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Review

Geo-Environmental Models of In-Situ Leaching Sandstone-Type Uranium Deposits in North China: A Review and Perspective

by
Fuxin Zheng
,
Yanguo Teng
,
Yuanzheng Zhai
*,
Jingdan Hu
,
Junfeng Dou
and
Rui Zuo
Engineering Research Center for Groundwater Pollution Control and Remediation of Ministry of Education of China, College of Water Sciences, Beijing Normal University, Beijing 100875, China
*
Author to whom correspondence should be addressed.
Water 2023, 15(6), 1244; https://doi.org/10.3390/w15061244
Submission received: 5 February 2023 / Revised: 8 March 2023 / Accepted: 20 March 2023 / Published: 22 March 2023
(This article belongs to the Special Issue River Ecological Restoration and Groundwater Artificial Recharge II)
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Abstract

:
Since the 1990s, sandstone-type uranium in the northern basin of China has become the main target for mining. Uranium mining can cause a series of impacts on the environment. A conceptual model of the geo-environment for sandstone-type uranium in northern China was described, which covers the changes in the geo-environmental characteristics in the natural state, in the mining process, during decommissioning and after treatment. Sandstone-type uranium is mainly distributed in the Songliao, Erlian, Ordos, Turpan–Hami and Ili Basins, which have arid climates and poor stratum permeability. Pitchblende is the main uranium-bearing mineral and is associated with iron, copper, coal, organic matter and other minerals. The mineral often has a low ore grade (0.01–1.0%) and high carbonate content (2–25%). Uranyl carbonate accounts for more than 90% of the total uranium in groundwater. The uranyl content is closely related to the TDS. The TDS of groundwater in the eastern and central ore belts is usually lower than 2 g/L, while in the western region, such as Xinjiang, it can exceed 10 g/L. In situ leaching (ISL) is the main mining method that results in groundwater pollution. Acid leaching leads to a pH decrease (<3), and heavy metals represented by U and Fe exceed the background values by hundreds of times, resulting in groundwater pollution. CO2 leaching is more environmentally friendly, and the excess ions are usually Ca2+, Mg2+, NO3− and HCO3−. Soil chemical anomalies originate mostly from wind erosion and precipitation leaching of decommissioned tailings. Uranium pollution is mainly concentrated within 20 cm of the surface, and the exceedance generally varies from two to 40 times. During ISL, a series of environmental measures will be taken to prevent pollution from being exposed to the surface. After treatment, the decommissioned uranium mines will likely have no impact on the surrounding environment. In the future, the protection of groundwater should be strengthened during production, and remediation methods based on electrokinetic, microbial and permeable reactive barrier (PRB) technology should be further researched.

1. Introduction

As the world’s largest carbon-emitting economy, the Chinese government has pledged to achieve carbon neutrality by 2060. To achieve this goal, the number of nuclear power plants in the next 30 years will be increased five times on the current basis [1]. Uranium is the main raw material for nuclear power generation, and the development of nuclear power is bound to significantly increase the mining of uranium [2,3]. The largest reserve uranium deposits in China are sandstone-type deposits, which account for approximately 43% of the total [4]. Since the 1990s, with the introduction of the in situ leaching (ISL) mining method, sandstone-type uranium in northern China basins has gradually replaced southern granite-type and volcanic-type uranium and has become the main mining body [5,6]. Compared with open pit mines, the ISL of uranium is mainly conducted underground, which is relatively friendly to the environment, especially in the atmosphere and soil. However, ISL often pollutes groundwater due to the use of geological drilling and chemical solvents [7,8]. In addition, heap leaching of tailings poses potential environmental threats [9,10,11].
A geo-environmental model is a collection of multiple conceptual models that contain information on geology, hydrogeology, geochemistry, engineering, etc. [12]. In 1998, the U.S. The Department of the Interior and the Federal Geological Survey first systematically described the concepts on their web pages and used the host deposit types as the main index to classify environmental models [12,13]. The construction of geo-environment models can provide reference targets and ideas for mine environment management and remediation [12,13,14]. Uranium deposits usually show some special hydrogeological and geochemical characteristics [15,16,17], which usually change during and after mining [18,19]. How to repair the geo-environment according to these changes presents new challenges for researchers.
Based on the above, this article describes geo-environmental models of sandstone-type uranium in northern China, which covers the changes in the Earth’s environment before, during and after mining, and discusses the prospects of geo-environmental protection and restoration methods in the future. Section 2 describes the distribution of sandstone-type uranium in northern China and summarizes the Earth’s environmental characteristics and baselines of these deposits. Section 3 concentrates on the soil, rocks and groundwater chemical anomalies, as well as radiation and vegetation environment changes. The environmental measures during and after mining are addressed in Section 4. Finally, Section 5 provides suggestions for the geo-environmental protection and remediation of sandstone-type uranium mine development.

