Water wells in Contra Costa County.

What the completion reports say

• Median depth

  185 ft

  The middle of 3,854 completion reports for newly drilled water-supply wells. Half were finished shallower, half deeper.

• Eighty per cent finish between

  80–342 ft

  A range wider than the median itself, which means the county figure is a weak guide to any one property. What the neighbours hit matters more than what the county did.

• Deepest on record

  1,235 ft

  One well. It says what the county has demanded of a rig at least once, not what it usually asks.

• Water level, where recorded

  30 ft

  How deep the water stood when the well was finished, on the 2,187 reports that recorded it — 57% of the county's wells. This is the measurement Texas's record does not carry at all. It describes the day it was written: levels move with the season, the year and the pumping around them.

• Tested yield, where measured

  20 GPM

  What the well made during the driller's own test, on the 1,286 reports carrying one in gallons per minute. A test is a few hours; it is not a guarantee of what the well sustained afterwards, and it is not a figure to size a system from without a hydrogeologist.

Decade by decade

Wells here have been finished deeper decade by decade: 140 ft in the 1970s, 255 ft in the 2020s — 1.8× the depth. That is what the filings show for the county as a whole. Completion depth follows where and why people drill as much as it follows a water table, so this describes the record rather than explaining it.

• 1970s140 ft218 reports
• 1980s160 ft1,147 reports
• 1990s200 ft831 reports
• 2000s224 ft722 reports
• 2010s255 ft308 reports
• 2020s255 ft171 reports

What this page cannot tell you

• Not the water table

  A record, not a reading

  The levels above were written down when each well was finished, some of them forty years ago. They describe those wells on those days. Nothing here is a current measurement, and no average of old filings becomes one.

• Not a yield you can plan on

  A test, not a guarantee

  A driller's test runs for hours. Sustained capacity over a season is a different quantity, measured differently, and most wells on record carry no test at all.

• Not a prediction for a parcel

  3,854 wells, one county

  The median describes the whole county. What matters for one property is the nearest wells in the most similar ground, which is the question Beneath exists to answer.

• Not every well

  Filings only

  Wells drilled before reporting, wells never filed and wells filed with a bad location are missing or misplaced. This is what was submitted, not a census of what exists.

Questions about Contra Costa County wells

How deep are water wells in Contra Costa County, California?

The median water well in Contra Costa County, California is 185 feet deep, based on 3,854 completion reports filed with the Department of Water Resources. Eighty per cent finish between 80 and 342 feet. Where drillers recorded the water level, the median was 30 feet below ground, across 2,187 of those reports. These figures describe wells that were drilled, not what a new well would find.

How deep is the water table in Contra Costa County?

The filed reports do not answer that, and the difference matters. What they carry is the static water level each well recorded on the day it was completed — a median of 30 feet below ground across 2,187 reports in Contra Costa County. Those readings span decades and were taken in different places under different conditions, so they describe wells that were drilled, not the water table today. A current level for one property comes from a measured well nearby, not from an average of old filings.

How many gallons per minute does a well in Contra Costa County produce?

Where drillers tested and filed a figure — 1,286 reports — the median was 20 gallons per minute. A driller's test runs for hours, so it says what the well made that day and not what it sustained through a dry summer. Wells with no filed test are absent from that median entirely, which is most of them.

Are wells in Contra Costa County getting deeper?

On the filed record, yes: a median of 140 feet in the 1970s against 255 feet in the 2020s. Contra Costa is not unusual in this — 54 of the 55 California counties with enough reports to compare show the same direction. What the record cannot say is why: deeper completions follow where people drill and what they are drilling for, as well as what is happening underground.

Does this tell me whether there is water under a specific property?

No. A county median describes 3,854 wells spread across the whole county, and the useful comparison for one parcel is the wells nearest it in the most similar ground. That is a different question, and it is the one Beneath answers — by scoring the whole parcel against the records around it and saying where the evidence is too thin to score.

Counties next door

• Alameda County — 2,388 wells, 225 ft
• Solano County — 5,199 wells, 201 ft
• San Joaquin County — 14,705 wells, 242 ft
• San Mateo County — 2,968 wells, 170 ft
• Marin County — 1,932 wells, 160 ft
• Napa County — 7,008 wells, 327 ft