3.1. Obtention and Elaboration of the Data
The results of the soil or solid material analysis obtained from 46 sampling points can be seen in
Figure 2.
Table 4,
Table 5,
Table 6 and
Table 7 contain the values of the specific parameters for the CS site, C
air-v, and C
air-p from Equations (5)–(10).
Table 4 provides representative statistical values for the concentration of arsenic and mercury in the soil: sample size, N; minimum, Min; maximum, Max; mean, Aver; and standard deviation, SD.
The data clearly show that the zones with the highest concentration of mercury and arsenic are those of the metallurgical plant, with the roasting furnace and the chimneys. This is because these concrete structures were in contact with the fumes produced and became impregnated with mercury and arsenic.
In the two waste dumps, the levels of As and Hg are very high, although they do not reach the contamination level of the two previous zones. However, beneath the lower waste dump, there is a platform where the concentration of arsenic is significantly higher than in the waste dumps, making it a special zone. This is because it collects all the leachates produced by the waste dump, and during dry periods, arsenic salt precipitates occur due to water evaporation.
Finally, in the other zones (mainly associated with transit areas and buildings), the concentrations of Hg and As are several times lower.
It is worth noting that there was a single point with a high concentration of As far from the most contaminated areas; this was the site of the arsenic loading station.
On the other hand, the concentration of mercury in the air was measured at different points within the facilities (Rodríguez et al. [
28]; Rodríguez et al. [
31]).
Table 5 presents the average concentration of gaseous mercury in the air in the different zones. No measurements were taken in the chimney area, and there is no data on Hg in that area, so an estimate will be used by assuming that the concentration of Hg in the air in the chimney area is about twice that of the demolition debris area. This is a very conservative value because this area is more exposed to the influence of wind than the demolition debris.
It should be emphasised that the concentration of mercury in air is only linked to the mercury present in the soil in the demolition debris zones and the chimneys, which act as emission sources. In the other zones, practically all the mercury in the air comes from these sources.
Regarding the concentration of As and Hg in suspended dust, it is worth noting that very little dust is generated at this site. Measurements conducted in the area yielded an airborne dust concentration ranging from 0.08 to 0.15 mg/m
3 [
27].
As a result, the measurements of arsenic and mercury in airborne dust were also low. As described by García et al. [
27], three measurement campaigns were carried out: in the demolition debris area, in a nearby area, and in the upper waste dump area. No further control campaigns were deemed necessary, so these measurements are considered for the data in the calculations according to
Table 6.
3.2. Analysis of the Results
Following the methodology described, arsenic was first analysed as a potentially carcinogenic element.
Table 7 presents the results of the calculation of the total cancer risk index,
CR. As observed, the values are several orders of magnitude higher than the reference limit of 10
−5 in Spain. Consequently, it can be inferred that under no circumstances could normal industrial activity be conducted over a full 40-year working life.
However, as will be seen later, in less contaminated areas (e.g., transit zones) and under certain restrictions, the risk decreases to values below the limit.
In order to complete the risk analysis, arsenic and mercury were analysed as pollutants to which exposure poses a risk of occupational diseases (non-cancer). Considering they do not affect the same target organ, the analysis was conducted separately and the corresponding HI indices are not summed.
Table 8 shows the health risks associated with the presence of As in the soil for different areas and the health risks associated with the presence of mercury, both in the soil and in the atmosphere, in different zones.
It should be noted that the greatest health risk to workers is associated with high concentrations of arsenic rather than mercury. Nevertheless, the volatility of mercury and the possibility of encountering high concentrations of Hg in the environment are unique features of these facilities that significantly affect the work.
The analysis of the results allows for the distinction of three main areas based on the risk indices. This division is perfectly coherent with our understanding of the nature and origin of these areas, the working conditions within them, and the tasks undertaken in the SUBproducts4LIFE project.
Figure 3 illustrates the three major zones defined based on CR and HI. As mentioned earlier, it is noteworthy that there is a point with an extremely high health risk within a zone of lower risk. This is because it is a singular point where the former arsenic loading platform was located.
In
Table 9, the three ranges of variations in the selected CR and HI indices are defined.
There is a less critical zone (in green in
Figure 3), although it remains a high-risk area in general, as the indices exceed the
HI > 1 and
CR > 10
−5 limit values. This means that, in this zone, regular work or industrial activity could not be carried out over a 40-year working life. However, conditions could be found where the
CR and
HI values decrease below 10
−5 and 1, respectively.
For instance, in the framework of a research project (SUBprofucts4LIFE), work could be conducted for 6 months (120 days) in areas where the concentration of As and Hg in the soil is below 500 mg/kg and 200 mg/kg, respectively, and the concentration of Hg in the air is below 10−3 mg/m3. Recalculating the former values using these values yields CR = 4.2 × 10−6 for As and HI = 0.98 and HI = 1.0 for As and Hg, respectively.
An example of such an area is the entrance zone to the facility (
Figure 4A). The average concentrations of As and Hg in the soil in the transit zones are 138 and 77 mg/kg, respectively, and the average concentration of Hg in the air throughout the year is 6.1 × 10
−4 mg/m
3. In theory, work could be conducted for up to 6 months without special protection. However, compliance with regulations requires the use of personal protective equipment (PPE), such as dust masks, gloves, boots, helmets, and so on.
This is coherent with the fact that some industrial activity was carried out in the past, with acceptable values for CR and HI, considering that this activity was developed 20 years ago (before establishing the legal limits).