2. Environmental Properties and Geochemical Baselines

2.1. Environmental Properties

Most of the sandstone-type uranium deposits in northern China are located in arid or semiarid areas [20]. The main ore belts are the Songliao Basin, Erlian Basin, Ordos Basin, Turpan-Hami Basin and Ili Basin from east to west (Figure 1) [21,22,23]. The largest deposit is Nuheting, located in the Erlian Basin, with a reserve of more than 50,000 t. There are more than ten medium-scale deposits of more than 3000 t. Limited by technical conditions and economic factors, scaled mining is currently mainly concentrated in the Mengqiguer area of the Ili Basin and Qianjiadian area of the Songliao Basin. Uranium ore is usually low-consolidated sandstone composed of detritus of different sizes. The main components of the detritus are quartz and feldspar, and there are also some cemented substances (such as calcareous cement) [24,25,26,27,28]. The ore has a high carbonate content and is rich in organic matter and other reducing substances (such as organic carbon, H2S and CH4), which have good reducing abilities [29].
The stratigraphic chronology of uranium deposits varies by region (Figure 2a). Affected by interlayer oxidation, the distribution of ore belts can be divided into oxidation zones, transition zones and primary zones, among which the transition zone is the main metallogenic zone (Figure 2b) [30]. The oxidation zone is mainly associated with pyrite, limonite or siderite. Therefore, the ore is dark yellow or brown, and some ore may even contain “red layers,” whose main component is iron oxide [31,32,33]. In the reduction zone and transition zone, ores are generally gray due to iron reduction (Figure 2) [4,34,35,36]. In the Ordos and Ili basins, uranium deposits are often associated with coal or copper mines. Paragenesis of uranium and coal is also one of the main resource development methods in these areas [37,38,39,40]. Other minerals, such as trace elements and rare earth elements, are usually characterized by abnormal abundance and have higher contents than other ores [41,42,43]. The main component of uranium minerals is pitchblende [27,28,44,45], while in the Turpan-Hami Basin, the U-bearing minerals are brannerite and coffinite [46]. Uranium minerals are usually found in the crevices of ore and cementitious material and are often associated with hematite and pyrite [45].
Multilayer aquifer systems are universal in all these deposits [47,48,49,50]. For example, the Qianjiadian uranium deposit in the Songliao Plain has four aquifers with an average thickness of 55 m [48], and the Shihongtan deposit in the Turpan-Hami Basin has seven aquifers with an average thickness of approximately 23 m [49]. The aquifer is often composed of light gray or reddish gravelly sandstone mixed with argillaceous cementation. Dense and continuous groundwater barriers are usually composed of mudstone with a thickness of 1–6 m [47,48,49,50]. These aquifers determine the ore occurrence characteristics through the control of redox conditions and then affect the geo-environment of the deposits (Figure 3). Similar to the metallogenic differences between the eastern and western Shihongtan ore belts [51], the uranium occurrence differences in the northern and southern Ili Basin [50] are all related to the interlayer oxidation of groundwater.
Precipitation is the main source of groundwater [36,47]. In Northwest China (Ili and Turpan-Hami Basins), there is also Quaternary phreatic recharge from mountains in the basin margins [50,52]. Controlled by the tectonic system and geological conditions, the groundwater flows from the basin edge to the central depression. The hydraulic gradient and flow velocity vary from different basins. In open basins (Songliao, Erlian or Ordos Basin), the hydraulic gradient is generally less than 2‰, and the groundwater flow velocity is low. Similar to the Erlian Basin, the hydraulic gradient is 0.3–1.4‰, and the groundwater flow velocity is 0.7–2.9 m/a. In intermountain basins, the hydraulic gradient is generally 4–5‰, and the groundwater velocity can reach approximately 4 m/a [53]. This value will be higher in areas with large local topographic fluctuations, such as the No. 513 mine in Sudongbulake and the No. 510 mine in Honghaigou, where the hydraulic gradient reaches 7–9‰ and 23–35‰, respectively. The groundwater flow velocity exceeds 17 m/a [53]. Due to the arid climate, groundwater discharge is usually dominated by evaporation [47,54,55]. Except for the Shihongtan area in the Turpan-Hami Basin, the groundwater in most deposits is of good quality, with low salinity and moderate to weakly alkaline pH. Ca2+, Na+, HCO3−, SO42− and Cl− are the main ions [38,47,53,56]. These ions come from rock dissolution (such as gypsum and calcite) under water-rock interactions. In addition to conventional ions, groundwater also contains sulfides (such as H2S, HS−, S2−, etc.), reducing gas (such as CH4), Fe3+/Fe2+, and some trace elements [57,58,59]. Uranium in groundwater often exists in the form of uranyl. U(VI) is dominant in the oxidation environment of shallow aquifers, while U(IV) is dominant in deep reduction aquifers [47,52,60]. The formation of the Ili uranium deposit is due to uranium valence changes in different aquifers. Uranium in the shallow aquifer is continuously oxidized and enters groundwater in the form of uranyl [such as UO2(CO3)22− and UO2(CO3)34−] (Table 1). The uranyl enters the deep aquifer with the recharge and flow of groundwater, where uranyl is restored to U (IV) precipitation and mineralization [50,55].

