Author Topic: Vadose devlopment in chalk?  (Read 7140 times)

Offline Pitlamp

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Re: Vadose devlopment in chalk?
« Reply #25 on: February 04, 2011, 06:36:33 pm »
H'm - I'm still not entirely sure why a water table and a peizometric surface are different. Is a piezometric surface a localised thing? Graham, you mention karst drainage routes crossing without mixing. Can you develop that example to help me understand?

Let me give you an example to comment on which might help me. In some areas (e.g. Peak District) there are lavas interbedded with the limestones. These act as aquicludes such that there may be passages at different levels (crossing in plan view) each carrying drainage and each (sometimes) under phreatic conditions. Let's assume there are two of these - the lower one is at the level of the permanent spring for a cave system and the higher one (also ultimately draining to the same spring but with vadose passages inbetween). Is the lower passage "below the water table"? Is the upper one below a different (higher) water table or below a piezometric surface? Or neither?

I've never understood this one and it's time I did!

Offline graham

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Re: Vadose devlopment in chalk?
« Reply #26 on: February 04, 2011, 07:52:46 pm »
OK Pitlamp let's give you an example to fathom out.

Consider Swildons sump 2 sump 3. These sections of passage are wholly filled with water, so they will develop a "phreatic" passage form, will they not. But immediately downstream there is an open streamway with cascadey bits an' all. This is obviously a vadose streamway and thus above the water table, isn't it?

So what happens to the water table at the downstream end of sump 3? Or is it just that the concept is not as useful in karst aquifers as it is in "lesser" rocks?
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Offline TheBitterEnd

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Re: Vadose devlopment in chalk?
« Reply #27 on: February 04, 2011, 08:39:53 pm »
I think you are understanding it pretty well. I suspect that the term "piezometric surface" really means the "surface" of the water as seen in piezometers (see http://en.wikipedia.org/wiki/Piezometer). In your example there are two piezometric surfaces for two aquifers and probably illustrates why "water table" is a difficult term to define.  If, as in your example there is a spring that then runs down hill to a river, I would say that neither of your aquifers is at the regional water table - but then again one of you aquifers may originate deeper than the spring ...

In the simplest model  a porous soil (sand or gravel) is sitting on a hard unfractured bed rock. It rains, the soil fills up with water to some depth - this is the water table - i.e the zone at which all pores are saturated. The water table goes up and down seasonally, if you installed piezometers they would show this water level.

Now suppose you go and install a line of piezometers across Chaple-le-Dale, some of these, say down at Gods Bridge will be in the reqional water table (typically about the level of the flow in the Doe), others will be dry (in solid unfractured rock), but some will hit vadose passage, with water in the bottom. This will give you a level in the piezometer at that point, it may be completely different to one just a few metres away.

Another thing to think about is something like Bull Pot of the Witches down stream sump - if you look at that small area then all the voids appear saturated and you could assume that this is the "water table". But we know that it drains into Wilf Taylors, so the actual "water table" must be lower. So possibly this could be thought of as a perched water table.

So although the term "water table" is useful in lots of types of ground, it is just plain difficult in karst. Hence the use of terms like phreatic, vadose and piezometric.
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Offline owd git

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Re: Vadose devlopment in chalk?
« Reply #28 on: February 04, 2011, 11:56:35 pm »
[

Let me give you an example to comment on which might help me. In some areas (e.g. Peak District) there are lavas interbedded with the limestones. These act as aquicludes such that there may be passages at different levels (crossing in plan view) each carrying drainage and each (sometimes) under phreatic conditions. Let's assume there are two of these - the lower one is at the level of the permanent spring for a cave system and the higher one (also ultimately draining to the same spring but with vadose passages inbetween). Is the lower passage "below the water table"? Is the upper one below a different (higher) water table or below a piezometric surface? Or neither?

I've never understood this one and it's time I did!

Really not a pedantic point, but often in 'the Peak, especially Masson. the lava flows are found as 'clay' way-boards. altered in chemistry and now an impervious 'way'.
I too would like a definitive delineation between 'table' and piezometric surface. especially on an incline.( or anticline!)
Thanks in anticipation.
Owd Git :thumbsup:
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Offline Pitlamp

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Re: Vadose devlopment in chalk?
« Reply #29 on: February 05, 2011, 07:40:08 am »
In my brain I have this very simple model whereby there is an altitude below which all voids in rock are water filled. This is my understanding of the term "water table" and is based on the general idea that water in caves flows in discrete passages.

Some passages above this simple model of a water table will water filled for localised geological reasons. In high discharge friction will impede the flow so there's a temporary backlog of water in many passages so the general "water table" (in my simple thinking) will rise until the various springs have allowed the excess water out of the limestone hill and the "water table" falls again.

If you punched a well through into any passages below the water table your piezometric surface would be the same as the "water table" wouldn't it? If you punched a well through into a higher level phreatic passage then I'd have thought that your piezometric surface might suddenly fall if you keep drilling and hit a lower level passage! Surely the concept of a piezometric surface in karst is an abstract idea which only helps understand the hydrology rather than being of any direct practical use as the drilling of wells would potentially alter the very drainage they were intended to provide evidence for?

In my simple understanding the term "water table" is a useful concept for cavers generally, even though we might be talking about the water table in a cave system relative to the level of the permanent resurgence rather than a regional water table of more use to the professional hydrologist. The 64,000 dollar question - is what we cavers call a "water table" in fact a piezometric surface (or is the idea of a piezometric surface itself actually not that useful to a caver anyway?) I'm still a little confused but please bear with me!

