Hydrochemical, Isotopic, and Geophysical Studies Applied to the Evaluation of Groundwater Salinization Processes in Quaternary Beach Ridges in a Semiarid Coastal Area of Northern Patagonia, Argentina

Quaternary sea level fluctuations have led to the development of beach ridges on many South Atlantic coasts. The objective of this paper was to asses from lithological, hydrochemical, isotopic, and geophysical studies the salinization processes affecting groundwater stored in Pleistocene and Holocene beach ridges of the northern Patagonian coast. A hydrogeomorphological characterization of the area was performed using digital elevation models, the interpretation of satellite images, and field studies. Vertical electrical soundings were performed on transects running perpendicular to beach ridges in order to define variations in the freshwater-saltwater interface position. The salinity, chemistry, and stable isotopes of the groundwater were analyzed. The results demonstrated that the groundwater salinization of Pleistocene ridges responds to processes associated with the geological-geomorphological evolution of the area. The cementation of these surface sediments limits rainwater infiltration, which consequently prevents the development of freshwater lenses. This suggests that saline water is the result of ancient marine ingressions. Freshwater lenses develop in Holocene beach ridges; however, slight water salinization is detectable in the most populated areas as a result of intensive exploitation. The data provided are useful for freshwater resource prospection along the arid coast of Patagonia, where beach ridge deposits abound and populations experience serious drinking water supply problems.

Impact of Island Urbanization on Freshwater Lenses: A Case Study on a Small Coral Island

Freshwater resources on small coral islands mainly exist in the form of freshwater lenses. The freshwater lens is highly vulnerable to salinization due to natural recharge variations and urbanization construction. However, it is unclear how a freshwater lens evolves under the influence of urbanization construction and which factors control its evolution. Based on the hydrogeological data of a small coral island in China, a corresponding 3D numerical model was established by the Visual MODFLOW software to investigate the formation and evolution of freshwater lenses under natural conditions. Thereby, the island reclamation scenario and impermeable surface scenario were set up and the changes in morphology and volume of the freshwater lens were analyzed. The results show the following: (1) After island reclamation and island building, the freshwater lens would reach a stable state after 25 years and the freshwater lens would also appear in the newly added part of the island with a thickness of 9.5 m, while the volume of the total freshwater lens would increase to 1.22 times that of the original island. (2) When the impermeable surface is built at different positions of the island, the reduction in the volume of the freshwater lens, in the order from large to small, is Scenario B (northeast side), Scenario A (southwest side) and Scenario C (central); with the increase in the impermeable surface area, the volume of the freshwater lens would gradually decrease and the volume of the freshwater lens would decrease by more than 50% with the impermeable surface exceeding 30% of the island area. The study has important implications for the conservation and rational development of subsurface freshwater resources on islands.

Pumping Well Layout Scheme Design and Sensitivity Analysis of Total Critical Pumping Rates in Coral Island Based on Numerical Model

Groundwater on small coral islands exists in the form of freshwater lenses that serve as an important water resource for local inhabitants and ecosystems. These lenses are vulnerable to salinization due to groundwater abstraction and precipitation variation. Determination of the sustainable yield from freshwater lenses is challenging because the uncertainties of recharge and hydrogeological characteristics make it difficult to predict the lens response to long-term pumping. In this study, nine pumping well layout schemes along a line are designed using the orthogonal experimental design method, and an optimal well layout scheme is determined by multi-index range analysis and comprehensive balance analysis method. The total critical pumping rates of the freshwater lens corresponding to different schemes are calculated by numerical simulation, and the sensitivity of the total critical pumping rates to hydrogeological parameters is analyzed. The results show that the calculation of the total critical pumping rates needs to be combined with the specific well layout scheme with consideration to the length of well screens, the number of wells and the distance between wells. The difference in total critical pumping rates between different schemes can be up to three times. The uncertainty of hydrogeological parameters has a great impact on the total critical pumping rates. Within the range of a 30% reduction in parameters, α and K are the key risk factors of pumping; within the range of a 30% increase in parameters, α, ne and K are the key risk factors; α-ne combined changes had the greatest impact. The management of freshwater lenses and the assessment of sustainable yield will continue to be important tasks for coral islands in the future, and this study can help with the sustainable exploitation of island freshwater lenses.

Laboratory Physical Experiments on the Saltwater Upconing and Recovery of Island Freshwater Lenses: Case Study of a Coral Island, China

The research of saltwater upconing is crucial for the development and utilization of island freshwater resources. In this paper, a laboratory physical experiment device was developed, and the saltwater upconing and recovery of island freshwater lenses were investigated using rainfall simulations and single-well and multi-well pumping tests with various pumping intensities. The results of the experiment revealed that: (1) The thickness of the freshwater lens increased continuously and linearly during the early stages of rainfall. As the rainfall continued, the upward trend slowed and eventually leveled off. (2) Under the same pumping duration, when the pumping intensity was less than the critical pumping intensity, the increasing height rate of the saltwater upconing increased linearly at a small gradient, and when the pumping intensity was greater than the critical value, the rising height rate of the saltwater upconing increased linearly at a large gradient. (3) Under the same pumping intensity conditions, the pumping duration of the multi-well was longer than that of the single-well pumping, and the pumping volume of the multi-well was also greater than that of the single-well pumping. The experiment results can provide support for the development and utilization of island freshwater lens.