2.2. Geochemical Baseline

The geochemical baselines in different regions are shown in Table 2. The proportion of detritus in the ore is generally more than 80%, and the detritus is composed of more than 60% quartz [26,63,64]. Approximately 90% of the ores have a carbonate content of 1–3% (carbonate-containing ores), and a few can reach more than 3% (carbonate ores) [65,66,67,68]. Some ore carbonate contents in the Ili Basin can reach up to 5–25% [64,69,70]. The uranium grade of the ore is generally between 0.01–0.05%. The Ili Basin has the highest average level in the phreatic oxidation zone of the southern basin margin (0.1–0.2%) and in the coal seam (88.7 μg/g) [27]. Approximately 70–90% of uranium minerals are UO2. The pH of the metallogenic belt (transition zone) is nearly neutral to weakly alkaline, and the organic matter is generally 0.1–1.5% [65]. The Songliao Basin has the lowest organic matter content, with an average of only approximately 0.2%, while the Ordos Basin has the highest organic matter content, and some ores can reach 5–10% [28,68].
The associated trace elements in ores of different regions have similarities and differences in element types and enrichment (Figure 4). For example, the main associated trace elements in the Songliao and Erlian Basins are Re, Th, Ba, Se, Co, Ni, Pb, V, Zn, etc., but the Songliao Basin has a stronger enrichment effect [43,73]. The ores in the Dongsheng mine (Ordos Basin) and No. 511 mine (Ili Basin) are both associated with and enriched with Se, Mo and Ge [24,25]. However, the No. 511 mine is additionally associated and enriched with Re and V. The contents of Se and Re in the No. 511 mine are higher, and the ore has a comprehensive utilization value, which is not available in the Dongsheng mine [42]. In addition, rare earth elements (REEs) are usually high in the Erlian Basin and the Ordos Basin. The ∑REE contents in the Dongsheng mine are 94.29 × 10−6~203.53 × 10−6, while those in the Bayintala mine are 105.01 × 10−6~169.80 × 10−6 [74].
The location of uranium deposits usually shows some hydrochemical anomalies (Figure 5). These chemical differences can be used as prospecting indicators for uranium deposits in the area and provide a reference for subsequent mining. The hydrochemical baselines of the same area show differences due to the different groundwater occurrences and types (Table 2). In the Songliao Basin, the groundwater chemical type in the upper aquifer is HCO3·Cl-Na, the TDS is approximately 580 mg/L, and the pH is 7.65. The groundwater in the lower aquifer has better quality than that of the upper aquifer, and it can reach class III for the “standard for groundwater quality (GB/T 14848-2017). The groundwater chemical type is HCO3·Cl-Na and Cl·HCO3-Na, the TDS is approximately 3.64–4.89 mg/L, and the pH is 7.15–7.79 [48]. In the Erlian Basin, the bedrock fissure water in the low mountain area is mostly HCO3-Ca·Na type water, and the TDS is less than 2000 mg/L. The bedrock fissure water in the hilly area is the HCO3·SO4-Na·Mg type or HCO3·SO4-Na·Ca type, the TDS is generally greater than 1000 mg/L, and some parts are as high as 2000–4000 mg/L. The pore water quality is mainly affected by evaporation and concentration, which increases the TDS of groundwater. From upstream to downstream, evaporation and concentration become stronger, and the TDS changes from low to high. The water type downstream is Cl-Na, and the TDS significantly exceeds 4000 mg/L [62]. The shallow groundwater (<20 m) in the northern Ordos Basin is of the HCO3-Ca and HCO3-Ca·Mg types, the pH is 7.53–8.06, and the TDS is 126–497 mg/L. Deep Cretaceous confined water with good quality ranges from 28 m to 300 m, and the TDS is 877–968 mg/L, which supplies local people and livestock drinking water [47]. The shallow groundwater in the Ili Basin is mainly the SO4·Cl-Na·Ca type, and the TDS is low (<500 mg/L) or medium (500–5000 mg/L) [50,55]. The groundwater in the Turpan-Hami Basin has a high TDS, the deposit aquifers are mainly Cl SO4-Na type water (TDS: 8–12 g/L), and the anticline area of the southern deposit is Cl-Na type water (TDS > 12 g/L) [52].
More than 90% of the uranium in groundwater is uranyl carbonate, followed by uranyl hydroxide [47,60]. The U content in groundwater is positively correlated with the U content in rocks [47,60]. In Shihongtan, Turpan-Hami Basin, the U content in the groundwater of the southern deposit (uranium-rich ore) is 44.5–4237 μg/L, while in the northern deposit (uranium-poor ore), the highest U content in the groundwater is only 65.1 μg/L, with an average of 29.3 μg/L [52]. In addition, the U content in groundwater is negatively correlated with TDS. In the Songliao Basin, the average U concentration in the upper aquifer with high TDS is approximately 4.6 μg/L and that in the lower aquifer with low TDS is 130.0–312.0 μg/L [48]. In the southeastern Alanor area, Erlian Basin, the groundwater discharge has high TDS, and the uranium concentration is 0.021–244 μg/L, while in the Nuheting area, which has low TDS, the concentration of U can reach 453 μg/L [92]. In the Ili Basin, the uranium concentration in SO4·Cl-Na-type water with high TDS is generally less than 10 μg/L, while in HCO3·SO4-Ca-type water with low TDS, it is more than 30 μg/L [50,55] (Table 1).
Surface water in sandstone-type uranium deposits is often of poor quality, and the water-rock interaction with uranium ore is small. For example, studies on streams and lakes in northern Ordos [47] found that the local surface water is weakly alkaline, with a pH of approximately 9.3. The TDS is 412–3710 mg/L, approximately 6.4 times that of groundwater at the same location. The main ions in water are Na+, Mg2+ and HCO3− and Cl−. The U concentration is 1.14–1.18 μg/L, which is only approximately 1/24 of the groundwater. There are no significant differences between uranium ore belts and other places for surface water. Compared with groundwater, uranium ore does not significantly affect the quality of surface water.

3. Environmental Impact of Uranium Mining and Extraction

3.1. Overall Characteristics of Environmental Impact

In situ leaching (ISL) of uranium directly injects the solvent into the underground mining layer and extracts uranium minerals through the reaction between the solvent and ore [93,94]. The most common ISL method is acid leaching, which dissolves and mines uranium by injecting H2SO4 + H2O2 [94,95]. Secondly, in the Songliao, Ordos and Ili Basins, the alkaline method (or CO2 method, CO2 + O2, weakly alkaline) has also been adopted [28,96]. Compared with open-pit mining, ISL has limited surface impact but a noticeable effect at depth [96]. Therefore, the mining process and tailings have a certain impact on the water and soil environment in and around the mine.
The pH of the deposits and groundwater are usually near-neutral. Acid leaching leads to pH reduction because of H2SO4 pollution. Meanwhile, leaching leads to the dissolution and release of other minerals, such as pyrite, which releases Fe3+ into the groundwater. Fe3+ as an oxidant further increases the release of other metal or alkali metal ions [94,95]. In alkaline leaching, CO2 reacts with carbonates, resulting in an increase in Ca2+ and Mg2+. Other heavy metals are also released by mineral dissolution. However, due to the near-neutral environment, insoluble metal sediment is formed quickly, so the content of heavy metal ions in groundwater is low [28,96].

