Geophysical Research LettersVolume 30, Issue 6 Free Access Contribution of time tracers (Mg2+, TOC, δ13CTDIC, NO3−) to understand the role of the unsaturated zone: A case study—Karst aquifers in the Doubs valley, eastern France H. Celle-Jeanton, H. Celle-Jeanton helene.celle-jeanton@univ-fcomte.fr Département de Géosciences, Universitéde Franche-Comté, Besançon, FranceSearch for more papers by this authorC. Emblanch, C. Emblanch Laboratoire d'Hydrogéologie, Universitéd'Avignon, Avignon, FranceSearch for more papers by this authorJ. Mudry, J. Mudry Département de Géosciences, Universitéde Franche-Comté, Besançon, FranceSearch for more papers by this authorA. Charmoille, A. Charmoille Département de Géosciences, Universitéde Franche-Comté, Besançon, FranceSearch for more papers by this author H. Celle-Jeanton, H. Celle-Jeanton helene.celle-jeanton@univ-fcomte.fr Département de Géosciences, Universitéde Franche-Comté, Besançon, FranceSearch for more papers by this authorC. Emblanch, C. Emblanch Laboratoire d'Hydrogéologie, Universitéd'Avignon, Avignon, FranceSearch for more papers by this authorJ. Mudry, J. Mudry Département de Géosciences, Universitéde Franche-Comté, Besançon, FranceSearch for more papers by this authorA. Charmoille, A. Charmoille Département de Géosciences, Universitéde Franche-Comté, Besançon, FranceSearch for more papers by this author First published: 26 March 2003 https://doi.org/10.1029/2002GL016781Citations: 22AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract [1] Time tracers (NO3−, TOC, δ13CTDIC, Mg2+) have been used to define the hydrodynamic behavior of a karst system: high values in NO3− and TOC reflect rapid infiltration and consequently a short residence time within the aquifer, whereas enriched δ13CTDIC and high Mg2+ are expected for “old water”. 9 Springs and 5 boreholes have been sampled during three field campaigns in the Doubs valley karst aquifer: low water, flood and recession periods. A clear differentiation can be highlighted between boreholes, characterized by a long residence time, and springs that show a rapid infiltration. Considering only the springs values, it appears that TOC and δ13CTDIC contents can easily be correlated to the sampling period. We show then the contribution of the unsaturated zone to the discharge during the low-water period, and the existence of reserves that seem badly connected to the drainage network, and that contribute poorly to the minimal flow. 1. Introduction [2] The vadose or unsaturated zone in a karst system typically consists of a soil layer, an epikarst and a transition zone (unsaturated zone s.s.), between the epikarst and the saturated zone, that could be several hundred meters thick. In the soil layer, water infiltrating through the soil matrix gets typical chemical and isotopic signatures by mineral and/or soil CO2 gas dissolution [Albéric and Lepiller, 1998]. The new rain water is able to recharge the aquifer rapidly, possibly through fractures or surface swallets, and mobilizes older, deeper water out of the aquifer that was residing in smaller fractures and pores. This older water is at or near the equilibrium with limestone carbonates, but the new water is not. Many approaches were then used to evaluate the mixing between the two types of water: oxygen-18 [Lakey and Krothe, 1996; Vallejos et al., 1997; Marc et al., 2001], carbon-13 [Emblanch et al., 1998a], Mg2+ [Wels et al., 1991; Blavoux et al., 1992] and Total Organic Carbon (TOC) [Emblanch et al., 1998b]. In this study, we have used a combination of these chemical and isotopic tracers to reduce the uncertainties experienced while using a single tracer. We focused on TOC, major ions, δ13CTDIC (Total Dissolved Inorganic Carbon), pCO2 measurements on springs and boreholes to understand the complexity of the Doubs valley aquifers [Abdelgader et al., 1996; Mudry et al., 2002]: a definition of the water origin and a determination of the rough mixing ratio of pre-existing water and rapid infiltration will be given. 2. Site and Analytical Method [3] The plateau of Besançon (Jura, Eastern France) spreads between the wide valley of the Ognon to the North and the enclosed valley of the Doubs to the South. The 200 m thick aquifer is composed of Bajocian and Bathonian limestones, that are unconfined on their greater part and become overburdened by Oxfordian marls towards the Doubs Valley. This aquifer has a great importance, as it is used for industrial purposes (paper mills) and drinking water supplies. [4] 9 springs and 5 boreholes have been sampled during three field campaigns that cover the main hydrological events: low water (June 2000), flood (November 2000) and recession (May 2001) periods. They were analysed for cations (Ca2+, Mg2+, Na+, K+), anions (Cl−, SO42−, NO3−), δ13CTDIC and TOC. Field parameters (pH, electric conductance, and HCO3−) were measured immediately after the sampling. Major elements were analysed at the Geosciences Department of the University of Franche-Comté at Besançon, analysis of carbon-13 and TOC were processed at the Hydrogeology Department of Avignon. 