Evolution of recharge conditions and rates. Isotopic approach

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Evolution of recharge conditions and rates: isotopic approach.

Beyond the hydrological processes that show a strong variability in time and space, groundwater recharge may be the most considerably affected by modifications in land use and land cover (e.g. Leduc et al., 2001; Scanlon et al., 2006). In semiarid Mediterranean, rainfall mostly occurs during strong and violent storm event, inducing important runoff and limited recharge on the land. On the other hand, despite water saving technologies like drip irrigation, artificial recharge by irrigation return-flow may occur and significantly increase the recharge of shallow aquifers. The quantitative assessment of the related additional water inflow to the aquifer is then a fundamental task for updating the regional water budget and defining a sustainable management of the water resource. Environmental tracers have been used for multiple topics in Mediterranean hydrology and hydrogeology. They provide information for understanding rainfall processes (e.g. Cruz San Julian et al., 1992; Celle-Jeanton, 2001; Fernandez-Chacon et al., 2010), groundwater flow (e.g. Celle-Jeanton et al., 2009), exchange between aquifers (Adar et al., 1992) or between aquifers and hydrographic network (e.g. Burnett et al., 2003). Regarding recharge assessment, environmental tracers are widely used for estimating recharge at integrated scale in time and space in semiarid areas (e.g. Le Gal La Salle et al., 2001; Klaus et al., 2008). Recent reviews on processes and techniques were proposed by Scanlon et al. (2006) and Herczeg and Leaney (2011). Studies using environmental tracers in groundwater to assess the recent evolution of recharge in semiarid areas as a consequence of changes in land use were mostly developed in the last decade, with applications, among others, in Niger (e.g. Favreau et al., 2002), Ivory Coast (Adiaffi et al., 2009), or China (e.g. Currell et al., 2010). Nevertheless, only a limited number of studies worldwide focused on the direct impact of irrigation on recharge (e.g. Horst et al., 2008; Qin et al., 2011) applying environmental tracers in groundwater.
In addition, the use of environmental tracers in complex multi-aquifer systems faces the difficulty of in situ identification of irreproachable sampling points, because of possible artificial mixing of water from the different layers. Assessing whether groundwater samples are representative of regional aquifer conditions or might result from local inside-borehole perturbations is a basic step before any interpretation. This tricky stage is rarely mentioned in the scientific literature. Several authors discussed the origin of a sample from long-screened boreholes by theoretical and modelling approaches (Lacombe et al., 1995; Elci et al., 2003) or with experimental approaches (Martin-Hayden, 2005; Mayo, 2010). Excepting authors like Barbecot et al. (2000), this topic has not been studied in details, and no in situ methodology exists.
In this investigation, despite the increase in the number of groundwater samples reliably usable for a geochemical study provided by section 4.7, the high homogeneity in the geochemical composition of the different aquifers did not allow to trace recharge, mineralization and mixing processes. I contacted with the IDES team from Univ. Paris Sud) to propose a collaboration using radiocarbon and tritium as complementary tracers to understand the evolution of recharge conditions since the set-up of agriculture and the mixings of water from different horizons. Once selected the most reliable sampling points, and found the first part of financial founding for the experiment, I organized the sampling campaign and performed part of the lab work (crystallization of dissolved inorganic carbon) at the IDES laboratory. The interpretation of the results was then discussed with Dr. Barbecot (IDES), Pr. Yves Travi (UAPV) and Dr. Jean-Denis Taupin (UM2), among others. Two scientific publications were extracted from of this chapter, one is published in Hydrological Processes (Baudron et al., 2013b) and the other one has been accepted in Radiocarbon (Baudron et al., 2013c). In addition, partial results were presented in four international congresses (Baudron et al., 2011; 2012b, c, d).

