Seepage Pits and Drywells: Why They Get Replaced (Oregon)

Quick Verdict

Seepage pit replacement in Oregon means retiring a deep, rock-filled disposal pit, often called a drywell in this context, and installing a modern shallow drainfield in its place. A seepage pit was historically used after a septic tank to dispose of effluent vertically, deep into the ground. That deep vertical disposal is now disfavored because it gets effluent too close to groundwater and tends to clog at depth where it cannot be maintained. Replacement looks like a new site evaluation, a conforming shallow drainfield (or whatever the soil now allows), and proper decommissioning of the old pit. In Oregon this is regulated work, a DEQ-licensed installer and a county permit are required, and a high water table makes deep pits especially problematic.

What a Seepage Pit Is

A seepage pit, sometimes called a drywell in the wastewater sense, is a deep, rock-or-rubble-filled vertical pit used to disperse septic effluent downward into the soil. It typically sat downstream of a septic tank, the tank settled solids, and the pit handled the liquid by letting it seep away vertically.

It is different from a cesspool, which received raw waste with no tank and no treatment. A seepage pit at least followed a tank. But both are deep-disposal relics, and both get replaced for related reasons. The cesspool version is covered in cesspool replacement; this article is the seepage pit. The full dig side is in our septic system excavation guide.

Why Deep Vertical Disposal Is Disfavored

Modern onsite systems use a shallow drainfield, effluent disperses through a wide, shallow area of soil where it is treated as it moves through the biologically active upper soil layers. A deep seepage pit does the opposite: it sends effluent straight down, deep, fast, and concentrated.

That creates two problems:

• Groundwater protection: deep disposal puts effluent closer to groundwater with less soil treatment in between, which is exactly what modern rules try to avoid
• Clogging at depth: the pit walls clog over time, and at depth there is no practical way to maintain or restore them, so the pit eventually fails

A shallow drainfield treats effluent better and spreads the load over more soil. That is why the standard of care moved away from deep pits, and why a failing seepage pit is usually replaced with a conforming shallow system rather than another pit.

What Replacement Looks Like

Replacing a seepage pit follows the regulated onsite-wastewater path, not a quick swap.

Step	What happens
Site evaluation	A new soil and site evaluation determines what system the ground now supports
Permit	A county/DEQ permit is pulled for the new system
Install conforming system	Typically a shallow drainfield sized and placed per the evaluation
Connect and inspect	The system is tied in and inspected
Decommission old pit	The old seepage pit is properly abandoned so it leaves no hazard

As with any onsite system, the evaluation drives the design. What the soil allowed decades ago is not what current rules require, so the replacement is designed around a fresh evaluation. The master context is the Oregon excavation contractor guide.

The High Water Table Problem in Oregon

A deep vertical pit is especially bad where the water table is high, and much of Oregon qualifies. In the wet valley and in coastal areas, groundwater can sit close to the surface for part of the year. A deep seepage pit may reach down into or near that groundwater, which defeats the treatment the soil is supposed to provide and risks contamination.

That is a core reason seepage pits get replaced in Oregon specifically, and why the replacement system has to be designed around the actual water table. How a high water table shapes a septic design is covered in septic on a high water table. Where the groundwater is high, a shallow, properly separated drainfield is the conforming answer, not another deep pit.

Current Market Reality

Seepage pit replacement cost depends on the site evaluation result, the system the soil dictates, depth, access, and haul-off, so it is never a fixed price. Industry Baseline Range: excavation runs $150 - $350+ per hour, haul-off runs $250 - $750+ per load, residential permits run $100 - $600+, and small jobs carry a $500 - $1,500+ minimum callout, with the new system itself a separate, larger line. These are industry baseline ranges for planning only -- actual pricing depends on site conditions, soil, access, depth, haul-off, and current market conditions. Get a site-specific quote. A high water table or poor soil that forces an alternative treatment system raises the cost substantially.

Decommissioning the Old Pit Safely

The replacement system gets the attention, but properly retiring the old seepage pit is its own task and an important one. A deep, rock-filled pit that is simply disconnected and forgotten does not disappear, it remains a void in the ground that can settle, slump, or collapse over time, and an old pit's contents are not something you want migrating.

Proper decommissioning means more than turning off the flow. The pit is typically pumped out, then handled so it cannot leave a hidden hazard, generally by collapsing or filling it so there is no open void, in line with what the county and DEQ require. This is regulated, because both the structural hazard and the groundwater concern are exactly what the rules aim to control.

A few points on doing it right:

• The old pit is pumped before it is closed
• It is filled or collapsed so no void remains to settle or cave
• The work follows county and DEQ requirements for abandonment
• The location is recorded so a future owner knows it is there

This is also why a seepage pit replacement is more involved than just installing a new drainfield, you are closing one system safely while opening another. A homeowner who only installs the new field and walks away from the old pit has left a hazard and may not satisfy the permit. The complete job retires the old and establishes the new, both done to the rules, which is part of why an evaluation and a licensed installer matter from the start.

The Bottom Line

A seepage pit or drywell sends septic effluent deep and vertical, which is disfavored today because it nears groundwater and clogs where it cannot be maintained, problems that are worse in Oregon's high-water-table areas. Replacement means a fresh site evaluation, a conforming shallow drainfield, and proper decommissioning of the old pit, all under DEQ and county rules with a licensed installer. The right first step is an evaluation. Cojo handles seepage pit replacement excavation across Oregon. See our excavation services or request a free estimate to start with a site evaluation.