Environmental impact of the abandoned coal mines on the surface water and the groundwater quality in the south of Bochum,Germany

Surface water and groundwater samples were collected from 20 locations, situated in the vicinity of the abandoned coal mine fields south of Bochum. The main objective of this research is to assess the environmental impacts of these mines on the surface water and groundwater quality as well as to determine the factors controlling these impacts. The water samples were collected from stream sources, groundwater, surface water and engineered channels during April 2011. Physicochemical parameters were measured during fieldwork. Water samples were analysed for major ions and aluminium, iron, manganese, ferrous iron, zinc and hydrogen sulphide. The hydrochemistry of the surface water and the groundwater of this area is characterized by near-neutral to alkaline conditions, represented by predominance of calcium bicarbonate and sometimes calcium sulphate water types. Hence, the surface water and the groundwater quality in this region is significantly affected by abandoned coal mines. These effects resulted from oxidation of iron disulphide minerals that release iron, sulphate and hydrogen. The presence of carbonate-rich materials, which contained within the landfilling materials, has led the releasing of calcium, magnesium and bicarbonate. These materials could be the main source responsible for raising the alkalinity of the affected water. The environmental hazard of the abandoned coal mines in this area is related to the high concentration of Fe especially in the groundwater that possess the highest Fe concentration compared to other water sources: the Fe is 18 times larger than the allowed value in drinking water. Significant spatial variations of the water pollution were noted in this study. For this reason, the environmental hazards of the abandoned coal mines in Germany should be considered at closure of coal mines in the near future. Otherwise, these mines will be sources of environmental threats unless all necessary measures are taken to reduce their impact.     

Assessment of environmental hazards at abandoned mining sites: A case study in Sardinia,Italy

In the last two centuries, several Pb–Zn mines were active in the Rio Mannu basin near Narcao (SW Sardinia, Italy), but are now abandoned. These abandoned mines pose a serious risk to the population and required an assessment of the hazard sources and the contaminant pathways in the area. The characterization plan of the Rio Mannu basin was carried out according to Italian protocols. Specifically, samples of soil, stream sediment, fine-grained ore-processing waste (from washery and flotation processes), surface water and groundwater have been analyzed in order to assess the levels of contamination in each mine area and the risks in the plains surrounding the site. Several potential chemical contaminants were considered both in solid materials and water samples. The Rosas mine was recognized as the most contaminated area in the basin, due to the presence of a large tailing pond, some fine-grained waste piles and two mine adits with concentrations of the toxic elements As, Cd, Cu, Pb, Sb and Zn exceeding Italian regulatory standards. The dispersion of contaminants occurs downstream from the small stream draining the area. In case of heavy rain the runoff into the streamlet transports the contaminated material far into the plain. The results of this study show that the characterization plan is a relatively cheap tool for establishing mitigation actions, prior to the realization of a complete, and usually expensive, remediation project at abandoned mine sites. Urgent recommended actions in the Rio Mannu basin include the treatment of the adit water prior to its discharge into the Rio Barisonis; the construction of drainage barriers on waste piles to reduce runoff and solid transport into the Rio Barisonis; the consolidation of the earthen dam containing the tailing pond of Rosas to avoid the deepening of erosion furrows that may threaten its stability.     

Domestic water supply impacts by underground coal mining in Virginia, USA

 Underground coal mining can affect wells and springs used as water supplies. In Virginia, concerns over such impacts are felt by both surface owners and coal-mining firms. Virginia's geologic history has caused faults and fractures to be common in its coalfield region, relative to other Appalachian coal-mining areas. The results of 73 investigations of alleged domestic water supply impacts by underground mining were analyzed; the investigations were conducted by the Virginia Division of Mined Land Reclamation (VDMLR). This analysis was conducted with reference to guidelines that define a primary zone of underground mining influence where dewatering of aquifers is to be expected. The VDMLR data set included 27 investigations of alleged water supply impacts by partial-extraction room-and-pillar mines, 41 investigations of high-extraction room-and-pillar mines, and 4 investigations of longwall mines. VDMLR investigators found that 14 of 16 water supplies within the primary zone of influence were likely to have been affected by pillar-retreat mining, but no water supplies within the primary zone of influence for longwall and room-and-pillar mines were represented in the data base. VDMLR investigators found 42 of 56 water supplies outside of the primary zones were likely to have been affected by mining; these cases represented room-and-pillar, pillar-retreat, and longwall mining. Geologic circumstances not directly related to subsidence were found to be responsible for 31 of these 42 impacts. These geologic circumstances included subsurface fractures and other geologic features acting as aquifers that were drained by underground mining operations. VDMLR investigators also found some of the investigated water losses to have been caused by factors other than mining. These results demonstrate the inherent difficulties of any attempt to rigidly define a "zone of underground mining impact" based solely on mine subsidence effects, especially in regions where geologic faults and fractures are common such as the southwest Virginia coalfield. Received: 4 August 1995 · Accepted: 23 October 1995     

