Using Managed Aquifer Recharge to mitigate saltwater intrusion in a Southern Malta Coastal aquifer
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Magar from PexelsWater is a scarce resource in the Mediterranean region. The Maltese archipelago is a relevant regional case study in which the available water sources do not provide sufficient volumes to meet demand, and sustainable water resources management measures are necessary to face the climatic variability and supply security. A national policy framework for the use of non-conventional water sources paved the way for the implementation of alternative methods such as Managed Aquifer Recharge (MAR).
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The Maltese Energy and Water Agency, through European Union Research Funding, implemented a series of well injection experiments using highly polished treated wastewater to curb the advance of saltwater intrusion into Malta’s island main source of freshwater, the karstic limestone lower aquifer. Groundwater overexploitation and the expected changes in rainfall patterns due to climate change drives the protection prioritization of this source, the most important in the region. The MAR scheme developed showed that, although no considerable results were achieved, together with other methods associated with agriculture and industrial water use reduction and treated wastewater reuse, MAR is a sound complementary methodology to help cope with near-future challenges.
The implementation of Managed Aquifer Recharge to mitigate the advance of saltwater in the Maltese aquifers was first suggested by Energy and Water Agency (EWA), a governmental agency from the Maltese Ministry of Energy, Energy and Sustainable Development. It aims to implement national water and energy sectors policies to ensure security, sustainability and affordability of energy and water in Malta.
EWA partnered with Malta’s Water Services Corporation (WSC), a public entity responsible for the production and distribution of drinking water and collecting and treating wastewater in the islands. It also designs, builds, and sells dedicated desalination and water processing equipment and provide accredited laboratory services to industry, hotels, and private individuals.
Malta National Climate Change Adaptation Strategy, the main driver for water management, is enforced through the basis of the Water Catchment Management Plans (WCMP). According to the Maltese Resources Authority, those plans, similar to other catchment plans implemented throughout the EU, represent a holistic approach to address water issues, considering impacts on health, biodiversity, landscape, soil and climate. The objectives related to groundwater include the following:
- Preventing deterioration in the status of groundwater bodies.
- Protecting, enhancing, and restoring all groundwater bodies.
- Prevention of and limitation of the input of pollutants to groundwater.
- Reversing any significant upward trend of pollutants in groundwater.
- Achievement of good groundwater qualitative and quantitative status by 2015 or in specific circumstances by 2021 and 2027
The second WCMP (2015-2021) (EWA, 2015) envisaged a reduction of groundwater use by increasing efficient use, recycling/reuse of water resources and supply substitution through the use of alternative resources such as water re-use, grey-water recycling, water efficiency measures, managed aquifer recharge and the optimization of water use by the commercial and agricultural sector (Fig. 1).
The Maltese archipelago consists of three inhabited islands (Malta, Gozo and Comino).
Aqueduct Water Risk Atlas characterizes Malta as an extremely high Water Stress Index area – the ratio of total water withdrawals to the available surface and groundwater resources – with medium drought risk and medium-high seasonal water variability (Hofste et al., 2019) with a semi-arid Mediterranean Climate (Sapiano, 2020). Freshwater is a scarce resource in the Maltese archipelago, as its geological conditions do not allow for efficient surface water storage. The islands comprise a succession of Tertiary limestones and marls with scarce quaternary deposits (Barbagli et al., 2021). The geological characteristics result in two main aquifer types: small, perched, groundwater bodies and the lower main sea-level groundwater bodies which sustain groundwater floating lenses in direct contact with saltwater (Lotti et al., 2021). The second is commonly referred to as the Malta Mean Sea Level aquifer (MSL).
Together with one of the highest population densities in the world and tourism pressure, with peak seasonal increase in the driest months with and around 550 mm/y mainly in September and April (Sapiano, 2020) this creates an increased challenge in water resources management. In 2014, around 60% of natural freshwater produced is extracted from groundwater sources to cope with national demand (EWA, 2015).
Climate change impacts are expected in the decrease in the mean annual rainfall, with extreme high run-off generating rainfall events and resulting in the decrease in aquifer recharge (EWA, 2015; Sapiano, 2020).
Desalination of seawater has become one of the most important sources of freshwater in Malta, today accounting for more than 60% of the public drinking water supply (Sapiano, 2020). Groundwater abstracted from the southern region of the MSL aquifer system exhibits characteristically high chloride contents. This deterioration in quality has resulted from the saltwater intrusion in response to the historically high groundwater abstraction rates registered in the area, particularly from the dense and widely distributed private abstraction for agricultural purposes. This situation has resulted in the discontinuation of groundwater abstraction for public supply in this region since the early 1990s.
