Bentley Village Pond Update
Published: 18 August 2020
There had been some social media speculation that the loss of water in the pond was the direct result of the remedial dredging work that was carried out at the end of last year. The purpose of this work was to remove the thick layer of silt that had built up and to put it on the banks to create a more interesting and wildlife-friendly environment. We believed this work had been successfully carried out, but then the drought arrived meaning the pond received little water inflow since March.
The surface layer of the bottom of the pond dried out and cracked and it appeared to onlookers that this was the cause of the water apparently leaking away. The contractors assured us that nothing they had done would have resulted in this. However, the social media speculation was advancing the theory that this was an ancient puddled clay pond and this leak proof layer had been removed in the dredging operation.
Puddled clay is in effect a natural waterproof liner that was indeed used by man over the centuries to create many artificial ponds in our agricultural and village landscapes. These are made by using natural valley and damming it or a hollow is dug by hand. The bottom is lined by clay imported from elsewhere. Typically, it was trampled by foot to spread out the layer and provide the water seal to prevent leakage. Such puddled clay would appear as a distinct 100-200 mm layer if the bed of the pond was investigated.
Therefore, we resolved to try and examine this possibility in a bit more detail. On Wednesday 12th August (luckily only hours before the rain arrived) a soil investigation of the pond bottom was carried out.
There was a residual layer of more than 50 mm of silt which in the high points had clearly dried out with large 10mm cracks. In lower areas of the pond there were deeper layers of silt but effectively little water was left.
Examination of the dredged material and the residual silt layer showed it did not contain clay and was a soil/organic constituency and not capable of providing a waterproof seal. Therefore, any waterproof layer had to be beneath this in the bed of the pond.
With expert geologists guidance a test pit was dug which revealed at least a 500 mm deep layer of gravel with no evidence of any clay. This proved very difficult to dig through and contained heavy flint content suggesting it had not been artificially created. This is most likely undisturbed ground in effect a natural river bed deposit from when the Wey river was a much larger ice age outflow.
There was no clay identified in this pit and it proved impossible to hand drive a probe to greater depth. The geological maps for the area do indicate there is a lower level clay bed which will prevent the water draining vertically. In this situation water may permeate slowly through the gravel horizontally but in the main the water runs off the surface into ditches which then flow downhill to the river. The soil at this depth was grey/blue coloured which lost colour on exposure to the air. This indicates an aerobic environment and is significant in that it indicates there is not significant water flow drainage through the gravel.
Thus in normal weather conditions water flows from the brook, through the pond to overflow. Water is held back in the pond by a dam in the outlet. Therefore we are sure that the loss of water level in the pond is entirely a result of the drought. As clay dries out in drought mainly as a function of trees extracting water, the water table will begin to drop. The drop in the water table then means that the water may slowly drain away through the gravel. Normally the continuous water flow in from the brook guarantees a surplus of water.
On Wednesday and Thursday the effect of a rainfall ( only about 30mm of rain) was enough to establish 150mm of water in the pond which has not immediately drained away. This proves that it does not take much rain to establish a good pond level. So, the water holding capacity of the pond is still intact and the wildlife can recover.
We are reasonably confident this drought is an extreme situation but will see what happens in future. If this phenomenon becomes a more regular event we have a few contingency ideas to mitigate a repeat. However, at the moment we are hoping this scenario will be unlikely in spite of climate change concerns.