Janet,
This is such an interesting subject and ever so complex.
After all we can only observe phenoma from the surface or by
collecting data through drilling and underground features we can
reach from the surface.
Re the water table. Under natural conditions it finds its
own level according to the imperveous or porous nature of the
underground strata as you say. Interestingly enough rainfall does
not change the water table.
Re springs created by saturated ground that drips. I don't
think that is correct (from anything I have ever read). Springs
rise to the surface through pressure, sub artesian below ground, and
artesian above the surface (as in Yellowstone), presumably restricted
by the narrowness of the fissures but with the force of water behind
them, and the falling ground creating pressure. On the other hand a
hillside might contain trickling water that enters the mountain or
surfaces like a small waterfall. Come to think of it we dont usually
differentiate these from artesian springs.
I would have thought if there is such a thing as overflowing from an
underground reservoir that the surrounding rocks either channel or absorb
water in the usual way. But with no force behind the spillage I cannot
see that springs would result.
In the 70's when I did most of my reading on this subject there was
controversy about the deposition of evaporites. Since then I try to keep
abreast by testing what I think I know with the professionals, and my
impression is that there is still much that earth scientists need to
resolve.
For me the subject is totally fascinating.
Bea
On 2/10/01 12:11 PM Janet Kaiser writes:
>Phil
>
>My only knowledge is based on ancient A-level
>geography, so it is definitely neither
>up-to-date nor dependable, especially
>considering the state of my little grey cells.
>But your question interested me, so please
>forgive me a few thoughts...
>
>I am not familiar with the lake you mention, but
>why do you believe the water cannot come from
>the surrounding higher ground, however little
>you consider that to be? Extinct calderas at the
>tops of mountains often contain lakes. Often
>very deep, yet apparently with little run off
>from the surrounding higher ground. And do not
>forget that a great deal of the water enclosed
>in Britain's lakes was probably left behind
>following the ice age and a minimal amount of
>water would be needed to "top up".
>
>Given our climate in the British Isles
>(especially 1999-2000) a postage stamp would
>collect a great deal! The amazing thing is that
>it does not overflow! Indeed, if there is no
>run-off by rivers or waterfalls, my primary
>question would be the opposite to yours: "Where
>does all the water go?" Evaporation is certainly
>not a major factor here, eh? :-)
>
>Apart from the site within the landscape, the
>local geology would be the most important thing
>to investigate when considering a particular
>example. The water table is going to be
>influenced by that and it is going to be the
>most important factor. Yes, water rises with the
>lay of the ground along with the water table,
>but I would think it is general and not very
>localised, as in a spring.
>
>For example, in the case of springs which have
>dried up, the felling of trees (which help to
>keep the water table near the surface) have
>often been a major influence, rather than a
>change in direction of water "flowing"
>underground.
>
>Water stored in underground aquifers should not
>be seen as a body of water like a lake. This
>complex water storage system is similar to a
>sponge, not a water container with holes in it.
>Squeeze or extract water from one point and the
>sponge dries out at that point, but remains wet
>elsewhere. Dig a well to extract water and the
>surrounding rock will dry out, creating a blip
>in the underground water level. The sponge will
>gradually even out the moisture through osmosis,
>but only very slowly, unless more water is
>added. I believe the same goes for rocks. Hence
>all the rain of the past couple of years has not
>replaced all the water removed by the water
>companies.
>
>When a sponge is saturated, it also drips... I
>have always thought of springs as being
>similar... The drips from the huge underground
>reservoir which is full and overflowing at the
>point of least resistance or along a
>particularly impervious layer of rock thus
>produce springs. Including or even producing the
>fissures someone else mentioned. This is usually
>a hillside spring.
>
>Of course underground "water pressure" must also
>be an added factor where thousands of gallons of
>water flow but there is no apparent stream or
>water course. Heat will be another factor... We
>probably underestimate that influence, except
>when contemplating hot springs.
>
>There is also the spring which bubbles up from
>the ground... I think this is usually when the
>surrounding geology creates a storage sponge
>which collects so much water, that the immense
>pressure results in the water being pushed
>upwards. Again at a point of least resistance.
>
>I wish more hydro-whatsits were part of this
>list! My schoolgirl memories are hardly going to
>add to the sum and total of mankind's knowledge!
>
>Janet Kaiser
>Criccieth
>
>----- Original Message -----
>From: Philip David
>> What I'm curious about is
>> water rising underground. One model that I
>have in mind is a very deep (and
>> quite large) lake only slightly below the two
>summits of Slieve Gullion, Co.
>> Armagh, which is very steep on all sides below
>the lake. It doesn't seem
>> plausible that all that water comes from the
>two summits. Does water rise
>> under the ground, and if so, how? Could
>"syphonic action" account for this?
>> Phil
>>
Beatrice Hopkinson 73071,327@compuserve
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