No one wants to see odd puddles of water over a buried septic tank except in the hour or two after a good rain. But if you're now dealing with these unwelcome sights, you need to figure out the cause, fast. Because unless it just stopped raining or someone aimed a garden hose at the spot where the tank is buried, you have a problem on your hands.
Poorly Draining Soil and Effluent Backup
This is the big one, the one you hope won't happen. Sometimes septic tanks can spring a leak, letting more effluent flow out than it should, and the soil doesn't drain well enough to handle the increased flow. Normally your tank does release some liquid that's been separated from solids; the liquid flows into pipes that have perforations, allowing the liquid to seep into the soil. This is why you never want to play on or grow anything edible in a field near a septic tank; it's likely the drain field for the liquid from the tank.
However, if the tank itself leaks, then more liquid -- and possibly solids, if the hole is big enough -- leach into the surrounding soil at an increased pace. If the soil doesn't drain well, the liquid can saturate the soil and pool on top where it begins to smell. If you suspect a leak, you need to have a septic company inspect your tank immediately. This is somewhat of an urgent issue.
Soil Shifting and Tank Destabilization
Another potential issue, though one that's not as common as the leak option, is that the soil around the tank has shifted. This could be due to a leak under the tank eroding soil there, or it could be due to repeated rain eroding nearby soil. If you've had recent seismic activity, that could do it, too; you can also see this shift if the soil was unstable to begin with and the people who installed the tank didn't bother to check.
The bad news here is that if the soil shifts, the tank might no longer be stable. It could shift, too, and open up. Again, it's time for a tank inspection.
Potential Foot Traffic
One more cause that doesn't necessarily affect the tank but is cause for concern is unexpected foot traffic over the tank, compacting the soil. If the soil is poorly draining and sprinklers hit the compacted areas, that could leave puddles. Stop people from running over the tank, if for no other reason than it will stop them from getting septic soil all over their shoes. But the leak and shifting soil are more likely culprits.
Call a septic tank maintenance company and arrange an inspection now. Do not mess around with septic because the longer you let this go on, the higher the risk that something will really start to go bad.