Drainfield Failure Assessment

Purpose

This SOP produces a written, evidence-ordered determination of WHY effluent stopped accepting, and a repair-versus-replace recommendation that survives a second opinion, a health department review and a property transfer two years later.

The failure it prevents is condemning a field on a wet spot. The evidence separating an exhausted field from a plugged filter, a tipped distribution box or a running toilet is all gathered in the first two hours, in a specific order, and out of order the cheap causes get buried under the expensive one. A shop replacing three recoverable fields a year will never find out, because a replaced field cannot come back to prove it was wrong.

Scope

Covers assessment of conventional gravity trenches, beds, chamber systems and pressure-dosed fields at residential and small commercial sites, up to the point a recommendation is written.

Does NOT cover the repair itself, mound construction (mound system construction SOP), tank structural defects (baffle inspection and replacement SOP), or the design work that follows a replacement (permit and inspection coordination SOP). Atmospheric testing before any lid comes off is owned by the gas testing SOP and is a prerequisite here, not something to improvise.

Roles and handoffs

Procedure

1. Stop the exposure before any diagnosis. Where effluent has surfaced, set a continuous physical barrier around the wet perimeter plus a pace of dry ground, and get children and pets indoors before the truck is unloaded. Acceptance: barrier closed on all sides, customer told not to cross it. Wrong is a tech walking through the wet area to reach the tank; if that has happened, boots come off outside the vehicle and hands get washed first. Surfaced effluent is raw sewage at grade, so nobody hoses, rakes or pressure-washes it, because all three put it in the air.

2. Read the record before you read the ground. Pull the permit, as-built, design flow, pumping history, occupant count and any water treatment discharging to the system. Acceptance: system age, design daily flow, trench count and the last two pumping dates written down before the first measurement. Wrong is an as-built putting the field on the opposite side of the house from the wet spot; when drawing and ground disagree, believe the ground and mark the record unverified. Probing to an unverified as-built is how a steel probe finds a pressurized lateral, so call 811 and treat every private line as unlocated.

3. Establish tank liquid level against the outlet invert first. This decides whether the restriction is upstream or downstream of the tank, and doing it after looking at the field is the most common way a good field gets condemned. Follow the gas testing SOP before the lid moves, then measure from a fixed datum on the tank wall to the liquid and to the bottom of the outlet tee. Acceptance: liquid within 1 in of the outlet invert. Wrong is more than 1 in above it; the stop rule is that you do not evaluate the field yet, you go to step 4. Well below the invert with no recent pumping is a leaking tank, a different job.

4. Pull and read the effluent filter before you clean it. The loading pattern is evidence and washing it destroys the evidence. Lift the cartridge into a bucket, photograph the face, estimate what fraction of the slot area is occluded by continuous mat rather than loose debris. Acceptance: slot face open across its full height, no mat bridging the slots. Wrong is mat over more than about a quarter of the slot area; clean it per the effluent filter cleaning SOP, let the tank draw down and re-measure step 3 before concluding anything. A loaded cartridge releases concentrated headspace and throws effluent at face height, so eye and face protection go on before the lid moves.

5. Open the distribution box and measure it, do not eyeball it. A box that has settled a fraction of an inch sends most of the household flow to one trench for years. Set a level across the outlet inverts and measure each against the lowest. Acceptance: all inverts within 1/8 in of each other, all outlets clear of solids, inlet not scoured into a channel. Wrong is any outlet more than 1/8 in low or packed with solids; that is a maldistribution finding, written down as a cause before any trench is called failed. Old fibre and thin concrete lids fail under a kneeling adult, so sound the lid with a bar and kneel on a plank spanning firm ground.

6. Measure ponding in every trench, and record the gravel depth it is measured against. A ponding figure without its trench depth beside it means nothing. Drop a probe to the trench bottom through the inspection port, mark the liquid line, express ponding as a percentage of gravel depth. Acceptance: under 25 percent in every trench. Wrong is over 50 percent, a saturated trench, and over 75 percent with surface breakout, a failed trench; one failed trench beside a dry one is a distribution finding, all of them together is a field finding. Do not walk a saturated field to reach a port, because wet sidewalls collapse under foot load and drop a leg into raw effluent.

7. Reconcile actual daily flow against design flow before blaming the soil. Read the water meter, leave it 7 full days at normal occupancy, read again, divide by 7. Acceptance: metered daily flow at or under the design flow the permit was issued for, which many state codes size around 150 gallons per day per bedroom, though your state's table governs and not that figure. Wrong is metered flow above design, or flow that never falls overnight, which is a leak rather than a field fault; a running toilet is a continuous dose that drowns a healthy field, so it gets fixed and the field re-measured before replacement is quoted.

