5.1. Distribution and Origin of Metals
The occurrence of interlayer levels in mining ponds enriched in trace elements is usually related to two causes: either there are areas of the mines with a local higher content of exploitable minerals or improvements in metallurgical processes that increase the utilisation of the ore minerals. In the case of Pond N of the San Carlos mine, the higher metal content shown by two different depth levels must be related to the different origin of the reworked materials, with different dump sites providing material with different initial concentrations. Another example of the presence of more enriched levels in mine ponds is found in the Brunita slurry pond in the Cartagena-La Unión Mining District, where several levels with different metal concentrations were defined by Martín-Crespo et al. [
27] and explained as resulting from the joint treatment of two different mines from the same district. In the case of Pond S at San Carlos, the thinner, more enriched level (sample HB2-32) at 3.5 m depth (
Figure 6) seems to be related to a higher sulfosalt content of that particular sample, due to the presence of some larger crystals in the sample.
With respect to the comparison between the two ponds, a slight generalised increase in the concentrations of almost all elements (except Fe
2O
3 total and S) is observed in the Pond S with respect to the Pond N. As mentioned above, these generalised increases in concentrations are usually related to a different origin of the minerals that were processed for the extraction of the ore. In fact, Pond S originates from the dumping of tailings from the last operating phase of the mine, between 1980 and 1996—that is, until the date of the mine’s definitive closure. In the latter period, old dumps from previous stages of mine operation were processed, which probably still contained significant quantities of Ag minerals that could not be effectively managed [
20]. To clarify this point,
Figure 7 shows the cluster analysis of compositional variables from both ponds.
Figure 7A shows the cluster analyses of the variables (analysed elements) of Pond N. The paragenesis of the mineralisation is clearly reflected, with a significantly low distance between Ag, Sb, Pb, Cu, As and Zn, i.e., the main mineral elements of the mineralisation: argentiferous sulfosalts such as pyrargyrite (Ag
3SbS), freieslebenite (AgPbSbS
3), stephanite (AgSbS
4) and/or proustite (Ag
3AsS
3), galena (PbS), and stibnite (Sb
2S
3). Fe
2O
3 total is the furthest apart, probably because it is part of unmined gangue minerals such as siderite. S and Ba also have a low distance defined by the barite of the paragenesis.
Figure 7B shows the cluster analysis results for elements in Pond S. In general, the behaviour resembles that of Pond N, although with slight differences. The fact that waste from the treatment of old dumps from at least two different mines, Santa Teresa and La Fuerza [
20], was stored in pond S may have caused these differences. The greater distance of Ba in this pond could be explained by the fact that barite from the dumps was not processed.
Figure 7C shows the two clusters of samples with close distances between them, the first cluster with samples mainly from Pond S (samples 21 to 38) and the second cluster with samples mainly from Pond N (samples 1 to 20). There is a greater distance between the two groupings, which indicates a different origin of the waste from one pond to the other. These results are very similar to those obtained by the authors in all mining districts analysed [
33], reflecting the metallic signature of each mining district. The small differences always depend on the specific origin of the minerals to be treated and the metallurgical technology of the time in which they were accumulated: the mine, old dumps, etc. [
20].
5.2. Mine Tailings Volumes from Aerial Imagery and LiDAR Data
The use of remote sensing techniques to measure surface elevation in time series has become widespread in recent years with the aim of estimating geomorphological changes and quantifying volumes of sediments and soils mobilized by wind, runoff, landslides or oceanic currents [
9,
34,
35,
36,
37]. These studies are benefited from the development of increasingly precise equipment and sensors whose usefulness is continuously tested and verified [
38,
39]. Terrestrial laser scanning and ground photography are applicable to small areas, the latter being very useful at small scales and rugged terrains such as gullies, although their use may disturb the ground surface that is surveyed. The non-destructive alternative, and more suitable for larger areas, are sensors mounted on unmanned aerial vehicle (UAVs). Many studies combine the use of several techniques to integrate the advantages of each of them [
35,
40].
