Septic dry well: what it is, how far it must be from your well, and what to test

A septic dry well is a buried pit with an open bottom or holes in its sides that takes the liquid from a septic tank and lets it seep into the soil around and below it. It does the job a leach field does, but in one deep hole instead of shallow trenches. Minnesota's well code defines it this way: "'Seepage pit,' 'leaching pit,' or 'dry well' means an underground pit, tank, or receptacle into which a septic tank discharges effluent or other liquid waste and from which the liquid seeps into the surrounding soil through the bottom or openings in the side."

Two things it is not. It is not a water well that has gone dry: that is a well that stopped producing enough water, and our well flow test guide covers it. And it is not always a stormwater dry well, the gravel pit that soaks up roof runoff, although some codes use the same words for both. This page is for the person with a drinking-water well near one of these pits, their own or a neighbor's.

What is a dry well in a septic system?

On an older property the setup usually looks like this: the house drains to a septic tank, the tank holds back solids, and the liquid that leaves it flows to one or more perforated concrete or block pits surrounded by gravel. The Licking County (Ohio) Health Department describes it as "a prefabricated concrete tank with uniform holes throughout, or could be manmade out of laying up bricks or blocks," with gravel outside. Some homes have more than one, and the county notes that some use dry wells "in conjunction with leaching."

The names vary by state, and the differences matter because each name can carry a different setback:

So a "dry well" in Minnesota is a sewage pit, and a "dry well" in New Jersey is a roof-drain pit. If a home inspection report or a seller's disclosure lists a dry well, ask which kind. The septic permit or as-built drawing usually answers it; our guide to finding well and septic records shows where those are kept.

Cesspool or dry well?

The difference is the tank. A cesspool gets raw sewage straight from the house. EPA's cesspool page says that although cesspools "are designed to capture sanitary waste, they do not treat waste." A septic dry well comes after a tank, so the solids have mostly settled out first. EPA Region 9 warns that if the tank and other parts "are not properly sized, constructed and maintained, seepage pits may receive sewage solids (essentially functioning like cesspools)."

Stormwater dry wells

EPA lists "dry wells" among the common names for Class V stormwater drainage wells, which "are used for subsurface infiltration to manage surface water runoff (rainwater or snow melt)." Allegany County, Maryland, describes a residential one as "an excavated pit filled with gravel and sand" for roof runoff and requires it to sit at least 50 feet from water supply wells and 25 feet from septic systems. It carries no sewage, but it still puts water into the ground near your well, and the county sets a distance for it.

Septic dry well vs leach field

EPA describes the drainfield (leach field) as "a shallow, covered, excavation made in unsaturated soil," where "the soil accepts, treats, and disperses wastewater as it percolates through the soil, ultimately discharging to groundwater." A dry well skips most of that shallow soil. EPA's Region 9 Ground Water Office listed the problems in a 2001 fact sheet, "Seepage Pits May Endanger Ground Water Quality":

• Seepage pits "disperse effluent in anoxic, or oxygen-poor, environments, where pathogens (especially viruses) may not be treated before they reach the water table."
• "They place fluids below the root zone," where plants can't take up the water and nutrients.
• "Water tables are not static, and may rise above the bottom of the seepage pit, flooding it and allowing direct contact of pathogens and nitrogen species with ground water."
• Building and using a pit "may open up pathways to cracks and fissures in rock, sending effluent directly to waterways."

The office discourages seepage pits "particularly on steep slopes, fractured rock areas, areas with shallow ground water, and/or areas where ground water provides the sole source of drinking water." If your household drinks from a private well, that last case may describe your area.

The Licking County sheet says that around a dry well "the gravel and soil act as biological filters," and they do. The issue is how much soil is left between the bottom of the pit and the water table. New York requires "a minimum three foot vertical separation" between a pit bottom and high groundwater, bedrock or another impervious layer, and allows a seepage pit only where soil and site conditions are not adequate for absorption trenches.

