No abstract

No abstract

No abstract

Previous studies of freshwater lenses in saline aquifers adjoining gaining rivers ("riparian lenses") have so far considered only rivers that fully penetrate the aquifer, whereas in most cases, rivers are only partially penetrating. This paper presents a new methodology for obtaining the saltwater discharge and the shape of a steady-state, non-dispersive riparian lens, where the river is partially penetrating, combining two previous analytical solutions. The resulting analytical solution is compared to numerical modeling results to assess assumptions and the methodology adopted to approximate the "turning effect," which is the change in groundwater flow direction (horizontal to vertical) near the partially penetrating river. Model parameters were taken from previous studies, representing simplified conditions in the River Murray floodplains (Australia). Consistency between analytical and numerical results and field observations highlights the capability of the proposed analytical solution to predict the riparian lens geometry and saltwater discharge into partially penetrating rivers. Sensitivity analysis indicates that larger riparian lenses are produced adjacent to the deeper and wider rivers, as expected. The change in width or depth of the river has more influence on the saltwater discharge and the horizontal extent of the riparian lens (and less effect on the vertical extent of the lens adjacent to the river) for shallower and narrower rivers. This research highlights the utility of the new method and demonstrates that the assumption of a fully penetrating river likely leads to significant overestimation of the saltwater discharge to the river and the riparian lens horizontal extent and vertical depth.

We propose a new concept that has the potential to mitigate seawater intrusion and increase the fresh groundwater storage of oceanic islands by creating a less permeable slice along the shoreline. We present a proof‐of‐concept study to examine its effectiveness through analytical and experimental studies. Analytical expressions for calculating the freshwater‐seawater interface location, water table elevation, fresh groundwater volume, and groundwater travel time are presented for both barrier and circular islands, which are found dependent on three different scenarios of interface locations. The analytical solution of the interface location in a barrier island is verified through sand‐tank experiments. Sensitivity analyses based on a simplified conceptual model of St. George Island in Florida, USA, indicate that the fresh groundwater volume monotonically increases with the decrease in the hydraulic conductivity of the coastal less permeable hydrogeologic unit. On the other hand, the increase of the coastal less permeable unit extent leads to an increased fresh groundwater volume. However, when the interface tip is on the aquifer bed of the coastal less permeable unit, a further increase of the less permeable unit extent only slightly increases the fresh groundwater volume, since the interface does not change any more and only the water table is elevated. We demonstrate here that the concept proposed has the potential in increasing the fresh groundwater storage of oceanic islands. Analytical expressions presented can improve our understanding of seawater intrusion in a dual‐unit oceanic island.

[1] The width of a mixing zone between freshwater and seawater is important primarily because it directly reflects the extent of mixing and the growth and decay of the mixing zone indicates changes of the flow regime and water exchange between freshwater and coastal seawater. Wide mixing zones have been found in many coastal aquifers all over the world. However, the mechanisms responsible for wide mixing zones are not well understood. This work examines the hypothesis that kinetic mass transfer coupled with transient conditions, which create the movement of the mixing zone, may widen mixing zones in coastal aquifers. The hypothesis is tested by conducting two-dimensional numerical simulations based on a variable-density groundwater model for a scaled-tank model and a field-scale model. Periodic water levels, representing periodic tidal motion and freshwater table fluctuations, are imposed at the seaward and landward boundary, respectively, which cause the movement of the mixing zone. Both the scaled-tank model and the field model show that the combination of the moving mixing zone and kinetic mass transfer may significantly enhance the extent of mixing and create a wider mixing zone than the models without kinetic mass transfer. In addition, sensitivity analyses indicate that a larger capacity ratio (immobile porosity/mobile porosity) of mass transfer leads to a wider mixing zone, and the maximum width of the mixing zone may be reached for a given capacity ratio when the mean retention time scale in the immobile domain (the reciprocal of mass transfer rate) and the period of water level fluctuations are comparable.

No abstract

No abstract