As far as minerals go, uranium is relatively soluble and is not uncommon in trace (non-hazardous) amounts in some groundwaters, and accumulates to hazardous or potentially hazardous levels under certain conditions.
The link between radon and CO2 in caves is currently tenuous at best; they both accumulate where there is less air circulation, but it is not yet certain they have a similar origin. A lot of work remains to be done on both topics. CO2 can be from deep sources, decomposition of organic material, poor air circulation, and other factors. I've observed that caves in the Austin Chalk (think of Robber Baron Cave) and Ellenberger Limestone (think of Colorado Bend) have the highest CO2 in the state, and both are oil-producing units where they are buried deep underground. That suggests to me that they contain higher levels of organic carbon which is possibly being released as the limestone is dissolved in the outcrops and shallow subsurface where we know caves occur. Then again, there could be some other reason and I may be completely wrong. What is needed is a set of analyses of the carbon isotopes in the CO2 in various high CO2 caves of central Texas. Those results will eliminate some possible sources and will likely point strongly to other sources. Someone pointed out earlier that the underlying granite in the Colorado Bend area is an unlikely source of CO2. I agree. For Gill and others interested in the hydrogeology of the area, download http://www.twdb.state.tx.us/publications/reports/GroundWaterReports/GWReport s/R346/R346.pdf (this is about a 10 Mb file) and look at Figures 4-7 which are geologic cross sections of the area. For other reports on Texas hydrogeology, check out http://www.twdb.state.tx.us/publications/reports/GroundWaterReports/GWReport s/GWreports.asp. Don't be too disappointed if you see little or no karst-specific information on the reports on Texas' karst aquifers. Historically, karst has not been considered much although it has been getting more and long overdue attention in the past 10 years. George From: [email protected] [mailto:[email protected]] Sent: Wednesday, December 22, 2010 13:52 To: [email protected] Subject: Re: [Texascavers] San Saba County/CO2/radon 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). Gill's prompts me to ask: anybody know where in hell the uranium and radon in Houston's water comes from? We aren't exactly known for our igneous (or any other) rock exposures. Roger again. -----Original Message----- From: Gill Edigar <[email protected]> To: Aimee Beveridge <[email protected]> Cc: Cavers Texas <[email protected]> Sent: Wed, Dec 22, 2010 1:48 pm Subject: Re: [Texascavers] San Saba County/CO2/radon 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.