Submarine Groundwater Discharge

Another illustration of the impact of increasing anthropogenic pressure on Mediterranean aquifer systems is the modification of hydrology and ecology of coastal areas by modifying submarine groundwater discharge (SGD) (e.g. Burnett et al., 2003; Moore and Arnold, 1996). In the Mediterranean Sea, such processes are a particular source of concern in wetlands (e.g. Rodellas et al., 2012) and lagoons (e.g. Gattacceca et al., 2011). SGD assessment is therefore a critical need for water resources management.
A series of methodologies were used in the last decades to locate and quantify SGD. One is a simple Darcy’s calculation of groundwater flow through the aquifer (e.g. Senent et al., 2009). Another one consists in direct measurements of groundwater seepage rates using a manual “seepage meter” (e.g. Israelsen and Reeve, 1944; Cable et al., 1997), i.e. a chamber inserted into the sediments and connected to a plastic bag. Submarine groundwater discharge can also be calculated from water balances of aquifers (e.g. Oberdorfer et al., 1996) or surface water masses (e.g. Martinez-Alvarez et al., 2012). Numerical modeling methods are also developed (e.g. Smith and Turner, 2001).
Another possibility is to use natural tracers. The radon and radium approaches rely on a global mass balance of the studied water masses, as pioneered by Moore (1996) or Cable et al. (1996). The interest is based on the simple field implementation and on the spatio-temporal integration. Along the last decade, numerous authors successfully applied this method in many places worldwide (e.g. Burnett et al., 2001; Mulligan and Charette, 2006) and to a lesser extend in the Mediterranean (Garcia-Solsona et al., 2010; Gattacceca et al., 2011; Rodellas et al., 2012; Weinstein et al., 2007).
The most sensitive part of the mass-balance method lies in a precise determination of the discharge rates and the radionuclide activities of the different end-members. Such calculation is particularly sensitive i) to the composition of discharging groundwater and ii) to the assessment of inputs from surface water.
The behaviour of radon and radium in coastal aquifers is complex (e.g. Burnett et al., 2003). No specific rule exist for assessing the composition of discharging groundwater, although it has a direct impact on the calculated SGD values. The discharge processes are not always well known, and the use of mean values as representative for a complex system is not completely satisfying.
Surface water fluxes often represent limited inputs of radionuclides. Nonetheless, in highly anthropized watersheds, surface water tributaries may carry unexpected high quantities of radon and radium to coastal lagoons. In such cases, their precise assessment remains a fundamental task for deciphering the influence on the tracer distribution in water masses and on the radionuclide mass-balance. This task is even more difficult when surface-water hydrodynamics is very reactive, inducing a fast dispersion of the tracers due to strong waves or tides (e.g. Ferrarin et al., 2008; Santos et al., 2009a; Liu et al., 2011). Nonetheless, their precise localization and sampling is not always an easy task, especially where artificial submarine emissary are present.

Geomorphology, climate and economy

The Campo de Cartagena is located in the South of the region of Murcia (Figure 1), in southeast Spain. It is characterized by a wide plain with a slight tilt southeastwards, rounded by small mountains, except on its eastern side where its borders are defined by the Mediterranean Sea and the Mar Menor lagoon.
The altitude of the mountainous relieves that separate the Campo de Cartagena from the Murcia plain culminates a 1065 m asl in the western part and progressively decreases to the east. In the Southeastern part, several peaks reach up to 551 m asl, at the 2 small mountains located eastern from Fuente Alamo culminate at 292 and 305 m asl. In the central part, the plain is interrupted by several hills, standing out the Cabezo Gordo (312 m asl) and the Carmoli (117 m asl), as detailed in section 2.2.

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Mar Menor lagoon

The main surface outlet of the watershed is the Mar Menor lagoon (135 km2), one of the largest coastal lagoons of the Mediterranean Sea (Figure 47). It represents a volume of 605 106 m3 with a mean depth of 4.5 m, with a maximum depth close to 6.5 m. Water temperature in the depth ranges between 7.8 °C in winter and 30.2 °C in summer (Lopez-Castejon, personal communication) in coherence with the atmospheric temperature variations.
The Mar Menor is separated from the Mediterranean Sea on its eastern side by a 22 km long narrow sandy bar system (La Manga; width between 100 and 1200 m) tied to four volcanic outcrops. Other volcanic outcrops in the lagoon form three small islands. Three inlets connect the lagoon with the Mediterranean Sea (Figure 47), although the main water exchange occurs through the central one, the Estacio channel that was dredged in 1973 to make it navigable.
Due to the scarcity of precipitations (300 mm.a-1), which mainly occur during storm events, the limited surface runoff does not compensate the high evaporation of the lagoon, requiring a net inflow from the Mediterranean Sea of about 130 106 m3.a-1 (Cabezas et al., 2009). The lagoon is therefore hypersaline, around 64 mS/cm. Calculated renewal time ranges from 0.66 to 1.2 year (Ruzafa, 1998; Gilabert. 2008; Cabezas, 2009; Martínez-Álvarez et al., 2011). Water circulation can be very dynamic and is mainly controlled by wind and atmospheric pressure (Arévalo, 1998). Despite a weak stratification in the early morning, the water column can be considered homogeneous (Lopez-Castejón, personal communication). Still, local stratification can be found in some areas close to the inlets and affected by the Mediterranean Sea water. Variations of the water level in the lagoon are limited to a few centimetres of amplitude, and are controlled by both tidal cycles and non-tidal phenomena like the variations of the atmospheric pressure.
Some authors suggested the existence of faults systems under the lagoon (Montenat, 1973; Lillo Carpio et al., 1979) or at its western limits (Rodriguez Estrella, 2004). However, García-Aróstegui et al. (2012) recently reviewed the existing information and did not identify such faults systems along the lagoon. In the absence of further indication, the Quaternary aquifer is assumed to fully underly the lagoon.

