3.1. Mine Waste
shows the evolution of the main variables in research into mine waste (MW) on a global scale from 1988 to 2017. The number of articles published on this subject (A) increased notably from 14 in 1988 to 279 in 2017. This trend indicates that research into MW has increased in importance, culminating in the maximum number of articles published in 2017. The comparison between the total growth of articles on MW and the total growth of articles within all of the disciplines would be highly interesting, but unfortunately, it was not possible to achieve these data under the current research framework. The remaining variables in Table 1
show a similar growing trend. The number of authors in this field (AU) grew from 28 in 1988 to 1221 in 2017. The number of references (NR) increased exponentially from 86 to 11,323. The number of journals (J) also increased during this period, from 13 in 1988 to 153 in 2017. The internationalization of the field is reflected in the number of countries (C), which increased from four in 1988 to 51 in 2017. The total number of cited articles (TC) on MW was three in 1989, increasing to 7413 citations in 2017. The number of citations per article increased from 0.10 in 1989 to 16.89 in 2017.
shows the evolution of the principle subjects under which Scopus classifies articles on MW. Note that one article may be simultaneously included in more than one category. During the period studied, 58.6% of published articles were classified under the Environmental Sciences category, 47.2% were in Earth and Planetary Sciences, 16.4% were in Agricultural and Biological Sciences, 8% were in Engineering, and 7% were in Chemistry. These were followed by the categories of Materials Science, Medicine, Social Sciences, Pharmacology, Toxicology, Pharmaceutics, and Energy, but none accounted for 4% of the total of articles. Since the beginning of the analyzed period, Environmental Science and Earth and Planetary Sciences have been the principle categories. However, since 2006, Environmental Science has become the leading discipline in this area, which indicates that MW research is being principally studied from an environmental perspective.
shows the 10 journals with the most publications on MW. This group is entirely made up of European journals, specifically British, Dutch, and German journals, with the exception of one Iranian publication. These journals publish 28% of all of the articles in the field, indicating no great concentration of publication in this area. In the first column, we can see the total number of articles published by each journal along the whole period. Moreover, the evolution of the article number per journal is shown during the three 10-year periods, into which the studied time was divided up. Applied Geochemistry was the most productive journal on this subject from 1988 to 2017, with 155 articles, followed by Science of the Total Environment with 130 articles, the Journal of Geochemical Exploration with 109 articles, and Environmental Earth Sciences with 88 articles. Environmental Earth Sciences was established in 2009 under that name; however, it was previously published under the name of Environmental Geology. This journal occupied the first position in terms of the number of articles published from 1997 to 2009, the year in which it changed its name. From this date, Applied Geochemistry took the first position. During the sub-period of 2008–2017, Environmental Earth Sciences established itself once again in first position. Both appear separately in the fourth and fifth position of the most productive journals, but if the publications were totaled, this journal would take first position with 172 articles and a total of 2875 citations.
Journals with a greater SJR index were: Environmental Pollution with 1.786, the Journal of Hazardous Materials with 1.727, and Science of the Total Environment with 1.621. Applied Geochemistry was the most cited journal, followed by Science of the Total Environment, Environmental Science and Technology, and Chemosphere. However, considering the average number of citations per article, Environmental Science and Technology was the journal with the greatest impact, with a total of 48.8 citations per article. Chemosphere took second position with 44.2 citations per article, and Environmental Pollution was in third place with 37.1 citations per article. This journal had the greatest record within the top 10, since it first published an article on this subject in 1989. Notably, the journals in the top 10 are of the highest quality; they all appear in the first two quartiles of the Scopus classification.
shows the 10 most productive countries in the publication of articles on MW. The United States led the group, followed by Canada, Spain, Australia, and China. The number of articles published per million inhabitants (APC) is also shown in this table. This variable is led by Canada with 13.75 articles per capita, followed by Australia with 12.47, Portugal with 10.07, and Spain with 7.29. The United States placed first in the total number of citations, followed by Canada, Spain, and the United Kingdom. However, considering the average number of citations per article, the United Kingdom placed first with 29 citations per article, followed by the United States with 22.3, Spain with 21.4, Portugal with 19.2, and Canada with 18.1. Figure 3
shows the elevated correlation existing between the H index and number of articles published by each country.
