How to Find a Septic Tank With a Metal Detector

How to Find a Septic Tank With a Metal Detector

In the United States, about 48% of households in outlying areas rely on septic systems to treat their sewage[1]. Many of these systems go without regular maintenance for decades.

If sewage is backing up into your house, or your main drain line has been clogged, you’ll need to find your septic tank. In this article, we’ll discuss how septic systems work, and then show you how to locate your septic tank in six steps.

How do septic systems work?

A septic system consists of two parts: the septic tank and a drain field.

Septic Tank

Liquid and solid waste from toilets, sinks, and showers drain through a main sewer line into the septic tank.

A septic tank is a large, buried container that serves as the first part of an on-site sewage treatment system. The tank is a water-tight container made from concrete, steel, plastic, or fiberglass.

Sewage remains in a septic tank until the solids and liquids separate into three distinct layers.

An illustration of a two-compartment septic tank system.
In this illustration, you can see that sewage flows from the house into a two-compartment septic tank. Here, liquid and solid wastes separate. Liquid wastes then flow into the drain field.

Light solids, such as oils, fats, and grease, float to the top of the sewage and form a layer of “scum”.

Heavy solids settle to the bottom of the tank into what's called “sludge”. Anaerobic bacteria (those that live without oxygen) break down the organic compounds of the solid layers. The bacteria create a “digested” sludge that remains in the tank until it’s pumped out.

Sandwiched between the light and heavy solids is a middle layer of liquid wastewater. This layer is commonly called effluent. The effluent flows out of the septic tank and into the drain field, where it gets filtered through gravel and soil.

Drain Field

In the drain field, liquid sewage trickles down through layers of gravel and soil. Eventually, it joins the groundwater supply. Aerobic bacteria (those that require oxygen to live) and other microorganisms break down the remaining organic matter.

What happens if a septic tank gets too full?

If a septic tank gets too full, sewage can back up into your house. Sludge accumulates at a faster rate than it decomposes. If the sludge isn’t removed, the solids will accumulate until they overflow into the drain field. This can clog pipes and cause a backup.

To prevent this, the sludge must be periodically removed. Many homeowners only get their tanks pumped out when the system fails. Waiting until that point can cost many thousands of dollars in repair costs.

We'd recommend pumping a septic tank every three to five years.

6 Steps to Locate a Septic Tank

1. Find Your Main Sewer Drain Line

Sewage from your toilets, sinks, and showers collects into a main drain line. Inside your basement or crawl space, this line exits your house and enters your septic tank. Locate this line. Then, go outside to find that same spot on the other side of the wall.

Mark this location, as you’ll need it in a bit.

2. Check Permits and Public Records

Most county health departments maintain public records of septic system installation permits. These permits include a diagram or drawing of the planned location for the septic tank and drainage fields. They also include a description of the size and material of the tank. This information can be immensely helpful in locating a buried tank lid.

Depending on the age of your septic system, and the digitization efforts of your county’s health department, you may be able to search online public records. Otherwise, you can file a public records request for a small fee.

If you’re in Colorado, we’ve included links below to search septic records in select counties.

3. Determine Septic Tank Material

If you’ve found your septic permit, there is information on the size, shape, and material of your septic tank. If your septic records are unavailable, however, don’t worry. We can do a bit of detective work to figure out what material your septic is made of.

First, let’s explore the materials.

Types of Septic Tank Materials

There are four major materials that septic tanks are manufactured with: concrete, steel, fiberglass, and plastic.

Concrete has been the most widely used septic tank material since the 1880s. These tanks are durable enough to last for roughly 40 years. They can crack, however, in climates that experience extreme temperature fluctuations.

Municipalities with regulations on septic system design tend to require concrete tanks.

Steel was a popular septic tank material from the 1940s to the 1970s, and is still in use today.

In most climates, steel septic tanks begin to rust within 20 to 25 years. As these tanks rust, the overall structure is weakened. If a person or animal walks over the weakened tank, it could collapse under the weight.

Like concrete septic tanks, those made of fiberglass can last about 40 years. Unlike concrete or steel tanks, however, they won’t crack or rust. Because they are much lighter, they’re less costly to install, but more likely to be damaged during installation.

Plastic septic tanks have been in use since the 1980s. They're lightweight and much easier to install compared to concrete, steel, or fiberglass tanks. They won’t rust, and are less likely to crack than concrete.