Using stable isotopes (δD, δ<Superscript>18</Superscript>O, δ<Superscript>34</Superscript>S and <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr) to identify sources of water in abandoned mines in the Fengfeng coal mining district,northern China

With depleted coal resources or deteriorating mining geological conditions, some coal mines have been abandoned in the Fengfeng mining district, China. Water that accumulates in an abandoned underground mine (goaf water) may be a hazard to neighboring mines and impact the groundwater environment. Groundwater samples at three abandoned mines (Yi, Er and Quantou mines) in the Fengfeng mining district and the underlying Ordovician limestone aquifer were collected to characterize their chemical and isotopic compositions and identify the sources of the mine water. The water was HCO₃·SO₄-Ca·Mg type in Er mine and the auxiliary shaft of Yi mine, and HCO₃·SO₄-Na type in the main shaft of Quantou mine. The isotopic compositions (δD and δ¹⁸O) of water in the three abandoned mines were close to that of Ordovician limestone groundwater. Faults in the abandoned mines were developmental, possibly facilitating inflows of groundwater from the underlying Ordovician limestone aquifers into the coal mines. Although the Sr²⁺ concentrations differed considerably, the ratios of Sr²⁺/Ca²⁺ and ⁸⁷Sr/⁸⁶Sr and the ³⁴S content of SO₄^2? were similar for all three mine waters and Ordovician limestone groundwater, indicating that a close hydraulic connection may exist. Geochemical and isotopic indicators suggest that (1) the mine waters may originate mainly from the Ordovician limestone groundwater inflows, and (2) the upward hydraulic gradient in the limestone aquifer may prevent its contamination by the overlying abandoned mine water. The results of this study could be useful for water resources management in this area and other similar mining areas.     

Environmental tracers as indicators of karst conduits in groundwater in South Dakota,USA

Environmental tracers sampled from the carbonate Madison aquifer on the eastern flank of the Black Hills, South Dakota, USA indicated the approximate locations of four major karst conduits. Contamination issues are a major concern because these conduits are characterized by direct connections to sinking streams, high groundwater velocities, and proximity to public water supplies. Objectives of the study were to estimate approximate conduit locations and assess possible anthropogenic influences associated with conduits. Anomalies of young groundwater based on chlorofluorocarbons (CFCs), tritium, and electrical conductivity (EC) indicated fast moving, focused flow and thus the likely presence of conduits. δ¹⁸O was useful for determining sources of recharge for each conduit, and nitrate was a useful tracer for assessing flow paths for anthropogenic influences. Two of the four conduits terminate at or near a large spring complex. CFC apparent ages ranged from 15 years near conduits to >50 years in other areas. Nitrate-N concentrations >0.4 mg/L in groundwater were associated with each of the four conduits compared with concentrations ranging from <0.1 to 0.4 mg/L in other areas. These higher nitrate-N concentrations probably do not result from sinking streams but rather from other areas of infiltration.     

瞬变电磁法在煤矿水文地质灾害调查中的应用

以探测某地小煤窑遗留的采空区为例,探讨了瞬变电磁法在探测采空积水区的应用效果。选用同一回线装置形式,结合矿区地质条件,具体分析了采空积水区的地球物理异常特征。结果表明,该方法在煤矿水文地质灾害调查方面具有分辨率高、经济快速、准确有效等优势,能够满足煤矿安全生产的要求。     

Laboratory testing of trace metals removal from mine drainage and arsenic removal from groundwater in the Black Hills of South Dakota