3.2. Environmental Impacts on Soil

Soil chemical anomalies usually originate from eroding and leaching tailings rather than production (Figure 6). Wind-borne pollution and water leaching are the main sources of pollution [79]. Wind propagation affects a wide range, but pollution is generally only distributed on the soil surface. For example, in a U-Th tailing in northern China [83], the radioactive elements U and Th were abnormally high in the southeast of the tailing due to the prevailing northwest wind. The content of U in the downwind soil was 2.65 times the background value and 1.61 times the upwind value. The soil pollution from tailings spread to approximately 4 km but was basically distributed within 10 cm of the surface. The water leaching range is small, but the vertical effect is deeper. For example, in a tailing pond [97], the U content reached its peak at 8.5 m from the surface due to perennial precipitation leaching. The maximum migration depth reached 14 m.
Open wastewater leakage can also cause soil pollution. For example, in a mine in the northern basin [98], due to the discharge of wastewater along the surface, heavy metals were carried downstream for more than 450 m but were mainly concentrated within 20 cm of the surface. The U content in the soil was 74.78–79.30 mg/kg, which was more than 27 times higher than the background value (2.72 mg/kg). In addition, other heavy metals have different degrees of pollution. The average exceedances of Cd, Cr, Cu, Mn, Ni, Pb and Zn were approximately 946.0, 5.7, 4.2, 1.7, 1.5, 4.7 and 4.4 times, respectively. Among them, Cd pollution is the most serious, and it is even worse than U pollution. Therefore, special attention should be given during production and remediation.

3.3. Environmental Impacts on the Groundwater Environment

Compared with soil, uranium ISL has a greater impact on groundwater. Similar to uranium deposits No. 737, No. 738 and No. 739 in Xinjiang [99,100], soil indicators remained within limits from mining to decommissioning. However, for groundwater, indicators were still in an abnormal state after years of decommissioning. Among them, ∑Fe (275 mg/L) and Mg2+ (252 mg/L) were more than 200 times higher than the background value, and SO42−, U, Ca2+, and NO3− also exceeded the standard numerous times. The pollution degree of groundwater varies with the mining method (Figure 7). Acid leaching has a significant influence on groundwater. The highest content elements in the groundwater of the acid-leaching mining area are usually SO42−, ∑Fe, Ca2+, Mg2+, and U. Other heavy metals will also be leached into the groundwater. Although the concentration is relatively low, it still far exceeds the groundwater quality standard, posing a potential threat to the groundwater environment. The low pH of groundwater is another problem. Compared with acid leaching, CO2 leaching is more friendly to the groundwater environment because the weakly alkaline environment leads to the precipitation of pollutants.
For example, in an acid-leaching area of a mixed mine in Ili (Table 3) [101], the pH of groundwater was 1.51–1.87, with an average of 1.73. The ∑Fe pollution was the most serious, exceeding the background value by more than 2400 times; the content of SO42− ranged from 11,900 to 18,000 mg/L, with an average of 15,200 mg/L, which was 120 times more than the background value. The content of U (11231 μg/L) was approximately 114 times that before mining (98 μg/L). Other elements, such as Ca, Mn, Ni, Zn, Pb, As, and Cd, all exceeded the background value by dozens or even hundreds of times. Similar to the CO2 + O2 leaching area in the same mine [101], the ∑Fe in groundwater also decreased slightly after mining due to precipitation. The content of SO42− was approximately 1/13 of the acid-leaching area, and Ca and Mg were approximately 1/2 and 1/3 of the acid-leaching area, respectively. The contents of other heavy metals were also significantly reduced. The U content increased, which was five times that of the acid-leaching area. However, CO2 leaching can also lead to other chemical anomalies that are not available in the acid-leaching area. For example, the content of Cl− (279 mg/L) was 3.2 times greater than that in the acid-leaching area (87 mg/L) and 14.5 times greater than the background value (19.43 mg/L). The HCO3− content was 148 mg/L, which was not detected in the acid-leaching area or before mining. In addition, ISL will use resin adsorption and nitrate leaching, resulting in nitrate exceeding the standard. Similar to a deposit in northwestern China, NO3− exceeded the background value by 140 times during the production period [80].
Horizontally, the pollution halo is distributed along the groundwater flow direction, mainly concentrated downstream of the mine with a spread to both sides. The impact on the upstream groundwater is minor [81,91,102]. Vertically, it can barely affect the aquifer beyond the mining layer due to the dense aquifer [102]. Monitoring wells in some Ili mines [81,91,102] showed that the content of SO42− downstream (606.5 mg/L) was 3 times that upstream (202.4 mg/L), U (315.34 μg/L) was 16 times that upstream (25.9 μg/L), and NO3− (144.49 mg/L) was 51 times that upstream (2.81 mg/L) (Table 3).