3. Results [5] A principal component analysis (PCA) was carried out to summarise the relations between chemical variables (Figure 1). The 2 principal axis of PCA explain 74% of the sampling variance. Axis 1 (47%) is mainly determined by SO42−, NO3−, Cl−, Na+ and electric conductance is the anthropogenic factor. SO42− could also be due to a contact with marls. Axis 2 (27%) is determined by TOC and Mg2+ and represents the residence time factor. In factor plan 1–2, 3 groups of water can be distinguished. Boreholes have the highest Mg2+ contents, which suggests a long residence time within the aquifer, according to the slow dissolution kinetics of dolomite. Conversely, springs, which are resurgent cave streams, closely associated with surface runoff through sinkholes, have the highest content in TOC. The third group is characterised by the springs that emerge in agglomerations undergoing human pollution. Figure 1Open in figure viewerPowerPoint Principal Component Analysis (29 measurements). [6] To confirm this preliminary result, all the data was plotted on a NO3−versus Mg2+ diagram (Figure 2). These two ions are good tracers of both residence time and pollution: (1) as ground waters flow through the same limestones, and according to the incongruent dissolution of dolomite, the increase of Mg2+ [Blavoux et al., 1992] corresponds to the increase of the residence time within the limestones (which have a low Mg2+ content); (2) NO3−, that comes from the lixiviation and mineralisation of the natural organic matter of the soil or from the dissolution of fertilizers, which characterises rapid infiltration. These two tracers highlight the difference between two groups. The springs are characterised by a low Mg2+ content (below 4.5 mg/l) and by variable values of NO3− ranging from 1.14 mg/l to 23.7 mg/l. The maximum values are attributed to Camping (CA) and Mouillère (MO), located in polluted areas. The boreholes have a low NO3− content and show variable Mg2+ values that increase from Thise (TH) to Amagney (AM) and Branne (BR) with the transit time in the aquifer. The highest Mg2+ values are those of Amagney (AM) and Branne (BR); these are the deepest boreholes of the sector (110 m), characterised by the longest residence time. As opposed, Thise (TH) is located in the shallowest part of the aquifer (piezometric level of −3 m) and then appears to be closer to the contents of the springs (4.72 mg/l and 6.85 mg/l). Figure 2Open in figure viewerPowerPoint Mg2+ (mg/l) versus NO3− (mg/l). [7] The water sampled from the springs, except Fourbanne (FO2) and Camping (CA1) (5.28 mg/l and 3.17 mg/l respectively) reveals a content of TOC ranging from 0.66 to 2 mg/l. These values have the same order of magnitude as typical groundwater that contains less than 2 mg/l [Drever and Stillings, 1997] and Southeastern France karst systems [Emblanch et al., 1998b] with values ranging from 0.5 mg/l to 2 mg/l. Figure 3 displays variations of TOC versus δ13CTDIC for the three campaigns. The right-hand inset indicates that boreholes are usually enriched in δ13CTDIC and depleted in TOC compared to springs. Considering the classical δ13CTDIC ratio from carbonate (0 ± 3‰), from atmospheric CO2 (−8‰) [Butcher et al., 1992] and from biogenic CO2 (−22‰) [Fleyfel and Bakalowicz, 1980; Dever, 1985; Eichinger, 1987; Merlot et al., 1996], enriched values in δ13CTDIC are expected for “old water” whereas infiltration has a depleted value. According to Drever and Stillings [1997], TOC in groundwater is usually derived from the organic layer in soils. Heterotrophic bacteria, present in the unsaturated zone, aerobically degrade TOC to produce CO2. Then, high values in TOC reflect a rapid infiltration and consequently a short residence time within the aquifer [Batiot et al., 2003]. Considering only the spring values, it appears that TOC and δ13CTDIC contents can easily be correlated to the sampling period. The low flow stage (BI1, FO1, MO1) is characterised by average TOC values, the most depleted δ13CTDIC (Figure 3) values and the highest Mg2+ values (Figure 2). The Camping spring (CA) has a high TOC content, compared to the other springs, due to its location in a polluted area. During the flood period (BI2, FO2, MO2), a δ13CTDIC and TOC enrichment and an Mg2+ impoverishment can be observed. The recession period (BI3, FO3, MO3) leads to a decrease of the TOC and Mg2+ contents and an enrichment in δ13CTDIC for all the springs (TR: Trébignon, AV: Avanne, CH: Chaney, MO: Mouillère, BI: Briseux, FO: Fourbanne). Figure 3Open in figure viewerPowerPoint TOC (mg/l) versus δ13CTDIC (‰) for the springs of the Doubs valley aquifer. On the right-hand inset, both springs and boreholes values are plotted. 