Stormwater and drainageGuide

What is a dry well? How it works, when it's legal, and why most Sonoma County yards need something else

By The Downspout · Posted Oct 22, 2026 · 11 min read Share

A dry well is a buried gravel pit or tank that soaks roof water into the ground. Most Sonoma County yards are better off with a rain garden. Our soil mostly takes in water slowly, and a pit that stays full for days breeds mosquitoes. Where the soil drains well, a dry well for roof water can work. Test your soil, and call the county or your city first.

A dry well is a hole in the ground that takes rain from a downspout or yard drain and lets it soak into the soil below. In Sonoma County, most yards are better off with a rain garden or a swale instead.

Here is why. About 7 in 10 acres of rated soil in the county take in water slowly.^[1] Water poured into a pit in that soil can sit there for days.

Where the soil does drain, a dry well for roof water can work. A single home's dry well does not need to be registered with the federal government. State guidance and local permits still apply.

Below: what a dry well is, a quick way to tell if one would drain on your lot, what to build instead, where a dry well should not go, the rules, and the groundwater question.

Looking for a water well that ran dry? See the note at the end.

What is a dry well?

It is a buried place for stormwater to go. Usually it is a pit filled with gravel, or a perforated tank, meaning one full of small holes. A pipe brings water from a downspout or drain, and the water seeps out into the soil around it.^[2][3]

The same name covers much bigger things. Cities and developers build deep dry wells: perforated shafts up to 4 feet wide and 20 to 50 feet deep, sometimes deeper.^[4] They punch through a tight top layer of soil to reach looser ground below.^[4]

The idea behind both is old. Before roofs and pavement, much of the rain here soaked in. Soaking it in again can refill groundwater and help keep nearby creeks flowing between storms.^[2]

The drawing compares a home dry well with the much deeper kind cities build.

Will a dry well drain on your lot?

This is the first question, and the soil answers it. A dry well only works if it empties before the next storm.

How long is too long? Sonoma Resource Conservation District (Sonoma RCD), a local district that helps landowners with soil and water, says a soak-in pit like a dry well should empty within 72 hours, or 3 days.^[2] The State Water Board's guidance allows up to 96 hours, or 4 days, unless local rules are stricter.^[4] The limit is about mosquitoes, which breed in standing water.^[4]

Our take: use 3 days. It is the local figure, and the stricter one.

The US Department of Agriculture sorts soils into four groups, A to D, by how fast soaked ground takes in water.^[5] The chart shows how long a foot of water in a pit takes to soak away in each group.

The dashed line is the 3-day limit. Slow group C soil can take longer than that. Very slow group D soil takes longer still, or has groundwater too close to the surface. Together they are about 7 in 10 acres of the county's rated soil. That is why most yards here need something else.

Our calculation from USDA soil group limits and the county soil survey. Method and data: data/what-is-a-dry-well.

One catch: the groups describe the top 20 inches or so.^[5] A dry well's floor sits deeper, where soil can be faster or slower.

Say your roof is 1,000 square feet. One inch of rain on it is about 623 gallons, or 83 cubic feet of water.

• In the slowest group C soils, a pit floor would need to be about 10 feet by 10 feet to soak that in within 3 days. That is rain garden size.
• In the slowest group B soils, a floor about 3 feet by 3 feet could do it. But the pit would also have to hold all 83 cubic feet while it drains: about 8.5 feet of open hole, deeper once filled with gravel. That is deeper than it is wide, which makes it a well under federal rules, covered below. Sonoma RCD advises against pits deeper than 3 feet.^[2]

So in most yards, build a rain garden instead: a shallow planted dip that does the same job at the surface.

Test before you dig. The county soil survey only gives a rough guide for your lot. Soil around a house has often been dug, filled or packed down.^[5] In the rainy season, dig a hole 6 inches deep, fill it with water and check it the next day. Our 24-hour soak test has the steps.

• Water gone the next day: a rain garden can work. A dry well needs much faster soil. By our math from the soil group limits, soil fast enough for a small pit would empty the test hole in about 4 hours. Have a professional test the deeper soil first. Sonoma RCD says it can help with early planning.^[2]
• Water still there: your soil is too slow for both. Our guide to sink it or shed it shows where that water can go instead.

What to build instead

In most Sonoma County yards, these do the dry well's job at or near the surface, where soil can do its cleaning. Our pick: for most roofs, start with a rain garden.

1. A rain garden, for roof water. A shallow planted dip that holds water while it sinks in. In clay, give it an overflow.^[2] See how big to dig one.
2. A swale, when water needs to move away slowly. A shallow planted or rock-lined ditch.^[2] Our swale guide covers how to build one.
3. A rain barrel or tank, to keep the water for summer. See rain barrels in California.
4. A French drain, for ground that stays soggy with water moving through the soil. It needs an outlet too. Read do you need a French drain.

Good news for these options: EPA's well rules, covered below, do not apply to a swale, pond or ditch, as long as nothing buried lets water out underground.^[6]

If your soil drains fast and none of these fit the space, a dry well for roof water may make sense. Plan it with a professional and the county.

