(2020) Gas and Groundwater Dynamics in an As Contaminated Aquifer
Lightfoot A, Brennwald M, Stopelli E & Kipfer R
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13e: Room 4, Thursday 25th June 08:21 - 08:24
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I found the relationship between TDGP and As very interesting. I'm wondering if you have looked at the TDGP versus the hydrostatic fluid pressure in the wells. In principle degassing can occur when the TDGP is equal to or greater than the fluid pressure. Thank you.
Dear Kip, Thank you for your comment. So, yes, I can make a rough approximation for the hydrostatic pressure to be at around 1.4 [pressure units] - which means the TDGP is indeed higher in some of the wells here. I do believe degassing is occurring in-situ from the production of biogenic methane - which, in turn, can be related to the As mobilization mechanism and hence why we see this relationship between TDGP and As.
Very interesting. You conclude that the CH4 (or other gas) bubbles slow down groundwater flow what gives more time to accumulate 4He. Could the source of CH4 (reducing organic sediments with precipitated U?) also be an enhanced source for 4He? With other words: that not the water age triggers 4He concentrations but the accumulation rate. Thank you
So initially it was not really clear why there is this increase of 4He, while the other noble gases show depletion, and the first idea I had was maybe it could accumulate simply because of the reduced groundwater flow. However, we (my project group) are also looking at the possible input of solutes from the overlying clay aquitard, which is know in another part of the village (~800m upstream) to contain more conducting sandy lenses consisting of peat that intercalate within the clay and potentially connects, at certain points, to the aquifer. Solute (OC) input from the fractured parts of the aquitard could then indeed fuel the iron-reducing bacteria, which produce CH4 and simultaneously mobilise As. The same pore water carrying these solutes, could also contain higher 4He concentrations. But, it is still a big question if this kind of input is supplying enough OC to drive As mobilisation - though if it does, it certainly could offer an explanation to the higher He concentrations we see here. I hope this answer does not confuse too much! In short, yes, would be the answer to your question. Many Thanks.
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