Those methods, however, cannot be applied directly to the case of anthropogenic landscape changes related to abandoned metal mining. Abandoned wastes have gained interest due to its environmental impact or the recovery of metals that were not extracted during mining stages [
41,
42]. Therefore, it is essential to quantify the volume of potentially toxic elements that pose a risk to ecosystem health or have an economic interest in the current strategies of circular economy. However, in these areas, high-resolution topography for the original landscape on which mining wastes were deposited may not be available. Therefore, that surface is obtained by means of electrical tomography [
18,
33] or archival topographic maps and historical aerial photographs [
7,
17,
43,
44]. A geophysical procedure is only valid if there is a good contrast in the physical properties of wastes and basement and the fieldwork have a high economic and time cost. The digitisation of historical contour lines done in the Hiendelaencina area has several obvious advantages instead. It presents a greater precision in the calculation of the volumes by using aerial photographs for the topographical control of the base of the ponds. It is common that tailings ponds from mining operations at the beginning of the 20th century (Cartagena-La Unión and Mazarrón Districts, Spain) were established in stream valleys. In these cases, where the original topography of the pond base was not horizontal, having a topographical control is essential. Moreover, the results obtained in this way are highly accurate, and if sufficient mapping is available, the size and location of tailings deposits in mines abandoned many years ago can be tracked historically.
On the other hand, terrestrial laser scanning, ground photography or UAVs have a high resolution to capture topography, but they also have a high economic cost in equipment, data acquisition and data processing [
38,
39]. These disadvantages can be overcome with aerial imagery and LiDAR data that are publicly available, as in this work. Satellite LiDAR data are commonly used in large and inaccessible areas, and although it has lower accuracy than terrestrial or UAV sensors, it provides good preliminary estimations at the local scale [
7,
17,
43,
44]. Another issue of UAVs is that they require specific weather conditions and cannot fly over restricted zones. In the study presented here, the existence of an aerodrome 1 km NE of Hiendelaencina mine ponds prevents its use.
5.3. Environmental Problems and Legislation
One of the major problems of this type of waste lies in its medium-fine granulometry and the low cohesion of its particles, which causes significant erosion rates that favour the dispersion of its elements in the environment. There are several causes, both natural (wind and water erosion) and anthropic (movement of people, livestock, recreational activities and sporting activities) by which a mining slurry pond can disperse its constituents into the surrounding environment, being taken up by organisms living there [
2]. One of the ways of assessing the potential ecological and anthropogenic impact is by means of the enrichment factor (EF). The EF is used to identify the anomalous concentration of hazardous metals in the environment [
45]. To calculate it, a geochemical normalisation of the analysed metals against those of a conservative element (Fe) was performed. Then, it was evaluated using the following formula: EF = (M/Fe)
sample/(M/Fe)
blank, where (M/Fe)
sample is the ratio of the concentration of an element over the Fe concentration of the sample, and (M/Fe)
blank is the ratio of the concentration of a metal over the Fe concentration of the blank (sample BUS-1.5;
Table 3). According to Sutherland [
45], there are five categories of contamination: minimal enrichment (EF < 2), moderate enrichment (2 < EF < 5), significant enrichment (5 < EF < 20), very high enrichment (20 < EF < 40) and extremely high enrichment (EF > 40).
The enrichment factors of the samples from both ponds are very similar to each other, and no changes in values with depth are observed (
Figure 8). The slurries show high to extremely high EF in Ag and Sb, significant enrichment in As and moderate enrichment in Pb and Zn. The EF values for Cr, Cu and Ni are the lowest and correspond to the minimum enrichment category. These results are to be expected from the paragenesis of this mineralisation, which is mainly composed of Ag sulfosalts and sulphides of Pb, Zn, Fe and Cu. If these results are compared with the EF of other districts analysed ([
28]; EF > 20), the latter show higher EF values than those of the Hiendelaencina ponds, reflecting the lower contaminant potential of this mining district. If the average metal contents in San Carlos are also compared with those published for similar slurries from other abandoned mining districts [
27,
29,
33,
46], the concentration of potentially hazardous elements such as As, Cu, Pb, Sb or Zn in other districts is significantly higher.