The Gilt Edge Superfund Site is a former heap-leach gold mine that currently is being remediated in the Black Hills of South Dakota. Mine runoff water is treated before release from the site. The field pH, before treatment, is about 3; the water contains arsenic at low levels and some trace metals at elevated levels, in addition to total dissolved solids concentrations of more than 1,900 mg/L. In the Keystone area of the Black Hills, naturally occurring arsenic has been detected at elevated concentrations in groundwater samples from wells. The City of Keystone’s Roy Street Well, which is not used currently, showed arsenic concentrations of 36 parts per billion and total dissolved solids of 320 mg/L. With field samples of water from the Gilt Edge site, a limestone-based method was successful in reducing trace metals concentrations to about 0.001 mg/L or less; at the Keystone site, the limestone method reduced arsenic levels to about 0.006 mg/L. The results are significant because previous research with the limestone-based method mainly had involved samples prepared with distilled water in the laboratory, in which interference of other ions such as sulfate did not occur. The research indicates the potential for broader applications of the limestone-based removal method, including scale-up work at field sites for water treatment.     

废弃矿井地下水淹没过程的水流与水位数值模拟

定量分析和研究废弃矿井淹没过程的水流特征对于预测和防止废弃矿井地下水淹没过程中诱发的次生灾害具有重要意义。建立了废弃矿井淹没的多种介质水流数学模型,得出了淹没补给水量随时间、矿井内淹没水位和补给水源含水层水位之间的相关关系,建立了淹没补给水量与相关影响因素之间的函数关系式,完成了矿井淹没过程中涌水量实时计算的FEFLOW模块设计和开发。进行了近水平煤层开采后废弃矿井充水淹没水位回弹过程的数值模拟计算,得到了不同充水介质在充水淹没过程中的水流和水位回弹特征。结果显示,FEFLOW二次开发模块运算效果良好,具有实用价值。      

废弃煤矿瓦斯资源估算与评价方法构建及应用

基于我国废弃煤矿瓦斯资源勘探开发现状,为合理评估我国废弃煤矿瓦斯资源,提出"资源估算+初步评价"的思路,建立废弃煤矿瓦斯资源估算和评价方法,该方法具有所需参数少和实用性强等优点;依据废弃煤矿瓦斯涌出特点及废弃时间,划分废弃煤矿类型,将其分为稳定型和非稳定型两大类,非稳定型进一步划分为初期、中期和后期3个阶段;根据废弃煤...     

沁水盆地南部潘庄区块废弃矿井煤层气地球化学特征及成因

为了研究废弃矿井中煤层气成因,以沁水盆地南部潘庄区块废弃矿井为例,抽采废弃矿井中煤层气并进行化学组分和同位素测试,并采集部分废弃矿井水样品测试水中离子浓度、pH值等进行研究。结果表明:潘庄区块废弃矿井中煤层气CH₄体积分数平均值为91.99%,CO₂为1.26%,N₂为6.73%;甲烷碳同位素（δ13C₁）值为-31.36‰~-33.53‰,平均-32.25‰,氢同位素（δD）值为-182.76‰~-193.20‰,平均-187.538‰。废弃矿井排采水中阴阳离子主要为Mg2+、K+、HCO₃-、Cl-、Na+、SO₄2-和NO₃-等,产出水型为Mg-（HCO₃）₂型,表明矿井水受到地表水的强烈影响。废弃矿井中煤层气主要以热成因气为主,少量次生生物气。与附近未开采煤储层相比,研究区废弃矿井中的环境更有利于次生生物气的生成。      

The impact of rockfalls on dwellings during the 2011 Christchurch,New Zealand,earthquakes

Rockfalls and debris avalanches triggered by earthquakes during the 2010–2011 Canterbury earthquake sequence killed five people and caused an estimated US$400 million in damages. In total, about 200 dwellings were directly impacted by some of the ~6000 rockfalls and debris avalanches that occurred across the Port Hills of Christchurch. This research presents the results of the analysis of a high-quality database of 61 individual rockfall impacts on 29 dwellings in the Port Hills of Christchurch, New Zealand. Dwellings in the Port Hills are typically simple timber-frame structures with wooden or unreinforced masonry cladding, comparable to most dwellings across New Zealand, North America, Australia, and elsewhere. Rockfall impacts on dwellings in this study were observed to follow a power law relationship between kinetic energy and (1) the runout distance into and through the dwelling and (2) the impacted area within the dwelling. The results have been quantified and are presented as a damage proportion, which is defined as the proportion of the area affected by an individual rock block inside the dwelling divided by the total area of the dwelling. These data provide a fundamental input for rockfall risk analysis and will allow the losses from rockfall impacts to be better constrained.     