3.4. Chemical Clogging and Its Effects

Most of the sandstone-type uranium ores in China have high carbonate content, low permeability and low grade, which readily cause chemical clogging during ISL, resulting in the reduction of rock porosity and permeability [28,34,36,61,66]. The dissolution of rocks leads to a large increase in Ca2+, Mg2+ and other heavy metal ions in groundwater. These ions react with the solvent to form CaCO3, CaSO4 or metal precipitates [28,103]. For iron or a kind of metal X, some of the main chemical reactions are as follows.
XS2 + 7/2O2 + H2O→X2+ + 2SO42− + 2H+
Fe2+ + 1/2O2 + 2H+→Fe3+ + H2O
CaCO3 + CO2 + H2O→Ca2+ + 2HCO3−
X2+ + CO32−→XCO3↓
X2+ + 2OH−→X(OH)2↓
2Fe3+ + 3CO32−→Fe2(CO3)3↓
Fe3+ + 3OH−→Fe(OH)3↓
Spatially, chemical clogging mainly occurs at the periphery of the injection well. The injection well is usually not clogged due to the continuous injection of the solvent. At locations far from the injection well, as the concentration of the solvent decreases or the pH increases, a precipitate will form in the solution (Figure 8a) [28,76]. The chemical precipitation moves along the groundwater and accumulates downstream, resulting in increased blockage along the groundwater flow direction (Figure 8b). This dissolution-precipitation-redissolution process leads to significant stratification of porosity around the injection well [68,76].
Temporally, chemical clogging has some hysteresis (Figure 8b). In the initial stage, due to the dissolution of the solvent, the porosity of the rock expands. Rock dissolution may increase porosity by 1–3% [68,104]. As the solvent content decreases, the pH decreases, and the metal cations react with CO3−, SO42− or OH− to form chemical clogging (Figure 8c) [103]. Clogging usually occurs after tens to hundreds of days if there is no human intervention [28].
Limited by chemical clogging, the spread of pollution halos is usually in a small area (Table 3, Figure 9). For high concentrations of pollutants (SO42−, Fe, U, etc.) Along the groundwater flow direction, the migration distance can exceed 1000 m (Figure 9a). Other minor pollutants (Cl−, NO3− and other heavy metals) are mainly concentrated within 200 m (Figure 9b). The maximum diffusion range to both sides was approximately 350 m [102]. In mining areas with a poor permeability coefficient, the effect of chemical clogging is more pronounced. The diffusion range of the pollution halo will be smaller. Similar to a mine in Qianjiadian [89], the deposit has low permeability (k < 0.2 m/d). The average diffusion rate of pollutants during mining was only 0.0843 m/d. After six years of mining, the diffusion range of the pollution halo was within 130 m, which basically controlled the area around the mine.

3.5. Radiation Environment Impacts

Radon release and radiation exposure in uranium mines can result in radioactive abnormalities. The dissolution of the rock and soil by the solvent will result in the release of 226Rn [105]. There are two main ways radon is released: evaporation of wastewater or waste liquid pools and precipitation of 226Rn-content dust on the solid surface after uranium mine decommissioning [106,107]. The released content is mainly related to the concentration of 226Rn in groundwater and leachate and the scale of the mine [106,107]. As in the No. 737 mine, mining produced approximately 1.22 × 1011 Bq of radon per 1 t of uranium produced. During the production of the Qian IV uranium mine in the Qianjiadian Basin, the 226Rn released from the liquid collecting tank and the evaporation pond were 1.06 × 10−12 Bq/a and 1.48 × 10−7 Bq/a, respectively [100]. Due to the ventilation system and human management, radioactivity is typically controllable and will not cause a significant impact on the environment during production. However, for decommissioned uranium mines, the radioactivity of 226Rn from heap leaching of tailings waste may far exceed the environmental limit. Similar to tailing in Liaoning Province, the 226Rn radiation dose on the surface was approximately 3.14 Bq/(m2·s), which was far beyond the management limits [108]. Another tailing’s radiation dose exceeded the background value by 8.21 times but was only concentrated within 10 cm of the surface [109].

3.6. Plant Accumulation of Nuclides

Sandstone-type uranium deposits in northern China are mostly distributed in areas with poor soils. In these areas, most of the plants are herbs, such as Poaceae, Compositae or Leguminosae [110,111]. Since these plants are usually not used as food or economic crops and have good heavy metal tolerance, they have little impact on human or ecological health. Plants are usually used for geo-environmental restoration due to their good enrichment [82,112]. The U distribution in plants is usually root > stem > leaf [83,113]. Similar to tailing [109], the U content in plant roots was 0.18–42.7 mg/kg. For another tailing, the highest activity of uranium in plants in the mining area was 784.58 Bq/kg, 291 times higher than the background value. The content of heavy metals in plants was consistent with the distribution of the soil pollution halo. Additionally, affected by wind direction, the U content in plants behaves as follows: inside the mining area > southeast > northwest.

4. Mitigation of Environmental Impacts

4.1. Environmental Mitigation during Mining

With the increasing emphasis on environmental protection in recent years, various measures have been adopted in the ISL process to protect the geo-environment of mines. First, in terms of mining methods, an increasing number of mines use weak-alkaline leaching. CO2 + O2 solvent with mild chemical properties has begun to replace acid leaching [28]. In Ili, Ordos, Qianjiadian, etc., most of the mines use the weak-alkali method when the geological conditions allow [28,102]. Second, closed-loop mining methods were used in the mining processes. In Qianjiadian, a closed-loop treatment system of “leachate extraction-surface treatment-tail liquid treatment and reinjection” was adopted [89]. At the same time, an anti-seepage device was introduced into the production to treat wastewater, which basically achieved no wastewater discharge and no waste residue stacking [114]. Finally, the government usually formulated environmental norms in the mining process and made demands on land area, waste storage and treatment, quality and safety, etc. An environmental monitoring system must be established around the mine (Figure 10).

