The Island of Gotland (3000 km2
), situated in the Baltic Sea 100 km from the mainland of Sweden (Figure 1
a), suffers from insufficient water availability to supply the ever-increasing demand from society, especially during the tourist season (June–August, [1
]). The annual precipitation on the island (ca 550 mm/year) is sufficient to cover a forecasted increase in water demand. However, intensive drainage of arable land, thin soil layers, and relatively impermeable rock lead to precipitation run-off and limited reservoir capacity in both surface water and groundwater reservoirs [2
]. The already constrained water supply will be further aggravated in the future because the total water demand on the island is estimated to increase by 40% by 2045 [1
]. The current water resources on the island will not meet this projected increase in demand. A high availability of water during the winter and a high demand for water in the summer makes MAR a suitable way to increase the water resources. Due to these factors, it is important to investigate the potential for MAR on Gotland.
The bedrock on Gotland consists mainly of Silurian limestone and marlstone, which represents the upper part of a 250–800-m-thick sequence with Palaeozoic sedimentary rocks overlying the crystalline basement [3
]. The Quaternary overburden is generally thin (less than 2 m) and is largely composed of till and postglacial sand deposits. The relief is low, and the highest point is 82 m a.s.l. The main land uses are agricultural and forestry. The main aquifers on Gotland are situated within the bedrock where cracks, fractures and dissolution cavities store and transport the groundwater. Nevertheless, the soil layers have an important role to play because areas with soil, especially sand and gravel, act as infiltration and storage systems for the bedrock aquifer. The island may be considered one large aquifer, but with groundwater divides (created by relief) producing 7 (sub)aquifers, according to the European water framework directive. Saline groundwater is a problem because relict saltwater occurs under the entire island at a depth of 20–100 m b.s.l.
The total water demand on the island is estimated to increase by more than 40% by 2045, with increases of 30% in tourism, 20% in domestic demand, 20% in animal keeping, 15% in industry, and 100% in irrigation [1
]. To enhance water resource security, and close the water supply and demand gap, several alternative water management measures are being examined. Managed Aquifer Recharge (MAR) is one of them. Today, the public water supply on Gotland relies on 14 well fields, two surface water catchments and a desalinisation plant. MAR is currently not used in any public water supply on Gotland, but may play an important role in the future if suitable areas can be found. Conversely, on the Swedish mainland, MAR has been in use for over 100 years, and accounts for approximately 20% of the public water supply [1
]. MAR can be explained as the intended recharge into and storage of water in an aquifer [5
]. It may be used to increase water security for uses including drinking water supply, irrigation, preventing saltwater intrusions, as well as providing environmental benefits [6
]. MAR is widely distributed and applied on various scales around the globe, as well as in Europe [7
]. The water source can be of varied origin, e.g., river water, seawater or sewer water. In some cases, there is a need for pre-treatment before groundwater recharge to minimize the risk of pollution or aquifer clogging [8
]. The recharge can be made by spreading methods in areas with high infiltration capacity; by deep infiltration direct into the aquifers via wells; or as induced infiltration due to withdrawal [9
]. Since the different MAR types are suitable for different conditions within hydrogeological settings (e.g., confined or unconfined aquifers), treatment opportunities, and land use, the selection of suitable recharge sites is crucial [10
]. In this study, we focus on areas suitable for recharge through infiltration basins and natural conditions for storage. Conditions for well or induced infiltration are expected to be of minor importance because of the geomorphology, geology and hydrology of the island. Hence, the suitability of those MAR types is not investigated in this study.
To prioritize between alternative measures to improve water resources security (e.g., increased groundwater abstraction and desalination), useful decision support is needed. GIS-MCDA (Geographical Information System Multi Criteria Decision Analysis) [12
] is a regularly applied method in MAR suitability assessment [13
]. There are several possible criteria for mapping MAR suitability, the three most common being aquifer storage capacity, geomorphology and soil [13
]. There are also concerns on limitations and discussion on the uncertainties of these GIS approach made visible by e.g., [14
]. Although a GIS analysis will show where MAR might be successful, field work and numerical modelling will be important tools for increasing the success of MAR [15
]. The economic assessment of MAR is an important question that has been studied previously [20
]. The aim of this paper is to explore the feasibility of MAR on the island of Gotland by: identifying potential areas for MAR in proximity to a fresh water source and which are available for recharge; estimate the possible increase in groundwater recharge and groundwater extraction at existing wellfields; and compare MAR to other alternative measures in terms of costs and water availability potential.
The presented data and analysis represent an early stage of mapping of MAR areas (focused on spreading methods in areas with high infiltration capacity) and estimates of potential and feasibility of this type of MAR on Gotland. Our project also includes mapping of good local conditions for a local source, e.g., water supply, which we believe further increases the utility of these study results for water management on the island. Mapping of suitable MAR areas with GIS is a widely used method [13
]. There are uncertainties in both data and accuracy in the analysis, but within these limitations there is now a detailed picture on possible MAR and source areas which can be used by the municipality, farmers, and other stakeholders. Concerns regarding the limitations of these GIS analyses have been raised by, e.g., [14
], who suggested that the use of sensitivity analyses of the factors used for MAR feasibility studies. In 2016, SGU made a first attempt to apply an overall assessment of the possibilities of MAR at existing abstraction areas [2
]. Results from the present study can now improve the prior prioritization between MAR and alternative water measures for the island. The results should also be addressed on more than the water quantity. Increased groundwater recharge by MAR may influence the quality of the groundwater, e.g., by dilution effects, but also change the salinity levels in the bedrock aquifers. The municipality can analyze the results and compare with areas that are not used today but where the presented method shows good potential for MAR. There will be need for validation with numerical modelling and field tests to increase the strength and substance of the results, as shown in, e.g., [15
This paper does not use any restrictions on unsuitable areas due to land use, a criterion that has been used in several analyses [13
]. This can preferably be made by water management authorities, who are more suited to deciding between conflicting interests. Mainly because of the low relief of Gotland, the often-used parameter “slope” [13
] is also not used here. A potentially much more important criterion is “closed depressions”. This criterion uses the bedrock surface and the slope to calculate where there are bedrock depressions where water has more time to infiltrate and be stored as groundwater.