Surface hydrology

Similarly to other semiarid areas of SE Spain and of the Mediterranean, a network of ephemeral  streams called “ramblas” drains the area, transferring rainwater only during the sporadic rainfall events. The main stream is the Rambla del Albujón. Flowing E-W, it constitutes the axial drainage of the Campo de Cartagena; its watershed (556 km2) has a shoreline of about 40 km long (García-Pintado et al., 2007), and covers almost half of the Quaternary aquifer surface area. It also artificially concentrates water from neighbouring watersheds that used to flow directly to the lagoon.
Since the 1980s’, a permanent flow has appeared in the last kilometres of the river bed; it represents nowadays around 5.106 m3.a-1 (IEA, 2011). This is explained by the natural drainage of the Quaternary aquifer, whose water table level has risen in response to the increased irrigation return flow. In addition, a high number of agricultural drains and artificial releases now increase the Rambla del Albujón flow. Artificial releases are mostly brines from private desalination plants and discharge of the Los Alcázares sewage water treatment plant.
Mean data from a 2.5 year survey (IEA, 2011) shows that two underground pipes (R2 and R3 in Figure 4), that cannot be related to any origin, discharge respectively 0.8 106 m3.a-1 and 2.4 106 m3.a-1 of water to the Rambla del Albujón.

Borehole inventory

The first task of this investigation was linked to the difficulty of assessment of reliable boreholes representative for each one of the 5 aquifers and for the different possible mixings.
Once reviewed all the available information (see section 4.4) the field review of 475 boreholes was conducted. Basic step before any interpretation of existing geochemical and piezometric data, it was also a requirement before any new sampling campaigns could be performed. Since the available data had not been updated since almost forty years (date of the last inventory), the owners of the well were rarely the same as in the inventory data, when available. Moreover, data on the localization of the boreholes (schematic maps, coordinates) was scarse and generally not precise enough. This task was spread over more than one year and partly realized with the contribution of Juan Guerra from IEA and Clemente Trujillo from IGME.
Finally, 70% of the boreholes (331) could be found (Figure 11), although half of them were abandoned, in bad conditions or even destroyed due to the construction of urban areas and golf resorts. The owner could be identified in close to 200 cases, and sampling was possible in 124 tubewells. Only a tenth of these boreholes were eligible for the water table survey of deep aquifers, due to the electrical and hydraulic equipment installed inside the boreholes, but more than 50 contributed to the design of a new piezometric network of the Quaternary aquifer.

Table of contents :

1 Introduction
1.1 Groundwater in arid and semiarid Mediterranean
1.2 Campo de Cartagena, a natural laboratory
1.3 Specific approaches
1.3.1 Review of historical information
1.3.2 Evolution of recharge conditions and rates: isotopic approach.
1.3.3 Submarine Groundwater Discharge
2 Study area
2.1 Geomorphology, climate and economy
2.2 Multi-layer aquifer system
2.3 Mar Menor lagoon
2.4 Surface hydrology
3 Data
3.1 Introduction
3.2 Data from official networks
3.2.1 Geochemistry
3.2.2 Water table levels
3.3 Data collected during this investigation
3.3.1 Precipitation
3.3.2 Borehole inventory
3.3.3 Groundwater
3.3.4 Surface water
3.3.5 Mar Menor and Mediterranean Sea
4 New interpretations of historical information
4.1 Introduction
4.2 3D Geological model
4.3 One century of groundwater exploitation
4.4 Boreholes inventory
4.5 Water table evolution
4.6 Geochemistry
4.7 Origin of groundwater samples
4.7.1 Methodology
4.7.2 Results and discussion
5 Evolution of recharge conditions and rates. Isotopic approach
5.1 Introduction
5.2 Methodology
5.3 Results
5.3.1 Temperature logs
5.3.2 Temperature of the samples
5.3.3 TDS
5.3.4 Majors ions
5.3.5 Isotopic data
5.4 Discussion
5.4.1 Mixing processes
5.5 Groundwater mean residence time
5.5.1 Recharge conditions
5.5.2 Recharge rates
5.6 Conclusions
6 Radon, Radium and hydrodynamic modeling for submarine groundwater discharge assessment
6.1 Introduction
6.2 Methods
6.2.1 Sampling
6.2.2 Sediments and pore water
6.2.3 Analytical techniques
6.2.4 Hydrodynamic modeling of the lagoon
6.3 Results
6.3.1 Geochemistry
6.3.2 Modeling of the currents
6.4 Discussion
6.4.1 Quantification of SGD
6.4.2 Location of Radionuclide inputs
6.5 Conclusions
7 Conclusions and future research
7.1 General conclusions
7.2 Limits and possible enhancements
7.3 Interest for other Mediterranean studies
Conclusions and future research– French
Conclusions générales
Limites et possibles améliorations
Intérêt pour d’autres études en Méditerranée
Conclusions and future research –Spanish-
Conclusiones generales
Límites y posibles mejoras
Interés para otros estudios en el Mediterráneo
8 References

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