Typical values from 212 sampled wells in San Diego County · Full groundwater profile
What Are You Noticing in Your Water?
Pre-selected from San Diego County well data. Untick anything you do not notice in your own water.
Based on the contaminants most commonly reported in this region. Always confirm with a certified lab water test before buying equipment.
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San Diego County has about 3,270,000 residents, and its largest communities are San Diego, Chula Vista and Oceanside. 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 Vallecito-Carrizo Valley.
Across 211 sampled wells, groundwater is very hard (15.1 gpg in the typical well). Arsenic exceeded the 10 µg/L drinking water limit in 11% of 210 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: San Diego County
Most groundwater pumped in San Diego County comes from the Basin and Range basin-fill aquifers (USGS Principal Aquifers map). Across 211 sampled wells, the median hardness is 15.1 gpg, which the USGS classes as very hard. The largest California DWR Bulletin 118 groundwater basins in the county are Vallecito-Carrizo Valley, Ocotillo-Clark Valley and Borrego Valley – Borrego Springs.
What shows up in local well samples:
- HardnessMedian 15.1 gpg; 75% of 211 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. Planning a salt-based softener? California Water Code Section 13148 lets some local sewer and water recycling agencies limit or ban new salt-discharging softeners, so check with yours first. Salt-free conditioners discharge no brine but do not soften water; see what salt-free systems can and cannot do.
Water softener sizing - Chloride (salinity)Median chloride 284 mg/L
A salty taste and faster corrosion, often a sign of saltwater intrusion. The EPA aesthetic guideline is 250 mg/L.
Reverse osmosis guide - Arsenic11% of 210 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
Salinity varies across the county: 47% of 215 sampled wells exceed California’s 1,000 mg/L upper limit for dissolved solids, even though the typical well (940 mg/L) is below it.
Hydrogen sulfide (the rotten-egg smell) was measured in only 8 San Diego County wells, too few to say how common it is. Limited local data (fewer than 20 samples): individual well testing recommended. If you notice the smell, a sulfide test confirms it.
Arsenic, Nitrate, Chromium-6 and Salinity in San Diego County
| Contaminant | Typical well | Wells above limit | California standard |
|---|---|---|---|
| Arsenic | 1.6 µg/L | 11% of 210 | 10 µg/L (primary MCL) |
| Nitrate (as nitrogen) | 0 mg/L | 5% of 204 | 10 mg/L (primary MCL) |
| Chromium-6 | 0 µg/L | 0% of 41 | 10 µg/L (primary MCL since Oct. 1, 2024) |
| Salinity (dissolved solids) | 940 mg/L | 47% of 215 | 500 mg/L recommended, 1,000 mg/L upper (secondary) |
| Hardness | 15.1 gpg | 75% of 211 | No limit; above 7 gpg is hard |
Typical well = median of sampled wells; salinity column counts wells above 1,000 mg/L and hardness wells above 7 gpg. Standards: California State Water Resources Control Board (Title 22 maximum contaminant levels). Samples: USGS and EPA Water Quality Portal, wells sampled since 2000.
How San Diego County Compares to California and the U.S.
See How San Diego County Compares to California & the US
| Share of wells with… | San Diego CountySan Diego | CaliforniaCA | United StatesUS |
|---|---|---|---|
| Hard water (above 7 gpg) | 75% | 64% | 60% |
| Iron above 0.3 mg/L | 22% | 9% | 19% |
| Manganese above 0.05 mg/L | 47% | 22% | 21% |
| Dissolved solids above 500 mg/L | 78% | 38% | 15% |
| Acidic water (pH below 6.5) | 2% | 2% | 14% |
| Nitrate above 10 mg/L (EPA limit) | 5% | 9% | 4% |
| Arsenic above 10 µg/L (EPA limit) | 11% | 14% | 7% |
| Chromium-6 above 10 µg/L (California limit) | 0% | 6% | — |
| Median hardness | 15.1 gpg | 9.9 gpg | — |
| Median pH | 7.5 | 7.6 | 7.3 |
San Diego County and California: share of sampled wells (all well types, samples since 2000) whose typical value exceeds the level, from the Water Quality Portal. United States: USGS Circular 1332, about 2,100 domestic wells in 48 states sampled 1991 to 2004. Well types and years differ, so read this as a broad comparison. Highlighted cells are well above the national share.
See what a typical San Diego County well needs
Opens the diagnostic calculator with San Diego County median values (15.1 gpg hardness, 0.02 ppm iron, pH 7.5) for a 3–4 person home. Swap in your own lab results for an exact plan.
Testing Your Well in San Diego 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 San Diego 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
- Chloride and sodium, to catch saltwater intrusion
- 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
Statewide guidance is on the State Water Board domestic well testing page, and our guide to how to test well water covers which tests to order.
Nearby Counties
San Diego County borders these counties with published reports:
Frequently Asked Questions
How hard is well water in San Diego County?
Groundwater samples from 211 wells in San Diego County have a median hardness of 15.1 grains per gallon (259 mg/L), which the USGS classes as very hard. About 75% 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 San Diego County well water?
The median iron level across 212 sampled wells is 0.02 mg/L, below the EPA aesthetic guideline of 0.3 mg/L, and 22% 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.
Why does my well water smell like rotten eggs in San Diego County?
Hydrogen sulfide (the rotten-egg smell) was measured in only 8 San Diego County wells, too few to say how common it is. Limited local data (fewer than 20 samples): individual well testing recommended. If you notice the smell, a sulfide test confirms it. The smell is a nuisance rather than a health risk at typical levels.
Which aquifer supplies wells in San Diego County?
Most groundwater pumped in San Diego County comes from the Basin and Range basin-fill aquifers (USGS Principal Aquifers map).
Is chromium-6 a concern in San Diego County well water?
Hexavalent chromium was above California’s 10 µg/L drinking water limit in 0% of 41 sampled wells in San Diego 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 San Diego 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 San Diego County?
Only if bacteria are the problem. A one-minute rolling boil kills germs, but it does not remove nitrate, arsenic or other chemicals, and it concentrates them as water evaporates. Test first, and see our boiling well water myth buster for when boiling helps.
How often should I test my well water in San Diego 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.