indicates the different variables related to the international collaboration between the group of the 10 most productive countries on the subject of MW. The United Kingdom had the largest percentage of articles produced in collaboration with other countries, with 46.9% of the total. The United States, Australia, Spain, Canada, and Germany were its main collaborators. These were followed by Germany with 42.8%, Portugal with 40.4%, and Australia with 38.9% of the total. The United Kingdom was also the country with the greatest number of collaborators, with 46 associates, followed by the United States with 45, and Australia with 38. The United States stands out as the principle collaborator among the remaining top 10 countries, being the foremost collaborator of these five countries: Canada, China, the United Kingdom (UK), Germany, and India. This table also shows the average number of citations (TC/A) per article produced in international collaboration (IC), and those produced without collaboration (NIC), for each country. The number of average citations per article, in every country, was greater with international collaboration, except in the case of the United States, Spain, and Portugal.
The principle characteristics of the institutions with the largest number of publications on MW are displayed in Table 5
. Half of these were found in Canada, with the remainder in Spain, China, Australia, and the United States. Canada’s University of British Columbia was the institution with the greatest number of articles published, followed by the Chinese Academy of Sciences, the University of Queensland, the United States Geological Survey, and Western University in Canada. The University of Waterloo (Canada) had the largest number of cited publications, followed by the University of British Columbia, the Chinese Academy of Sciences, and the United States Geological Survey. The University of Waterloo also took first position in terms of the average number of citations per article with 37.7, followed by Spain’s National Research Council with 27.2, the United States Geological Survey with 24.5, and Western University with 19.3. Spanish institutions were those with the largest percentage of research completed with international collaboration.
shows the authors with the largest number of MW articles. The four most prolific authors were affiliated with Canadian institutions. David Blowes of the University of Waterloo was the most seasoned of the ranking with a paper from 1994. He was the most cited, with a total of 1394 citations and the highest H index (20). Ernest K. Yanful of Western University had 481 citations and an H index of 14. Following this were Mostafa Benzaazoua and Bruno Bussière of the Université du Quebec. The most recent author to join the ranks was R. Hakkou of the University Cadi Ayyad Marrakech of Morocco, with the first paper published in 2008. Even so, Hakkou managed to place ninth. Karen A. Hudson-Edwards of the University of Exeter was the author with the largest average number of citations per article with a total of 48.7. Figure 4
shows a network map illustrating the collaborative relationships of co-authorship between the different authors of MW articles. The size of the circle indicates the number of articles, whereas the thickness of the line indicates the number of collaborations between authors. The formation of different clusters can be observed through the colored representation. The group made up by Blowes, Smith, Ptacek, and Jambor stands out. Yanful leads a cluster that includes Simms, Hendry, Morris, and Song, among others. In the Benzaazoua group, we also find Bussière and Hakkou, whereas Craw, Lottermoser, and Schippers create another cluster. Next to Öhlander we can find Nason, Mäkitalo, Alakangas, and Maurice. Conesa shares the group with Jiménez-Cárceles, Robinson, Schulin, Álvarez-Rogel, and Elbaz-Poulichet. Hudson-Edwards builds a group together with Macklin, Bird, and Kossoff, among others.
We analyzed keywords to identify trends in MW research, which was necessary in order to previously remove duplicities. This pre-treatment of keywords has been undertaken with the SciMAT software. Words such as “article” and “priority journal” were excluded from this process, as they were irrelevant for our purposes. Table 7
shows the 20 most frequently used keywords in articles during the period of 1988 to 2017. This table also shows the evolution of these words through the three different 10-year sub-periods, into which the complete period may be divided. The values refer to the number of articles in which each keyword appears (A), the position the word occupies in relation to the others in terms of the number of repetitions (R), and the percentage of appearances with respect to the total number of articles analyzed in the period (%). Among the most often-used keywords were mining products (zinc, lead, copper, metals, heavy metal, iron, and arsenic), different terms relating to the processes and elements of mining (tailings, acid mine drainage, concentration, industrial waste, oxidation, and environmental monitoring), and soil contamination (soils, soil pollutants, pH, and soil pollution).
