Sources of information
For the compilation of the MAR inventory, a comprehensive survey on MAR case studies was conducted over a period of about 12 months. The data were extracted from freely available publications, including journal articles (22%), reports (31%), conference papers (10%), presentations (5%), books and book sections (9%), academic theses (5%), and partially websites (10%), newspaper articles (1%), and other documents (7%). The first attempts of online search using specific evident keywords gave very discouraging results, indicating that most case studies are not readily available via common search engines. In addition, the search in specialized databases such as Web of Science, Academic Search Complete, ScienceDirect and similar was not giving satisfactory results due to very different terminology used in articles’ metadata. The most efficient search method was proven to be the manual screening of references listed in already available papers. This led to a systematical search with results either geographically localized or grouped by similar technologies and objectives. Once a certain case study was geographically localized, further sources were identified in available databases using the location’s name as keyword. This led to a cross verification and validation of the original paper and often enabled the harvesting of additional information about the site.
Given the international scale of the survey, it was obvious that a search in local languages might reveal supplementary information. Using the same methodology as described above, the screening was thus continued, beside English, in German, Spanish, Portuguese, French, and Chinese by MAR experts fluent in these languages. In addition, the search for European case studies was conducted by DEMEAU project team also in Polish and Dutch.
All the papers collected were stored in Zotero (2016), a reference management software that allows to easily collect, manage, and organize bibliographic information (metadata).
Criteria for site selection
A set of criteria was formulated for the selection of suitable references for the review. The first condition was that the MAR scheme is either in the advanced planning phase with clear perspectives for implementation or at different construction or operational stages. The category included projects that were shut down by the time of publication but operated in the past, as well as pilot projects followed or not by full-scale implementation. Not included were simulation studies conducted for the estimation of feasibility of MAR schemes and optimisation of existing applications. If at one site, more infiltration tests were conducted and at least one parameter was different (MAR type, objectives, influent type or end use of effluent), then multiple entries were assigned to the site following the number of distinct infiltrations. A second criterion was the description of either main or specific MAR types in the paper analyzed, with papers only mentioning aquifer recharge but without description of the method being excluded. Further criteria included public availability of the reference, either as direct web reference or through institutional subscription at local university and state library.
A special situation was created by the identification of very large amounts of case studies concentrated in small regions such as the “Catalog of recharge basins on Long island, New York, in 1969” published by the U.S. Geological Survey in its 70th Bulletin in 1973 (Seaburn and Aronson 1973). The catalog contains descriptions of 2162 recharge basins in Long island, New York, constructed between 1923 and 1969. Another example is the personal communication received from the Brazilian Institute of Geography and Statistics (Instituto Brasileiro de Geografia e Estatística) with technical data and locations of 3914 subsurface dams constructed between 2008 and 2015 in nine federal states of north eastern Brazil. These kinds of bulk data sets were not included in the present edition of the inventory, but they might be included in future updates after a more consistent review.
Data collected was structured in four main categories, following the general structure of the DEMEAU catalog with minor modifications and extensions (Table 1): (1) general information (18 parameters); (2) operational parameters (18); (3) hydrogeological properties (9); and (4) water quality parameters (water quality monitoring with 6 fields and hydrochemical data with 106 fields). Mostly, general information about the site was available in the collected references with some relevant aspects remaining unclear. For example, the exact number of MAR schemes in operation is currently unknown, same for the operational capacity of the schemes, existing monitoring programs or pre- and post-treatment measures. Nevertheless, more parameters are planned to be added in the future but these must be extracted from other sources (e.g., national hydrogeological maps, climate databases, etc.).
Due to data inconsistency (values within brackets indicate the number of sites where the parameter was reported, as percent from the total number of sites), the web-based portal contains at this stage mostly general information about the MAR sites (parameters marked with “*” in Table 1), including the original reference. In future updates, additional parameters are planned to be added, including financial and economic data from selected MAR schemes.