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.

https://i.postimg.cc/LsY2FJLj/Scan.jpg shows a side view of the before and
after, with the spoil from the trench dumped in the centre of the pond. I've
exaggerated the depth of water afterwards: in practice it is shallow puddles
on the surface which weren't there before I started digging.

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.

On 25/04/2022 10:46, NY wrote:
> 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. ..

Chalk is not impermeable. It is rain falling on the Downs that seeps
through the chalk there, which is layered between two impermeable
layers, to create a large reservoir of water under London.

....
> 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?
....

That is probably the water table; the natural level of water under the
surface which does vary. In winter, before I had my garden raised, the
water table could be above the surface by an inch or so. So, it would
flood without any recent rain.

--
Colin Bignell

On Mon, 25 Apr 2022 10:46:07 +0100, "NY" <me@privacy.invalid> wrote:

>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.

As Colin says, chalk is quite permeable due to its well developed
interconnected network of fractures. https://tinyurl.com/3y2s247n

>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?
>

I suggest the reason your trench has filled to overflowing is that the
natural water table is higher than the top of the trench, but the
water had been held back by the impermeable sludge in the bottom of
the pond. Now you've cut through the sludge, it has allowed the water
to rise to it's equilibrium level.

--
Chris

Chris Hogg <me@privacy.net> wrote:
> On Mon, 25 Apr 2022 10:46:07 +0100, "NY" <me@privacy.invalid> wrote:
>
>> 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.
>
> As Colin says, chalk is quite permeable due to its well developed
> interconnected network of fractures. https://tinyurl.com/3y2s247n
>
>> 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?
>>
>
> I suggest the reason your trench has filled to overflowing is that the
> natural water table is higher than the top of the trench, but the
> water had been held back by the impermeable sludge in the bottom of
> the pond. Now you've cut through the sludge, it has allowed the water
> to rise to it's equilibrium level.
>

It is also possible you have an unground spring in the area.

Our house is built on the side of a valley. I used to work with someone who
is about 70 now at an estimate and he can recall a small stream in the
bottom of the valley. There is no trace of it now, there are houses and a
road there. It was fed by a small spring, also long gone, which ran through
the gardens of some houses in our road.

As more water has been extracted from the under ground aquafas, the water
table has dropped and all traces have gone.

A neighbour made some unauthorised modifications to his drive- he removed a
LOT of earth leaving the front gardens either side unsupported. The Council
stepped in an forced him to replace it as there was a danger, in the event
of heavy rain, there would be a land slide.

There was also a natural pond / lake - know as a mere- which gave the
adjoining area its original name Wigmere. It was changed / corrupted to
Wigmore. It dried up long ago - I suspect a hundred years plus- before the
area started to be built on post WW1 when small plots were sold as small
holdings. It is now a residential area. We used to have a house built in
the 1930s there.

It isn’t unusual to find water a few feet down.

I’m not sure re the legality of sinking a well, although it would be useful
to water the garden etc.

I’d certainly contemplated it to water the veg plot. Currently, we collect
water from the roof etc and pipe it to butts.

When I dug a deep hole for an large block of concrete for an antenna base,
I went down about 1.2 metres. I was just starting to reach definite signs
of water. There was also hard rock so I was happy the base was deep enough.
Plus, I was resting on the edge of the house foundations. If the house
isn’t moving, the antenna won’t ;-).

It has been up 20 years so I think it is OK.

On 25/04/2022 15:32, Brian wrote:
.....
> I’m not sure re the legality of sinking a well, although it would be useful
> to water the garden etc.
>
> I’d certainly contemplated it to water the veg plot. Currently, we collect
> water from the roof etc and pipe it to butts.
....

AIUI, you need a licence from the water authority if you want to extract
more than 20 cubic metres (5,283 gallons) a day.