Los Angeles County ties the distance to depth directly. Since November 1, 2018, it has required 200 feet between a septic system with a seepage pit and a public water well "where depth of effluent dispersal system is more than 10 feet," and 150 feet between a septic tank and a public water well where the dispersal system is 10 feet deep or less. That rule is for public wells, not household wells, but it shows the reasoning: deeper release, bigger distance.

Why codes keep dry wells farther from your well

Where a code lists a septic dry well separately, its distance to a drinking-water well is usually larger than the tank's and often larger than the leach field's. Three rules we read on November 4, 2026:

• Minnesota (Minn. R. 4725.4450): 50 feet from a septic tank, 50 feet from a drainfield, and 75 feet from "a cesspool, seepage pit, leaching pit, or dry well." For a sensitive well, the drainfield distance becomes 100 feet and the dry well distance 150 feet.
• New Jersey (N.J.A.C. 7:9D-2.7, for household wells): 50 feet from a septic tank, 100 feet from a disposal field, 150 feet from a seepage pit or cesspool, and 50 feet from a dry well, which in New Jersey means a roof or basement drainage pit.
• New York (Appendix 75-A, Table 2): 50 feet from a septic tank, 100 feet from an absorption field, and 150 feet from a seepage pit, increased by 50% (to 225 feet) when water enters the well less than 50 feet below grade, and 200 feet when the system is uphill in the drainage path to the well.

Distances for other states, including the ones that list seepage pits at 100 feet or put them in the same row as the tank, are in the state table on our septic setback distances page. That page also covers measuring, older layouts and loan rules; this one doesn't repeat them.

The sensitive-well rule

Minnesota doubles the distance for a "sensitive water-supply well": one "with less than 50 feet of watertight casing where the casing does not penetrate a confining layer or multiple layers of confining materials with an aggregate thickness of ten feet or more." The rule says the distances "are doubled between a sensitive water-supply well and a contamination source directly entering the soil." A dry well is such a source; a watertight septic tank is not, so its 50 feet doesn't change.

New Jersey's rule adjusts in both directions. Its well rule lets the Department reduce the 150-foot seepage-pit distance for a new well "where the well casing is sealed into a confining layer separating the aquifer from the stratum of soil used for sewage disposal or where the well is constructed with additional casing." It also lets the local authority require more distance where "gravel, limestone, or fractured, creviced or fissured rock formations are encountered." Both rules tie the distance to casing and geology. Your well log shows the casing depth; how a private well works explains what casing does.

Worked example

You live in Minnesota. Your well log shows 40 feet of casing and no confining layer, so the well is sensitive. The neighbor's septic tank drains to a dry well about 110 feet from your wellhead. For a sensitive well the rule's minimum is 150 feet, so the pit is closer than today's code allows for new construction. Minnesota's rule says a contamination source "must not be placed, constructed, or installed any closer" than those distances; it doesn't, by itself, order an old pit moved. What you can do is test: coliform with E. coli and nitrate every year, and keep every report (row 1 of the table below).

Is a septic dry well an EPA Class V injection well?

For one single-family home, no. EPA's Underground Injection Control (UIC) rules define a "well" broadly enough to include a dry well, and they list "dry wells used for the injection of wastes into a subsurface formation" as Class V wells. But 40 CFR 144.1(g) excludes "individual or single family residential waste disposal systems such as domestic cesspools or septic systems," and 40 CFR 144.81 says the UIC requirements "do not apply to single family residential septic system wells."

The federal rules do reach a dry well or cesspool in these cases:

• More than one household, or a business. EPA treats a septic system as a large-capacity septic system, a type of Class V well, if it "receives solely sanitary waste either from multiple dwellings or from a non-residential establishment and the system has the capacity to serve 20 or more persons per day." EPA's minimum requirements: the owner submits inventory information, and the injectate "cannot endanger" underground sources of drinking water.
• A cesspool shared with a home business. EPA says "a cesspool receiving sanitary waste from a business in addition to a residence is a large-capacity cesspool," and gives examples including "a home-based beauty salon, childcare center, auto repair shop." EPA banned new large-capacity cesspools on April 5, 2000, and required existing ones to close by April 5, 2005.
• Anything other than sanitary waste. EPA: "a septic system that receives vehicular repair or maintenance waste is known as a motor vehicle waste disposal well," and "toxic chemicals can pass through these systems untreated." The rules also cover hazardous waste disposed of "into what would otherwise be septic systems and cesspools, regardless of their capacity" (40 CFR 144.1(g)).

