Renewal rate estimation of groundwater based on radioactive tracers (3H, 14C) in an unconfined aquifer in a semi-arid area, Iullemeden Basin, Niger
Introduction
In the Sahelian area of Niger, there is no permanent surface runoff, with the exception of the Niger River. Thus, domestic and pastoral water supply depends on groundwater. Currently, the sustainability of these resources is threatened by both increasing population (3% per year) and drought periods occurring since the 1970s. Hydrogeological information such as total resource and renewal rate of groundwater (i.e. the ratio between the annual recharge and the total reserve) is essential for improving water resources management. Estimation of recharge is difficult, however, due to the very low amounts of recharge, and due to temporal and spatial variability of rainfall and recharge processes. This investigation aims to estimate the recharge rate of the unconfined aquifer in the Iullemeden basin (western Niger). This project was carried out under a general hydrogeological study reported by Desconnets et al., 1997, Leduc et al., 1996, Leduc et al., 1997.
The Iullemeden basin system is characterised by high aquifer heterogeneity and complex discontinuity of the recharge mechanism. As a result, estimations of the local recharge rate vary widely. Early studies estimated the recent recharge of the unconfined aquifer based on pool water budgets in 1991 and 1993 to be in the order of 50–60 mm a−1 (Desconnets et al., 1997, Leduc et al., 1996, Leduc et al., 1997). In a subsequent investigation, the renewal rate, expressed in percentage of the aquifer volume that is renewed annually, was estimated on the basis of the tritium contents (Leduc et al., 1996). The annual renewal rates calculated in this study, using results above the analytical threshold, ranged from 0.05 to 5% of the aquifer volume. Considering all samples, the overall average rate appeared to be not more than 0.5%. The radioisotope (tritium and carbon-14) approach provides natural time tracers of groundwater and should allow an estimation of local renewal rates integrated over a much longer period of time than the two years of observation. The tritium method is limited here by the short half-life of this isotope, 12.43 a. With a longer half-life, 5730 a, carbon-14 should allow us to confirm and extend the previous calculation of the recharge rate to smaller values. In order to estimate the mean renewal rate of the shallow aquifer, the models considered in this paper take into account the variations of the radioactive tracer input and the mixing processes occurring within the aquifer. The radiotracer input to the groundwater varies according to both (i) variations in atmospheric 3H contents and 14C activities with time and (ii) variations in the amount of annual precipitation.
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Hydrogeological setting
The study area (2–3°E, 13–14°N) is situated in the Iullemeden basin and covers a region limited by a large dry fossil valley in the east, the Dallol Bosso, and by the Niger River in the south-west (Fig. 1).
The Iullemeden basin is covered by late Tertiary continental deposits, the Continental Terminal (CT). Derived from the erosion of the crystalline formations surrounding the basin, the CT formations mainly consist of sands, silts, clays and sandstones including indurated lateritic layers. The
Sampling and measurements
During 1994 and 1995, groundwater samples were collected from both the shallow and the confined aquifers. Most shallow groundwater samples were collected from open village wells while the samples from the confined aquifer were collected from boreholes equipped with manual pumps.
Physical and chemical parameters, i.e. depth to water table, temperature, pH, electrical conductivity (EC) and alkalinity, were measured in the field. The total dissolved inorganic carbon (TDIC) is calculated from
Mineralisation, pH and oxygen-18 contents
The groundwater temperature of the shallow aquifer ranges from 28 to 32°C reflecting the mean annual temperature. Most water samples are characterised by low total mineralisation, with EC ranging from 20 to 300 μS cm−1, although locally a few of them show higher values. The shallow groundwater is characterised by low pH, ranging from 5.3 to 6.5, and low TDIC contents, around a mean of 1.6 mmol l−1 (ranging from 0.4 to 3.6 mmol l−1). According to a large survey carried out in 1994 and 1995 (Leduc and
Modelling of carbon-14 activity and the calculation of renewal rates
Annual renewal rates of groundwater into the unconfined aquifer can be estimated from both tritium and carbon-14 activities taking into account both (i) the annual input of the radio-tracers and (ii) the radioactive decay. Two models of groundwater mixing based on successive annual recharge within the aquifer have been considered. The model of a well-mixed reservoir assumes that a complete mixing of groundwater issued from successive recharge events occurs within the aquifer. This model was
Sensitivity of models
As the two models represent two extreme views of the processes, which can occur in the aquifer system, the calculated renewal rates should represent extreme values. However, from one model to the other, the calculated renewal rates show little variation for a given carbon-14 activity (Fig. 4). These variations are negligible compared to the wide range of renewal rates suggested by the tritium (less than 0.05–5%) and carbon-14 data (0.03–3%). Moreover, most shallow groundwater samples show
Conclusion
The oxygen-18 signature of the groundwater emphasises the heterogeneity of the recharge processes that is also shown by the highly variable carbon-14 activity in the unconfined aquifer. The variations of the carbon-13 signature show the relative influence of native vegetation and millet on the geochemical signature of soil carbon dioxide.
The model of a well-mixed reservoir is in good agreement with the experimental data of tritium and carbon-14 content of the groundwater in the studied aquifer,
Acknowledgements
The authors gratefully acknowledge the contributions of the staff at IRD (Institut de Recherche pour le Developpment), Niamey, for their help during fieldwork, and of the staff at the LHGI (Laboratoire d'Hydrologie et de Géochimie Isotopique-Université de Paris-Sud) for their continuing support and constructive comments. We also thank technical staff at the LHGI, A. Filly and A. Raguideau, for their constant help and assistance during laboratory analyses.
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2020, Journal of HydrologyCitation Excerpt :Hence, if there is preferential recharge by high 3H winter rains, the actual recharge rates would be lower than those in Table 2. Hb is difficult to estimate and is likely to be spatially variable (Le Gal La Salle et al., 2001; Cartwright et al., 2007); however, given the observations of vertical differences in groundwater geochemistry, it is unlikely that Hb is significantly greater than the 5 to 10 m used in the calculations. The renewal rate model also assumes a homogeneous aquifer.