Typical values from 87 sampled wells in Madera County · Full groundwater profile
Quick Answers for Madera County Well Owners
Arsenic above the drinking water limit. Arsenic exceeded the 10 µg/L limit in 10% of 87 sampled Madera 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.
Typical treatment, in plumbing order:
- Point-of-use reverse osmosis filtration for drinking water, but only after a lab test confirms the problem in your well.
If your water comes from a utility, its annual Consumer Confidence Report is the right source; the figures here are for private wells. Find your utility's report
On this page (8 sections)
Where to Test Well Water in Madera County
- Your county environmental health department handles private well questions in Madera 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 Madera County and nearby counties
Madera County has about 163,000 residents, and its largest communities are Madera and Chowchilla. Homes outside city and community water systems rely on private domestic wells. Wells here tap the Central Valley aquifer system, and the largest DWR groundwater basin in the county is San Joaquin Valley – Madera.
Across 87 sampled wells, groundwater is moderately hard (5.3 gpg in the typical well). Arsenic exceeded the 10 µg/L drinking water limit in 10% of 87 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: Madera County
Across 87 sampled wells, the median hardness is 5.3 gpg (moderately hard). At 5.3 gpg, Madera wells run softer than Merced (9.5 gpg) and Fresno (10.5 gpg) and about as soft as Mono (5.8 gpg) next door. The largest California DWR Bulletin 118 groundwater basins in the county are San Joaquin Valley – Madera, San Joaquin Valley – Chowchilla and San Joaquin Valley – Delta-Mendota.
Another address? Check hardness by ZIP code.
What shows up in local well samples:
- Arsenic10% of 87 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 Madera County
| Contaminant | Typical well | Wells above limit | California standard |
|---|---|---|---|
| Arsenic | 2 µg/L | 10% of 87 | 10 µg/L (primary MCL) |
| Nitrate (as nitrogen) | 2.02 mg/L | 9% of 86 | 10 mg/L (primary MCL) |
| Chromium-6 | 0 µg/L | 0% of 35 | 10 µg/L (primary MCL since Oct. 1, 2024) |
| Salinity (dissolved solids) | 231 mg/L | 2% of 87 | 500 mg/L recommended, 1,000 mg/L upper (secondary) |
| Hardness | 5.3 gpg | 35% of 87 | No limit; above 7 gpg is hard |
Typical well = median. Standards: State Water Board; samples: Water Quality Portal, since 2000.
How Madera County Compares to California and the U.S.
See How Madera County Compares to California & the US
| Share of wells with… | Madera CountyMadera | CaliforniaCA | United StatesUS |
|---|---|---|---|
| Hard water (above 7 gpg) | 35% | 64% | 60% |
| Iron above 0.3 mg/L | 3% | 9% | 19% |
| Manganese above 0.05 mg/L | 12% | 22% | 21% |
| Dissolved solids above 500 mg/L | 14% | 38% | 15% |
| Acidic water (pH below 6.5) | 3% | 2% | 14% |
| Nitrate above 10 mg/L (EPA limit) | 9% | 9% | 4% |
| Arsenic above 10 µg/L (EPA limit) | 10% | 14% | 7% |
| Chromium-6 above 10 µg/L (California limit) | 0% | 6% | — |
| Median hardness | 5.3 gpg | 9.9 gpg | — |
| Median pH | 7.4 | 7.6 | 7.3 |
Sources: Water Quality Portal (since 2000); US: USGS Circular 1332. Highlighted: well above the US share.
See what a typical Madera County well needs
Pre-filled with the Madera medians. Swap in your own results.
Testing Your Well in Madera 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 Madera 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, work out the bleach dose and shock the well and retest 10 to 14 days later.
Nearby Counties
Madera County borders these counties with published reports:
Fresno County
10.5 gpg hardness · Hardness, NitrateMariposa County
4.9 gpg hardnessMerced County
9.5 gpg hardness · Hardness, NitrateMono County
5.8 gpg hardness · ArsenicTuolumne County
3.8 gpg hardnessCounties with similar well water
Not next door, but the groundwater is much the same:
Frequently Asked Questions
Is well water in Madera County safe to drink?
Not without a test. Arsenic exceeded the 10 µg/L limit in 10% of 87 sampled wells and nitrate exceeded the 10 mg/L limit in 9% of 86 sampled wells. Only a lab result shows whether your well is one of them; treat or switch to bottled water until you know.
How hard is well water in Madera County?
Groundwater samples from 87 wells in Madera County have a median hardness of 5.3 grains per gallon (91 mg/L), which the USGS classes as moderately hard. About 35% of sampled wells are above 7 gpg, the point where a water softener is usually worthwhile. The California statewide median is 9.9 gpg.
Is iron a problem in Madera County well water?
Only in some wells. 3% of 87 sampled Madera wells top the EPA’s 0.3 mg/L guideline; the typical well has 0 mg/L. Orange or black stains are the sign to get a lab test for iron and manganese.
Is arsenic a problem in Madera County well water?
Arsenic topped 10 µg/L in 10% of the 87 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.
Is nitrate a problem in Madera County well water?
9% of 86 sampled wells had nitrate above 10 mg/L. Infants under six months are most at risk, so never mix formula with untested well water, and do not boil it: boiling concentrates nitrate.
Why does my well water smell like rotten eggs in Madera County?
The data cannot say: just 11 Madera wells were tested for hydrogen sulfide. If your water smells, test for sulfide, and if the smell is only in hot water, check the water heater anode rod. It is unpleasant but rarely a health risk.
Is chromium-6 a concern in Madera County well water?
Hexavalent chromium was above California’s 10 µg/L drinking water limit in 0% of 35 sampled wells in Madera County. The limit took effect on October 1, 2024. It has no taste or smell, so only a lab test shows whether your well is affected; reverse osmosis is a common household treatment.
Does boiling well water make it safe to drink in Madera County?
No, not for the arsenic found in some Madera 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 Madera 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.