SELECTED PAPERS
3. IMPACT OF MINING RESIDUES ON WATER RESOURCES
Hereafter, the presented findings are based on a monitoring program including 50 sampling stations (Fig. 1) in conjunction with scarce hydrogeological data about the Mailuu Suu area [6].
Sampled water types comprise seepage from tailings, river and creek waters as well as groundwater (deep and shallow aquifers) sampled from springs, dug wells, artesian wells and 11 new observation wells. The artesian wells are screened within Cretaceous or Tertiary hard rocks. Shallow wells tap groundwater from Holocene alluvial sediments. Reliable well data are restricted to the new observation wells drilled in frame of this study (M1–M11, see Fig. 1).
However, even in case of an uncertain origin of a water sample due to lacking screen depth data, interpretation of major and trace constituents (hydrochemical fingerprinting) provides indications of the host material as well as specific interrelations with other water resources as described hereafter (Fig. 2).
FIG. 1. Sketch map of Mailuu Suu valley with locations of dumps, tailings as well as surface and groundwater sampling stations. In frame of this study installed shallow wells are labelled ‘Mx’.
The Mailuu–Say River, flowing NE to SW, represent a Ca–Mg–HCO3 water type with low solutes and, thus, a typical meteoric composition. In contrast, the major tributary Kulmin–Say with a Na–Mg–SO4 composition indicates a dominating impact of other sources than meteoric.
In Fig. 2, river samples plot along a mixing line bordered by the northernmost Mailuu–Say water on one end and Kulmin–Say water on the other. This indicates a southward increasing impact of tributaries which drain water sources rich in sulphate and sodium.
The sampled Holocene alluvial aquifer shows a remarkable resemblance to the Mailuu–Say River water with respect to their major composition (Ca–Mg–HCO3). Similarly to Mailuu–Say River water, shallow groundwater has a southward increasing impact of sodium and sulphate dominated fluids. Thus, the associated water composition is arranged along the mixing line discussed above (see ‘GW–Holocene’ in Fig. 2). This indicates a hydraulic interconnection between shallow aquifer and Mailuu–Say River. Therefore, the shallow aquifer seems to be subject to locally influx of deep groundwater and contaminated seepage water as well as tributaries, additionally to a continuous exchange with Mailuu–Say River water.
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FIG. 2. Major water composition of sampled water resources (n=91), modified after [7].
In contrast to natural water resources, the term ‘technogene’ water is applied when influence from mining residues is likely, such as by tailings or rock dumps. The predominating water composition (Na, SO4 and HCO3) in this group might be explained by dissolution of sulphide and carbonate minerals, commonly present within the tailings together with a high fraction of gypsum precipitated during the leaching with sulphuric acid. Especially, the sampled seepage water leaking from tailings have a remarkably high fraction of dissolved solids (total dissolved solids (TDS) up to 10 g/L).
3.1. Uranium in natural water resources
Analyses of water samples points out that most water resources in Mailuu Suu area are at least locally affected by elevated dissolved uranium contents. From a chemotoxic point of view, uranium is the most problematic parameter. In more than 50% of all water samples, the provisional guideline value for drinking water quality recommended at the period of this study by WHO (3rd edition: 15 µg U/L) has been exceeded with maximum uranium concentrations found in seepage water up to 36 mg/L. Please note that the WHO raised the guideline value to 30 µg/L in the 4th edition published 2012, based on new data for human exposure to elevated uranium in drinking water. Further guideline values respective solutes such as sulphate (maximum 5 g/L), fluoride (maximum 10 mg/L) and arsenic (maximum 1.8 mg/L) were locally exceeded and therefore increase the risk for adverse health effects in case of consumption.
Repeated monitoring revealed that contaminants’ load of the observed water bodies varies seasonally mainly depending on precipitation and inflow of other water sources. The observed variability was within 30%, while being highest in case of surface waters (rivers, seepage
Hereafter, the sampled water types are discussed with respect to the observed dissolved uranium content (Fig. 3).
FIG. 3. Distribution of dissolved uranium in 108 water samples, grouped in terms of their origin (modified after [7]). Dashed lines indicate WHO guideline values for uranium in drinking water in red (4th edition: 30 µg/L) and yellow (3rd edition: 15 µg/L) color.
The seepage from tailings is generally dominated by high dissolved uranium (up to 36 mg/L), as a result of the tailing materials fine grain size combined with high residence time and evaporation effects (Fig. 4). Seepage discharges into receiving creeks and rivers or directly infiltrating into the subsurface.
North of Mailuu Suu valley, the main river Mailuu–Say is low in dissolved uranium (0.4 µg/L).