There are other areas (brown colour in
Figure 3) where the soil is contaminated with arsenic and mercury but the concentration of mercury in the air is not too high; specifically, less than 10
−3 mg/m
3. Normal activity cannot be conducted in these areas; they involve special tasks where workers must wear more protection, such as full-body suits to protect the skin from contact with materials, or special gloves. However, work can be carried out in these areas for the entire day (8 h/day) and throughout the year (240 days/year). An example of this is presented in
Figure 4B, where a worker is shown collecting samples in the upper waste dump, where the average concentrations of As and Hg in the soil are 16,107 and 3646 mg/kg, respectively, and the average concentration of Hg in the air throughout the year is 5.1 × 10
−4 mg/m
3.
Finally, there are two areas (highlighted in red in
Figure 3) where, for different reasons, the risk is extremely high. One of them is the platform at the foot of the lower waste dump, where the precipitation of arsenic salts results in much higher concentrations of this element than in other areas.
Another is the demolition debris areas of the metallurgical plant along with the smoke ducts and chimneys which, due to high concentrations of gaseous mercury in the air, form another critical area in terms of health risks to workers. This is due to the extremely high concentrations of Hg in the air, for which the use of the PPE, as mentioned earlier, is not sufficient, and a half-face mask must also be used for protection against Hg gases; here, work must be conducted according to a strict safety protocol that limits working hours based on temperature, as described by Rodríguez et al. [
31].
This situation is reflected in
Figure 4C, where work is being carried out in the demolition debris area. As observed, the average concentrations of As and Hg in the soil can reach values as high as 136,160 and 17,199 mg/kg, respectively, and the average concentration of Hg in the air throughout the year is 1.3 × 10
−2 mg/m
3. In these areas, the work shift is at most 6 h/day at temperatures below 15 °C, which can be reduced to 3 h/day at a maximum temperature of 25 °C.
As described above, the As and Hg concentrations in the soils in different parts of the site are clearly related to the mining activities developed over the life of the mine. The paragenesis of the ore deposit is composed of cinnabar, orpiment, realgar, pyrite (usually with high concentrations of As), arsenopyrite, marcasite, and pararealgar in a gangue of quartz and calcite [
32].
First, the mineral had to be mined out from the rock mass. In this process, two materials were produced: one rich with a high concentration of cinnabar, which was sent to the metallurgical process, and the other poor in Hg, which was stockpiled in a waste dump. This waste dump, in the lower part of the site, is an area with high As and Hg contamination, although it is not the most contaminated area. We should keep in mind that both As and Hg are chemically combined in the minerals (they are sulphates); with this, the mobility and the contaminating potential are lower than in other areas.
The rich minerals were collected at the metallurgical plant, where As and Hg were produced on one side and waste was produced on the other. As the efficiency was not 100%, part of the As and Hg remained in this waste, which was stored in a waste dump in the upper part of the site. The concentration of As and Hg is of the same order of magnitude as in the other waste dump.
The metallurgical plant is divided into two different zones. At first, there were the buildings and structures that supported the roasting furnace. At present, only the debris from the demolition of these structures and buildings is located in this zone. In the other zone, there are the ducts and the chimneys for the smoke. These two zones are the ones with the highest concentrations of mercury and arsenic. This is because these concrete structures were in contact with the fumes produced in the metallurgical process and became impregnated with mercury and arsenic.
There is a singular area—the platform in the lower part of the site—where the concentration of arsenic is significantly higher than in the waste dumps, making it a special zone. This is because it collects all the leachates produced by the waste dump and, during dry periods, arsenic salt precipitates occur due to water evaporation.
Finally, in the other zones, mainly associated with transit areas and buildings, the concentrations of Hg and As are several times lower than in the previous areas. In these areas, the contamination was produced by the dispersion of PTEs existing in the other areas.
A number of studies have been carried out recently using the US EPA model, demonstrating that it is useful under different conditions. A summary of the research is shown in
Table 10, which lists the areas that pose a low or high risk to the health of adults. In general, it analysed the effect of more PTEs than As and Hg; although, in all cases, the weight of As in the total values of the indices for cancer risk (CR) and non-cancer illness (HI) is significant.
The results show that there are two different scenarios. The first is related to agricultural activities or general industry (Cases 1 to 5). In general terms, the contamination is moderate, and the CR and HI indices have acceptable values.
The second is related to mining activities (Cases 6 to 9), gold mines (in which Hg was used in the past), and former mercury mines. Under these conditions, both cancer and non-cancer illness risks are over the legal limits. The worst conditions (Case 9) are present in former abandoned mercury mines and facilities in which the waste from metallurgical processes and debris from the demolition of metallurgical plants are present. The case studied here is similar to this last case.
Only in Case 9 (related to the Terronal Mine in Spain) was there a comparable situation to that of La Soterraña concerning the elevated CR and HI index values. When comparing the CR values associated with arsenic from La Soterraña Mine (
Table 7) with Terronal Mine in Mieres (Asturias) [
12], notable differences emerge. In Terronal, there are two areas (25 × 25 m) with CR levels of 2.5 × 10
−1, which is significantly higher than the maximum observed at La Soterraña, although it is still within the same order of magnitude as the chimneys, which register levels of 1.4 × 10
−1. In La Soterraña, the second focus of concern lies in the demolition debris, with CR levels reaching 8.8 × 10
−2. Interestingly, in Terronal, there is a grid exhibiting similar levels to those found in La Soterraña, with a CR value of 3.2 × 10
−2.