Using Managed Aquifer Recharge to mitigate saltwater intrusion in a Southern Malta Coastal aquifer

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).

Photo of Som Thapa

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.

Responsible entity

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.

Institutional setting

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).

Geographical setting

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.

Detailed explanation

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.