4. Discussion [8] PCA and Mg2+versus NO3− diagrams have shown that the springs are mainly characterised by recent infiltrations whereas boreholes have a longer residence time. We then consider that boreholes catch “old” water whereas springs are characterised by a rapid infiltration (high anthropogenic components and low Mg2+). Mg2+, TOC and δ13CTDIC values over time should give some information on old versus new water entering the system during three stages : low flow, flood and recession periods. The low stage flow has the longest residence time according to the Mg2+ (Figure 2) and TOC (Figure 3) values. The 13CTDIC (Figure 3) and TOC values, relatively high (ranging from 0.67 to 3.17 mg/l), suggest an origin from the unsaturated zone, where the system is open to the soil CO2. The flood period shows a decrease of the residence time (increase of TOC and lowering of Mg2+) due to the rapid infiltration of new water. A tiny enrichment in δ13CTDIC, compared to the low stage flow, could have three origins: (1) a participation of the saturated zone, where groundwater is closed off from the source of soil CO2; (2) CO2 a degassing due to the two-phase turbulent flow (this hypothesis cannot explain TOC and Mg2+ variations); (3) an infiltration of surface waters, characterised by δ13CTDIC from atmospheric CO2. This latter hypothesis only explains the variations observed at the outlet of the Fourbanne cave stream (FO). The recession stage reveals an absence of rapid infiltration, according to the decrease in TOC and the average values in Mg2+ (similar to the autumn ones). As in the previous stage, the δ13CTDIC enrichment should reflect an increase in the participation of the transmissive part of the saturated zone. [9] The phenomenon observed during the low stage flow shows a significant contribution of the unsaturated zone to the discharge during the low-water period in this system. The water seems to have a long residence time through the unsaturated zone, as the content in Mg2+ ranges from 3.15 mg/l to 4.45 mg/l. Emblanch et al. [2003] found values for the unsaturated zone waters of the Ventoux karst systems from 3 to 5 mg/l. Ratios Mg/Ca measured on the Ventoux karst system range between 0.06 and 0.11. For the Doubs karst system, we found values range between 0.008 and 0.025. In first estimate, we should expect a maximum value of 5mg/l in the unsaturated zone. This leads to the conclusion that the summer campaign (low flow) corresponds to waters that have circulated in the unsaturated zone (δ13CTDIC, TOC) where they acquired their Mg2+ content. According to the values found in boreholes with an Mg2+ increase when the residence time increases, we assume that reserves could be found but seem badly connected to the drainage network, and contribute poorly to the minimal flow. 5. Conclusion [10] By using time tracers (TOC, NO3−, δ13CTDIC, Mg2+), we have shown that the Doubs valley karst aquifer is a complex system, composed of an unsaturated zone that is badly connected with the saturated zone. The latter is characterised by two types of fractures, related respectively to transmissive and storage functions. [11] The behaviour of the system could be summarized as follows: during the flood period, rainwater infiltrates through the soil (very negative δ13CTDIC), enriched in water-soluble organic matter (high TOC content) and provokes a high pressure transfer within the aquifer that connects the saturated zone (two types of fractures providing the transmissive and capacitive functions). This mixing between pre-existent water and recent infiltration leads to an enrichment in δ13CTDIC. During the recession period (springtime), the stock of water-soluble organic matter decreases and then the waters have a lower content in TOC. The pressure excess homogenises between the conduits and the fissures in the saturated zone. The unique flow participation is then linked to the mixing of recent infiltrations with stored waters (δ13CTDIC enriched). During the depletion period, the saturated zone is poorly connected and the minimal flow can be roughly attributed to the unsaturated zone. 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