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Oxidation of organic matter in a karstic hydrologic unit supplied through stream sinks (Loiret, France)

Abstract : The aim of this paper is to appraise the ability of the oxidation of riverine organic matter in the control of limestone dissolution, in a karst network. Biogeochemical processes during infiltration of river water into an alluvial aquifer have already been described for an average flow velocity of 4-5 m d−1 (Jacobs, L. A., von Gunten, H. R., Keil, R. and Kuslys, M. (1988) Geochemical changes along a river-groundwater infiltration flow path: Glattfelden, Switzerland. Geochim. Cosmochim. Acta 52, 2693-2706; Von Gunten, H. R., Karametaxas, G., Krähenbühl, U., Kuslys, M., Giovanoli R., Hoehn E. and Keil R. (1991) Seasonal biogeochemical cycles in riverborne groundwater. Geochim. Cosmochim. Acta 55, 3597-3609; Bourg, A. C. M. and Bertin, C. (1993) Quantitative appraisal of biogeochemical processes during the infiltration of river water into an alluvial aquifer. Environ. Sci. Technol. 27, 661-666). Karstic drainage networks, such as in the river Loire-Val d'Orléans hydrologic system (Fig. 1), make possible flow velocities up to 200 m h−1 and provide convenient access to different water samples several tens of km apart, at both extremities of the hydrologic unit (Chéry, J.-L. (1983) Etude hydrochimique d'un aquifère karstique alimenté par perte de cours d'eau (la Loire): Le système des calcaires de Beauce sous le val d'Orléans. Thèse, Université d'Orléans; Livrozet, E. (1984) Influence des apports de la Loire sur la qualité bactériologique et chimique de l'aquifère karstique du val d'Orléans. Thèse, Université d'Orléans). Recharge of the karstic aquifer occurs principally from influent waters from stream sinks, either through coarse alluvial deposits or directly from outcrops of the regional limestone bedrock (Calcaires de Beauce). Recharge by seepage waters from the local catchment basin is small (Zunino, C., Bonnet, M. and Lelong, F. (1980) Le Val d'Orléans: un exemple d'aquifère à alimentation latérale. C. R. somm. Soc. Géol. Fr. 5, 195-199; Gonzalez R. (1992) Etude de l'organisation et évaluation des échanges entre la Loire moyenne et l'aquifère des calcaires de Beauce. Thèse, Université d'Orléans) and negligible in summer. This karstic hydrologic system is the largest in France in terms of flow (tens to hundreds of m3/s) and provides the main water resource of the city of Orléans. Chemical compositions of influent waters (River Loire) and effluent waters (spring of the river Loiret) were compared, in particular during floods in summer 1992 and 1993 (Figs 2-4). Variation of chloride in the River Loire during the stream rise can be used as an environmental tracer of the underground flow (Fig. 2). Short transit times of about 3 days are detectable (Fig. 2) which are consistent with earlier estimations obtained with chemical tracers (Ref. in Chéry, J.-L. (1983) Thèse, Université d'Orléans). Depending on the hydrological regime of the river, organic carbon discharge ranges between 3-7 and 2-13 mg/l for dissolved and particulate matter respectively (Fig. 3). Eutrophic characteristics and high algal biomasses are found in the River Loire during low water (Lair, N. and Sargos, D. (1993) A 10 year study at four sites of the middle course of the River Loire. I -- Patterns of change in hydrological, physical and chemical variables in relation to algal biomass. Hydroécol. Appl. 5, 1-27) together with more organic carbon rich suspended particulate matter than during floods (30-40 Corg % dry weight versus 5-10%). Amounts of total organic carbon and dissolved oxygen (Fig. 3) dramatically decrease during the underground transport, whereas conversely, dissolved calcium, alkalinity and inorganic carbon increase (Fig. 4). Anoxia of outflows may start in April. Dissolution of calcium carbonates along the influent path outweighs closed system calcite equilibrium of inflow river waters (Table 3). The impact of organic matter oxidation on calcite dissolution may be traced by variations of alkalinity and total carbonates in water. Following, Jacobs, L. A., von Gunten, H. R., Keil, R. and Kuslys, M. (1988) Geochemical changes along a river-groundwater infiltration flow path: Glattfelden, Switzerland. Geochim. Cosmochim. Acta 52, 2693-2706), results are shown graphically (Fig. 5). Extent of reactions is controlled by the consumption of dissolved O2 and nitrate for organic matter oxidation and by the release of Ca2+ for calcite dissolution (Table 2). The karstic network is considered to behave like a biological reactor not exchanging with the atmosphere, with steady inhabitant microbial communities (Mariotti A., Landreau A. and Simon B. (1988) 15N isotope biogeochemistry and natural denitrification process in groundwater: Application to the chalk aquifer of northern France. Geochim. Cosmochim. Acta 52, 1869-1878; Gounot, A.-M. (1991) Ecologie microbienne des eaux et des sédiments souterrains. Hydrogéologie, 239-248). Thus, energy requirements only are considered, not carbon assimilation. Moreover, there is no necessity to invoke any delay for nitrification enhancement, as observed elsewhere, after waste water discharge into the river (Chesterikoff, A., Garban, B., Billen, G. and Poulin, M. (1992) Inorganic nitrogen dynamics in the River Seine downstream from Paris (France). Biogeochem. 17, 147-164). 05n microbial processes are assumed to be aerobic respiration, nitrification and denitrification. Reactions with iron and manganese, real but not quantitatively important, were neglected. Sulphate reduction and methane formation, certainly not active, were not considered. Denitrification, which is suggested by low nitrate and ammonium concentrations and anoxia in the outflow, is known to be rapid enough to be achieved in a short time (Dupain, S. (1992) Dénitrification biologique hétérotrophe appliquée au traitement des eaux d'alimentation: Conditions de fonctionnement et mise au point d'un procédé. Thèse, Université Claude Bernard, Lyon). Reaction are somewhat arbitrary but conform to general acceptance (Morel, M. M. and Hering, J. G. (1993) Principles and Applications of Aquatic Chemistry. Wiley, New York). Anaerobic ammonium oxidation (Mulder A., van de Graaf, A. A., Robertson, L. A. and Kuenen, J. G. (1995) Anaerobic ammonium oxidation discovered in a denitrifying fluidized bed reactor. FEMS Microbiol. Ecol. 16, 177-184), although possible, was not considered. In fact, C/N ratio of the reactive organic matter has only mild repercussions on the results; i.e. in the same range as the analytical errors for alkalinity and total carbonates. The objective was simply to roughly confront characteristics of outflowing waters and the calculation. Respective roles of aerobes and denitrifiers, for instance, are not certain. Several periods during low water or floods were selected with various ranges for calcium dissolution or nitrate and oxygen concentrations. The result is that in most cases simulation and data are in reasonable accordance (Fig. 5). Amounts of organic matter in River Loire are generally sufficient to sustain the process (Table 3). Particulate organic matter is probably the most reactive. The balance of oxidation of organic matter indicates that about 65 μg Corg/l*h are oxidized during the transport without much variation with the river regime or organic discharge. It is concluded that limestone dissolution is directly dependent on organic matter oxidation, but variation occurs (7-29 mg CaCO3/l) with the level of bases that can be neutralized in the influent River Loire water.
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Patrick Albéric, Michel Lepiller. Oxidation of organic matter in a karstic hydrologic unit supplied through stream sinks (Loiret, France). Water Research, IWA Publishing, 1998, 32, pp.7, 2051-2064. ⟨10.1016/S0043-1354(97)00439-9⟩. ⟨hal-00112911⟩

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