Figure 9 shows the average values for San Carlos together with the average values for ponds in three important mining districts in Spain: Brunita and Las Matildes (Cartagena La Unión District), San Quintín (Valle de Alcudia District) and Monterromero (Riotinto District). It can be clearly seen that the levels of potentially hazardous elements in the San Carlos Pond (Hiendelaencina) are significantly lower than those measured in the ponds of the other three mining districts studied. This may be due to the fact that the San Carlos slurries come from more modern metallurgical tailings processes, with more efficient metallurgical technologies, which optimised the utilisation of the metals present in the tailings dumps [
20]. In some of the ponds in the other districts, the measured values could make them exploitable, depending on the world price of the substance. Furthermore, the samples from the Brunita, Las Matildes and Monterromero Ponds have an acidic pH, while the San Carlos samples have an alkaline pH, which means a lower capacity for the mobilisation of potentially hazardous substances in the case of Hiendelaencina Pond. All these data reflect the lower pollution potential of this district compared to the rest of the mining districts studied.
Generally, the parameter that usually determines the higher or lower degree of mobility of metals in the environment is the pH. Although no direct relationship between the physicochemical parameters and the composition of the pond was observed at first sight, it is known that sulphides with a metal/sulphide ratio = 1 (e.g., sphalerite, galena or chalcopyrite) do not normally produce acidity when the oxidising agent is oxygen [
47]. Therefore, slurries from this type of mineralisation usually have alkaline characteristics. Even so, considering the significant percentage in weight of siderite estimated by XRD (
Table 1), it is important to study the pH of the slurry, as sulphides can generate acidity upon weathering [
2]. With regard to the Spanish applicable legislation on slurry pond contamination, it is necessary to consult Royal Decree 1310/1990, which establishes the limit concentrations of potentially dangerous elements that soils may contain depending on the pH (
Table 6).
The pH values of all slurry samples are >7; therefore, Pb is the only metal exceeding the permitted values in 4 of the 38 samples analysed. The contents of the other elements do not exceed the permitted values. Apart from the metal contents envisaged by the present legislation, two other elements (As and Sb) are present in significant amounts in the mine ponds and could pose a risk for the environment. Moreover, As and Sb are metalloids whose degree of mobilisation increases with the pH, as is the case for the study area, where alkaline pH values have been measured. However, the pseudo-total As concentrations are not a good indicators of mobilisation and the potential environmental impact, because As fractions (As(III) and As(V)) differ in their solubility, particularly under different environmental conditions, e.g., [
49]. In order to properly discriminate the degree of arsenic mobilisation and potential risk of transfer to aqueous systems, fractionation studies and leaching tests should be used. Similar conclusions can be obtained for Sb, as their degree of mobilisation also depends on the different Sb fraction (Sb(III) and Sb(V)) concentrations. This is beyond the scope of this work and, consequently, has not been considered here, but they would be envisaged for future works in the area. However, it is worthy to say that a recent work about the physicochemical properties of both surface and groundwater discharge points around the mine ponds [
50] has revealed very low (<1 µg/L) concentration values of the total As and Sb, independently of the climate conditions (different field sampling campaigns were carried out to evaluate potential differences between the wet and dry periods during the year). The results obtained by the authors seem to confirm that, despite the high concentration values of the metalloids found in the pond samples, its degree of mobilisation towards the aqueous systems is very reduced. This seems to exclude the polluting potential of these ponds in relation to the surrounding aqueous systems, but its dispersion from mobilisation agents different from water cannot be completely ruled out. As stated in [
7], the entire pond perimeter is exposed to the combined effects of water, wind, gravity and anthropogenic actions. Consequently, measures to restore and/or stabilise the ponds should be undertaken to reduce the high rate of erosion and the dispersion of potentially toxic metals.
In summary, although the minor polluting potential of these ponds is evident, the quantities are significant enough to require a detailed geo-environmental study and the implementation of appropriate management to minimise the risk both for the environment and for the native population and visitors.