Geochemical approach to estimate the quality of water entering abandoned underground coalmines

Geochemical modeling was employed to estimate the chemistry of water infiltrating into abandoned underground flooded and unflooded coalmines of the Uniontown syncline, Fayette County, Pennsylvania. This was done to help evaluate factors governing the long-term evolution of coalmine discharge water quality in the Uniontown syncline area. The subsurface structure and lithology was delineated using borehole, mine shaft, and stratigraphic information. Hydrogeologic analyses indicated that most of the recharge to mine voids occurs in the zones of shallow overburden cover of less than 20 m in thickness. The water–rock contact period in the recharge areas of the overburden was estimated to be 5 days or greater for the flooded mines, and a day or less for the unflooded mines. Flow-through reaction-path models were applied to the topsoil and shale–sandstone lithological units identified in the recharge areas. The model predicted water entering the flooded mines at a pH of 6.65, alkalinity of 6.92 mequiv. l^–1 and a total sulfate concentration of 7.33 mM, and the unflooded mines at a pH of 6.68, alkalinity of 6.99 mequiv. l^–1 and a total sulfate concentration of 3.08 mM. The model predictions for the flooded and unflooded mines are consistent with groundwater data from the study site, indicating the usefulness of this approach in evaluating the contribution of overburden chemistry to the evolution of mine discharge quality.     

矿山地质环境影响评价及修复研究——以汶川某废弃露天矿山为例

矿山正常的生产经营活动势必形成一定的矿山地质环境问题。为全面贯彻生态文明思想,牢固树立“绿水青山就是金山银山”的理念,按照恢复生态、兼顾景观总体要求,因地制宜、多措并举,扎实开展废弃露天矿山地质环境修复。依据矿山地质环境保护与恢复的相关规范与要求,以汶川某废弃露天矿山为例,从矿山环境角度介绍了矿山地质环境的影响因素,开展矿山斜坡整体稳定性、采场边坡稳定性、弃渣堆稳定性及地形地貌景观破坏等地质环境影响评价,划分治理重点、次重点和一般3个基本的矿山地质环境保护与恢复防治区。结果表明: 矿山斜坡及弃渣堆现状基本稳定、采场边坡稳定性较差、地形地貌景观影响和破坏程度中等; 研究区矿山地质环境保护与恢复治理可划分为一般防治区和较严重防治区,其中一般防治区面积约554 m²(矿区无因采矿活动诱发的地质灾害,影响区内无居民居住,无威胁对象,矿区及周围地表水体未漏失,矿业活动对水土环境影响程度为影响程度小,未影响到矿区及周围生活供水)、较严重防治区面积186 m²(矿区与影响区地质灾害发生可能性小,地形地貌景观影响和破坏程度较严重)。针对评价结果提出的“坡面浮石清理+清除建筑垃圾、块石+植被重建”等修复建议,为同类型矿山绿色开采及地质环境保护措施提供了的重要技术参考依据。     

山西省废弃矿井煤层气地面钻井开发关键问题与对策

随着我国煤炭去产能政策的有力实施,一批资源枯竭及产能落后矿井将陆续关停废弃。废弃矿井仍赋存着大量的煤层气资源,其开发利用是实现煤炭产业清洁安全高效低碳发展、促进煤矿安全生产、优化能源结构、实现温室气体减排等方面的重要举措。基于山西省煤基重点科技攻关(煤层气产业链)项目相关研究,系统阐述了废弃矿井煤层气开发面临着资源量评价不准、钻进体系不健全、井上下联合缺失等关键问题。针对这些问题提出以下几点对策:废弃矿井精准地质探测是采空区地面钻井轨迹设计的重要依据,尤其是炮采等落后采煤工艺的废弃矿井,地球物理勘探精度应达到米级才能有效降低钻遇煤柱风险;优选废弃矿井煤层气地面“L”型钻井思路,即选采空区周边一定距离的保安煤柱作为L型井位,并配套特殊钻进工艺;煤矿企业应将废弃矿井资源开发利用纳入煤矿全生命周期规划,尤其是矿井废弃前应确保煤层气抽采通道畅通,以实现煤层气井“一井多用”的新型井上下联合开采模式,提高废弃矿井煤层气开发效率;采用防回火、各种传感器等装置,并对关键参数设置自动报警停机界限值,从而使废弃矿井煤层气地面开采工艺安全、高效;对不同浓度废弃矿井煤层气,需要采取相应的梯级利用模式,从而提高整体开发利用价值。以山西省废弃矿井为示范区,研究认识对推动全国煤矿区废弃矿井煤层气开发利用具有重要的指导和示范意义。      

