I would opt for installing some perforated pipe under the liner to
vent the gas. There could be some rotting vegetation buried there
creating the gas or just naturally occurring do to the geology of the
area.

Mike Barnes wrote in message ...
We have a problem with gas under our pond liner. I'd like to know what
causes it and what to do about it.

The pond is about 15,000 litres (which I make about 3,200 gallons) over
a butyl liner. The gas has been coming and going over the three years
since the pond was dug. It lifts the liner, sometimes above the level of
the surface - it looks bad and it tips over the plant tubs. I know it's
gas and not water because if I sweep it right to the edge it bubbles out
with a fetid smell.

My best guess at what to do is to install some kind of pipe work under
the liner, to allow the gas out before it collects and becomes a
nuisance. Any suggestions?

The pond is 100 cm (3') deep in the middle, with shelves at 60 cm (2')
and 30 cm (1'). The gas collects mainly over the 60 cm shelf and at the
edges. I suspect that the 60 cm shelf was dug with a slight slope away
from the centre, otherwise the gas would find its own way to the edge.