It would be good to actually understand the processes that occur with CO2 in caves. Our Vurley trip yesterday revealed that we have around %1 near the entrance with the figure gradually increasing with depth. Below 100m depth the figure hit %3 with %4 plus at the very bottom. This seems a bit odd to me as the cave is pretty dry now after the long Summer we have experienced. We are in an unusual situation. A closed depression half a mile wide. 7m of loess soil and a verdant meadow which has grass nearly a foot high now. Clearly all of those aspects can have a bearing on CO2. I wonder how other caving regions vary with different circumstances of climate , soil and vegetation. Is this primarily a Mendip problem ?
Human tolerance is variable too. One of our team dare not go down the cave now as he wakes the next day with extreme headaches. In fact he can be ill for two days. Others seem hardly to notice any effect within the cave. Two team members spent two hours or so in the %4 zone with little effect though they were not working hard. I heard that some Australian cavers can tolerate %8 " having built up a tolerance to it ". ( Perhaps this is a myth. ). Personally I can work fairly normally in %3 though at 71 I get used to puffing a bit. I do recall in my early deep air diving days that extreme workload ( hauling down a shot line in tide ) resulted in immediate headache that made me vomit through the regulator.
An interesting feature of Vurley is the number of mini vadose trenches in the deeper parts. These are sometimes carved in near vertical walls by occasional stream flow and clearly demonstrate the highly " corrosive " nature of the water even at depth. The cave gets bigger with depth but my guess is that much of that is the result of drip rather than phreatic or extreme vadose flow. The whole cave has the nature of sharply eroded limestone with very little in the way of flowstone or stal. deposits. I doubt very much that Vurley has the age of classic Mendip Swallets like GB etc. What happens below 100m OD might be another matter as then we have reached the former phreas level of High Country in Reservoir Hole and associated fossil risings above Gough's Cave.
Our " scientist " hopes eventually to have a CO2 logging system that can be left in the cave to give variable readings. ( Does the CO2 decrease or increase with rain and higher water flow. What relationship is there between the grass in the field. Will CO2 decrease after the grass is cut and what are the " safe " times in the cave? ). One thing that can impair CO2 metre readings is condensation within the unit so time must be made to let it adjust to ambient temperature.

Eroded rock from below the loess.



Loess overlay at Vurley with thin boulder layer at 6m depth.



The soil is slightly darker below the boulder layer.



Vurley closed karst basin in Winter. Beyond the tree the low wide col that leads to a dry valley. Possibly the overflow route for surface water when the depression was a glacial lake.

Excavation of the cave and the pipe has considerably altered the hydrology. All surface water used to permeate through the loess resulting in collapses on the surface. Now the water ponds and finds its way directly down the pipe to flow though much of the known cave. One would assume that this would result in less CO2 formation. At Vurley solid rock is not met until a depth of some 66m. Most of the cave above this consists of small and huge boulders associated with the South West Overthust fault system. In all probability all surface water from the depression eventually finds its way to the lower reaches of Vurley

CO2 is heavier than air, so will follow the same route as water to the lowest point. Water running from the surface will absorb it, whilst percolation water is likely to be already saturated.

Mike

The odd thing Mike is that there is so little percolation water at the moment.

Consensus seems to be that CO2 comes from soil bacteria & chemicals added by farmers probably increases production.

Vurley closed basin is an SSSI with special protection. I doubt any chemicals have been added there for decades. There is a magnesium deficiency for cattle which I guess is corrected back at the farm. Its possible that the CO2 has arrived earlier this year as last year we only reached %2.8 in LaLaLand on 19th Sept. By 26th Sept levels in the upper cave reached  %5 so further digging was abandoned. A recce trip on November 14th revealed %2 near the entrance but %6 at LaLaLand. Some strong water flow had recently washed the cave clean but not reduced CO2. We did manage to cap and remove 20 loads from the bottom. Full digging activities resumed in late November with no record of CO2 in my log until recently. Our poor " air " period seems to be September to around January though with various chokes now removed there is an entirely different airflow. I gather Brimble Pit experienced similiar problems. They are also in a closed karst basin.

Vurley pipe taking surface water in January this year.

Eroded base rock at about - 65m

I have mentioned that I attended lectures on this last year at UIS. It is a world wide issue.