Numerical analysis of physical barriers systems efficiency in controlling saltwater intrusion in coastal aquifers

Numerical investigation of mixed physical barriers for saltwater removal in coastal heterogeneous aquifers

Saltwater intrusion is a prevalent global environmental issue that detrimentally impacts coastal groundwater aquifers. This problem is exacerbated by climate change and increased groundwater abstraction. Employing physical barriers proves effective in mitigating saline water intrusion. In this study, a validated numerical simulation model is utilized to assess the impact of aquifer stratification on the effectiveness of mixed physical barriers (MPBs) and their response to structural variations. Additionally, the performance of MPBs was compared with that of single physical barriers in a laboratory-scale aquifer. Three different configurations were replicated, comprising two stratified aquifers (HLH and LHL) and a homogenous reference aquifer (H). The results demonstrate that MPBs are efficient in decreasing the saltwater penetration length in the investigated cases. The reductions in penetration length were up to 65% in all cases. The removal efficacy of residual saline water for MPBs exceeded that of the subsurface dam by 2.1-3.3 times for H, 2.1-3.6 times for HLH, and 8.3 times for LHL conditions, while outperforming the cutoff wall by 38-100% for H, 39-44% for HLH, and 2.7-75% for LHL. These findings are of importance for decision-makers in choosing the most appropriate technique for mitigating saline water intrusion in heterogeneous coastal aquifers.

Cost-effective management measures for coastal aquifers affected by saltwater intrusion and climate change

Sustainable management of natural water resources and food security in the face of changing climate conditions is critical to the livelihood of coastal communities. Increasing inundation and saltwater intrusion (SWI) will likely adversely affect agricultural production and the associated beach access for tourism. This study uses an integrated surface-ground water model to introduce a new approach for retardation of SWI that consists of placing aquifer fill materials along the existing shoreline using Coastal Land Reclamation (CLR). The modeling results suggest that the artificial aquifer materials could be designed to decrease SWI by increasing the infiltration area of coastal precipitation, collecting runoffs from the catchment area, and applying treated wastewater or desalinated brackish water-using coastal wave energy to reduce water treatment costs. The SEAWAT model was applied to verify that it correctly addressed Henry's problem and then applied to the Biscayne aquifer, Florida, USA. In this study, to better inform Coastal Aquifer Management (CAM), we developed four modeling scenarios, namely, Physical Surface Barriers (PSB), including the artificial aquifer widths, permeability, and side slopes and recharge. In the base case scenario without artificial aquifer placement, results show that seawater levels would increase aquifer salinity and displace large amounts of presently available fresh groundwater. More specifically, for the Biscayne aquifer, approximately 0.50% of available fresh groundwater will be lost (that is, 41,192 m³) per km of the width of the aquifer considering the increasing seawater level. Furthermore, the results suggest that placing the PSB aquifer with a smaller permeability of <100 m per day at a width of approximately 615 m increases the available fresh groundwater by approximately 45.20 and 43.90% per km of shoreline, respectively. Similarly, decreasing the slope on the aquifer-ocean side and increasing the aquifer recharge will increase freshwater availability by about 43.90 and 44.50% per km of the aquifer. Finally, placing an aquifer fill along the shallow shoreline increases net revenues to the coastal community through increased agricultural production and possibly tourism that offset fill placement and water treatment costs. This study is useful for integrated management of coastal zones by delaying aquifer salinity, protecting fresh groundwater bodies, increasing agricultural lands, supporting surface water supplies by harvesting rainfall and flash flooding, and desalinating saline water using wave energy. Also, the feasibility of freshwater storage and costs for CAM is achieved in this study.