In contrast, significant dissolved uranium has been observed in its tributaries (e.g. Kulmin–Say 170–220 µg/L). Consequently, the dissolved uranium content of the Mailuu–Say is increasing downstream but still remains below the WHO guideline for drinking water quality. The dissolved solute concentration varies in dependency of seasonal flow rate fluctuations of the Mailuu–Say and its tributaries. Consequently, the maximum uranium concentration has been observed in the south of Mailuu Suu valley at the end of the dry season in October 2006 (up to 11 µg/L, Kok–Tash area).
In Holocene alluvial sediments, lowest dissolved uranium levels have been identified north of Mailuu Suu valley (~3 µg/L). Further downstream, uranium content increases up to a maximum within the city area (30 µg/L), exceeding the current WHO guideline value for drinking water.
Nevertheless, this aquifer is utilized by numerous domestic household wells. In the southernmost sampling station (Kok–Tash area) uranium has been observed to be slightly elevated but below the WHO threshold (~7 µg/L).
Artesian groundwater of Tertiary and Cretaceous aquifers can reach high uranium levels (up to 140 µg/L), probably associated with uranium ore mineralization in the host rocks. An impact of flooded mining excavations cannot be confirmed with the available data. In contrast, a deep
Hardrock
well in the very northern area (Sarabiya) taps an unspecified, possibly Tertiary hardrock aquifer which bears very low dissolved uranium (0.05 µg/L).
FIG. 4. Fraction of the stable water isotopes δ2H and δ18O in selected water samples (modified after [7]). Lowest values mark artesian hardrock groundwater from Sarabiya. U–rich seepage waters and receiving tributaries plot along a dashed line diverging from the local meteoric water line.
3.2. Assessment of contamination sources
Superficial mining residues such as dumps and tailing impoundments obviously represent contamination sources. However, the contamination path for specific natural water resources is quite complex since an interaction of different sources should be considered. In addition to seepage discharge from numerous dumps and tailing impoundments, artesian groundwater from Cretaceous aquifer containing dissolved uranium and metal sulphides and/or crude oil might be a significant impact when seeping into alluvial aquifers and rivers (as observed at Cretaceous outcrops north Mailuu Suu city). Moreover, uranium oxidation and dissolution in flooded mining excavations might enhance dissolved uranium levels.
Another perspective to the specific relevance of contamination sources for superficial water resources in Mailuu Suu area is provided by analysis of stable hydrogen and oxygen isotope data (Fig. 4). While the natural water samples generally plot along the local meteoric water line, high uranium seepage waters as well as the sampled tributaries (including Kulmin–Say) are characterized by an increased δ18O fraction. This hints to evaporation effects, which take place in poorly capped tailings and dumps and resulting in accumulation of heavy oxygen isotopes as well as non-volatile solutes (r(δ18O, TDS) = 0.62). Based on the available stable isotope data, the water composition of sampled tributaries including Kulmin–Say is dominated by uranium rich seepage water from dumps and tailings. This should be confirmed in future monitoring campaigns including stable isotope analysis.
Radiochemical leaching experiments with local tailing material (Tailing 3) as well as thermodynamic calculations have been carried out to get insight into the uranium mobilization
Seepage water
process within the tailing impoundments [8]. In summary, formation of dissolved uranium is generally controlled by pH/Eh conditions as well as the availability of the ligands bicarbonate and calcium [8, 9]. The dominating U species are calcium uranyl carbonates (Ca2UO2(CO3)3
and CaUO2(CO3)32-). These are mobile species in sediments with neutral or negatively charged mineral surfaces. Moreover, a significant proportion of uranium has been mobilized as uraninite colloids (<200 nm) with 20 ± 5% of total mobilized uranium.
Vandenhove et al. [10] identified Tailing 3 as the dominating environmental hazard with a total radiation inventory of 650 TBq, as much as 60% of the total radiation of all tailings impoundments. Consequently, recent remediation activities relocated Tailing 3 to a safe disposal site. However, considering the water transport path other sources are found to be more relevant as shown in Table 1. Based on our field observations, in 2006 seepage from Tailing 5 represented a major contaminant for Mailuu–Say River with a calculated load of 122 g uranium per day. Notably, the relocation of Tailing 3 has limited impact on improving water quality due to the comparatively low daily uranium discharge into the Mailuu–Say.
TABLE 1. APPROXIMATE URANIUM RELEASE OF SELECTED SOURCES DISCHARGING INTO THE MAILUU–SAY RIVER, CALCULATED FROM OBSERVATIONS DURING FIELD CAMPAIGN IN 10/2006 (MODIFIED AFTER [7])
Source U [mg/L] Discharge
[L/min] U release [g/d]
Seepage Tailing 3 1.8 0,5 1
Seepage Tailing 5 8.5 10 122
Seepage Tailing 16 36 0.1 5
Kara Agach River 0.04 500 28
Kulmin–Say River 0.17 100 25