The Lower Coralline Limestone formation represents the most important aquifer formation of the Maltese islands and supports the MSL aquifer. The primary porosity of the formation is highly variable and suggests that a large part of the primary pore‐space is not interconnected, and the effective porosity of the formation is mainly connected with fracture permeability. The fractures range from micro-fissures to Karst cavities (Barbagli et al., 2021; Sapiano, 2020).
The Malta demonstration site is located in the south-eastern coastal area, Ta Barkat, 130 m from the coastline, near a wastewater treatment plant (WWTP) which provide a highly polished treated effluent for injection (Fig. 2). Geologically, the area shows karstic cavities, and which are influenced by the tidal effects and with freshwater-saltwater interface depth at around 25-30 m (Sapiano, 2015).
Injection water was transported through a 75 mm diameter HDPE pipe from the polishing plant to the recharge wells through a 300 m network connecting all the recharge wells and which was developed by the project partner, the Water Services Corporation (Fig. 3). Monitoring of the MAR scheme was conducted thru four boreholes located further inland. Experimental injection procedures were initiated in October 2016 and maintained over the subsequent six‐month period, up to the end of March 2017 using six recharge wells (Sapiano, 2017).
Electrical Conductivity and Temperature readings didn’t show relevant impacts from the experiment and a local increase in the hydraulic head was observed around the injection wells. A long-term flush out of the interface towards the coastline is expected with the continuity of the injection. Third-party abstraction activities in the area possibly made changes in water level due to the artificial recharge event more difficult to identify.
Technically, MAR implementation in Ta Barkat was intended as a test site to assess the practical impact of MAR in a coastal groundwater system and help guide the development of an upscaled MAR system further inland in a region where the aquifer system shows better characteristics, namely, where an increase in the hydraulic head would thus change hydraulic gradients and limit the outflow of groundwater from the central regions.
Alternative methods for managing water are historically common in Malta. Harvesting of rainwater is an ancient practice with the first structures being traced back to the Neolithic. At a national level runoff is collected and managed through small dams constructed along with the main valley systems, which structures also permit a certain level of MAR to the underlying sea-level aquifer systems (Sapiano, 2020).
Treated sewage effluent offered a potential alternative to freshwater in irrigation if correct salinity treatment is applied (Mangion et al., 2005) but also a source to other alternative uses such as MAR.
The MAR pilot was initiated during the 1st Catchment Management Cycle (EWA, 2015), financed by EU MARSOL Project (Framework Programme 7) and aimed to develop a regulatory framework for MAR authorization under EU Groundwater and Environmental Impact Directives.
Other experimental MAR schemes were tested thru numerical modelling parallel to the experimental Ta Barkat site. A large-scale 3D groundwater model of the southern sector of MSL Aquifer was developed to study two scenarios: business as usual, where no injection of water occurs; injection in a hypothetical well‐field with a uniform injection rate of 2 hm3/year for 10 years (Monteiro et al., 2016). Those volumes correspond to the surplus in polished wastewater (New Water). The results were promising as the groundwater level increased and a decrease in seawater-freshwater ratio was observed, although effects were geographically limited to the areas around the injection wells (Fig. 4).
Particle tracking modelling showed that the injected wastewater flows towards the sea, as opposed to the direction of the existing public supply wells, not affecting the quality of the water abstracted for supply.
Evidence of benefits from implementation
From a technical perspective, the expected benefits of a MAR scheme in such an aquifer system are of two types (Sapiano, 2017). Firstly, the MAR system will lead to an increase in water levels in the coastal zone resulting in a global lowering of the freshwater‐saltwater interface. A thicker freshwater floating lens will limit the advance of intrusion beneath the abstraction stations protecting or improving the quality of the extracted groundwater (Fig. 5).
Secondly, injection creates a mound that limits the outward flow of freshwater from the central regions of the aquifer system. Higher hydraulic heads around the MAR site would modify groundwater flow conditions by inducing an inward groundwater flow component blocking the naturally infiltrated groundwater (better quality) in inland regions of the aquifer system, preferentially discharging the recharge waters. This can also flush out contaminants from the coastal aquifer system, having also a beneficial ‘cleaning’ effect downstream of the MAR site.
Replication potential in SUDOE region
The use of MAR as a tool to curb saltwater intrusion has been extensively investigated in numerical modelling in case studies around the world with favourable results (Allow, 2012; Masciopinto, 2013; Lu et al., 2017; Armanuos et al., 2019; Armanuos et al., 2020; El Alfy et al., 2020).
Real-scale implementation of such schemes usually referred to as saltwater barriers is common in the USA, particularly in California, with some degree of success. In Los Angeles, injection wells have been used since early 1950 to hinder the advancement of saltwater intrusion (Herndon and Markus, 2014). In 1975 the Orange County Water District established a “water factory” in which treated wastewater is also used to counter saltwater intrusion (Kiparsky et al., 2021). In Europe, a seawater injection barrier recharge scheme using reclaimed water was tested in Llobregat delta aquifer (Barcelona, Spain) with a significant groundwater quality improvement (Ortuño et al., 2012).
