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Thread: What lies underneath groundwater?

  1. #1 What lies underneath groundwater? 
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    Maybe this is a na´ve question, but what lies beneath groundwater?

    I'm not talking about cases of perched water tables, where groundwater sits on top of an impermeable aquitard. In such cases it's obvious what lies beneath the groundwater (an aquitard).

    But in general, what is beneath groundwater? The uppermost extent of groundwater is referred to as the water table. Is there an equivalent lowermost extent?

    Does groundwater extend into the mantle, or is it confined to the crust?

    Every diagram I've seen that explains groundwater is cut off at the bottom and doesn't show what's underneath and doesn't show groundwater in a larger context (i.e. its deepest extent)


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    Forum Freshman R1D2's Avatar
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    I would say that in my opinion, ground water does not extend past the mantle. There is clay around there. And the deeper you go it warms up.


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    Forum Radioactive Isotope Paleoichneum's Avatar
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    No, aquifers do not extend to the mantle. The are always bounded on the bottom at some point with an impermeable rock unit, such as a basalt intrusion or a metamorphic dyke.
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    Forum Professor arKane's Avatar
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    Quote Originally Posted by pikkiwoki View Post
    Maybe this is a na´ve question, but what lies beneath groundwater?

    I'm not talking about cases of perched water tables, where groundwater sits on top of an impermeable aquitard. In such cases it's obvious what lies beneath the groundwater (an aquitard).

    But in general, what is beneath groundwater? The uppermost extent of groundwater is referred to as the water table. Is there an equivalent lowermost extent?

    Does groundwater extend into the mantle, or is it confined to the crust?

    Every diagram I've seen that explains groundwater is cut off at the bottom and doesn't show what's underneath and doesn't show groundwater in a larger context (i.e. its deepest extent)
    As you go deeper into the crust, the temperature rises. Right off hand I don't know at what depth water will start to boil, but at that depth is as deep as water can be before it becomes steam and starts percolating back up.
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    Forum Professor arKane's Avatar
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    Quote Originally Posted by R1D2 View Post
    I would say that in my opinion, ground water does not extend past the mantle. There is clay around there. And the deeper you go it warms up.
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    Northern Horse Whisperer Moderator scheherazade's Avatar
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    This is merely my own 'local' observation and does not answer the OP.

    We have a drilled well as our domestic water supply. Water was found at 66 feet (as predicted by my father-in-law of the time who was a dowser) but could not be utilized as there was too much sediment flowing wth the water. We drilled down to 260 feet and encountered a slow seep, ten feet above bedrock. At the suggestion of the driller, we drilled 100 feet into the bedrock to create an underground reservoir for the seep. This gives us 140 gallons of standing water in the pipe at any one time and a refill rate of just over a gallon per minute.

    This has proven to be a very good well, slow but steady, and the flow, to date, has not been affected by our changing weather or gradually increasing population density.

    The age of groundwater is quite fascinating to me and I thought this might be an appropriate thread to post this item on:

    This new study from the USGS, the Maryland Geological Survey (MGS) and the Maryland Department of the Environment (MDE) documents for the first time the occurrence of groundwater that is more than one million years old in a major water-supply aquifer along the Atlantic Coast. The oldest groundwater was found in the deepest parts of the aquifer, but groundwater even in shallower parts of the aquifer is tens to hundreds of thousands years old.
    There are relatively few aquifers in the world in which million-year-old groundwater has been documented, including the Nubian aquifer in the Sahara Desert, Canada's Alberta Basin, and the Great Artesian Basin in Australia
    Million year old groundwater in Maryland water supply
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    Quote Originally Posted by Paleoichneum View Post
    No, aquifers do not extend to the mantle. The are always bounded on the bottom at some point with an impermeable rock unit, such as a basalt intrusion or a metamorphic dyke.
    This.

    Eventually, the water hits impermeable rock.
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    The intuition behind this (for me at least) is that as you go deeper the pressure increases. As the pressure increases gaps in the rock get closed off. Both the networks between gaps, and the gaps themselves. Such that eventually there are no gaps in which the water can live.

    You still get water in the mantle though, but it is incorporated into the mineral structure at an atomic scale, and so is not "free", like groundwater is.
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  10. #9  
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    Quote Originally Posted by arKane View Post
    Quote Originally Posted by pikkiwoki View Post
    Maybe this is a na´ve question, but what lies beneath groundwater?

    I'm not talking about cases of perched water tables, where groundwater sits on top of an impermeable aquitard. In such cases it's obvious what lies beneath the groundwater (an aquitard).

    But in general, what is beneath groundwater? The uppermost extent of groundwater is referred to as the water table. Is there an equivalent lowermost extent?

    Does groundwater extend into the mantle, or is it confined to the crust?

    Every diagram I've seen that explains groundwater is cut off at the bottom and doesn't show what's underneath and doesn't show groundwater in a larger context (i.e. its deepest extent)
    As you go deeper into the crust, the temperature rises. Right off hand I don't know at what depth water will start to boil, but at that depth is as deep as water can be before it becomes steam and starts percolating back up.
    Is it accurate that water boils the deeper into the crust you go? Temperature does increase the deeper into the crust you go, but so does pressure. The boiling point of water is 100 degrees C at 1 atm of pressure, but the boiling point of water under more than 1 atm of pressure is more than 100 degrees C. To find out if water boils at a certain depth, you'd need to know both the pressure and temperature at that depth, then consult a phase diagram.

    I don't have that information on hand though, so I don't know.
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  11. #10  
    Universal Mind John Galt's Avatar
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    Groundwater is contained within the pore spaces in sediments and rocks. As noted, with increasing depth comes increasing pressure, compaction of the rock and reduction of pore space. Also at depth rocks, of any type become metamorphosed, their chemical and mineralogical composition changing and recrystallisation taking place. At that point any remaining pore space is wholly eliminated. Again, as noted, any remaining water will be incorporated in the structure of the stable minerals. (It is a little more complex than that: in some instances ultra-thin, intercrystaline water layers may exist and may facilitate the metamorphic process.)
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  12. #11  
    The Enchanter westwind's Avatar
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    I'm interested in this. While I'm thinking about the million year old groundwater situation, please do not be making coffee or bathing. westwind.
    Words words words, were it better I caught your tears, and washed my face in them, and felt their sting. - westwind
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