Decomposition of granite would not produce any significant
amount of CO2 on its own. Granite is mostly silica and alumina and
contains very little carbonate. Maybe the heat from radioactivity in
granite could bake CO2 out of overlying limestone, though.
Mark Minton
At 02:48 PM 12/22/2010, Gill Edigar wrote:
OK. But I'm interested in seeing a cross-section of the
sequence--including ground water conditions) above the granite (also
known to have radiation levels above EPA MCLs). That granite is pretty
hot, as I understand it, and decomposition of it could be producing a
lot of CO2 & radon. I don't think there would be much geothermal
influence left in the pluton but don't really know that and is one of
the questions I'm looking for somebody to answer. Also, was that area
not part of the Llano Uplift? If not, it is very near it. The caves of
that area have always had a CO2 content higher than other Texas caves
and which would seem to me to be unjustified by the decay of organic
surface material--especially since some caves don't take run off but
still have a lot of CO2.
--Ediger
On Wed, Dec 22, 2010 at 1:25 PM, Aimee Beveridge <[email protected]> wrote:
> Gill,
>
> I'd bet that San Saba caves could could have slightly higher radon than
> background, especially if they are connected to groundwater enriched in
> radon and have little air turnover. Radon gas is denser than air.
>
> You'd need long term exposure for it to be a health risk. Living
in a poorly
> ventilated house with a water well producing from igneous, metamorphic or
> coal or hcarb-bearing strata would be of much greater concern naturally.
>
> Most of Texas (including San Saba), has very low radon gas compared to many
> other parts of the country. Risk increases over the Llano Uplift and El
> Paso area. See map.
>
> http://www.city-data.com/radon-zones/Texas/Texas.html
>
> The ash layers (bentonite beds) in our local Austin Chalk are somewhat
> radioative. Recent studies of groundwater from Houston have shown elevated
> levels (above EPA MCLs) of uranium and radium. Tangentially, no level of
> radioativity is really safe but avoiding cigarretts, air travel and x-ray
> machines is helpful.
>
> Aimee
> ________________________________
> From: Gill Edigar <[email protected]>
> To: Cavers Texas <[email protected]>
> Sent: Tue, December 21, 2010 5:18:05 PM
> Subject: [Texascavers] San Saba County/CO2/radon
>
> For a long time after I first visited caves in San Saba County in the
> 1960s I assumed that the source of the high levels of CO2 were due to
> decomposition of organic materials that originated on the surface. At
> some point (years ago) after radon became a hot topic I started
> associating radon with CO2 out gassing and more recently wondering if
> any radon studies have been done at CBSP. So I asked my caver friend
> and geologist Steve Hall of Virginia about those relationships. He is
> a radon specialist. He said that the source of out gassing CO2 is
> often igneous rocks which are also a radon source. I started to wonder
> what the rock sequence was in the Gorman Falls area. How thick is the
> limestone/dolomite? What formations lay between the top layer and the
> granite below? Could that be a source for the CO2? Should cavers there
> take any special precautions regarding radon? I tried the internet
> without getting much satisfaction so am wondering if anybody has
> information on any of that handy. It would make an interesting
> discussion.
> --Ediger
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