Long and short-term cation exchange linked to a negative hydraulic barrier in a coastal aquifer

Saltwater extraction in coastal aquifers generates a negative hydraulic barrier that prevents marine intrusion and produces a general freshening landward from this barrier. In the Andarax delta aquifer, SE Spain, two instances of saltwater extraction were performed and their effect on the aquifer hydrochemistry was studied. ¹⁴C groundwater dating, together with chemical analysis, reflects the presence of waters with different infiltration ages. Old marine groundwater (~10 ky) must be the remains of marine intrusion generated during the Holocene transgression at the same time the delta was formed. The freshening induced by the saltwater extraction triggers cation exchange between the aquifer substratum and groundwater. Unlike what is described in other examples of cation exchange in coastal aquifers, in the Andarax delta the freshening causes an exchange between Mg, which is released into the groundwater, and Na, which is held in the clay mineral structural unit. This process is reverted the moment the hydraulic barrier stops acting. Short saltwater pumping-stopping cycles generate fast inversions in this exchange chemical reaction. At the same time, a clear excess of Ca ion can be seen in all the groundwater samples. This excess is attributed to the release of this ion resulting from the overall marine intrusion in this area during the Holocene transgression. Contrasting what occurs with the Na-Mg exchange, the Na-Ca exchange process is more long-lasting in time.

Influence of the subsurface physical barrier on nitrate contamination and seawater intrusion in an unconfined aquifer

Coastal areas are facing not only environmental problems associated with seawater intrusion (SWI) but also health and ecological problems caused by excessive nitrate (NO₃^-) contamination. The installation of a subsurface physical barrier (SPB) is one of the common methods employed to reduce or prevent SWI, but there are few studies on the impact of SPBs on NP in groundwater. Through laboratory experiments and numerical simulations, the effects of the hydraulic gradient (HG), the nitrate concentration of the set groundwater nitrate pollution source, the relative height of the SPB (H_P') and the relative distance between the SPB and the saltwater boundary on the NP of groundwater in the presence of SWI, subsurface dams and cut-off walls were studied. Evaluation indicators were established to evaluate the degree and shape of the SWI and NP. To better describe the relationship between the velocity distribution and changes in the velocity distribution area and the degree of NP and SWI, the velocity distribution in the presence of SWI and a SPB was summarized separately. The results showed that when there was SPB, low-velocity zones were formed on both sides of the SPB, which not only slowed the migration of NO₃^- but also changed the shape of the NO₃^--contaminated area. The closer to the SPB area the pollutants were, the more obvious the obstruction effect. The obstruction effect of adding the cut-off wall on NP was more obvious than that of adding the subsurface dam wall. The selected HG and H_p' were important factors affecting NP and SWI. The higher HG was, the more serious the NP, the lower the HG, and the stronger the degree of SWI. Adding SPBs reduces the impact of HGs on NP and SWI. Therefore, the design of SPBs in coastal areas should focus on aspects related to these two factors.

Mitigation of seawater intrusion in coastal aquifers using coastal earth fill considering future sea level rise

Saltwater intrusion (SWI) is a physical problem that threatens many coastal aquifers all over the world. Saltwater intrusion is increasing with abstraction and rise in sea level. Coastal aquifer protection is essential to protect groundwater resources in these areas. A number of methods have been developed to protect coastal aquifers from SWI. This paper presents the impact of sea level rise on SWI in coastal aquifers and application of coastal earth fill as a new technique to control SWI. Different future sea level rise scenarios were studied and different coastal earth fill with an appropriate soil to extend the coastline towards the sea in order to control SWI was studied using SEAWAT model. The proposed control measure is numerically assessed by Henry's problem and then applied to a real case study of Biscayne aquifer, Florida, USA. For each aquifer, the corresponding relation was developed between the intrusion length of saltwater wedge and the width of fill. The results showed that increasing the fill width resulted in decreasing the intrusion length. In the case of Biscayne aquifer, increasing the fill width by 10, 20, 30, and 40% of the aquifer length resulted in retarding the intrusion to 329, 192, 42, and - 48 m respectively. Using 150- and 300-m fill widths retards the intrusion length by 32.3% and 60.5%. In addition, increasing the fill width to 465 m can retard SWI by 91.3%. This approach is capable to control the future risks of SWI and sea level rise.