The main obstacle associated with MAR is primarily one of an economic kind instead of the urge to immediately solve an environmental or scarcity problem. MAR schemes in coastal regions are expected to lose a significant fraction of the injected volume by aquifer discharge the sea. From a policy and planning perspective, the energy spent in generating polished water as a replacement to groundwater that ultimately will not be recovered may be seen as a poor investment and other methods can be applied, such as adopting water-saving policies, prior to the undertaking of any MAR scheme. Visible benefits will only be achieved after a long period of MAR implementation due to the common groundwater dynamics (i.e., very low velocities compared with surface water). Considering Malta’s hydrogeological conditions, current water demand patterns and the quality of water being produced by the Ta Barkat polishing plant, MAR is to be considered as a ‘last’ solution to maximize the resource benefits or a complementary solution to very specific management challenges (Sapiano, 2017).
MAR to prevent saltwater intrusion in Malta is a near-future reality. An EU LIFE Integrated Project is ongoing, aiming to drive the implementation of Malta’s 2nd River Basin Management Plan (RBMP), and the achievement of the environmental objectives of the Water Framework Directive (). The project revisits the challenges of water scarcity, high population density and saltwater intrusion vulnerability challenges and has, as one of the sixteen actions, developed a MAR pilot scheme in the Pwales groundwater body, in the northern part of Malta island. The MAR scheme will inject polished wastewater, initially used for irrigation, from a nearby WWTP during periods when the water demand is low during rainier months. The plant, owned and operated by the WSC, receives water from the Northern part of Malta and following biological treatment is submitted to ultrafiltration, reverse osmosis, and an advanced oxidation process to produce reclaimed water which is suitable for unrestricted irrigation and will not compromise both soils and aquifers.
This action will at first be based on numerical modelling, similarly to MARSOL, which will evaluate the feasibility of the method. Field investigations and modelling of this pilot MAR scheme will be conducted by an early-stage researcher as part of the Managed Aquifer Recharge Solutions Training Network (MARSoluT), a four-year Marie Skłodowska-Curie Actions (MSCA) Innovative Training Network (ITN) funded by the European Commission.
Key points of the innovative method
- The pilot scheme is expected to curb saltwater intrusion caused by overexploitation of groundwater and declining natural recharge.
- Makes use of high-quality treated wastewater that does not compromise the groundwater quality if injected.
- Monitoring showed residual impacts in terms of groundwater level improvements
- The methodology is most effective as a complimentary water resources tool, together with water-saving policies.
- This implementation paved the way for further MAR experiments that are expected to be conducted in another site in the Northern region of Malta using reclaimed water (polished treated wastewater).
- Similar saltwater intrusion “barriers” are known in Europe and the United States of America with success.
This innovative practice was suggested by João Simão Pires from the Portuguese Water Partnership (PWP).
Allow, K.A., 2012. The use of injection wells and a subsurface barrier in the prevention of seawater intrusion: a modelling approach. Arab J Geosci 5, 1151–1161. https://doi.org/10.1007/s12517-011-0304-9
Armanuos, A.M., Ibrahim, M.G., Mahmod, W.E., Takemura, J., Yoshimura, C., 2019. Analysing the Combined Effect of Barrier Wall and Freshwater Injection Countermeasures on Controlling Saltwater Intrusion in Unconfined Coastal Aquifer Systems. Water Resour Manage 33, 1265–1280. https://doi.org/10.1007/s11269-019-2184-9
Armanuos, A.M., Al-Ansari, N., Yaseen, Z.M., 2020. Assessing the Effectiveness of Using Recharge Wells for Controlling the Saltwater Intrusion in Unconfined Coastal Aquifers with Sloping Beds: Numerical Study. Sustainability 12, 2685. https://doi.org/10.3390/su12072685
Barbagli, A., Guastaldi, E., Conti, P., Giannuzzi, M., Borsi, I., Lotti, F., Basile, P., Favaro, L., Mallia, A., Xuereb, R., Schembri, M., Mamo, J.A., Sapiano, M., 2021. Geological and hydrogeological reconstruction of the main aquifers of the Maltese islands. Hydrogeol J 29, 2685–2703. https://doi.org/10.1007/s10040-021-02406-z
El Alfy, K., El-Ghandour, H., Abd-Elmaboud, M., 2020. Controlling of Saltwater Intrusion Using Injection Wells (Case Study: Quaternary Aquifer of Delta Wadi El-Arish, Sinai). Bulletin of the Faculty of Engineering. Mansoura University 40, 74–92. https://doi.org/10.21608/bfemu.2020.101079
EWA, 2015. The 2nd Water Catchment Management Plan for Malta Water Catchment District 2015-2021. Sustainable Energy and Water Conservation Unit of the Environment and Resources Authority.
