We have a large pond, maybe about 50 feet diameter, in our garden. It isn't lined, so it relies on the natural impermeability of the chalky ground to retain (some of) the water. The only inlets are rainwater from the gutters of the garage, and the rising water main which can be turned on to feed the same pipe from the rainwater gutters to top up the pond when there hasn't been any rain. We've seen the level of water in the pond gradually fall over the past few months, so it is draining away into the ground faster than rainwater is refilling it (apart from brief occasions when there is heavy rain).
We've let the pond dry out because we wanted to dig out some of the bed of the pond to build up the spoil in the middle to create an island for ducks etc. The surface was dry (ie no surface puddles of water) but the soil underneath was( about 2 feet of damp, gooey, blancmange-texture mud which is holding a lot of water, and below this is harder chalky ground.
I've dug a trench, roughly 18" to 2 feet down to the chalk, and about 2 feet wide at present, from the edge towards the centre and then one from there round the area that we want to turn into an island, creating a "moat". The intention is to gradually widen this moat and generate more spoil to raise the island. I've put all the spoil in the centre, raising the level of what was already a low mound - so the spoil is going onto ground which is more dry than the blancmange-texture soil on the bed of the pond.
Here's the weird thing. The trench has gradually filled with water. I presume it is draining from the surrounding soil into the trench. Fair enough. But the level of water in the trench has now risen *above* the sides of the trench and has started to spill over, creating puddles on top which weren't there before I dug out the trench. How is this happening?
There has not been any rain for several weeks, so presumably no more water is getting into the pond. The soil will be capable of holding a certain amount of water: if there is more water than the soil can hold, it will rise above the surface of the soil and the pond will fill with water; if there is less water, it will be retained entirely within the soil and the surface will dry out. I've removed a large volume of soil and placed it above the water line. The water that this soil contained will have drained back into the pond and water from the surrounding soil will also drain into the "moat". How can the removal of a lot of earth, effectively just retaining the water that it held, cause the level of water in the pond to rise
*higher* than it was before? I'd expect the level of water in the soil to be lower than before because I've created extra space by removing some of the soil.
The before and after states are about 1 week apart, so with no rainfall it's unlikely that the surrounding land in the garden is draining much into the pond.