Leach Field Replacement Cost: Repairs to Full Drain Fields
A full leach field replacement runs $3,000 – $15,000 for a conventional field; partial repairs land at $1,500 – $5,000. Jetting and bio-remediation attempts run $500 – $2,500 and sometimes buy time. If the soil itself has failed and you need a mound conversion, expect $10,000 – $25,000. Here is how to tell which one you actually need.
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| Scope | Typical range | When it applies |
|---|---|---|
| Jetting / bio-remediation | $500 – $2,500 | Early biomat, line cleaning, additive treatment |
| Partial field repair | $1,500 – $5,000 | One failed trench or line replaced or rerouted |
| Full conventional field replacement | $3,000 – $15,000 | New trenches sized to soil and bedrooms |
| Mound conversion | $10,000 – $25,000 | Native soil failed the retest, field built above grade |
| Soil retest / perc test | $450 – $1,500 | Required before any new field is designed |
| Permits & design | $600 – $2,500 | Engineered plans for difficult lots |
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Why fields fail: the biomat
The drain field is the part of a septic system with a clock on it. As clarified effluent soaks into the soil, a layer of bacteria called the biomat forms along the trench bottoms. A healthy biomat is good: it does the final treatment. The problem is that it keeps thickening, and eventually it seals the soil so water can no longer pass, which is hydraulic failure. The field stays soggy because effluent has nowhere to go.
The fastest way to kill a field early is to send it solids. When a tank is overdue, sludge escapes and clogs the trenches in years instead of decades, which is why staying on a septic tank pumping schedule is the most affordable field insurance there is. Excess water from leaks, downspouts draining over the field, or a household that simply uses more than the field was sized for also drowns it. Most fields reach hydraulic failure at 20 – 30 years; the ones that fail at ten almost always had a solids or water problem upstream.
Repair or replace: making the call
The most affordable interventions target the field before the soil is fully sealed. Jetting clears the line from the tank and the distribution box, and bio-remediation adds bacteria or oxygen to break down a young biomat: together $500 – $2,500, and worth trying when symptoms are new and limited to one area. These buy time; they do not reverse a field that has failed for years.
A partial repair ($1,500 – $5,000) makes sense when one trench or one lateral has failed while the rest of the field still absorbs, often paired with adding a few new trenches in reserve area. Full replacement ($3,000 – $15,000) is the answer when the whole field is saturated, effluent is surfacing, and the biomat has sealed the soil everywhere. The honest tell is time and area: a single wet spot on an otherwise dry field may be a repair, while a field that is wet end to end and smelly after a septic inspection flow test is a replacement.
The soil retest changes the price
You cannot design a new field without a current soil evaluation, and the result can move the budget more than any other factor. A perc test ($450 – $1,500) at field depth measures how fast water percolates. If the native soil still drains well, a new conventional field at the bottom of the range is feasible, sometimes in a fresh reserve area beside the old one.
If the soil itself has degraded, has too high a water table, or never drained well to begin with, a conventional field is off the table and you move to a mound system ($10,000 – $25,000), which builds the field above grade in imported sand with a pump to dose it. This is the same fork that drives new installs, covered in our septic system cost guide. Spend the soil-test dollars first: everything downstream is a guess until that number exists.
Protecting the new field
A new field is a 20 – 30 year asset, and the habits that kill the old one will kill the new one on the same schedule. Pump the tank on a 3 – 5 year cycle so solids never reach the new trenches, and replace or add an effluent filter at the tank outlet if there is not one, since a $50 filter is the single strongest guard against the failure you just paid to fix.
Keep traffic off it: no vehicles, no parking, no sheds or patios over the field, because compaction crushes the trenches and roofs over it concentrate water. Route gutters and surface runoff away from the field, plant only grass over it rather than trees whose roots chase the moisture, and spread out water-heavy laundry days. A field that surfaces fast despite good habits often points back to the tank, so confirm the tank and baffles are sound before celebrating, and pump-and-clean it on schedule going forward.
- ·Pump the tank every 3 – 5 years, no exceptions
- ·Keep vehicles, structures and trees off the field
- ·Divert gutters and surface water away from the trenches
- ·Add an effluent filter at the tank outlet if missing
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