4.2. Environmental Mitigation after Mining

At present, the main objects of remediation and treatment of decommissioned uranium mines in China are tailings storage, abandoned wells, wastewater and vegetation restoration. The assessment indicators of remediation and treatment usually focus on radiation reduction [79]. For tailings storage and abandoned wells, removal, landfill and plugging are the main methods for treatment, while for wastewater, artificial excretion is the main method [78]. The impact of the repaired mine on the surrounding environment can be reduced to a negligible level. Similar to the No. 440 uranium mine in Northeast China [115], there were 11 tailings heaps, 30 abandoned wells and tunnels, and more than 100,000 tons of waste residue after decommissioning. The average radiation dose in the area before treatment was approximately 557 nGy/h, 5 times more than the background value. By removing the waste and covering the site with compacted soil, the average environmental radiation dose after treatment was 60.54 nGy/h, which was even better than the level before mining. In addition, at the No. 781 mine in Henan Province, after decommissioning, U in groundwater exceeds the background value by 4.6 times and that in soil by 82 times. After the polluted water and soil were removed, 1.3 m thick planting soil was used for covering. Due to these remediation measures, the content of radioactive pollutants in water decreased by 82%, and the content of U in soil decreased by 98%, reaching the level before mining.

5. Environmental Management for Prospective Mining in the Future

5.1. Environmental Management during Uranium Mining

As described above, the pollution of uranium ISL is mainly concentrated in groundwater. In view of the difficult treatment of groundwater after pollution, it is necessary to strengthen groundwater protection during the mining process [77]. Measures can be taken from the following three phases. Before mining, the hydrogeological characteristics and chemical indicators of groundwater in the mining area should be investigated in detail, and the mine construction plan and decommissioning restoration goals should be formulated based on this. The drilling design of the mine should consider the groundwater flow direction and layout of wells from upstream to downstream, which can reduce the spread of the pollution halo to both sides. During operation, the volume of leaching solution extraction should be considered larger than the solvent injection. This can lead to negative pressure in the mining area and prevent pollutants from spreading outward. After the mine is exhausted, a monitoring system should be set up in the surrounding area, especially downstream. A periodic monitoring plan should be formulated to determine the range of pollution halo diffusion.

5.2. Environmental Management for Uranium Mine Closing

At present, the mainstream remediation methods of decommissioned uranium mines are to ensure the nonproliferation of pollution. Physical methods are used to isolate the contaminated area to prevent further damage to the nearby environment. However, U itself has not been repaired or removed, which should be considered in the future, especially under complex conditions. Conventional physical removal and chemical leaching are usually expensive and can easily cause secondary damage to the environment; phytoremediation is economical and environmentally friendly but is inefficient and limited by the capacity of enrichment [109,116,117]. These factors make it difficult to commercialize such methods on a large scale. Three methods can be considered for future application. The first is electric repair. Through electrodialysis, electromigration and other electrodynamic effects, uranium compounds migrate along the direction of the electric field and accumulate at the electrode, and then they can be removed by physicochemical methods (adsorption, ion exchange, precipitation, etc.) [75,84]. Especially in an acidic environment with a high iron content, the removal efficiency can reach 58–88% [75,84]. Second, microbial remediation can be considered. As a major participant in the biogeochemical cycle, microorganisms can reduce U(VI) to insoluble U(IV) precipitation through biological reduction, which can retard uranium migration in the environment [116,117]. Similar microbial reduction processes also exist in many uranium mineralization processes [118]. For example, with ethanol as the electron donor, the uranium concentration in groundwater can be reduced by more than 99.9% by microbial reduction [119]. Finally, for heavily polluted sites, PRB can be used for remediation. By placing the PRB wall downstream of the pollution halo, U can be removed by reacting with the medium in the wall. For the use of PRB, material selection is a challenge, such as zero-valent iron, activated carbon, slaked lime, quartz sand, etc. [120], which are all very promising materials but still need to be further verified in field trials.

6. Conclusions

In the context of China’s carbon peak and carbon neutrality, the development of nuclear energy will reach its peak in the next few decades. As uranium is the main raw material for nuclear energy, environmental problems caused by mining should be taken seriously by government agencies. Sandstone-type uranium in northern China is mainly distributed in the Songliao Basin, Erlian Basin, Ordos Basin, Turpan–Hami Basin and Ili Basin from east to west. Most of the deposits are located in arid or semiarid regions with poor aquifer permeability, slow groundwater flow velocity and low hydraulic gradient. The type of uranium ore is usually dominated by pitchblende and is accompanied by pyrite, hematite, coal and organic matter. The main form of U is UO2 in ore and uranyl carbonate in groundwater. In situ leaching is the main method of sandstone-type uranium mining. The dissolution of the rock by the solvent will increase the porosity in a short period of time. However, as leaching continues, the rock crevice becomes clogged by metal precipitation, resulting in a decrease in porosity. Moreover, dissolution also leads to the release of other enriched metals, which cause groundwater pollution. The groundwater pollution in the mining area is mainly SO42−, Fe and U. Other elements, such as Ca, Mg, NO3− and TDS, also have a significant increase. The pollution halo mainly diffuses along the groundwater flow direction. Due to chemical clogging, the diffusion range usually does not exceed 1000 m. Compared with groundwater, soil pollution is often much lighter. Soil chemical anomalies usually come from heap leaching and wind erosion of tailings. The heavy metals in the tailings can spread for several kilometers by wind, but they are mainly concentrated within 20 cm of the surface. Finally, uranium mining and heap leaching also cause radiation anomalies and radioactive enrichment in plants. Through treatment, decommissioned mines can have a negligible impact on the surrounding environment. Treatment methods are mainly physical partitioning, and pollutants have not been removed. At present, the attention and treatment of uranium tailings are still in their infancy. In the future, scholars should focus on the impact of decommissioned mines on the environment, including collecting more data for treatment assessment and providing statistical data support for environmental recovery. Economic and feasible methods should be widely reviewed to assess their practicality, such as electric remediation, microbial remediation or PRB, which are worth considering.

