Hydraulic communication and impact on the quality of groundwater in the North Djeffara coastal aquifer, Tunisia

The studied aquifer system is located in the south east of Tunisia bordering on the Gulf of Gabes. It extends over an area of 1000 km2. This coastal Djeffara basin is characterized by an undulating landscape with a semi arid climate. The average annual rainfall is about 180 mm, the potential evaporation is about 1300mm/year, and the average monthly temperature ranges from 16 deg. C (coldest month) to 30 deg. C (warmest month). Two sandy layers between 0 and 120 m depth, which belong to the Mio-Pliocene, form the aquifers. The results of geochemical and isotopic studies, presented in this paper, have shown that the groundwater in the south of the study area (El Hicha) originates mainly from the Djeffara aquifer. The groundwater is replenished by discharge of the Continental Intercalaire in the fault system of the el Hamma region. North of a drain that crosses the El Hicha area at Oued el Akarit, the groundwater contains varying proportions of additional local recharge, which range between about 10% and more than 50%. At some sites the presence of bomb-tritium indicates a modern component of recharge in the groundwater. This suggests that the aquifer is unconfined, at least in its upper layer. 14C data have been used to estimate the rate of local recharge (in the order of 1mm/a). The salinity of the groundwater appears to originate from dissolution of minerals in the aquifer system. The higher values of the dissolved anions and cations in the northern part of the El Hicha area are due to higher sodium and chloride concentrations. The changes of the latter concentration seem to be associated with changes of the proportion of local recharge. The paper is related to a technical cooperation project with the International Atomic Energy Agency, Vienna, Austria, aimed at the utilization of saline groundwater to grow salt-tolerant plants. In this regard, a better understanding of the recharge and flow regime as well as the origin or salinity of the groundwater was requires. To reach this goal, isotope and geochemical investigations were carried out. Water samples were taken from wells and boreholes in the study area in the Djeffara coastal plain between Metouia in the south and El Hichia in the north. The samples were analysed for their chemical and isotopic compositions. In the following, the results of these analyses are presented and discussed in terms of the replenishment and flow regime of the groundwater and the origin and evolution of its salinity. On the basis of a detailed evaluation of combined isotope and chemical data the following can be concluded. 1. Groundwater in the south of the El Hicha study area originates mainly from the Djeffara aquifer. It has been formed by the discharge of the Continental Intercalaire in the fault system of the El Hamma region (Aranyossy and Mamou, 1985). 2. North of the drain that crosses the El Hicha area at Oued el Akarit, the groundwater contains varying proportions of additional local recharge, which range between about 10% and more than 50%. Usually the higher proportions of local recharge have been found in dug wells or in dug wells combined with pumped boreholes. 3. At some sites bomb-tritium has been found, which indicates a modern component of recharge in the groundwater. This suggests that the aquifer is unconfined, at least in its uppers layer. 4. The few 14C data have been used to estimate the rate of local recharge. The obtained value is in the order of 1 mm/a and agrees with modelling results based on the Darcy law. 5. So far, there are no indications of seawater infiltrations. The salinity of the groundwater appears to be mainly due to dissolution of minerals in the aquifer system involved (Edmunds et al. 1997). The higher values of the dissolved anions and cations in the northern part of the El Hicha area are due to higher sodium and chloride concentrations. The changes of the latter concentration seem to be associated with changes of the proportion of local recharge