"Richard Holub" wrote in message
...
Hello,

I am going to construct a 3000 gal pond in my back yard and have a question
about construction in/on a high water table location.

I have filled in my back yard which is on a sloping hill towards a marsh. One
side of the proposed pond area is approximately 4 ft. deep (2 ft. above water
table) and the other side has been filled approximately 6 ft. (0 ft. above
water table) to level off the land. The fill is 90% heavy clay. Prior to
filling in the yard I had surface rain water flowing by towards the marsh
area.

Now for my question...I was planning of keeping the shallow side of the pond
at 4 ft. and the deeper side at 5 ft. I have read that if a liner pond is
constructed directly on the top of a wet land, the liner will bubble up. But I
have also read in my new book "Koi: Living Jewels of the Orient" by Steve
Hickling that if the water table is high one can dig a sump, pump it out
during construction and lay the liner. After filling the pond one can fill
the sump back in. Am I missreading this section? If the liner is layed out
and the pond is filled then there should not be a problem with bubbling
underneath the liner?

I know that I can build the pond a few feet higher to remedie this problem
but, BUT...

Rich

Hi Rich:

I'm a hydrogeologist, so the best advice I can give you is to establish a datum
elevation near your pond site (you can do this simply by driving three solid
steel rods or pipes into the ground with a mallet. Place these rod/pipes
immediately outside the area where you want your pond. Make a mark on the side
of one rod/pipe with a permanent marker. Then using a length of string and a
line level, run the string to each of the other rods/pipes, using the line level
to make sure that the string is level. Make sure the string is tied tightly,
and doesn't sag. Then mark the other two rods/pipes where the sting is
attached. Call that elevation of the string the 100 ft mark. Then rent a hand
auger and auger a boring down to about a foot below the water table in three
places immediately outside the area where you want to place your pond
(preferably close to the rods/pipes). After letting the borings sit open for
about an hour, water should come up in the borings and acheive approximate
static water level conditions. Then measure the depth of the water in the
borings relative to the marks on the rods/pipes (you may need an assistant to
help you make the level measurements). Subtract each measurement from the 100
ft elevation you've establish for your string to establish ground water
elevations for each location. For instance, if in one boring you encounter
groundwater 4 feet below the datum elevation of 100 ft, then the ground water
elevation at that location is 96 ft. Say the second and third measurements give
you readings of 4.2 and 4.3 feet, respectively. Then at those points, your
ground water elevations will be 95.8 ft and 95.7 feet, respectively. Obviously,
by looking at these numbers, your lowest ground water elevation is a 95.7 ft.
Do the same for each boring, By doing this you can figure out the ground water
elevation and flow direction (yes, groundwater does flow) beneath the area where
you want to put the pond. Make these measurements over a period of a couple of
weeks (especially before and after a good rain) so that you know what kind of
fluctuations in water levels to expect. When you are finished, be sure to
backfill the borings and tamp the soil in very firmly.

Where is all of this going? By doing the above, you will be able to determine
before you dig the first shovel full of dirt if there is going to be a problem
with rising water levels beneath the pond. It will also tell you the direction
that the ground water is going so you can plan the placement of your sump (the
sump should be at the lowest ground water elevation nearest to the pond - in my
example, above, as close to the 95.7 feet ground water elevation as possible).
And if it was me, I'd install a permanent sump adjacent to your pond so that if
the water level does rise (say, after a heavy rain), you can pump it out before
it becomes a problem. You may even want to install a french drain beneath your
liner, and run it to the sump. The above data will also give you an indication
of how deep you can go and still leave a cap of clay on top of the ground water
to minimize any seeping beneath the liner (I'd advise at least a foot of clay
left in place above the water table). Good luck, and if you have any questions,
feel free to ask.