A septic system is only as good as the soil under its drain field. The tank does the settling, but it's the soil that finishes treating the wastewater before it reaches groundwater. In the Tennessee Valley that soil sits on limestone, and limestone country has its own rules. Here's what's under Huntsville-area yards and why it shapes the septic system you can have.
The red clay
A good example of the valley's red subsoil is the Decatur soil series, whose official description, kept by USDA's Natural Resources Conservation Service, puts its type location in Limestone County, near Greenbrier. Decatur soils formed in material weathered from limestone. The subsoil is dusky red silty clay to clay, typically 35 to 60 percent clay in its upper part. Even so, the series is rated well drained with moderate permeability, and limestone bedrock is usually more than 6 feet down. The setting it describes gets more than 49 inches of rain a year on average.
So the red clay isn't automatically a deal-breaker. The questions for any lot are how deep the good soil goes, where water perches or rock shows up, and how fast effluent moves through it. That's what the required soil evaluation measures, and it's why two lots on the same road can end up with very different systems.
The limestone underneath, and the sinkholes
Under Redstone Arsenal, geologists describe the bedrock as Mississippian limestone and related carbonate rock, including the Tuscumbia Limestone and the Fort Payne formation, and that kind of rock slowly dissolves. That's karst: sinkholes, springs, caves and open channels in the rock. A 2006 study of Redstone Arsenal, covering 154 square kilometers, counted 1,886 mapped sinkholes, 424 springs and 26 caves, and found solution cavities in about 70 percent of bedrock borings. The USGS mapped the bedrock of the Huntsville quadrangle decades ago with the city and the Geological Survey of Alabama, as part of a study of Madison County's groundwater.
Open channels in rock move water fast and filter it poorly. That's why Alabama's onsite rules are strict about sinkholes:
- A site with caves, sinkholes or similar depressions within 300 feet of the drain field or its reserve area, where sewage would likely affect them, is rated extreme.
- The county may allow system parts closer than 300 feet only with a report certified by a geologist addressing the risk to surface water and groundwater.
- No part of a system is allowed within 50 feet of the rim of a sinkhole, period.
The rules define a sinkhole as a closed depression from collapse of soil or rock below, usually shown by closed contour lines on a USGS topographic map, with the rim set by a professional geologist in the field.
Tell us the town and the trouble, and we will put you through to a licensed septic contractor. (256) 692-6131
Hillsides and mountains
Lots on the mountains and ridges around Huntsville run into the slope rules: a drain field on ground steeper than 25 percent requires an engineer's design, and over 40 percent is unsuitable for a system. Steep lots often end up with pumps sending effluent to a better spot, which brings in the pumps and alarms that need regular service.
What this means for your system
| If your lot has | Expect |
|---|---|
| Deep, well-drained red clay, gentle slope, no sinkholes nearby | Often a conventional tank and gravity field |
| Shallow rock, a perched water table, or slow soil | An engineered design, and possibly a mound or dosed field |
| A sinkhole within 300 feet of the field area | An extreme rating; a geologist's report or a different field location |
| Slopes over 25 percent | An engineer's design; over 40 percent, no field on that slope |
For an existing system, the lesson is protecting the field you have: keep roof drains and runoff away from it, don't drive or build on it, plant only grass over it, and keep up with pumping so solids never reach it. Replacing a field in this ground means a new evaluation under the permit process.
FAQ
Can you put a septic system in red clay?
Often, yes. Red clay soils like the Decatur series are well drained with moderate permeability. The required soil evaluation decides what kind of system a given lot can use.
How close can a septic system be to a sinkhole in Alabama?
Within 300 feet of a sinkhole that sewage could affect, the site is rated extreme and needs a geologist's report. No part of a system may be within 50 feet of the rim.
Why do some Huntsville-area homes have septic pumps?
Steep lots, shallow rock or slow soil can push the field uphill or into an engineered design, and a pump moves the effluent there.