Author Contributions

All authors contributed to the study’s conception and design. Material preparation, data collection and analysis were performed by Y.T. and Y.Z.; The draft of the manuscript was written by F.Z.; Figures and tables were made by J.H. and R.Z.; Proofreading and perfecting of the manuscript were performed by J.D. All authors commented on previous versions of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by The National Natural Science Foundation of China (41831283 and 42077170), and The National Key Research and Development Program of China (2020YFC1806604).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors have no relevant financial or non-financial interest to disclose.

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Water 15 01244 g001 550
Figure 1. Distribution of sandstone-type uranium deposits in northern China.
Figure 1. Distribution of sandstone-type uranium deposits in northern China.
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Figure 2. Stratigraphic chronology of stone-type uranium deposits in northern China (a) and a sketch of the oxidation-reduction zone (b) in the metallogenic belt. T-H means Turpan-Hami Basin.
Figure 2. Stratigraphic chronology of stone-type uranium deposits in northern China (a) and a sketch of the oxidation-reduction zone (b) in the metallogenic belt. T-H means Turpan-Hami Basin.
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Figure 3. Aquifer and interlayer oxidation in the metallogenic belt.
Figure 3. Aquifer and interlayer oxidation in the metallogenic belt.
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Figure 4. Statistics of the main trace elements and ∑REEs in each ore zone (10−6) (a) for Qianjiadian in the Songliao Basin; (b) for Dongsheng in the Ordos Basin; (c) for Shihongtan in the Turpan-Hami Basin; and (d) for the southern edge of the Ili Basin [38,41,54,75,76,77,78].
Figure 4. Statistics of the main trace elements and ∑REEs in each ore zone (10−6) (a) for Qianjiadian in the Songliao Basin; (b) for Dongsheng in the Ordos Basin; (c) for Shihongtan in the Turpan-Hami Basin; and (d) for the southern edge of the Ili Basin [38,41,54,75,76,77,78].
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Figure 5. Representative constant elements of groundwater in the ore belt (a) for the Songliao Basin; (b) for the Ordos Basin; (c) for the Turpan-Hami Basin; and (d) for the Ili Basin. The pH is unitless, and * represents μg/L [14,75,79,80,81,82,83,84,85,86,87,88,89,90,91].
Figure 5. Representative constant elements of groundwater in the ore belt (a) for the Songliao Basin; (b) for the Ordos Basin; (c) for the Turpan-Hami Basin; and (d) for the Ili Basin. The pH is unitless, and * represents μg/L [14,75,79,80,81,82,83,84,85,86,87,88,89,90,91].
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Figure 6. Soil pollution under wind erosion and precipitation leaching.
Figure 6. Soil pollution under wind erosion and precipitation leaching.
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Figure 7. The statistical data of the main pollution ions in the acid-leaching (a) and CO2-leaching (b) mining areas [89,90,91,100].
Figure 7. The statistical data of the main pollution ions in the acid-leaching (a) and CO2-leaching (b) mining areas [89,90,91,100].
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Figure 8. Chemical clogging during ISL. (a) Shows that chemical clogging occurs in space, while (b) shows that chemical clogging occurs over time; the red line shows the porosity changes; a positive value on the vertical axis indicates an increase, while a negative value indicates a decrease; (c) shows the yellow blockage in the gray rock, and the sample was collected in a decommissioned uranium mine in Northwest China.
Figure 8. Chemical clogging during ISL. (a) Shows that chemical clogging occurs in space, while (b) shows that chemical clogging occurs over time; the red line shows the porosity changes; a positive value on the vertical axis indicates an increase, while a negative value indicates a decrease; (c) shows the yellow blockage in the gray rock, and the sample was collected in a decommissioned uranium mine in Northwest China.
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Figure 9. Schematic view of the groundwater pollution halo. The black square delineates the range of the mine, and the dots indicate the mining wells. (a) shows the pollution halo of primary high-concentration pollutants, such as SO42−, Fe, and U; (b) shows the horizontal pollution halo of low-concentration pollutants, such as Cl−, NO3− and other heavy metals; (c) shows the vertical pollution halo; the data were interpolated from the measurement of a decommissioned uranium mine in Northwest China.