Even though there is a clear picture of the best areas from the perspective of infiltration/groundwater storage and source/dam, other areas can also take advantage of the presented data. Local conditions—for example, where the distance to the public water network is long—can make MAR solutions profitable in those more remote areas. The outcome of favorable areas (from 2000 ha (20 km2) up to at most 43,000 ha (430 km2)) in the analysis of infiltration areas, source areas and the combination of these should be compared to the area of the island (3000 km2). The designated areas constitute only a small part of the island, and therefore care must be taken so that these are not destroyed by over-exploitation.
The degree of detail in the results is determined by the available data sets. SGU is working on an update on a few of the data sets, which will improve the certainty of the result. There are also several ongoing and future investigations associated with some of the data sets. For example, a few of the designated areas with a closed depression may be particularly suitable areas for groundwater dams. This is also the case with depressions that are not completely closed, not used in our investigation, and hence an interesting subject for future analysis for the viability of this MAR technique on Gotland. A groundwater dam is a man-made structure that obstructs the natural flow of groundwater and thereby can store larger quantities of water in the aquifer [31
]. The results from this study are not validated by field studies. The data sets are; however, delivered to the local authorities for water resources and water management, and for analyses. Once the data sets are updated, the results may be integrated into the hydrogeological 3D model and calibration of parameters from field studies may improve future work with MAR on Gotland. The presented data sets may be tested in the newly developed online tools for suitability mapping, e.g., https://dss.inowas.com/tools
]. The resulting suitability maps will be shared at the international MAR portal (https://apps.geodan.nl/igrac/ggis-viewer/viewer/globalmar/public/default
The economic aspect of implementing MAR systems to improve potable and agricultural water supply has previously been investigated in different parts of the world (e.g., [21
]). The associated capital costs are highly system specific, influenced by, e.g., hydrogeological, socioeconomic and legal factors [35
]. As the economic analysis of MAR in this paper was based on rather small-scale complimentary infiltration of surface water at existing municipal well fields, no additional costs for new wells, treatment plants or pre-treatment were considered. The economic analysis was based on cost estimates associated with infiltration basins, raw water intake and new piping, resulting in a unit cost of approximately 5 SEK/m3
. This can be compared to cost estimates for MAR in Spain ranging between €0.08–0.58 per m3
] (approximately 0.8–6 SEK/m3
The total water demand on Gotland is forecasted to increase by more than 40% by the year 2045 [1
]. This will require water currently not available on the island. To make well-founded decisions on how to meet this forecasted demand and concurrently increase the preparedness for water scarcity situations, thorough decision support is needed. The presented data and analyses can be used to inform decision-making on measures to increase the amount of water that can be recharged on the island. Considering the entire island as a single groundwater aquifer would permit a holistic approach to groundwater management, and the water situation more robust. This is particularly important for the forecasted increase in public water demand but also for individuals, industry and farmers relying on private wells. By comparing MAR to alternative measures, in terms of costs and water availability potential, this paper also provides support in assessments of the measures’ economic viability, usually an important decision criterion for municipalities, corporations and individuals alike.
This paper contributes results from analyses of possible MAR areas and their potential to increase water availability on the island of Gotland, Sweden. The method can be used to evaluate the MAR potential in similar areas with the same data sets. The results can be used on different scales, by authorities, the public water producer, farmers, industry and by people with private wells, for improved water resource security and further validated with field tests and more detailed models such as the afore mentioned hydrogeological model. Comprehensive field tests are probably the best for better understanding the problems in general. However, they are time consuming, and only a limited number of sites are available to accommodate the tests. In contrast, the GIS analysis allows us to explore and assess multiple sites with relative ease, yet the validity needs to be carefully checked. The main results are listed below.
The best conditions for infiltration and groundwater storage occur in a total area of ca 2000 ha (0.7% of Gotland), second best in 14,400 ha (4.8% of Gotland), and third best in 43,000 ha (14% of Gotland).
Areas with both proximity to a raw water source and conditions for storage in dams occur in a total area of ca 10,000 ha (3.3% of Gotland).
An area of ca 7700 ha (2.5% of Gotland) has good local conditions for MAR and an area of ca 22,700 ha (7.5% of Gotland) has moderate local conditions for MAR.
Decision support is provided by comparing MAR with other measures in a marginal abatement cost curve, contributing to informed prioritizations and decisions on water resource improvement on Gotland.
MAR is not the alternative with the largest water availability potential, but it has significantly lower marginal costs compared, for example, with desalination, and the potential will increase if also considering new well fields and in preventing adverse consequences of increased abstraction.
The water supply potential of MAR in existing well fields (public water supply) was estimated to be about 35% of the forecasted drinking water supply and 7% of the total water demand gap in year 2045. The total water supply potential of MAR on Gotland is much larger and is expected to exceed the demand.