As expected, the term most used during the entire study period was mining. The rest of the keywords varied their positions in accordance with the research preferences of each period. Although the words in the table were the most used, their importance oscillated over time. From 1988 to 1997, the most common keywords were mining, contamination, heavy metal, copper, soil, lead, water pollution, soil pollution, zinc, and environmental impact. During this time, the materials that were most studied were heavy metals, copper, lead, zinc, cadmium, and uranium. Attention was focused on both soil and water contamination (contamination, environmental impact, industrial wastes, waste disposal, sediment). The most frequently named countries in keywords were Canada, the United States, and Australia.
The most relevant keywords during the 1998–2007 sub-period, apart from mining, were: “tailings”, “heavy metal”, “acid mine drainage”, “copper”, “zinc”, “arsenic”, “lead”, “soil pollution”, and “soils”. The principle elements that were analyzed were copper, zinc, lead, and arsenic, with the latter attracting more attention in this period compared with the previous. Acid drainage received particular attention, moving from position 19 in the previous period to fourth. Notably, the amount of attention paid to tailings in this period rose from 30th position to second place. However, the use of monitoring to study the environment experienced the greatest boost in this period, entering the list of 20 principle themes, from position number 72 during the 1988–1997 period. Conversely, studies on water contamination were no longer among the most numerous. In terms of geographic location, the regions with most studies on MW were Eurasia and Europe, and the countries were the United States, Spain, and Canada. The term “world” appears for the first time, indicating the gaining global significance of the research in this field.
The largest number of articles was published from 2008 to 2017; therefore, the greatest number of keyword repetitions were concentrated in this period. This conditioned the current framework of keywords. The principle keywords during this time were “mining”, “tailings”, “heavy metal”, “lead”, “soil pollution”, “soils”, “zinc”, “concentration”, “copper”, and “metals”. The two things of note during this period were the consolidation of a preference for studies of the ground rather than water, and the emergence of the term “concentration” in MW articles. From no presence at all in the previous periods, “concentration” became the eighth most common keyword. Studies of abandoned mines began to appear more frequently. Geographical reference takes 52nd place among keywords, with the United States closely followed by Spain and China.
shows a network map of the co-occurrence of the main keywords. The size of the circle represents the number of repetitions, and the color shows the different clusters in which the words are grouped according to the number of ties between the different words. Three main groups were found. The first (green) is titled “Contamination and public health”. In this cluster, elements such as potassium, arsenic, aluminum, antimony, cobalt, copper, zinc, and lead are analyzed. The cluster includes terminology related to health, both human and animal, such as “health risk”, “health hazard”, “public health”, “drinking water”, “animals”, “fish”, “human”, “pollution exposure”, etc. The principle methodology terms gathered here are: “multivariate analysis”, “principal component analysis”, and “risk assessment”. The main countries in this line were the United States, the United Kingdom, Spain, and Portugal. The second group (red) is called “Waste management”, and includes the terms: “metal recovery”, “heavy metal removal”, “neutralization”, “waste management”, and “waste treatment”. This group presents a perspective from the fields of Hydrology and Hydrogeology. The most significant methodology terms were “analytic method”, “analytical geochemistry”, “chemical analysis”, “computer simulation”, and “experimental study”. Brazil, Canada, Germany, South Africa, and Sweden appear in this group. The last cluster (blue), called “Ecological restoration”, includes China, Australia, and India as the foremost countries with an environmental orientation. Terminology relating to the ground appears in this cluster, including: soil composition, microbiological activity, and revegetation (ecology, plant restoration, revegetation, ecosystem restoration, soils, soil microbiology, soil conservation, soil analysis, soil remediation, microbiology, microbial activity, etc.). The outstanding methodology terms are: “microbial analysis”, “controlled study”, and “comparative study”.