Herndon, R., & Markus, M. (2014). Large-Scale Aquifer Replenishment and Seawater Intrusion Control Using Recycled Water in Southern California. Boletin Geologico y Minero, 125(2), 143-155. https://olemiss.edu/sciencenet/saltnet//swica1/liles-thomas-sovich-exabs.pdf; https://www.ocwd.com/media/1857/large-scale-aquifer-replenishment-and-seawater-intrusion-control-using-recycled-water-in-southern-california.pdf
Hofste, R., Kuzma, S., Walker, S., Sutanudjaja, E., Bierkens, M., Kuijper, M., Faneca Sanchez, M., Van Beek, R., Wada, Y., Galvis Rodríguez, S., Reig, P., 2019. Aqueduct 3.0: Updated Decision-Relevant Global Water Risk Indicators. WRIPUB. https://doi.org/10.46830/writn.18.00146
Kiparsky, M., Miller, K., Blomquist, W., Holtzapple, A., Milman, A., 2021. Groundwater Recharge to Address Seawater Intrusion and Supply in an Urban Coastal Aquifer. Case Studies in the Environment 5, 1223118. https://doi.org/10.1525/cse.2021.1223118
Lotti, F., Borsi, I., Guastaldi, E., Barbagli, A., Basile, P., Favaro, L., Mallia, A., Xuereb, R., Schembri, M., Mamo, J.A., Demichele, F., Sapiano, M., 2021. NECoM (Numerically Enhanced COnceptual Modelling) of two small Maltese Aquifers: Mizieb and Pwales. AS-ITJGW 10. https://doi.org/10.7343/as-2021-496
Lu, C., Shi, W., Xin, P., Wu, J., Werner, A.D., 2017. Replenishing an unconfined coastal aquifer to control seawater intrusion: Injection or infiltration? Water Resour. Res. 53, 4775–4786. https://doi.org/10.1002/2016WR019625
Mangion J., Micallef P., Attard G. Treated sewage effluent: an alternative water supply for the Maltese Islands. In : Hamdy A. (ed.), El Gamal F. (ed.), Lamaddalena N. (ed.), Bogliotti C. (ed.), Guelloubi R. (ed.). Non-conventional water use: WASAMED project. Bari : CIHEAM / EU DG Research, 2005. p. 305-310 (Options Méditerranéennes: Série B. Etudes et Recherches; n. 53)
Masciopinto, C., 2013. Management of aquifer recharge in Lebanon by removing seawater intrusion from coastal aquifers. Journal of Environmental Management 130, 306–312. https://doi.org/10.1016/j.jenvman.2013.08.021
Monteiro, J.P., Costa, L.R.D., Hugman, R., Sapiano, M., Schembri, M., 2016. MARSOL – Demonstrating Managed Aquifer Recharge as a Solution to Water Scarcity and Drought Project Deliverable 10.4 – Regional Groundwater Model of the Malta South Region. http://www.marsol.eu/files/marsol_d10-4_malta-groundwater-model.pdf
Ortuño, F., Molinero, J., Garrido, T., Custodio, E., 2012. Seawater injection barrier recharge with advanced reclaimed water at Llobregat delta aquifer (Spain). Water Science and Technology 66, 2083–2089. https://doi.org/10.2166/wst.2012.423
Sapiano, M, 2015. MARSOL – Demonstrating Managed Aquifer Recharge as a Solution to Water Scarcity and Drought Project Deliverable 10.1 – Characterisation of the sea‐level aquifer system in the Malta South Region. http://www.marsol.eu/files/marsol_d10-1_msla-characterisation.pdf
Sapiano, M, 2017. MARSOL – Demonstrating Managed Aquifer Recharge as a Solution to Water Scarcity and Drought Project Deliverable 10.6 – Combating sea‐water intrusion by managed aquifer recharge of treated effluent at the Malta South Demonstration Site. http://www.marsol.eu/files/marsol_d10-6_malta-mar.pdf
Sapiano, M., 2020. Integrated Water Resources Management in the Maltese Islands. AS-ITJGW 9. https://doi.org/10.7343/as-2020-477
Aqueduct Water Risk Atlas (https://www.wri.org/aqueduct)
Malta Resource Authority Climate Change Strategy (https://mra.mt/climate-change/adaptation-to-climate-change/)
MARSOL Demonstrating Managed Aquifer Recharge as a Solution to Water Scarcity and Drought (http://www.marsol.eu/)
MARSolut Managed Aquifer Recharge Solutions Training Network (https://www.marsolut-itn.eu/)
Project participating members:
Energy and Water Agency (https://www.energywateragency.gov.mt/)
Water Services Corporation (https://www.wsc.com.mt/)
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