Even where EPA steps back, states don't. EPA's large-capacity septic page notes that states and tribes "can always adopt more stringent requirements," and its cesspool page says small-capacity cesspools "may be regulated by state and local governmental agencies (e.g., departments of health)." For a household dry well, the state onsite-wastewater code and the county health department are the rules that count.

Where new septic dry wells are banned or limited

Several codes no longer allow a dry well for new septic systems, and others allow one only under conditions. Examples from the rule text and county documents we opened:

Counties differ even inside one state. A survey of California counties, reproduced in EPA Region 9's 2001 sheet, found 28 of the responding counties allowed seepage pits and 25 did not. That survey is old; it shows how local the rule is, not where things stand now. Your county health department can tell you what applies to a pit on your road.

Old dry wells and pits on your property

An old pit doesn't stop being a risk when the house is connected to a new system. Ohio's private water system rule (OAC 3701-28-07, Table 1) keeps a new water source 100 feet from "a leaching pit or drywell that has not been properly abandoned to render it inoperable," twice the 50 feet it sets for any working part of a sewage system. Minnesota's well rule says its distances "must be maintained between a new well and a source of contamination no longer in use, unless all contaminants have been removed from the source, and visibly contaminated soils have been removed," and its septic rule counts "a system not abandoned in accordance with part 7080.2500" as failing to protect groundwater.

A forgotten pit can be hard to find. The Licking County sheet says a dry well "may also have a riser or inspection port to grade with the surface," and some older systems "may not have a distribution device." Old pits and old wells often turn up together on the same farm lot; our guide to an old water well on your property covers finding and sealing the well side, and the county can tell you how it wants an old septic pit abandoned.

What to test if your well is near a dry well

A pit doesn't send a warning; a lab result is how you find out whether septic effluent is reaching your well. EPA's baseline advice for every private well: "Test your private well annually for total coliform bacteria, nitrates, total dissolved solids, and pH levels." Near a septic dry well, two of those carry most of the weight:

• Coliform bacteria with E. coli. EPA Region 9's concern with pits is pathogens that "may not be treated before they reach the water table." Total coliform is the screen; E. coli points to fecal contamination. See coliform bacteria in well water.
• Nitrate. EPA says cesspool wastewater "often" has "higher levels of nitrates and coliform bacteria than are allowed in drinking water." The federal limit for nitrate is 10 mg/L as nitrogen (40 CFR 141.62). That limit applies to public water systems, not private wells, but it is the usual yardstick. See nitrate in well water.

If a shop, garage or basement floor drain ever ran to the pit, add volatile organic compounds (VOCs). EPA's advice to well owners: "Never dispose of harsh chemicals, solvents, petroleum products, or pesticides in a septic system or dry well," and "if you suspect the presence of other contaminants, you should test for those also."

A single clean report covers the day the sample was taken. Several years of reports show whether nitrate is rising. How to read the pattern, and why one result can't prove the septic system is the cause, is covered in our guide to the septic system and well water. For the full schedule of extra triggers, see how often to test well water.

A second example. Your report shows total coliform present, E. coli absent, and nitrate at 6 mg/L as N, with the neighbor's pit 120 feet away in New York, where the seepage-pit minimum is 150 feet. That lands in row 1: coliform with E. coli and nitrate every year. For the coliform result, the next steps are on our coliform page and in how to shock chlorinate a well. Nitrate at 6 mg/L is below the 10 mg/L public-system limit; the yearly series shows whether it is moving toward it. If anyone in the home is pregnant or caring for an infant, ask a doctor or the health department what result should change your plans.

Next step

Find out what the pit near your well is (septic or roof drain), how far it is from the wellhead, and what your state's code calls it. Then order coliform with E. coli and nitrate from a state-certified lab: find one for bacteria or nitrate in our lab finder, or see which tests your well needs for the full list.

Questions people ask