3.2. Management of Mining Waste
In this section, we present the main analysis results of the evolution of worldwide research into the management of mining waste (MMW) during the period of 1988 to 2017. Table 8
shows the evolution of the principle indicators of research in this field. The number of articles on MMW (A) increased from one in 1988 to 100 in 2017. To contextualize the increase in articles in this line of research, Figure 6
displays the growing trend in the number of research articles on mining, mine waste (MW), and the management of mine waste (MMW). To facilitate the comparison and homogenize variables, logarithms were applied to them, and the annual average accumulated growth rate was calculated. As a result, although articles on mining increased by 7.54% on average per year, those on MW increased by 10.87%, and those on MMW increased by 17.52%. Figure 6
shows a great variability regarding the growth trends of the published articles on these three fields of research until the last decade of the 20th century. As far as the growth trend of mining articles (in green) is concerned, it did not start to be positive until 1998. Therefore, in the field of mining research, MW and particularly MMW have become increasingly important.
To analyze the contribution of research into MMW to MW, Table 8
shows a variable indicating the percentage of MW articles corresponding to research on MMW (AMW). Research into MMW gained importance within the field of MW in terms of the number of articles. In 1988, articles on management represented only 7.1% of the total; in 2017, they represented 38.7%.
shows the main variables of the most productive journals on MMW. If we compare this group of journals to Table 2
, we find a group of journals that published both on MW and MMW, their position in the ranking of most productive journals changed. In both cases, Applied Geochemistry was the most productive journal. The articles on MMW make up 29% of the total number of articles on MW. The Journal of Hazardous Materials and Chemosphere occupied the first and fourth positions, respectively, and are the only publications that improved their positions with respect to research on MW. For the former, articles on MMW accounted for 66.2% of the total number of articles on MW, whereas the latter accounted for 55.2% of the total articles. The Journal of Environmental Management, Environmental Science and Pollution Research, and Ecological Engineering were placed seventh, eighth, and 10th
, respectively. These three publications did not appear in the most productive group on MW. Similarly, Environmental Geology, Environmental Science and Technology, and Environmental Pollution were not among the most productive journals on MMW research.
The group of the 10 most productive journals on MMW included 29% of the total, indicating a wide distribution of publications on this theme. All of the publications included were among the first or second quartile in SJR ranking. The journal with the most citations was Science of the Total Environment with 1887; it also had the greatest average number of citations per article, with 43.9. The Journal of Hazardous Materials was the publication with the greatest SJR index (1.727).
shows the list of the 10 most productive countries publishing articles on MMW. Once again, the United States was the country with the most articles, followed by Canada, Spain, China, and Australia. This means that the five most productive countries on MMW coincided with those on MW. In terms of the number of citations, the United States was the most important, followed by Canada, Spain, and the United Kingdom. As with research on MW, taking into account the average number of citations per article, the United Kingdom was placed first, with 27.5 citations per article, followed by Portugal with 22, Sweden with 21.5, and the United States with 21. The table also includes the percentage of articles on MMW compared with the number of articles on MW (AMW) of each of the nine most productive countries in both research fields. The country with the largest percentage of articles on management of the total works on MW was India with 40%, followed by Portugal with 36.5%, the United States with 33.4%, and Spain with 32.4%.
shows the main characteristics of the institutions with the greatest number of articles on MMW. Sweden’s Lulea Tekniska Universitet had the most publications. This institution does not stand out for its production of articles on MW; however, it is a reference for MMW. Spain’s Universidad Politécnica de Cartagena had the same number of publications, and was included among the group of most important producers of MW articles. However, its first-place ranking in MMW research means this is one of its most important areas of research. Other institutions that gained ground with respect to Table 5
are the United States Geological Survey, which was ranked third, and Spain’s National Research Council in fifth place. Other institutions that did not appear among the most productive in MW, but did for MMW, were the Universidade de Lisboa (Portugal), the United States Environmental Protection Agency, the Universidad Politécnica de Cataluña (Spain), and the Universidade de Aveiro (Portugal). The United States Geological Survey is the institution with the greatest number of citations, followed by the National Research Council of Spain, the United States Environmental Protection Agency, and the Universidad Politécnica de Cartagena (Spain). In the ranking of average citations per article, these institutions remained in the same order. Those with the largest percentage of articles produced in collaboration were: the Universidade de Lisboa and the Université du Quebec in Abitibi-Temiscamingue with 50% of the total. These were followed by Spain’s National Research Council with 43.8%, and Sweden’s Lulea Tekniska Universitet with 38.9%.