/L health guideline |
| Radon 1991 to 1995 | no limit | 16 | 100% | 295 | 740 | 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 | 2%100 samples | 0%96 samples | 2%44 samples | n/a |
| Fluoride | 0%67 samples | 4%51 samples | 0%26 samples | n/a |
| Arsenic | 0%53 samples | 4%48 samples | 0%15 samples | n/a |
| Copper | 0%53 samples | 0%18 samples | 0%15 samples | n/a |
| Lead | 0%55 samples | 0%10 samples | 0%11 samples | n/a |
Nitrate, fluoride, arsenic 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 addressWhat this means
The contaminants most often above their limit here are arsenic (2% of 119 samples), fluoride (1% of 147 samples), and nitrate (1% of 243 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 Clark County
- 1Arsenic2% of 119 local samples were above the limit.
- 2Fluoride1% of 147 local samples were above the limit.
- 3Nitrate1% of 243 local samples were above the limit.
- 4Coliform bacteriaRecommended for every private well, every year.
- 5RadonMedian local reading is 295 pCi/L.
- 6ChlorideDetected in 100% of local samples.
Test your Clark County well for arsenic, fluoride and nitrate
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 $209Already 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 Clark County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Clark County
Northern Rocky Mountains Intermontane Basins aquifer system
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
Arsenic
Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
Fluoride
Point-of-use reverse osmosis; Activated alumina; Distillation
Nitrate
Point-of-use reverse osmosis; Anion exchange; 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 arsenic
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For arsenic
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.
Free, no obligation
Get on the list for a free in-home water test in Clark County
No water treatment company serves Clark 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 Clark County, and you get an email naming it first.
Common questions
Is there arsenic in well water in Clark County?
Arsenic was detected in 89% of 119 public well samples in Clark County, and 2% 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 Clark County?
The median radon level across 16 public samples in Clark County was 295 pCi/L, with a high of 740 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Clark 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 Clark 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: 1,055 samples in Clark 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): 251 samples in Clark 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 330 people on private wells in Clark 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.