Where a dry well should not go

The State Water Board's 2020 dry well guidance lists places that are not suitable. It was written for deep dry wells, but the board says most of it also applies to shallow ones.^[4]

• Within 10 feet of the groundwater. Keep at least 10 feet of dry soil between the bottom of the pit and the highest winter groundwater.^[4] The county soil survey shows about 148 square miles, roughly 9 percent of the county, with groundwater within 4.5 feet of the surface for part of the year.^[1] In those spots, a dry well cannot keep 10 feet of dry soil above the groundwater, so it is not an option there. Elsewhere, groundwater can still be within 10 feet: the survey only records the top few feet.
• Within 10 feet of bedrock. Keep 10 feet between the bottom of the pit and solid rock.^[4]
• Within 150 feet of a drinking water well.^[4] A dry well sends water deeper than a rain garden, so keep it farther from a well.
• Within 100 feet of a septic tank or leach field, the buried pipes that spread septic water into the soil.^[4] Our septic guide shows how to find the county records for yours.
• Close to the house. At least 20 feet when the dry well is downhill of the foundation, and 100 feet when it is uphill.^[4]
• On a slope steeper than 15 percent, a 15 foot rise over 100 feet, unless a licensed geotechnical engineer, a soil and slope specialist, has checked it.^[4]
• In contaminated soil, or within 250 feet of auto shops, nurseries and other places that handle chemicals.^[4]

A licensed professional can approve shorter distances from wells, the house and slopes after studying the site.^[4] The 10 feet of dry soil above groundwater has no such exception.^[4]

Is a dry well legal in California?

Often yes, for roof water at a home. But federal, state and local rules all have a say.

• Federal. A dry well for one family home does not need to be registered with the US Environmental Protection Agency (EPA). That changes if a home business, such as car repair, uses it.^[6] It still must not pollute groundwater people could drink.^[6] EPA counts a dry well as an injection well: any hole deeper than it is wide, or buried perforated pipe that spreads water underground.^[7] A pit wider than it is deep, with no perforated pipe or buried tank in it, is not one.^[7]
• State. The state test is different from the federal one. California's water law requires anyone planning an "injection well" to file a report with their regional water board, and a hole counts if it is deeper than the distance around it.^[4] A narrow pit can meet that. If yours will, call your regional water board first. Two cover parts of Sonoma County: North Coast, which includes the Laguna de Santa Rosa,^[8] and San Francisco Bay, which includes Petaluma.^[9]
• County. Outside city limits, drainage work needs a county drainage permit unless the county code exempts it.^[10] Call Permit Sonoma's engineering desk at (707) 565-2268 to ask before you dig.^[10]
• Cities. Rules differ from city to city. In the City of Sonoma, a single-family home that creates or replaces 2,500 square feet or more of hard surface needs a stormwater plan.^[11] Ask your city's public works or building department.

Our drainage permits guide covers the rest of the county rules.

Do dry wells pollute groundwater?

This is the real argument about dry wells, and both sides have a point.

• The worry. Soil near the surface breaks down many pollutants. A dry well skips past that layer, so whatever is in the water gets a shortcut toward the water table.^[12] That is why many California regulators have been reluctant to allow them.^[12] The Bay Area's regional water board restricted them back in 1951 to protect groundwater.^[3]
• The evidence so far. The City of Elk Grove, near Sacramento, and OEHHA, the state agency that studies health risks from pollution, tested it. They built two dry wells. Water passed through a settling area and plants first, to clean it. Test wells beside them let the team sample storm runoff and groundwater for two years.^[12] Most of the more than 200 pollutants they tested for rarely or never showed up. Metals and some pesticides were in the runoff, but were not traced into the groundwater. They found no sign of groundwater pollution from the dry wells.^[13]
• The limits. Pesticides that dissolve in water, such as imidacloprid, could move fast through soil, and the study called for more research on them. The city also decided to close its dry well at its corporation yard, where vehicles are serviced, because of the risk there.^[13]
• Roof water is the easy case. State guidance rates runoff from a home or other non-industrial roof as low risk.^[4] Sonoma RCD says downspout water is often the best water for a soak-in pit.^[2]

Where most experts land: a dry well that is well placed, cleans the water first and is kept up poses little risk to groundwater.^[13][4] That view could change as more is learned about newer pesticides.^[13]

Our take: send only roof water to a dry well. Never driveway, garage or patio runoff, which can carry oil and chemicals. Keep gutters clean, because leaves and grit clog a pit.^[2]

What it means for you

• Test the soil first. If a 6 inch test hole still holds water the next day, skip the dry well.
• Start with a rain garden. It does the same job at the surface.
• Roof water only. Keep driveway and garage water out of anything that goes underground.
• Measure the distances: 150 feet from a drinking water well, 100 feet from a septic tank or leach field, and 20 feet from the house if downhill, 100 feet if uphill. Then call Permit Sonoma or your city.

Looking for a water well that ran dry? That is the other meaning of the words. If your household well has gone dry, report it to the state's Dry Well Reporting System, which passes reports to state and local agencies.^[14] Then see our guide to wells, tanks and pumps.

Next: the rain calculator shows how much your roof sheds in a storm, and sink it or shed it helps you decide where that water should go.

Photos by Devan Hemmings unless noted.

Related reads

Stormwater and drainageSlow it, spread it, sink it: downspouts, rain gardens and how big to digTurn your downspouts toward planted ground, put gravel under the roof edge, and slope the soil away from the house. Then, if a test hole drains within a day, dig a shallow rain garden: about 56 square feet for 1,500 square feet of roof draining to it.Updated Oct 19, 2026Stormwater and drainageWhen you need a county permit to change drainagePlanning to regrade a yard, add a drain or change how water leaves your property? In unincorporated Sonoma County, drainage work needs a county permit before you start, and moving more than 50 cubic yards of soil needs a grading permit.Updated Oct 18, 2026Stormwater and drainageSink it or shed it? When to keep rain on your land and when to drain itMost properties need both. Sink clean water where the soil can take it and nothing will be harmed. Shed water, slowly and safely, where soaking it in would threaten a foundation, a slope, a septic system or a neighbor. The craft is deciding which water goes where.Updated Oct 18, 2026