MANAGED AQUIFER RECHARGE IN SALINE GROUNDWATER REGIONS FOR ENHANCED RECOVERY OF FRESHWATER

Abstract

Groundwater is a vital freshwater source in arid and semi-arid regions due to scanty/erratic rainfall, longer dry seasons, and insufficient availability of surface water resources. Increasing groundwater salinity in these regions makes freshwater availability more challenging. Managed Aquifer Recharge (MAR) techniques are increasingly used to ensure freshwater availability throughout the year by aquifer recharging using surface water collected during rainy seasons and subsequently recovering it during the dry periods. However, the performance of an MAR in saline groundwater regions is adversely affected due to the mixing/diffusion of recharged freshwater with ambient saline groundwater which reduces the amount of recoverable freshwater. The losses in recoverable freshwater with respect to its injected volume in saline aquifers occur primarily by processes like fresh-saline interface interactions, displacement of stored freshwater due to buoyancy, and horizontal/lateral groundwater flow displacement. These processes are mainly controlled by aquifer hydrogeology, MAR well design, and the operational factors of MAR. Therefore, a detailed study is required to explore the potential of MAR in saline aquifers by investigating the processes affecting the freshwater recovery efficiency (RE). The main focus of this study is to propose an efficient MAR for enhanced recovery of freshwater in saline aquifers through suitable site selection and optimized operational and design factors. A meta-analysis of the literature studies was conducted first to study the general influence of site hydrogeological conditions and operational factors on MAR performance in saline groundwater regions. The findings from previous field and numerical studies of MAR were reviewed, grouped, and analyzed systematically for each influencing factor. The results showed that higher freshwater recovery can be achieved at sites with moderate to low horizontal groundwater flow velocity, lower hydrodynamic dispersivity, lesser confined aquifer thickness, and low groundwater salinity. Further results demonstrated that high injection rate, larger freshwater volumes, smaller storage durations, multiple partially penetrating well (MPPW) design, and an optimal number of consecutive injection-recovery cycles of MAR lead to its higher RE. It was found that the site selection for the MAR application is crucial for achieving high RE which should be finalized based on characterization of site hydrogeological properties. The MAR site selection should be carefully decided based on favorable hydrogeology of the area as hydrogeological factors are difficult to alter. At the other hand, the operational and MAR well design factors are adjustable and can be optimized for achieving high RE. Therefore, the findings of this section are crucial to aid the decision-making process for MAR site feasibility and operational management in saline groundwater regions. After examining the factors influencing MAR performance in general, a series of practical experiments were performed using multi-dimensional sand tank setups to investigate the influence of each MAR operational factor in detail. The experiments were conducted using a three-dimensional (3-D) sand tank setup representing a confined (100 cm length x 30 cm width x 60 cm depth) aquifer conditions. The movement and spreading of the stored freshwater in confined formation were examined with the time under different operating conditions. To explore the potential of MAR in unconfined saline aquifers, a similar experiment using another 3-D sand tank (60 cm length x 30 cm width x 60 cm depth) setup was performed under controlled conditions. The impact of freshwater injection/release depth (shallow, moderate, and deep) on total RE was investigated using the developed system. The influence of groundwater salinity level, injection/extraction rates, freshwater storage duration, and the volume of injected freshwater on RE were investigated for both the aquifer conditions. The results demonstrated that higher injection/extraction rates, larger freshwater volume, and smaller storage durations lead to higher freshwater recovery in both confined and unconfined aquifers. The results from the investigation for unconfined aquifers suggested that deeper freshwater injection leads to higher losses and, thus, the injection/release depth should be kept moderately shallow to avoid losses caused by the freshwater displacement under the buoyancy effect. Finally, the performance of MAR at characteristic field scale was investigated using variable-density groundwater flow model runs. The 3-D finite difference MODFLOW linked with SEAWAT 4 model runs were used to evaluate the performance of an MAR on the basis of site hydrogeological conditions along with operational and well-design factors of MAR. The operational (injection/recovery rates, volume of injected freshwater, storage duration, and successive number of injection-recovery cycles) and well design (single fully penetrating well - SFPW, and multiple partially penetrating wells - MPPW) conditions of MAR were investigated to maximize the total freshwater recovery. The results showed that losses in injected freshwater due to mixing with saline water are most influenced by the number of successive MAR cycles followed by injection/recovery rates, injection volume, and storage duration. In addition to these operational factors, further improvement in RE can be achieved by replacing the MAR well design from SFPW to MPPW type by allowing efficient recovery of upwardly displaced freshwater. The modeling results depicted that with a MPPW-type well design in place of the SFPW for a specific storage duration, 11 % higher RE was achieved for a considered site hydrogeology. In general, it was found that MPPW is more suitable for locations where a comparatively lower amount of freshwater is available for injection, particularly where freshwater storage is required for longer durations. This study clearly highlighted the key factors governing the performance of an MAR system in saline aquifers and identified their favorable ranges to enhance the freshwater recovery. It is found that a careful site selection with suitable well design and maintaining favorable ranges of operational factors ensures an efficient MAR altogether. Through this work, the developed understanding on hydrogeological factors will help in appropriate site selection for MAR, while the optimized operational and well-design factors will help in improving the performance of a working MAR in salt-affected aquifers. Keywords: Managed Aquifer Recharge (MAR), MODFLOW and SEAWAT, recovery efficiency (RE), well design, saline aquifers, hydrogeological and operational factors.