8. Open one observation pit at the trench sidewall and read the soil. Hand-dig or auger alongside, never into, the trench, deep enough to expose the sidewall interface. Acceptance: pit no deeper than 4 ft, spoil set back from the edge, nobody in the hole. Wrong is a machine-dug pit someone steps into; egress is required at 4 ft under 29 CFR 1926.651(c)(2) and a protective system at 5 ft under 1926.652(a)(1), so a pit needing a person inside is a different job with a different plan. Black, greasy, structureless material at the interface is mature biomat; clean structured soil behind a wet trench says the soil never had the chance to clog.

9. Classify, then write the recommendation with evidence attached to each element. State one of four findings: tank-side restriction, maldistribution, hydraulic overload, or soil exhaustion, citing the step and measurement behind it. Acceptance: every element traceable to a numbered step, and any replace recommendation countersigned by the service manager. Wrong is a recommendation reading "field is shot"; a finding with no measurement behind it goes back and the visit is rebooked at the shop's cost. Where two causes are present, name which is corrected first and when ponding is re-measured, because a rested field that recovers is the only proof the cheaper fix worked.

When the site does not match

No inspection ports and no as-built means step 6 cannot be performed: say so, and quote port installation as its own visit rather than guessing ponding from a probe pushed blind into gravel. A frozen or snow-covered field gives no valid ponding read, so book it for thaw and document the limitation. A customer who declines the 7-day meter read gets step 7 marked "declined by customer", and the recommendation states which conclusion that missing measurement would have changed. A field buried under a driveway or an addition is a design problem: stop and hand it to the designer.

The record this produces

One drainfield assessment record on the work order: date and technician, permit and as-built reference or "no record found", system age and design flow, tank liquid level relative to outlet invert in inches, filter condition with occluded fraction and photograph, every D-box outlet invert measured against the lowest, ponding and gravel depth per trench with the percentage computed, metered 7-day flow as gallons per day, observation pit depth and interface description, the classification, the recommendation, and the countersignature on any replacement.

The service manager reads the classification column across a quarter and learns whether the shop is diagnosing or defaulting. The designer reads the ponding and flow figures instead of re-measuring them. The tech on the recheck visit reads the original percentages and can say whether the field moved.

Worked pass: four-bedroom home, 1993 gravity trenches, soggy ground over one trench

Barrier set, dog indoors. Record showed a 1,250 gallon tank, three gravity trenches at 18 in of gravel, design flow 600 gallons per day on a four-bedroom permit at the state's 150 gallons per day per bedroom, last pumped 4 years ago.

Step 3 failed. Liquid stood 5 in above the outlet invert against the 1 in acceptance, so under its stop rule the field was not evaluated yet. Step 4 found continuous mat across roughly 80 percent of the filter slot face, well past the quarter-face threshold; the cartridge was cleaned and the tank drew down. Re-measuring step 3 gave 3 in above the invert, still failing, so the filter was one restriction and not the only one.

Step 5 found the trench 2 outlet 1 in below the other two, eight times the 1/8 in gate, with the trench 3 outlet half buried in solids. Step 6 put trench 2 over the 75 percent failed threshold and trenches 1 and 3 inside the 25 percent acceptance. Step 7 metered 1,610 gallons over 7 days, which is 230 gallons per day, 38 percent of the 600 gallon per day design, so hydraulic overload was ruled out on measurement rather than impression. Step 8 showed mature black biomat at the trench 2 sidewall and clean structured soil behind trench 3.

Classification: maldistribution, with one trench failed as its consequence. The box has fed trench 2 nearly the whole household flow instead of a third of it, so across 33 years that trench absorbed on the order of three times the cumulative volume its sidewall was sized for while two thirds of the field sat unused. Recommendation: level the box, fit flow equalizers, clear the trench 3 outlet, rest trench 2, re-measure ponding at 90 days, and quote replacement only if trench 2 has not fallen below 50 percent.

Checking the run against the rules above: each ponding percentage is stated against the gravel depth it was measured on, and the 230 gallons per day is metered and compared against the same 600 gallon per day design the permit carries. No soil loading rate appears anywhere, because no percolation test was run and none was invented.

References

• EPA Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008) for soil absorption behaviour, biomat formation and hydraulic loading.
• 29 CFR 1926.651 and 1926.652 (construction excavation): egress at 4 ft, protective system at 5 ft, for any observation pit a person would enter.
• Your state onsite sewage code for design flow per bedroom and soil loading rate tables; these vary by state and the issued permit governs.
• See related: the gas testing SOP, the effluent filter cleaning service SOP, the baffle inspection and replacement SOP, and the drainfield repair versus replace reference card.