Drainfields fail gradually and then visibly, and the early signs are easy to explain away.
1. Wet or soggy ground over the field
Standing water or persistently soft ground above the trenches, particularly when it has not rained.
This is effluent that the soil can no longer accept, rising to the surface. It is the clearest sign and it is a public health matter as well as a plumbing one.
2. Grass greener and growing faster over the field
A stripe of noticeably lush grass following the trench lines.
On its own it is not proof — a field does deliver nutrients and moisture — and combined with any of the others it is confirmation.
3. Slow drains throughout the house
Not one fixture, which suggests a local blockage, but everything at once.
Slow drains across the house point at the system rather than the plumbing, and the tank or the field is where it is.
4. Sewage odours
Outdoors over the field, or indoors around floor drains.
Indoor odours can also be a dry trap or a venting fault, so it is worth ruling those out. Outdoor odours over the field generally are not.
5. Backing up
Fixtures backing up, starting with the lowest — a basement floor drain or a ground-floor toilet.
An alarm on a pump system going off is the equivalent signal on a system that has one.
6. The tank refills immediately after pumping
A telling one. Pump the tank and check it a few days later: it should have filled to its normal operating level from household use.
If it is full to the outlet within a day or two, effluent is not leaving for the field, or the field is pushing back into the tank.
What else could it be
Before concluding the field has failed, three cheaper faults produce similar symptoms:
A blocked effluent filter, which is a ten-minute rinse.
A collapsed or blocked pipe between the tank and the field, or a crushed distribution box.
A distribution box out of level, sending all the flow to one trench.
Hydraulic overload from a leaking toilet, a running tap, or a water softener regenerating into the system. A single leaking toilet can add hundreds of gallons a day and drown a field that would otherwise be fine.
Each of these is worth ruling out first, because each is inexpensive and the alternative is not.
What can be done for a genuinely failing field
Reduce the load. Fix leaks, spread out laundry, install low-flow fixtures, divert the softener discharge where permitted. Sometimes enough on a marginally overloaded field.
Rest it, where a second field or an alternating arrangement exists.
Rehabilitation — jetting, aeration or chemical treatments — which have mixed evidence and are worth trying only on the advice of someone who has examined the system.
Replacement, in the reserve area, which is why the reserve area is required.
The thing not to do
Ignore surfacing effluent. It is untreated sewage at ground level, it is a genuine health hazard for the household and for anything downhill, and in most jurisdictions it is a reportable failure.
Getting a licensed inspector to look at it produces a diagnosis and, frequently, a cheaper answer than the assumption.
Season matters
Fields fail visibly in the wet season, when groundwater is high and the soil is already carrying water.
A system that surfaces in March and behaves all summer is not fixed; it is a field at the limit of its capacity, and next spring will be worse. That is the window in which reducing the load or planning a replacement is still a choice rather than an emergency.