Gas supersaturation, <sup>222</sup>Rn and CO<sub>2</sub> as tracers in karst water
Karst aquifers are among the most important water resources world-wide. Nevertheless, due to their intrinsic properties consisting in fast transport processes and reduced contaminant attenuation capacity, they are proved to be highly vulnerable. <br> Consequently, it is important to discriminate between water originating from the soil reservoir, the epikarst, the low permeability volume and the freshly infiltrated rainwater. To do so, the Prédernier artificial drainage gallery (Gorges de l'Areuse, Swiss Jura Mountains) was investigated by a combined continuous monitoring of radon, carbon dioxide and total dissolved gas pressure. Electrical conductivity, turbidity, total organic carbon and dissolved ions were also followed. The survey of various seepages spread throughout the drainage gallery, offered the unique feature of allowing to compare the dynamics of natural parameters characterized by different storage origins and watershed scales. <br> Radon and carbon dioxide are two gases produced in soil, hence their input function is well delimited. They both are characterized by good solubilities and can be dissolved in percolating water and transported to system outlets. Even tough originating from the same production area, these gases have different chemical and physical properties. On the one hand, radon is an inert radioactive noble gas produced through α-decay of radium present in soil and is characterized by a half-life of 3.8 days. It can be used to assess fast transport processes, as after 20 days its concentrations pass under detection limits. On the other hand, carbon dioxide reacts with carbonates on its way down to the saturated zone. <br> Total dissolved gas pressure (TDGP) represents water vapour pressure in addition to the individual dissolved gases partial pressures. When TDGP in water exceeds the atmospheric pressure, supersaturation occurs. Following a precipitation event, soil air entrapped in pore space, undergoes a newly applied hydrostatic pressure, allowing more gas to be dissolved. Hence, the percolating water acquires a supersaturation signature, which can easily be followed at system outlets. <br> These gases, characterized by good solubility, different chemical and physical properties and naturally and abundantly produced in soil or during rainfall events, were used as natural tracers. The temporal variations of their respective concentrations in underground water not only allow to gain valuable information about fast transport processes in karst systems, but also to identify the different water reservoirs contributing to the discharge. <br> Results demonstrated that the dissolved gases approach, revealed the importance of the soil sub-system with regards to its influence on the recharge of karst aquifers during high-flow conditions, and on the sustainability of its influence to the flow regime. The supersaturation base-level, a soil thickness specific feature, was used as a relevant surrogate to assess the temporal distribution of soil contribution to the selected karst system. Whereas, codependent radon, carbon dioxide and supersaturation peaks depict the influence of soil stored water, enriched in dissolved gases during rainfall events.
The Gorges de l'Areuse study area is situated some 20 km east of the city of Neuchâtel in the Neuchâtel adminstirative district (Switzerland) Thèse de doctorat : Université de Neuchâtel, 2014
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