Septic in Cochise County: The Soil Evaluation, the 100-Foot Rule and Caliche

Septic is the step that most often reshapes a site plan, because its constraints are unusually rigid and they are checked by someone else.

It is also the step where a parcel occasionally turns out to be a great deal more expensive than it looked, and that discovery is made in a test pit rather than on a spreadsheet.

Who runs it

Cochise Health and Social Services Department, Environmental Health Services Division. Septic inspection line 520-432-9441. Environmental Health on 520-586-8206, or envhealth@cochise.az.gov.

Alternative systems — composting toilets and similar — may be reviewed by the Arizona Department of Environmental Quality or by the county Health Department depending on the system.

The composting toilet question, answered up front

People arrive with a plan that avoids septic entirely. It usually does not work, and the reason is specific.

The county states it plainly: even with a composting toilet, you will still be required to install a small septic system, because kitchen drain water is not considered greywater in the State of Arizona.

Arizona has genuinely permissive greywater rules — shower, bath, sink and laundry water can be reused under a general permit with conditions. But kitchen water is excluded from that definition, and so it has to go into a permitted wastewater system. A composting toilet reduces the size of the system you need. It does not remove the requirement.

The soil and site evaluation, which comes first

This is the part that surprises people who assume they will apply for a permit and then get tested.

Cochise County requires a soil and site evaluation by a state-certified evaluator before you apply. The evaluation requires at least three test pits — two in the primary sewage disposal area and a third in the reserve disposal field area — dug to a depth of twelve feet.

Note that a reserve field is required. You are not siting one drainfield, you are siting two: the one you build and the one you would build if the first fails. Both need to fit within the setbacks, on suitable soil, on your parcel.

Who can do the evaluation: an Arizona registered professional engineer, a registered professional geologist, a registered sanitarian, or somebody holding an ADEQ-recognised training certificate.

Perc test or test pit?

The state rule allows three subsurface investigation methods: the ASTM test-pit practice, percolation testing, or seepage-pit performance testing.

The ASTM test-pit method becomes mandatory where slopes exceed 15 percent, where bedrock outcrops, where there is a high water table, or where prior percolation results were outside acceptable ranges.

Test pits are dug to twelve feet below land surface or to an impervious layer, whichever is shallower.

That clause is the one to underline.

Where caliche comes in

Caliche is soil cemented by calcium carbonate, and in this valley it occurs both as scattered nodules and as a continuous layer anywhere from a few inches to several feet thick. An impenetrable caliche layer restricts water movement — that is the University of Arizona extension service's own description of it.

A conventional drainfield works by letting effluent percolate downward through soil into which it can disperse. If there is a cemented layer at four feet that water cannot get through, that dispersal has nowhere to go.

The connection between "impervious layer" in the septic rule and the caliche layer under a lot of Cochise County parcels is one that nobody spells out for you, because the septic regulation is written by one agency and the caliche literature by another. But on a great many parcels here they are the same thing.

You can get an early indication for free. NRCS Web Soil Survey gives the depth to a restrictive layer for your specific parcel. It is regional-scale mapping rather than a site investigation, but if it shows a shallow restrictive layer, budget for a harder septic conversation.

The setbacks, which constrain everything else

These come from the state rule on minimum horizontal separations, and they are the numbers that shape your whole site plan:

From the well side, Cochise County mirrors this: wells at least 5 feet from all property lines and at least 100 feet from all septic system components, verified by an inspector who takes GPS coordinates on parcels of five acres or less.

Why this constrains the site plan so hard

Put those numbers on a four-acre parcel with a wash through it and a driveway coming in from one corner, and the workable area shrinks quickly. You need a primary field and a reserve field, both 100 feet from the well, both 50 feet from washes over 20 acres of catchment, both clear of the shared property line by 50 feet unless a neighbour agrees otherwise.

On the 1.4-acre lots common in some of the older subdivisions, this is not a theoretical squeeze. It is the reason those lots are hard to build on.

Sizing

A typical three or four bedroom home is generally in the range of a 1,200 to 1,500 gallon tank with a drainfield sized by the soil's percolation rate and the number of bedrooms. Poor-percolating soil means a larger field, which is another way soil conditions turn into cost.

Permit validity and fees

Septic permits in Cochise County expire after two years. If your build is slow — and owner-builder projects frequently are — check the date rather than assuming.

On fees: we are not printing a number. The county links a fee schedule but does not publish the amounts on its septic pages, and one of the application documents referenced online is no longer retrievable. Call Environmental Health on 520-586-8206 or email envhealth@cochise.az.gov for the current schedule. Anyone publishing a specific Cochise County septic fee without a source is guessing, and the same is true of the installed-cost estimates you will find — every one we could locate came from a programmatically generated cost-estimator site with no methodology.

The realistic sequence

1. Corners found, so setbacks can be measured from a real boundary.
2. Soil map pulled, so you know whether to expect a restrictive layer.
3. Well sited.
4. Certified evaluator engaged; three test pits dug in the proposed primary and reserve areas.
5. System designed against what the pits found.
6. Permit application to Environmental Health.
7. Installation and inspection.

Note that steps one to four all happen before you have a septic design, and all of them are cheaper than the mistake of skipping them.

Where we fit

We do not design or permit septic systems — that is the certified evaluator and the licensed installer. What we do is the excavation and the site work around it: access for the equipment, the grading that keeps surface water off a drainfield, the trenching, and the layout work that gets the well, the septic, the building and the driveway to coexist within the setbacks on the land you actually have.

The single most useful thing we do at this stage is dig the test holes while a machine is already on site. Twenty minutes, and it converts the biggest unknown in your project into a number before you have committed to anything.