Figure 9. Schematic view of the groundwater pollution halo. The black square delineates the range of the mine, and the dots indicate the mining wells. (a) shows the pollution halo of primary high-concentration pollutants, such as SO42−, Fe, and U; (b) shows the horizontal pollution halo of low-concentration pollutants, such as Cl−, NO3− and other heavy metals; (c) shows the vertical pollution halo; the data were interpolated from the measurement of a decommissioned uranium mine in Northwest China.
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Figure 10. A demonstration security management system of ISL ore mine in Ordos.
Figure 10. A demonstration security management system of ISL ore mine in Ordos.
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Table
Table 1. Hydrochemical baseline of the representative ore belt in each region.
Table 1. Hydrochemical baseline of the representative ore belt in each region.
AreaLocationWater TypeHydrochemical TypepHTDS (mg/L)Uranyl TypesConcentration of U (μg/L)References
Songnen BasinQianjiadianConfined groundwater of Yaojia FmHCO3· Cl-Na, Cl·HCO3-Na7.15–7.793.64–4.12UO2(CO3)34−, UO2(CO3)22−130–312[48,61]
Erlian BasinYihegaoleBedrock fissure waterHCO3·SO4-Na·Mg, HCO3·SO4-Na·Ca7.12–10.87<2000UO2(CO3)34−, UO2(CO3)22−0.02–244[62]
Ordos BasinNorthern basinSurface water/9.31–9.32412–3710/1.14–1.18[47]
Jurassic confined waterSO4·Cl-Na8.58–12.34157–1060UO2(OH)3−, UO2(CO3)34−, UO2(OH)20.44–1120
Turpan-Hami BasinSouthern ShihongtanBedrock fissure waterCl·SO4-Na, Cl-Na, SO4-Ca·Mg8.02–8.258250–12,160UO2(CO3)34−, UO2(CO3)22−44.5–4237[51,52]
Ili BasinSouth edge of the basinJurassic interlayer porous confined waterHCO3-Ca, HCO3-Ca·Na, HCO3·SO4-Na·Ca, SO4·HCO3-Ca·Na, SO4·Cl-Na·Ca7.8–8.550–2790UO2(CO3)34−, UO2(CO3)22−10–1200[50,55]
Table
Table 2. Geochemical baselines of representative ore belts in each region.
Table 2. Geochemical baselines of representative ore belts in each region.
AreaLocationOre TypesGrade (%)Lithic Fragments Ratio (%)Carbonate Content (%)Organic Matter Content (%)Associated MineralsEnriched Trace ElementsReferences
Songnen BasinQianjiadianCoarse brownish red feldspathic sandstone, fine–coarse gray sandstone, red mudstone0.01–0.03902Ave: 0.2
Max: 1.0		Pyrite, sideriteV, Cr, Co, Ni, Zn[26,36,66,68]
Erlian BasinNuhetingDark gray silty mudstones, siltstone0.03–1.060–901.90.1–0.82PyritV, Mo, Cu, Pb, Zn, Se, Ni[34,63]
Ordos BasinNalinggouRed, green and gray mudstones, siltstones and sandstones0.01–0.0580–902.67Ave: 0.5
Max: 5–10		Coal, pyrite, chalcopyriteRb, Ba, Th, K, Pb[28,65,70]
Turpan-Hami BasinShihongtanGray white, light gray, light brown, yellow sandstone0.01–0.0920–502–30.1–1.5PyriteV, Se, Mo, Re[56,67,71]
Ili BasinSouth edge of the basinGritstone, gravel sandstone, sandstone, siltstone, mudstone, and coalstone0.01–0.246–795–250.1–2.8Pyrite, coalAs, Ag, Sb, Se, Fe, S, Mo[64,69,72]
Table
Table 3. Hydrochemical anomalies in some representative mining areas.
Table 3. Hydrochemical anomalies in some representative mining areas.
LocationMining MethodpHConcentration of Main Abnormal Elements
(Background Value, mg/L)		Other Abnormal IndicesReferences
Ca2+Mg2+USO42−Fe
Ili Basin Mine No. 737, 738 and 739Acid
(Decommissioned)		/360
(17)		252
(1)		3.9 (<0.1)3920 (127)275 (<1)K, Na, Re, Th, Mn, Cu[99]
Inner MongoliaCO2 + O27.62 (8.76)16.10 (5.24)8.17 (3.35)43.05 (0.14)272 (64)0.09
(0.09)		TDS, HCO3−, Cl−[90,91]
Ili Basin
(A mixed leaching mine)		Acid1.7376634211.215222363NO3−, K, NaAl, Cu, Zn, Cd, Pb, Th, As[101]
CO2 + O27.32 (8.32)342 (78)99 (13)60.0 (<0.1)1139 (127)0.04 (0.15)
Pollution halopHCa2+Mg2+USO42−FeNO3−Cl−[102]
Ili BasinVertically adjacent aquifers7.55
(8.04)		104.3
(78)		23.8
(0.03)		0.03
(/)		216.8
(83.5)		3.13
(60)		0.73
(0.84)		75.1
(24.8)
Upstream7.4391.528.60.026202.42.382.8142.35
Mining area3.42266.5232.626.284029.8209.83111.0474.67
200 m downstream7.03183.252.160.315606.52.1344.49185.35
1000 m downstream7.581.627.360.054109.40.570.0652.48
350 m from the side7.37121.133.010.022414.51.130.0677.25
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Zheng, F.; Teng, Y.; Zhai, Y.; Hu, J.; Dou, J.; Zuo, R. Geo-Environmental Models of In-Situ Leaching Sandstone-Type Uranium Deposits in North China: A Review and Perspective. Water 2023, 15, 1244. https://doi.org/10.3390/w15061244

AMA Style

Zheng F, Teng Y, Zhai Y, Hu J, Dou J, Zuo R. Geo-Environmental Models of In-Situ Leaching Sandstone-Type Uranium Deposits in North China: A Review and Perspective. Water. 2023; 15(6):1244. https://doi.org/10.3390/w15061244

Chicago/Turabian Style

Zheng, Fuxin, Yanguo Teng, Yuanzheng Zhai, Jingdan Hu, Junfeng Dou, and Rui Zuo. 2023. "Geo-Environmental Models of In-Situ Leaching Sandstone-Type Uranium Deposits in North China: A Review and Perspective" Water 15, no. 6: 1244. https://doi.org/10.3390/w15061244

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