U–Pb ages of Neoarchean granitoids from the Black Hills, South Dakota, USA: implications for crustal evolution in the Archean Wyoming province

Zircons in basement rocks from the eastern Wyoming province (Black Hills, South Dakota, USA) have been analyzed by ion microprobe (SHRIMP) in order to determine precise ages of Archean tectonomagmatic events. In the northern Black Hills (NBH) near Nemo, Phanerozoic and Proterozoic (meta)sedimentary rocks are nonconformably underlain by Archean biotite–feldspar gneiss (BFG) and Little Elk gneissic granite (LEG), both of which intrude older schists. The Archean granitoid gneisses exhibit a pervasive NW–SE-trending fabric, whereas an earlier NE–SW-trending fabric occurs sporadically only in the BFG, which is intruded by the somewhat younger LEG. Zircon crystals obtained from the LEG and BFG exhibit double terminations, oscillatory zoning, and Th/U ratios of 0.6±0.3—thereby confirming a magmatic origin for both lithologies. In situ analysis of the most U–Pb concordant domains yields equivalent ²⁰⁷Pb/²⁰⁶Pb ages (upper intercept, U–Pb concordia) of 2559±6 and 2563±6 Ma (both ±2σ) for the LEG and BFG, respectively, which constrains a late Neoarchean age for sequential pulses of magmatism in the NBH. Unzoned (in BSE) patches of 2560 Ma zircon commonly truncate coeval zonation in the same crystals with no change in Th/U ratio, suggesting that deuteric, fluid-assisted recrystallization accompanied post-magmatic cooling. A xenocrystic core of magmatic zircon observed in one LEG zircon yields a concordant age of 2894±6 Ma (±2σ). This xenocryst represents the oldest crustal material reported thus far in the Black Hills. Whether this older zircon originated as unmelted residue of 2900 Ma crust that potentially underlies the Black Hills or as detritus derived from 2900 Ma crustal sources in the Wyoming province cannot be discerned. In the southern Black Hills (SBH), the peraluminous granite at Bear Mountain (BMG) of previously unknown age intrudes biotite–plagioclase schist. Zircon crystals from the BMG are highly metamict and altered, but locally preserve small domains suitable for in situ analysis. A U–Pb concordia upper intercept age of 2596±11 Ma (±2σ) obtained for zircon confirms both the late Neoarchean magmatic age of the BMG and a minimum age for the schist it intrudes. Taken together, these data indicate that the Neoarchean basement granitoids were emplaced at 2590–2600 Ma (SBH) and 2560 Ma (NBH), most likely in response to subduction associated with plate convergence (final assembly of supercontinent Kenorland?). In contrast, thin rims present on some LEG–BFG zircons exhibit strong U–Pb discordance, high common Pb, and low Th/U ratios—suggesting growth or modification under hydrothermal conditions, as previously suggested for similar zircons from SE Wyoming. The LEG–BFG zircon rims yield a nominal upper intercept date of 1940–2180 Ma, which may represent a composite of multiple rifting events known to have affected the Nemo area between 2480 and 1960 Ma. Together, these observations confirm the existence of a Paleoproterozoic rift margin along the easternmost Wyoming craton. Moreover, the 2480–1960 Ma time frame inferred for rifting in the Black Hills (Nemo area) corresponds closely to a 2450–2100 Ma time frame previously inferred for the fragmentation of supercontinent Kenorland.     

A method to mitigate acid-mine drainage in the Shamokin area,Pennsylvania, U.S.A.