Offline graham

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Re: Vadose devlopment in chalk?
« Reply #30 on: February 05, 2011, 08:45:25 am »
Quote
I have this very simple model whereby there is an altitude below which all voids in rock are water filled.

That holds good in rocks with a high primary porosity, such as sandstones but not in limestones where there is a very low primary porosity but a high secondary (fracture) porosity. When water flow is highly localised within the rock, as it is confined to fractures, that model does not stand up well. There is no mechanism whereby head (water pressure) can be equalised "sideways" between flow routes in independent voids and local geologic features, such as the Derbyshire clay wayboards, will have a more marked effect.

Of course on a regional scale, there will be a degree of corroboration, given that many springs will be a similar heights, at the base of limestone outcrops for example, and sinks will be at similar heights, at the top of a limestone outcrop, but that could almost be described as coincidence rather than anything else. I am thinking here of the geologically relatively simple Yorkshire dales.

To take the example I know best (and which has probably been most intensively studied in these respects) of Mendip, if the "water table" was a real concept and drainage was thus simply controlled by hydraulic gradient, then the flow pattern on Eastern Mendip shown on page 7 of this paper by Dave Drew could not exist. This was the first example of flows crossing without mixing. Other examples include the flow from swallets in the Hillgrove area which reaches Rodney Stoke Rising having crossed, without mixing, flow to Wookey Hole. Probably the best example is Wigmore, where a group of swallets "should" send water to Wookey, or even north towards Sherborne, were it controlled by the steepest available hydraulic gradient, but instead the water flows all the way to Cheddar. This is probably a relatively old flow route, given that the limestone at Cheddar was stripped of it impermeable overburden much earlier than the flanks of the Hill further east, allowing springs to develop. However, the important point, here, is that even though those steeper flow routes became available later they were not used. This is because there is not a simple water table that acts in a simple fashion in limestone.

As always the devil is in the detail.
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Offline Rich West

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Re: Vadose devlopment in chalk?
« Reply #31 on: February 05, 2011, 12:02:15 pm »
Water Table - a theory without a leg to stand on

Offline graham

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Re: Vadose devlopment in chalk?
« Reply #32 on: February 05, 2011, 12:05:51 pm »
 ::)
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Offline John B

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Re: Vadose devlopment in chalk?
« Reply #33 on: February 05, 2011, 12:15:10 pm »
I found this quote for a definition of a piezometric surface, and I think it's a good one: "The level at which the hydrostatic water pressure in an aquifer will stand if it is free to seek equilibrium with the atmosphere. For artesian wells, this is above the ground surface."

Both "piezometric surface" and "water table" are difficult to define in a limestone area where there are aquicludes in the form of clay wayboards and interbedded shales, and areas of solid unfractured rock. I think of the area between East Canal in Giants Hole and Main Rising in Speedwell as a "water table", but it is at a different level to that between the Lower Streamway and Russett Well. In both cases there can't be any air space below it. Perhaps cavers need some new terminology!

Offline John B

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Re: Vadose devlopment in chalk?
« Reply #34 on: February 05, 2011, 12:26:45 pm »
Just thought of a better illustration. If you drilled down into a water filled passage in the area between East Canal and Main Rising the water would rise up the borehole to the level of the sump pool at each end. This would be a "piezometric surface".

Offline graham

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Re: Vadose devlopment in chalk?
« Reply #35 on: February 05, 2011, 12:29:32 pm »
Probably the best definition of piezometric surface is "the water level measured by a piezometer." That makes it ontologically similar to the definition of IQ that I was taught at college. "IQ is that which is measured by IQ tests." It shows that both concepts are only of very limited value in our contexts.
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Offline Pitlamp

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Re: Vadose devlopment in chalk?
« Reply #36 on: February 05, 2011, 06:34:38 pm »
Thank you John B; I'm glad I'm not the only one who thinks some new terminology might be germaine.

Your example of Giants Hole's East Canal to Speedwell's Main Rising is a most interesting one to choose. I thought the sump pools at each end were at different elevations (not allowing for the extra complication of the level of East Canal fluctuating massively when Main Rising doesn't - and Main Rising pulsing when East Canal isn't!). I wonder just what level the alleged "piezometric surface" would be at in your hypothetical well!

Yes - new terminology may well be a good way forwards.

Offline Pitlamp

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Re: Vadose devlopment in chalk?
« Reply #37 on: February 05, 2011, 06:37:00 pm »
P.S. - what's the story behind that wonderful photograph of you that appears with your posts John?!

Offline graham

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Re: Vadose devlopment in chalk?
« Reply #38 on: February 05, 2011, 06:42:18 pm »
Pitlamp what's the difference in elevation between East Canal and Main Rising?
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Offline graham

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Re: Vadose devlopment in chalk?
« Reply #39 on: February 05, 2011, 06:43:31 pm »
Oh, and when you talk about new terminology, what, exactly, is it that you thinks needs a new word?
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Offline John B

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Re: Vadose devlopment in chalk?
« Reply #40 on: February 05, 2011, 10:00:46 pm »
Pitlamp - the answer to the photo question is that Bill Whitehouse happened to stop by just as we arrived from Merlin Sump 5 in the 1976 drought at the time when the sumps were full of black mud. The kit is a wetsuit with the top half of a dead goon suit to stop the mud penetrating too far. Looked stupid, but it worked.

You are right about the levels - It would be interesting to see if the piezometric surface was in accord with East Canal or Main Rising! Whatever level it ends up at, it's still a piezometric surface.