Alright, let’s break this down. Southern California, for all its beaches and movie magic, isn’t exactly teeming with public sewers in every nook and cranny. Out in the hills, the canyons, or those weird patchwork neighborhoods, people have gotten used to relying on onsite wastewater treatment systems (OWTS) for the acronym lovers. And, look, among all the ways to deal with your, uh, “personal plumbing,” seepage pits are kind of the unsung heroes. Basically, imagine a vertical hole where partially cleaned water goes down deep and gets filtered by good old dirt, right?

Seepage pits come in clutch when your property’s all sloped, squished, or stuck on uncooperative surface soils. For example, it is the only thing that’ll fit or work. But don’t just start digging a hole and call it a day.

LA and San Bernardino County (those two love a good rulebook) want you to do your homework. Check out the site, poke around in the soil, and make sure you’re not about to break a bunch of local laws.

Here’s the thing—if you botch the design, you’re not just risking a funky backyard. We are talking about groundwater pollution, busted property, and the kind of health code violations. These will have county inspectors breathing down your neck. Not the vibe.

This guide explains how to correctly plan, test, and permit seepage pit systems. It will assist in following the standards. These standards are outlined by LA County Environmental Health and the California State Water Board.

Seepage pits are sort of the evergreen trend when you just cannot get your yard to behave. Soil with rocks everywhere, extremely narrow lots, or hills. It tends to remind more of ski mountains than nice lawns to roll up and down as a play activity in your yard. You’ll see this all over Malibu, Topanga, and Altadena. The places where finding a flat spot for a regular leach field is like searching for a unicorn.

Out in LA, San Bernardino, Riverside, and Ventura counties, especially way out where the city pipes don’t bother to reach, these pits pop up everywhere. It’s a deep hole stuffed with gravel.

To ensure safe operation, systems must follow strict California OWTS Policy guidelines. One can follow any additional county-level oversight. Improper implementation can lead to health hazards, system overload, and environmental fines.

The first major step in determining whether a seepage pit is viable is a percolation test. It measures the rate at which water infiltrates the soil.

Testing Process:

Drill 3–4 test borings, typically 15–30 feet deep. Presoak each borehole to mimic actual wastewater conditions

Here is how it would sound if an actual person were scribbling notes for someone else:

1. Note down water drop readings every so often for, like, six hours straight. 
2. Go have a legit geologist or engineer there to sign off. Otherwise, it’s just playing in the dirt. Best time to do this? After it’s been pouring for a while. 
3. Since you want the ground to be soaked, not bone dry. 
4. If the soil’s drinking water is super slow, or if you hit water way too close to the surface, forget about using a seepage pit. Because rules are rules. 
5. Note the soil texture, any weird color shifts, and how deep you hit water. 
6. Compile all that into a geotech report for people to read. 

These are the LA County Soil Requirements for Seepage Pits.

LA County enforces specific soil composition and permeability standards for seepage pit approval.

Check the LA County OWTS Manual to check site-specific regulations. The following are the basic requirements:

• Granite decomposed into coarse sand.
• Inappropriate materials: clay, silt, or fractured rock with perched water.
• Vertical distance of 10 feet and above the groundwater, with at least 100 feet of setback from water wells, 10-foot buildings, and 5-foot property lines.

Once a site is approved, the seepage pit must be designed to accommodate projected daily flows and match the soil’s absorption capacity.

Core Design Elements:

• Diameter: Typically 4-6 feet, Depth: Normally 15-30 feet
• Liner: Reinforced concrete rings or perforated PVC pipe, 
• Gravel Fill: 3- 4 inch clean gravel
• Access Lid: watertight concrete cover with a riser in it to allow inspection

Example Calculation:

A 3-bedroom home generating 450 gallons/day. It is with an absorption rate of 1 gal/ft²/day and would need 450 ft² of dispersal area. A 5-ft wide, 20-ft deep pit provides ~314 ft². Thus, two pits may be necessary.

Designers use: Area = π × Diameter × Depth

Engineering calculators can assist with volume and surface area estimates.

All seepage pit systems require review and permitting. In Los Angeles, it is required by the LA County Environmental Health Division.

Permit Submittal Must Include:

These documents are a must for a permit from the LA County Environmental Health Division

1. Certified percolation test report.
2. Site map showing structures, slopes, and setbacks.
3. Final pit design with engineering calculations.
4. A stamped letter from a licensed engineer or geologist

Timeline:

Reviews take 4 to 6 weeks. Delays may occur during peak construction season. Corrections are common if groundwater, sizing, or setback requirements are not met.

The geologist does more than collect soil samples. Their input is key to ensuring long-term system reliability.

Geologist Responsibilities:

In unstable or complex sites, a geotechnical engineer may assist in assessing landslide risk. He can also assess the structural compatibility. The following are the responsibilities of a geologist:

• Classify and describe soil horizons
• Record seasonal groundwater depth.
• Evaluate infiltration suitability.
• Make final recommendations on location, depth, and layout.
• Certify findings per California state licensure.

If the site fails to qualify for seepage pits, other OWTS alternatives must be considered:

• Shallow trench systems are horizontal flow designs in permeable upper soils. 
• Pressurized dispersal systems promote even effluent distribution. 
• Advanced Treatment Units (ATUs) are designed to pre-treat waste to higher standards
• Evapotranspiration beds are ideal for desert zones with sandy soil, holding tanks

It is used only for short-term or emergency purposes. These alternatives are governed under Tier 2 and Tier 3 requirements of California’s OWTS Policy.

A well-designed seepage pit still requires routine care to prevent failure.

Maintenance Tips:

Get someone out to check things every few years—like, every 3 to 5. 

Don’t let trees or big trucks hang out on top of the dispersal zone unless you want to deal with a mess later. 

Don’t pour grease or gnarly chemicals down there, unless you’re into expensive repairs. If your drains start acting weird, that’s a red flag. 

In some touchy areas, the folks in charge might make you file reports every year. 

If you have one of these systems, you should have a manual from whoever designed it. If you don’t, bug them until you get it.

Putting in a seepage pit in SoCal? Yeah, that’s not just some weekend DIY project—it’s a whole engineered thing. You got a jump through all sorts of hoops. These may be hydrogeology tests, nerdy design math, or paperwork for the powers-that-be. 

Every step has its pile of details. You can’t just gloss over it unless you want to end up with a massive headache (or, you know, a visit from the county). Work only with licensed professionals. Follow LA County OWTS guidelines, and design for long-term performance and compliance.

What’s the ideal percolation rate for approval? Under 30 minutes per inch. The faster, the better.

Can seepage pits be used on clay soils? Usually not. Clay retains water, which can cause system failure.

What’s the typical depth for a seepage pit? 

15 to 30 feet, depending on soil layers and groundwater separation.

Who can conduct a perc test in California? 

Only a licensed engineer or geologist certified in the state.

What happens if my system is denied? 

Your engineer may explore pressurized systems, ATUs, or trench designs.

Do I need a permit to replace an existing pit? 

Yes. All work must be reviewed and approved before installation.