Redox potential (𝐸𝐸ℎ) measurements are widely used as indicators of the dominant reduction-oxidation reactions occurring underground. Yet, 𝐸𝐸ℎ data are mostly used in qualitative terms, as actual values cannot be used to distinguish uniquely the dominant redox processes at a sampling point, and should therefore be combined with a detailed geochemical characterization of water samples. In this work, we have intensively characterized the redox potential of the first meter of soil in an infiltration pond recharged with river water using a set of in-situ sensors measuring every 12 min during a one-year period. This large amount of data combined with hydrogeochemical campaigns allowed developing a reactive transport model capable of reproducing the redox potential in space and time together with the site hydrochemistry. Our results showed redox processes were mainly driven by the amount of Sedimentary Organic Matter in the system as well as by seasonal variation of temperature. As a subsidiary result, our work emphasizes the need to use a fully coupled model of flow, heat transport, solute transport and the geochemical reaction network to fully reproduce the 𝐸𝐸ℎ observations in the topsoil.

Managed aquifer recharge (MAR) is a well-known technique for improving water quality and increasing groundwater resources. Denitrification (i.e. removal of nitrate) can be enhanced during MAR by coupling an artificial recharge pond with a permeable reactive layer (PRL). In this study, we examined the suitability of a multi-isotope approach for assessing the longterm effectiveness of enhancing denitrification in a PRL containing vegetal compost. Batch laboratory experiments confirmed that the PRL, installed in 2011, was continuing to enhance denitrification. At the field scale, changes in redox indicators along a flow path and below the MAR-PRL system was monitored over 21 months during recharge (RP) and non-recharge (NRP) periods. Our results showed that the PRL was still releasing non-purgeable dissolved organic carbon five years after installation. Nitrate concentration and isotope data indicated that denitrification was occurring under and close to the infiltration area where recharge water and native groundwater mix. Furthermore, longer operational periods of the MAR-PRL system increased denitrification. Multi-isotope analysis might be useful in identifying and quantifying denitrification in MAR-PRL systems.

Subsurface microorganisms must deal with quite extreme environmental conditions. 13The lack of light, oxygen, and potentially nutrients are the main environmental stresses 14 faced by subsurface microbial communities. Likewise, environmental disruptions 15 providing an unbalanced positive input of nutrients force microorganisms to adapt to 16 varying conditions, visible in the changes in microbial community diversity. In order to 17 test microbial community adaptation to environmental changes, we performed a study 18 in a surface Managed Aquifer Recharge facility, consisting of a settlement basin (two-19 day residence time) and an infiltration pond. Data on groundwater hydrochemistry, soil 20 texture, and microbial characterization was compiled from surface water, groundwater, 21 and soil samples at two distinct recharge operation conditions. 22Multivariate statistics by means of Principal Component Analysis (PCA) was the 23 technique used to map the relevant dimensionality reduced combinations of input 24 variables that properly describe the system behavior. The methodology selected allows 25 including variables of different nature and displaying very different range values. Strong 26 differences in the microbial assemblage under recharge conditions were found, 27 coupled to hydrochemistry and grain-size distribution variables. Also, some microbial 28 groups displayed correlations with either carbon or nitrogen cycles, especially showing 29 *Manuscript Click here to view linked References abundant populations of denitrifying bacteria in groundwater. A significant correlation 30 was found between Methylotenera mobilis and the concentrations of NO 3 and SO 4 , and 31 also between Vogesella indigofera and the presence of DOC in the infiltrating water. 32 Also, microbial communities present at the bottom of the pond correlated with 33 representative descriptors of soil grain size distribution. 34 , 2009). Actually, the unsaturated zone, and more specifically the topsoil, 39 supports the highest microbial activity and biomass of all compartments within the 40 subsurface environment (Lapworth et al., 2012). Likewise, microorganisms are 41 responsible for most biological processes in aquifers (Stein et al., 2010). 42 LuedersSeveral studies evidence microbial adaptation to groundwater extreme environments 43 (thermal or hypersaline) (e.g., Rothschild and Mancinelli, 2001) or disturbed by human 44 activities (Meckenstock et al., 2015). Human activities have caused disruption in 45 aquifer dynamics to some extent (Griebler and Lueders, 2009;, with 46 biological implications as indigenous microorganisms can acclimate (Pett-Ridge and 47Rodriguez-Escales and Sanchez-Vila, 2016), (2) determining the physical and 56 hydrochemical conditions that can govern the behavior of specific microbial groups 57