How close can a well be to a septic system in Arizona?

State law is direct: no well may be drilled within 100 feet of any septic tank or sewage disposal area, unless the Arizona Department of Water Resources (ADWR) director approves an exception in writing. That 100 feet is the statewide floor, not a target. Your county health authority can require a larger separation based on soil and site conditions, so confirm the exact setback with the county before you place the well or the septic field. Getting this right on the site plan protects your drinking water and keeps both permits on track.

The 100-foot rule and who sets it

The core rule is 100 feet between a well and a septic system, set by state water law in A.A.C. R12-15-818, the well construction rule. ADWR states it plainly: except for monitoring wells, "no well shall be drilled within 100 feet of any septic tank system, sewage disposal area, landfill, hazardous waste facility, storage area of hazardous materials or petroleum storage areas and tanks, unless authorized in writing by the Director." So a private domestic well and your leach field must keep at least 100 feet apart in every direction. To build closer, you would have to request a written variance from the ADWR director, which is the exception, not the plan.

The reason is contamination. A septic system treats household wastewater by letting it filter through soil in a disposal field. A well pulls drinking water from underground. Put them too close and treated or partially treated wastewater can reach the water you drink. The 100-foot buffer gives the soil room to filter the wastewater before it could reach the aquifer near the well.

Two agencies share authority over this. ADWR governs wells, including the 100-foot drilling rule. The Arizona Department of Environmental Quality (ADEQ) owns the septic rules under the Arizona Administrative Code, and it delegates the day-to-day permitting to the counties. The septic setback table in rule R18-9-A312 also requires the disposal works to keep its own distance from a well. The two rules point the same direction: keep the well and the septic field well apart.

Why the distance can be more than 100 feet

The 100-foot figure is the statewide minimum, and your county can require more. Arizona counties issue the septic permit on ADEQ's behalf, and they review the actual soil, slope, and lot layout. If conditions call for a larger buffer, the county sets it. So the honest answer to "how close can they be" is "no closer than 100 feet, and possibly farther, depending on your county and your soil."

A few site factors push the distance up. Coarse or fast-draining soil moves water quickly, so a site evaluation may call for more separation. Shallow bedrock or a high water table can do the same, because there is less soil to filter the wastewater. Slope matters too, since wastewater can travel downhill toward a well below it. This is why the percolation test and site evaluation come before you finalize where the well and septic go.

The setback also runs to neighboring lots. ADWR's well rules count any septic tank or sewer "on the property or within 100 feet of the well location, even if on neighboring properties." So a neighbor's septic field near your line can limit where your well can go, and your septic field can limit a neighbor's future well. On smaller lots this is one of the tightest constraints on the whole site plan.

How this affects siting your well and septic

Plan the well and septic together on one site plan, because the 100-foot rule and the other setbacks have to all work at once. Your well, your septic tank, your disposal field, your reserve field, the house, and the property lines all compete for room. The septic field and the well cannot overlap, and both have to clear the required distances from the house and the lines. On a tight lot this is a real design puzzle, and it is cheaper to solve on paper than after a failed inspection.

For domestic wells on small parcels, the state adds a paperwork step that locks the 100 feet in. ADWR requires that an exempt domestic well on a parcel of 5 acres or fewer come with a site plan showing the well is at least 100 feet from any septic tank or sewer, plus approval by the county health authority before the filing goes to ADWR. So on a small lot you must draw and prove the separation up front, and the county signs off on it.

Order the work so the distances drive the design, not the other way around. Start with the perc test and site evaluation during your due diligence, ideally before you buy the land, because a failing perc or poor soil can change your system type, push the setbacks out, or shrink your buildable area. Then lay out the septic field, the well, and the house together so every required distance is met on the first submittal.

Common siting mistakes that trigger a redo

Most well and septic setback problems come from siting one system without planning the other. These are the mistakes we see most often on rural lots, and each one is avoidable on paper.

• Placing the well first, then squeezing in the septic. If you drill before the septic field is laid out, you can box yourself in. Lay out both at once.
• Forgetting the reserve field. Many systems need a second, backup disposal area. That reserve also has to clear the well by the required distance, so plan for it now.
• Ignoring the neighbor's septic. A septic field across the property line still counts. A neighbor's system within 100 feet can block your well location.
• Skipping the perc test until after closing. Soil that drains poorly or too fast can push your setbacks out and shrink your buildable area. Test during due diligence, before you buy.
• Assuming flat 100 feet is always enough. On a slope or in coarse soil, the county can require more. Build in margin so a tighter ruling does not force a redesign.

Catch these on the site plan and you avoid the worst outcome, which is a failed inspection or a relocated well after money is already spent. A clean layout that meets every distance the first time keeps both the septic permit and the well filing moving.

Local rules for the Phoenix metro

In Maricopa and Pinal counties the same 100-foot floor applies, enforced through the county that issues your septic permit. Most Jematell Homes service areas that rely on well and septic, including Rio Verde Foothills, sit in unincorporated Maricopa County, where the county environmental division permits the septic system and ADWR authorizes the well. For the full county process, see our reference entry on Maricopa County septic and well requirements.

Casa Grande, Apache Junction, and other rural lots in Pinal County follow the same state rules through the Pinal County Aquifer Protection Division, which reviews the well on small parcels and confirms the 100-foot setback before ADWR signs off. Our Pinal County septic and well requirements entry walks through that path. Wherever your lot sits, the rule starts at 100 feet and only grows from there. Confirm the current setback with the Arizona Department of Water Resources and your county health authority before you site either system, since site conditions and county rules vary.

Building with Jematell Homes

Vetting whether a parcel is truly buildable is where we start every project. Reach out and we will talk through how this applies to your specific lot and plan.

Sources

• Arizona Department of Water Resources: Permitting and Wells FAQ
• ADEQ Onsite Wastewater Treatment Program (Delegation Agreements)
• ADEQ Onsite Wastewater Treatment Facility (septic) program