However, they’re not quite as durable. Plastic tanks can collapse from the weight of the soil above them, or if a vehicle drives over the location where they’re buried.

How Old is Your House?

Next, let’s figure out the age of your house. In some instances, all you need to do is look at the exterior of the house to ballpark its age.

But for a more accurate assessment, you can search your county’s tax assessor records. Much like searching for septic records, your mileage may vary based on your particular county’s digitized records.

Let's walk through an example search using Boulder County's Property Search tool, so that you know what to look for.

  1. Search for your address. Some property search tools have separate fields for your number and street name, while others (like Boulder County) combine them into one search box.
    Boulder County's Property Search tool.
  2. Look for Deeds and/or Sales Records information. Here, you'll find a list of transactions indicating when the property was bought and sold.
    List of Deeds and Sale Records
  3. Locate the earliest transaction date. This is likely when your home was built.

 With this information, you can make an educated guess about the type of material your septic tank is made from. 

4A. If it’s Concrete or Steel, Use a Metal Detector

Based on your conclusions in Step 3, if your septic tank is likely made from concrete or steel, a metal detector can make the task of locating it much easier. But not just any metal detector will do.

Standard metal detectors can reach 6 - 8” (15 - 20 cm) below the surface. As we mentioned earlier, most septic tanks are buried 1’ to 3’ (0.3 m to 1 m) underground, placing them out of that range.

There is a specialized type of metal detector called a Magnetic Locator that can reach up to 16’ (4.8 m) deep. It only detects iron and steel, making it ideal for detecting the iron handles and rebar in concrete septic tanks, or the walls of steel tanks.

How to Use a Metal Detector to Search for a Septic Tank

Referring to the diagram in your septic permit record, measure the distance from the point at which the drain line meets the house, to the point at which the diagram indicates the septic tank would be installed.

Remember, this diagram indicates the planned installation area, and may not reflect the actual conditions in the ground.

We’ll look at the diagram below.

A diagram from a septic permit

In the bottom left corner, we see that 1” (2.5 cm) is equal to 50’ (15 m).

By measuring the diagram with a ruler, we’ve concluded that the septic tank should be approximately 13’ (3.96 m) from the house.

Since the sewage drain line was planned to be installed at an angle, lets expand our search area to between 10’ (3.05 m) to 20’ (6.1 m).

  1. Start at the point you marked in Step 1, where the sewer drain line exits the house.
  2. From here, turn the locator on and set the Gain to High. Sweep the locator from side to side as you walk across the search area.
  3. Mark the areas of the highest signal strength as you go. These are the most likely locations of the tank-lid’s iron handles.
  4. Once you’ve exhausted the search area in one direction, sweep across it perpendicularly, and mark areas of the highest signal strength.
  5. Proceed to Step 5.

4B. If it’s Plastic or Fiberglass, Probe Gently

Plastic or fiberglass septic tanks tend to be buried one to two feet (0.3 to 0.91 m) deep. They have circular lids that are made of green or black plastic, and are approximately two feet (0.91 m) wide.

Because these tanks are entirely manufactured from plastic, a metal detector won’t help you locate them. In this case, a soil probe comes in handy.

Soil probes are a low-tech soil-sampling tool that consists of a 4’ (1.2 m) metal rod with a pointy tip on the end. They’re used by agronomists, soil scientists, farmers, and geologists to quickly determine soil conditions.

Using a soil probe, gently poke the ground every 2 to 3 feet (0.61 to 0.91 m). Mark any spots where you feel resistance, and continue poking across your search area. A light touch is key here, as plastic septic tanks (and sewer lines) can be broken by the probe’s metal tip if too much force is applied.

5. Time to Dig

Once you’ve marked the locations of highest signal strength with a magnetic locator (or probed the ground), you’re ready to dig.

Septic tank lids can be between 4” (10 cm) to 4’ (1.2 m) below the surface.

6. Mark the Location for Future Maintenance

Now that you’ve found your septic tank, you’ll want to make sure it can be easily located and accessed for future maintenance. To do this, you can install a septic tank riser.

Septic Tank Risers provide easy access to the septic tank at ground level. They are plastic or concrete shafts that connect the top of the tank to the surface of the ground. You won’t need to dig to reach the tank lid every time maintenance is required.


About the Author

Gary Iverson

Gary Iverson is a staff writer at Metro Metal Detectors covering all things metal and metal-adjacent.

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