Typical values from 112 sampled wells in Merced County · Full groundwater profile
What Merced County Wells Usually Need
Arsenic above the drinking water limit. Arsenic exceeded the 10 µg/L limit in 15% of 74 sampled Merced 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 nitrate.
Typical treatment, in plumbing order:
- A whole-house sediment filter first, so sand and scale do not foul the stages after it.
- A 24,000-grain softener for a family of four at 9.5 gpg.
- Point-of-use reverse osmosis for drinking water, but only after a lab test confirms the problem in your well.
See how a well water filtration system is laid out. Your own test result decides the final list.
Not sure what to install? Get a filter recommendation for Merced County homesOn 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 Merced County
- Your county environmental health department handles private well questions in Merced 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?
Pre-selected from Merced County well data. Untick anything you do not notice in your own water.
Confirm with a lab test before buying equipment.
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Explore Merced County and nearby counties
Merced County has about 292,000 residents, and its largest communities are Merced, Los Banos and Atwater. 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 – Merced.
Across 112 sampled wells, groundwater is hard (9.5 gpg in the typical well). Arsenic exceeded the 10 µg/L drinking water limit in 15% of 74 sampled wells; nitrate exceeded the 10 mg/L drinking water limit in 23% of 127 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: Merced County
Across 112 sampled wells, the median hardness is 9.5 gpg (hard). At 9.5 gpg, Merced wells run harder than Madera (5.3 gpg) and softer than Fresno (10.5 gpg), Santa Clara (15.3 gpg) and San Benito (18.5 gpg) next door. The largest California DWR Bulletin 118 groundwater basins in the county are San Joaquin Valley – Merced, San Joaquin Valley – Delta-Mendota and San Joaquin Valley – Turlock.
Another address? Check hardness by ZIP code.
What shows up in local well samples:
- HardnessMedian 9.5 gpg; 67% of 112 sampled wells above 7 gpg
Calcium and magnesium dissolved from limestone. Leaves scale on fixtures and water heaters and reduces soap lather. Not a health risk.
Water softener sizing - Nitrate23% of 127 sampled wells above 10 mg/L
A health risk for infants above the EPA limit of 10 mg/L, usually from fertilizer, septic systems or livestock. Only a lab test detects it.
How to test your well - Arsenic15% of 74 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 Merced County
| Contaminant | Typical well | Wells above limit | California standard |
|---|---|---|---|
| Arsenic | 3.2 µg/L | 15% of 74 | 10 µg/L (primary MCL) |
| Nitrate (as nitrogen) | 2.22 mg/L | 23% of 127 | 10 mg/L (primary MCL) |
| Chromium-6 | 1 µg/L | 5% of 57 | 10 µg/L (primary MCL since Oct. 1, 2024) |
| Salinity (dissolved solids) | 324 mg/L | 4% of 105 | 500 mg/L recommended, 1,000 mg/L upper (secondary) |
| Hardness | 9.5 gpg | 67% of 112 | No limit; above 7 gpg is hard |
Typical well = median. Standards: State Water Board; samples: Water Quality Portal, since 2000.
How Merced County Compares to California and the U.S.
See How Merced County Compares to California & the US
| Share of wells with… | Merced CountyMerced | CaliforniaCA | United StatesUS |
|---|---|---|---|
| Hard water (above 7 gpg) | 67% | 64% | 60% |
| Iron above 0.3 mg/L | 10% | 9% | 19% |
| Manganese above 0.05 mg/L | 31% | 22% | 21% |
| Dissolved solids above 500 mg/L | 28% | 38% | 15% |
| Acidic water (pH below 6.5) | 0% | 2% | 14% |
| Nitrate above 10 mg/L (EPA limit) | 23% | 9% | 4% |
| Arsenic above 10 µg/L (EPA limit) | 15% | 14% | 7% |
| Chromium-6 above 10 µg/L (California limit) | 5% | 6% | — |
| Median hardness | 9.5 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 Merced County well needs
Pre-filled with the Merced medians. Swap in your own results.
Testing Your Well in Merced 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 Merced County:
- Bacteria (total coliform and E. coli), nitrate and arsenic, at least once a year (California State Water Resources Control Board recommendation)
- Hardness in grains per gallon (gpg) to size a softener
- 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-chlorinate the well and retest 10 to 14 days later.
Nearby Counties
Merced County borders these counties with published reports:
Fresno County
10.5 gpg hardness · Hardness, NitrateMadera County
5.3 gpg hardness · ArsenicMariposa County
4.9 gpg hardnessSan Benito County
18.5 gpg hardness · Hardness, Total dissolved solidsSanta Clara County
15.3 gpg hardness · HardnessStanislaus County
19.1 gpg hardness · Hardness, NitrateCounties with similar well water
Not next door, but the groundwater is much the same:
Frequently Asked Questions
Is well water in Merced County safe to drink?
Many wells are fine, but in county records nitrate exceeded the 10 mg/L limit in 23% of 127 sampled wells, arsenic exceeded the 10 µg/L limit in 15% of 74 sampled wells and chromium-6 exceeded the 10 µg/L limit in 5% of 57 sampled wells. Have your water tested for these and for bacteria before you drink it untreated.
How hard is well water in Merced County?
Groundwater samples from 112 wells in Merced County have a median hardness of 9.5 grains per gallon (162 mg/L), which the USGS classes as hard. About 67% 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 Merced County well water?
The median iron level across 105 sampled wells is 0 mg/L, below the EPA aesthetic guideline of 0.3 mg/L, and 10% 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 nitrate a problem in Merced County well water?
Nitrate was over the 10 mg/L limit in 23% of 127 wells sampled here, usually from fertilizer or septic systems nearby. Reverse osmosis or anion exchange removes it; boiling makes it worse.
Is arsenic a problem in Merced County well water?
Arsenic topped 10 µg/L in 15% of the 74 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 Merced County?
Public lab records include sulfide results for only 19 wells in Merced, not enough to judge. The smell comes from hydrogen sulfide; a sulfide test confirms it, and at household levels it is a nuisance, not a health hazard.
Is chromium-6 a concern in Merced County well water?
Hexavalent chromium was above California’s 10 µg/L drinking water limit in 5% of 57 sampled wells in Merced 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.
Can I install a salt-based water softener in Merced County?
Usually, but check local rules first. California Water Code Section 13148 lets some local sewer and water recycling agencies limit or ban new salt-discharging softeners to protect recycled water. Homes on septic systems should ask the county environmental health department about brine. A salt-free conditioner discharges no brine, but it reduces scale rather than softening the water.
Does boiling well water make it safe to drink in Merced County?
No, not for the arsenic and nitrate found in some Merced 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 Merced 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.