Typical values from 29 sampled wells in Mono County · Full groundwater profile
What Mono County Wells Usually Need
Arsenic above the drinking water limit. Arsenic exceeded the 10 µg/L limit in 46% of 35 sampled Mono wells. It has no taste or smell, so only a lab test shows whether your well is affected.
Is it safe? Not untreated until a lab has checked it for arsenic.
What a typical home here installs:
- Point-of-use reverse osmosis filtration for drinking water, but only after a lab test confirms the problem in your well.
On city or county water instead? Your utility publishes an annual water quality report; this page covers private wells. Find your utility's report
On this page (8 sections)
Where to Test Well Water in Mono County
- Your county environmental health department handles private well questions in Mono County; for a water test, use a state-accredited lab, many of which accept mailed or dropped-off samples.
- Certified labs: State Water Board list of accredited labs (ELAP)
Ask the lab for sample bottles and how quickly the sample must reach it before you collect.
What Are You Noticing in Your Water?
Confirm with a lab test before buying equipment.
Explore California Interactive Map
Explore Mono County and nearby counties
Mono County has about 13,000 residents, and its largest communities are Mammoth Lakes. Homes outside city and community water systems rely on private domestic wells. Wells here tap the Basin and Range basin-fill aquifers, and the largest DWR groundwater basin in the county is Mono Valley.
Across 26 sampled wells, groundwater is moderately hard (5.8 gpg in the typical well). Arsenic exceeded the 10 µg/L drinking water limit in 46% of 35 sampled wells. These have no taste or smell, so only a lab test shows whether your own well is affected.
In this guide
Groundwater Profile: Mono County
Across 26 sampled wells, the median hardness is 5.8 gpg (moderately hard). At 5.8 gpg, Mono wells run harder than Inyo (4.5 gpg), softer than Fresno (10.5 gpg) and about as soft as Madera (5.3 gpg) next door. The largest California DWR Bulletin 118 groundwater basins in the county are Mono Valley, Owens Valley – Owens Valley and Long Valley.
Another address? Check hardness by ZIP code.
What shows up in local well samples:
- Arsenic46% of 35 sampled wells above 10 µg/L
Odorless and tasteless, with an EPA limit of 10 µg/L. Only a lab test detects it.
How to test your well
Arsenic, Nitrate, Chromium-6 and Salinity in Mono County
| Contaminant | Typical well | Wells above limit | California standard |
|---|---|---|---|
| Arsenic | 6.2 µg/L | 46% of 35 | 10 µg/L (primary MCL) |
| Nitrate (as nitrogen) | 0.06 mg/L | 0% of 25 | 10 mg/L (primary MCL) |
| Chromium-6 | 0.5 µg/L | Limited data (6 wells) | 10 µg/L (primary MCL since Oct. 1, 2024) |
| Salinity (dissolved solids) | 378 mg/L | 7% of 29 | 500 mg/L recommended, 1,000 mg/L upper (secondary) |
| Hardness | 5.8 gpg | 35% of 26 | No limit; above 7 gpg is hard |
Typical well = median. Standards: State Water Board; samples: Water Quality Portal, since 2000.
How Mono County Compares to California and the U.S.
See How Mono County Compares to California & the US
| Share of wells with… | Mono CountyMono | CaliforniaCA | United StatesUS |
|---|---|---|---|
| Hard water (above 7 gpg) | 35% | 64% | 60% |
| Iron above 0.3 mg/L | 17% | 9% | 19% |
| Manganese above 0.05 mg/L | 34% | 22% | 21% |
| Dissolved solids above 500 mg/L | 24% | 38% | 15% |
| Acidic water (pH below 6.5) | 9% | 2% | 14% |
| Nitrate above 10 mg/L (EPA limit) | 0% | 9% | 4% |
| Arsenic above 10 µg/L (EPA limit) | 46% | 14% | 7% |
| Chromium-6 above 10 µg/L (California limit) | Limited data | 6% | — |
| Median hardness | 5.8 gpg | 9.9 gpg | — |
| Median pH | 7.2 | 7.6 | 7.3 |
Sources: Water Quality Portal (since 2000); US: USGS Circular 1332. Highlighted: well above the US share.
See what a typical Mono County well needs
Pre-filled with the Mono medians. Swap in your own results.
Testing Your Well in Mono County
In California, county environmental health departments handle private well permits and can advise on testing. For bacteria, nitrate, arsenic or a full panel, use a laboratory accredited by California’s Environmental Laboratory Accreditation Program (ELAP); the State Water Board’s certified lab map lists them.
What to test for in Mono County:
- Bacteria (total coliform and E. coli), nitrate and arsenic, at least once a year (California State Water Resources Control Board recommendation)
- Arsenic, which appeared above the 10 µg/L limit in some sampled wells here
- After any flood, new well work or a change in taste, color or smell
Which tests to order and how: how to test well water.
After a positive coliform test, shock the well with the right bleach dose and retest 10 to 14 days later.
Nearby Counties
Mono County borders these counties with published reports:
Alpine County
4.6 gpg hardnessFresno County
10.5 gpg hardness · Hardness, NitrateInyo County
4.5 gpg hardness · ArsenicMadera County
5.3 gpg hardness · ArsenicTuolumne County
3.8 gpg hardnessCounties with similar well water
Elsewhere in California, these counties have well water like Mono County:
Frequently Asked Questions
Is well water in Mono County safe to drink?
Many wells are fine, but in county records arsenic exceeded the 10 µg/L limit in 46% of 35 sampled wells. Have your water tested for these and for bacteria before you drink it untreated.
How hard is well water in Mono County?
5.8 gpg (100 mg/L) in the typical well, based on 26 samples: moderately hard on the USGS scale, against a California median of 9.9 gpg. 35% of sampled wells top 7 gpg, where a softener usually pays off.
Is iron a problem in Mono County well water?
The median iron level across 29 sampled wells is 0.03 mg/L, below the EPA aesthetic guideline of 0.3 mg/L, and 17% of wells exceed it. Most wells will not stain, but if yours does, a lab test tells you whether iron or manganese is the cause.
Is arsenic a problem in Mono County well water?
Arsenic topped 10 µg/L in 46% of the 35 wells sampled here. You cannot see or taste it, so test for it, and use reverse osmosis or an arsenic filter if your result is high.
Why does my well water smell like rotten eggs in Mono County?
Too few Mono wells (0) have hydrogen sulfide results to say how common the smell is. If you notice it, a sulfide test confirms it. At typical levels it is a nuisance rather than a health risk.
Is chromium-6 a concern in Mono County well water?
Only 6 wells in Mono County have been sampled for hexavalent chromium, too few to say how common it is. California’s limit is 10 µg/L, and only a lab test of your own well can show whether it is affected.
Does boiling well water make it safe to drink in Mono County?
No, not for the arsenic found in some Mono wells. Boiling kills bacteria, but chemicals stay behind and become more concentrated as water evaporates. Here is why boiling cannot remove nitrate or arsenic.
How often should I test my well water in Mono County?
The California State Water Resources Control Board recommends testing domestic wells every year, including for bacteria, nitrate and arsenic, and again after flooding or well repairs.
Written by Well Water Advisor Editorial Team.