Abstract: Seawater intrusion (SWI) has led to salinization of fresh groundwater reserves in coastal areas worldwide and has forced the closure of water supply wells. There is a paucity of well-documented studies that report on the reversal of SWI after the closure of a well field. This study presents data from the coastal carbonate aquifer in northeast China, where large-scale extraction has ceased since 2001 after salinization of the main well field. The physical flow and concomitant hydrogeochemical processes were investigated by analyzing water level and geochemical data, including major ion chemistry and stable water isotope data. Seasonal water table and salinity fluctuations, as well as changes of δ2H–δ18O values of groundwater between the wet and dry season, suggest local meteoric recharge with a pronounced seasonal regime. Historical monitoring testifies of the reversibility of SWI in the carbonate aquifer, as evidenced by a decrease of the Cl− concentrations in groundwater following restrictions on groundwater abstraction. This is attributed to the rapid flushing in this system where flow occurs preferentially along karst conduits, fractures and fault zones. The partially positive correlation between δ18O values and TDS concentrations of groundwater, as well as high NO3− concentrations (\textgreater39mg/L), suggest that irrigation return flow is a significant recharge component. Therefore, the present-day elevated salinities are more likely due to agricultural activities rather than SWI. Nevertheless, seawater mixing with fresh groundwater cannot be ruled out in particular where formerly intruded seawater may still reside in immobile zones of the carbonate aquifer. The massive expansion of fish farming in seawater ponds in the coastal zone poses a new risk of salinization. Cation exchange, carbonate dissolution, and fertilizer application are the dominant processes further modifying the groundwater composition, which is investigated quantitatively using hydrogeochemical models.

Abstract: Despite that fractured coastal aquifers are widespread, the influence of fracture characteristics on seawater intrusion (SWI) has not been explored in previous studies. This research uses numerical modelling in a first step towards understanding the influence of fracture orientation, location and density on the extent of seawater and accompanying patterns of groundwater discharge in an idealised coastal aquifer. Specifically, aquifers containing single fractures or networks of regularly spaced fractures are studied using modified forms of the Henry SWI benchmark problem. The applicability of equivalent porous media (EPM) models for representing simple fracture networks in steady-state simulations of SWI is tested. The results indicate that the influence of fractures on SWI is likely to be mixed, ranging from enhancement to reduction in seawater extent and the width of the mixing zone. For the conceptual models considered here, vertical fractures in contact with the seawater wedge increase the width of the mixing zone, whereas vertical fractures inland of the wedge have minimal impact on the seawater distribution. Horizontal fractures in the lower part of the aquifer force the wedge seaward, whereas horizontal fractures located within the zone of freshwater discharge enhance the wedge. Inclined fractures roughly parallel to the seawater-freshwater interface increase the landward extent of seawater and fractures perpendicular to the interface inhibit the wedge. The results show that EPM models are likely inadequate for inferring salinity distributions in most of the fractured cases, although the EPM approach may be suitable for orthogonal fracture networks if fracture density is high and appropriate dispersivity values can be determined.