© 2017 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/\ud This author accepted manuscript is made available following 24 month embargo from date of publication (Aug 2017) in accordance with the publisher’s archiving policyThe inland migration of seawater in coastal aquifers, known as seawater intrusion (SWI), can be categorised as passive or active, depending on whether the hydraulic gradient slopes downwards towards the sea or the land, respectively. Despite active SWI occurring in many locations, it has received considerably less attention than passive SWI. In this study, active SWI caused by an inland freshwater head decline (FHD) is characterised using numerical modelling of various idealised unconfined coastal aquifer settings. Relationships between key features of active SWI (e.g., interface characteristics and SWI response time-scales) and the parameters of the problem (e.g., inland FHD, freshwater-seawater density contrast, dispersivity, hydraulic conductivity, porosity and aquifer thickness) are explored for the first time. Sensitivity analyses show that the SWI response time-scales under active SWI situations are influenced by both the initial and final boundary head differences. The interface is found to be steeper under stronger advection (i.e., caused by the inland FHD), higher dispersivity and hydraulic conductivity, and lower aquifer thickness, seawater density and porosity. The interface movement is faster and the mixing zone is wider with larger hydraulic conductivity, seawater-freshwater density difference, and aquifer thickness, and with lower porosity. Dimensionless parameters (Peclet number and mixed convection ratio) from previous steady-state analyses offer only limited application to the controlling factors of passive SWI, and are not applicable to active SWI. The current study of active SWI highlights important functional relationships that improve the general understanding of SWI, which has otherwise been founded primarily on steady-state and passive SWI

In this study, abstraction, desalination and recharge method and SEAWAT numerical model are used to investigate seawater intrusion repulsion in a hypothetical two-dimensional coastal aquifer to understand the relation of seawater intrusion with abstraction, desalination and recharge parameters (i.e. abstraction/recharge rate, wells distance and depth). Abstraction, desalination and recharge consists of abstraction and desalination of brackish water and recharge of desalinated water. The results of different defined scenarios showed that increase of recharge rate has a significant effect on the seawater intrusion mitigation (e.g. more than 80% variation in saline water volume) while the increase of abstraction rate does not have specific impact on seawater recession (e.g. less than 3% variation in toe position). The method efficiency in reducing seawater intrusion is increased when freshwater is recharged by well at outside of saltwater wedge and close to its toe position. Moreover, it is shown that the abstraction, desalination and recharge performance has slightly improved when the recharge and extraction wells are placed deeper into aquifer and close to aquifer bottom (almost 15% for all characteristics of salt wedge). Ultimately, dilution of saline water with recharged freshwater will widen the mixing zone but as salt wedge recedes toward the sea simultaneously, the mixing zone thickness cannot follow the steady reduction trend.

In this study, groundwater potential and aquifer characteristics were analysed and investigated for the first time in an integrated manner using pumping test and resistivity method in all areas within Makassar City, Indonesia. It is identified that the soil layer in Makassar consists of alluvial sediment, clay sand, sandy clay, tuff, and volcanic breccia. Productive aquifers were found at a depth of 15 to 50 m in the northern, southern, and eastern areas of the city and at a depth of 51 to 120 m in the central areas. The storativity values showed that the type of aquifer in Makassar is dominated by unconfined and semi-confined. The largest optimum pumping discharge was identified in Panakkukang and Manggala Districts with values of 0.102 and 0.061 m³/min, respectively. It was found that the distribution of aquifer transmissivity corresponds to the distribution of the optimum pumping discharge where the largest transmissivity values are located in Panakkukang and Manggala Districts. Among all districts in Makassar City, Panakkukang and Manggala Districts have the greatest groundwater potential. This is most likely due to the position of these two areas which are situated in a groundwater discharge zone identified in the previous study.