Well water in Gilliam County, Oregon

Name: Private well water quality in Gilliam County, Oregon
Creator: Well Water Nerd
License: Public domain (US federal government data)

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Testing strongly recommended

tested well samples in Gilliam County exceeded the EPA limit for arsenic (10 ug/L), based on public data from USGS Water Quality Portal.

729 test results across 13 contaminants from at least 47 wells, 1983 to 2021; sodium has the most at 148. Data refreshed September 15, 2026.

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

An estimated 526 people in Gilliam County get their drinking water from a private well, per USGS 2015 county water use. That is about 28% of residents.

Separately, the USGS national arsenic model estimates that 3% of the 526 people on private wells in Gilliam County draw water above 10 ug/L (the model's range is 2% 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 Gilliam 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 1989 to 2021	56%	121	100%	10.6	38.8	10 ug/L EPA limit
Lead 1989 to 2016	15%	13	92%	4.9	174	15 ug/L EPA limit
Uranium 2003 to 2021	2%	56	100%	1.05	58.8	30 ug/L EPA limit
Copper 1989 to 2016	0%	68	100%	0.003	0.13	1.3 mg/L EPA limit
Fluoride 1983 to 2016	0%	16	100%	0.5	2.2	4 mg/L EPA limit
Sodium 1983 to 2021	no limit	148	100%	24.4	225	20 mg/L health guideline
Chloride 1983 to 2021	no limit	136	100%	11	330	250 mg/L nuisance level
Iron 1983 to 2016	no limit	82	99%	56.9	88,500	300 ug/L nuisance level
Manganese 1983 to 2021	no limit	74	97%	3.78	5,810	300 ug/L health guideline

Sampled fewer than 10 times, too few to summarize: Nitrate (7), Radium (226 + 228) (4), E. coli (2), Radon (2).

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
Arsenic	60%58 samples	50%48 samples	54%13 samples	n/a
Copper	0%36 samples	0%30 samples	n/a	n/a
Uranium	n/a	2%41 samples	0%13 samples	n/a

Arsenic above the limit in Gilliam County fell from 60% of 58 samples before 2000 to 50% of 48 in the 2000s.

Copper has 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 arsenic (56% of 121 samples), lead (15% of 13 samples), and uranium (2% of 56 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, Iron, and Manganese are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.

The median radon reading (5,965 pCi/L) is above the 2,000 pCi/L level at which treatment is recommended. Radon in water raises indoor air radon when it is used for showers and laundry.

What to test for in Gilliam County

1
Arsenic
56% of 121 local samples were above the limit.
2
Lead
15% of 13 local samples were above the limit.
3
Uranium
2% of 56 local samples were above the limit.
4
Coliform bacteria
Recommended for every private well, every year.
5
Radon
Median local reading is 5,965 pCi/L.
6
Nitrate
Recommended for every private well, especially near farms or septic systems.
7
Copper
Detected in 100% of local samples.

Test your Gilliam County well for arsenic, lead and uranium

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 Oregon: certified labs, state programs and prices.

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

The rock under Gilliam County

Columbia Plateau basaltic-rock aquifers

Igneous and metamorphic-rock. Granite and similar crystalline rock hold water only in fractures. Wells are deep and yields modest, and the rock itself is the source of arsenic, radon and uranium, which is why those three head the test list in bedrock country.

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 Oregon

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.

Domestic Well Safety, Oregon Health Authority
Oregon Environmental Laboratory Accreditation Program, Oregon Health Authority

Contamination sources on record in Gilliam County

Oregon DEQ active tank sites records 2 regulated sites in Gilliam County, among 2 regulated sites in total. Groundwater does not follow property lines, so a well within a few hundred feet of one of these is worth testing for what that site could release.

2: registered underground storage tank sitesRegistered tanks are regulated and inspected; they matter most when they are old or near a shallow well.

Treatment that fits the local profile

Arsenic

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

Lead

Point-of-use reverse osmosis; Certified lead-removal carbon filter; Neutralizing filter to raise pH and stop leaching

Uranium

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

Radon

Aeration (bubble-up or diffused) whole-house; Granular activated carbon whole-house (lower levels only)

Iron

Oxidizing filter whole-house; Water softener (low levels); Aeration plus filtration

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.

For lead

Aquasana Claryum 3-Stage under-sink filter

under-sink or countertop, about $175

Certified to NSF/ANSI 53 to reduce 99% of lead and 99.7% of PFOA and PFOS at the tap. Not a fix for arsenic, nitrate or bacteria.

For lead

Acid neutralizing filter

whole-house, at the point of entry, about $1,200

Raises pH so plumbing stops leaching lead and copper.

For iron

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 Gilliam County

No water treatment company serves Gilliam 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 Gilliam County, and you get an email naming it first.

Common questions

Is there arsenic in well water in Gilliam County?

Arsenic was detected in 100% of 121 public well samples in Gilliam County, and 56% 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 often should I test a private well in Gilliam 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 Gilliam 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 Oregon dataset as CSV, every county and town, with the sources named.

Sources

USGS Water Quality Portal: 729 samples in Gilliam 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.
USGS national arsenic model (Ayotte 2017): estimated share of 526 people on private wells in Gilliam 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.
Oregon DEQ active tank sites: 2 regulated sites located in Gilliam County. Active underground storage tank sites from the Oregon Department of Environmental Quality. Retrieved September 18, 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.