To date, there has been no agreement on the best way to simulate saltwater intrusion (SWI) in karst aquifers. An equivalent porous medium (EPM) is usually assumed without justification of its applicability. In this paper, SWI in a poorly karstified aquifer in Lebanon is simulated in various ways and compared to measurements. Time series analysis of rainfall and aquifer response is recommended to decide whether quickflow through conduits can be safely ignored. This aids in justifying the selection of the exemplified EPM model. To examine the improvement of SWI representation when discrete features (DFs) are embedded in the model domain, the results of a coupled discrete-continuum (CDC) approach (a hybrid EPM-DF approach) are compared to the EPM model. The two approaches yielded reasonable patterns of hydraulic head and groundwater salinity, which seem trustworthy enough for management purposes. The CDC model also reproduced some local anomalous chloride patterns, being more adaptable with respect to the measurements. It improved the overall accuracy of salinity predictions at wells and better represented the fresh–brackish water interface. Therefore, the CDC approach can be beneficial in modeling SWI in poorly karstified aquifers, and should be compared with the results of the EPM method to decide whether the differences in the outcome at local scale warrant its (more complicated) application. The simulation utilized the SEAWAT code since it is density dependent and public domain, and it enjoys widespread application. Including DFs necessitated manual handling because the selected code has no built-in option for such features.

This study demonstrates groundwater quality differences between a limestone and a dolomitic limestone, (sub)oxic coastal aquifer in the Eastern Mediterranean (Lebanon), with and without ongoing moderate salinization since the last decades. For this purpose, 8 major and 50 trace elements (TEs) were analyzed in 80 water and 65 rock samples, and interpreted with a quad-fold approach utilizing: (1) nonparametric statistical tests, (2) concentration deviations from ideal conservative freshwater–seawater mixing lines, (3) a new parameter called Mixing Enrichment Factor to assess the mobility of chemical constituents under salinizing conditions, and (4) 1-D dual porosity flow path modeling with PHREEQC. Dissolution/precipitation of Ca_xMg_ySr_zCO₃ and cation exchange were the main disclosed hydrogeochemical processes besides minor signs of organic matter oxidation. In the dolomitic limestone aquifer, less carbonate dissolved as compared to the limestone aquifer, partly because of lower pCO₂ in addition to seawater inflow triggering Mg-calcite precipitation by cation exchange. Saltwater intrusion led to mobilization of As, Ba, Cu, Ni, Rb, Sr and U in both aquifers, sometimes likely by cation exchange (e.g. Ba and Sr). Some of these TEs (notably Cu and Ni) recorded higher concentrations in the dolomitic limestone regardless of salinization. Other elements like Al, Be, Ce, Cr, Nb, Pb, V, Y and Zr revealed no or a low mobilization tendency. The concentration of all TEs in groundwater remained below drinking water limits notwithstanding moderate salinization. This classifies carbonate rocks as a weak geogenic source of TEs, whereas encroaching seawater appears to be a more important source.

Most countermeasures to mitigate saltwater intrusion in coastal, karstic or fractured aquifers are hindered by anisotropy, high transmissivities and complex dynamics. A coupled strategy is introduced here as a localized remedy to protect shallow freshwater reserves while utilizing the deeper intercepted brackish water. It is a double sourcing application where fresh-keeper wells are installed at the bottom of a deepened borehole of selected salinized wells, and then supported by high recovery RO desalination. The RO design has < 1 kWh/m³ energy consumption, and up to 96% recovery in addition to low scaling propensity without use of any anti-scalant. A feasibility study is presented as an example for a salinizing, brackish well (TDS ~ 1600 mg/L) in the Damour coastal aquifer in Lebanon. The concept is expected to produce ca. 1000 m³/d of freshwater from this well by pumping 250 m³/d of fresh groundwater from the top well screen and 800 m³/d of brackish groundwater (to be later desalinized) from the fresh-keeper well screen below. Cost analysis shows that the capital cost could be returned back in 1 to 4 years depending on the choice of produced water (bottled or tap) and available market. As an alternative, water from the RO plant could be blended with lower quality water, for instance untreated brackish groundwater (if unpolluted), to supply 3 more volumes for domestic use. The usage of brackish groundwater from integrated fresh-keeper wells thus serves 3 purposes: production of high quality drinking water, financial gain and mitigation of water stress by overpumping.