--
Colin Bignell

Colin Bignell <cpb@bignellREMOVETHIS.me.uk> wrote:
> On 25/04/2022 15:32, Brian wrote:
> ....
>> I’m not sure re the legality of sinking a well, although it would be useful
>> to water the garden etc.
>>
>> I’d certainly contemplated it to water the veg plot. Currently, we collect
>> water from the roof etc and pipe it to butts.
> ...
>
> AIUI, you need a licence from the water authority if you want to extract
> more than 20 cubic metres (5,283 gallons) a day.
>

That would be more than enough for our veg plot ;-)

On 25/04/2022 12:56, Chris Hogg wrote:
> On Mon, 25 Apr 2022 10:46:07 +0100, "NY" <me@privacy.invalid> wrote:
>
>> 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.
>
> As Colin says, chalk is quite permeable due to its well developed
> interconnected network of fractures. https://tinyurl.com/3y2s247n
>
>> 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?
>>
>
> I suggest the reason your trench has filled to overflowing is that the
> natural water table is higher than the top of the trench, but the
> water had been held back by the impermeable sludge in the bottom of
> the pond. Now you've cut through the sludge, it has allowed the water
> to rise to it's equilibrium level.
>

I was going to suggest exactly the same thing - but you beat me to it!
--
Cheers,
Roger

"Brian" <noinv@lid.org> wrote in message news:t46be7$n3f$1@dont-email.me...
> Chris Hogg <me@privacy.net> wrote:
>> I suggest the reason your trench has filled to overflowing is that the
>> natural water table is higher than the top of the trench, but the
>> water had been held back by the impermeable sludge in the bottom of
>> the pond. Now you've cut through the sludge, it has allowed the water
>> to rise to it's equilibrium level.
>>
>
> It is also possible you have an unground spring in the area.
>
> Our house is built on the side of a valley. I used to work with someone
> who
> is about 70 now at an estimate and he can recall a small stream in the
> bottom of the valley. There is no trace of it now, there are houses and a
> road there. It was fed by a small spring, also long gone, which ran
> through
> the gardens of some houses in our road.

Thanks for the suggestions. Both are very plausible.

It hadn't occurred to me that the water table might naturally be slightly
above the top of the bed of the pond and that the water was being held to an
artificially lower level by the partial porosity of the mud - until I dig a
trench through that layer... And springs are definitely a possibility. There
is a long ridge of chalk hills about 1/4 mile from our house which drains
into a stream at the foot of our garden, with an earth bank (landscaped as
part of the garden) about 100 feet thick between the stream and the pond.
Water could be seeping sideways from the stream and from springs that feed
the stream from aquifers in the hill. The stream is well known locally (The
Gipsey Race) for having a watercourse that is many miles long, but which has
long sections that are dry for a lot of the year (*) and only flow after a
lot of rain: the "here today, gone tomorrow" concept led to the association
with gips(e)ys. You don't have to go more than about half a mile upstream of
us (further from the ridge) and the flow is a lot less, so a lot of what
flows past us is joining by springs that aren't even marked on maps.

I'll probably have to leave the pond for a while to let the water table
level to go down a bit, so it's easier to dig earth which is not quite so
soggy and slimy (**). And probably not dig as far down as the rocky layer,
in case it makes the pond too likely to drain instead of (at least in the
winter) being deep enough to attract ducks. I have wondered whether now I
have a moat around the beginnings of the duck island, I might wake up to see
some of the local ducks (which frequent our pond in the winter) swimming
round and round the moat ;-) That might well have happened if I'd dug the
moat earlier in the year, before the females started laying eggs and the
males congregated in boys' gangs.

(*) There was a case a few years ago of people whose houses border the
stream a few miles upstream of us, dumping grass cuttings in the "dry stream
bed" and the village then flooding later in the year when the water which
occasionally filled the stream bed had nowhere to flow. Ooops!

(**) Wading through water that it almost over-topping my wellies, with them
sticking in the mud every time I stop to dig out a section, makes for
difficult work. I need to hose off my boots, gloves and spade, and strip off
outside the back door to put my clothes in the washing machine, every time
I've finished digging.