The three anthracite coal fields of eastern Pennsylvania occur in large synclinal basins surrounded by sandstone hogback ridges. The streams draining the coal regions are heavily contaminated by sulfuric acid due to the weathering of pyrite in the abandoned mines and culm. Dams could be built in the water gaps of the streams leaving the basins, forming huge reservoirs. For example, in the western part of the Middle Coal Field (Shamokin-Mt. Carmel area), the construction of four dams would form a reservoir that would inundate practically all the culm banks, abandoned mines, and environmentally abused lands in this coal field. The reservoir (at approximately 410 m elevation) would be approximately 140 sq km in area and could be the source of water for pumped-storage hydroelectricity generation. The water should have moderately good quality and could serve the recreational needs of a vast population of the Middle Atlantic states.     

The Oued Mellègue: Mining activity,stream sediments and dispersion of base metals in natural environments,North-western Tunisia

In the Northwest of Tunisia, mining works have occurred in an area of vital water reserves and resources (Oued Mellègue and Oued Mejerda), which represent nearly 81% of total potential of water in the region. Demographic growth, insufficient drinkable water, agriculture and industrial plants are factors responsible for the strong demand of this resource. Water supply is done by an interconnected battery of dams built on the major watercourses, the first of which, the Mellègue dam, was erected in the 1960s. Nowadays, most of the mines are closed for almost two decades although one is still active: (Jerissa for Fe). It is important to emphasise the fact that there are millions of cubic metres of abandoned tailings, mainly from Pb–Zn–Ba mines, which are not surveyed, representing a serious threat to the environment.     

Abandoned mines drainage: impact assessment and mitigation of discharges from coal mines in the UK

The UK has a legacy of pollution caused by discharges from abandoned coal mines, with the potential for further pollution by new discharges as groundwaters continue to rebound to their natural levels. In 1995, the Coal Authority initiated a scoping study of 30 gravity discharges from abandoned coal mines in England and Scotland. Mining information, geological information and water quality data were collated and interpreted in order to allow a preliminary assessment of the source and nature of each of the discharges. An assessment of the potential for remediation was made on the basis of the feasibility and relative costs of alternative remediation measures. Environmental impacts of the discharges and of the proposed remediation schemes were also assessed. The results, together with previous Coal Authority studies of discharges in Wales, were used by the Coal Authority, in collaboration with the former National Rivers Authority and the former Forth and Clyde River Purification Boards, to rank discharge sites in order of priority for remediation.     

Arsenic and antimony contamination of waters,stream sediments and soils in the vicinity of abandoned antimony mines in the Western Carpathians,Slovakia

Environmental contamination with As and Sb caused by past mining activities at Sb mines is a significant problem in Slovakia. This study is focused on the environmental effects of the 5 abandoned Sb mines on water, stream sediment and soil since the mines are situated in the close vicinity of residential areas. Samples of mine wastes, various types of waters, stream sediments, soils, and leachates of the mine wastes, stream sediments and selected soils were analyzed for As and Sb to evaluate their geochemical dispersion from the mines. Mine wastes collected at the mine sites contained up to 5166 mg/kg As and 9861 mg/kg Sb. Arsenic in mine wastes was associated mostly with Fe oxides, whereas Sb was present frequently in the form of individual Sb, Sb(Fe) and Fe(Sb) oxides. Waters of different types such as groundwater, surface waters and mine waters, all contained elevated concentrations of As and Sb, reaching up to 2150 μg/L As and 9300 μg/L Sb, and had circum-neutral pH values because of the buffering capacity of abundant Ca- and Mg-carbonates. The concentrations of Sb in several household wells are a cause for concern, exceeding the Sb drinking water limit of 5 μg/L by as much as 25 times. Some attenuation of the As and Sb concentrations in mine and impoundment waters was expected because of the deposition of metalloids onto hydrous ferric oxides built up below adit entrances and impoundment discharges. These HFOs contained >20 wt.% As and 1.5 wt.% Sb. Stream sediments and soils have also been contaminated by As and Sb with the peak concentrations generally found near open adits and mine wastes. In addition to the discharged waters from open adits, the significant source of As and Sb contamination are waste-rock dumps and tailings impoundments. Leachates from mine wastes contained as much as 8400 μg/L As and 4060 μg/L Sb, suggesting that the mine wastes would have a great potential to contaminate the downstream environment. Moreover, the results of water leaching tests showed that Sb was released from the solids more efficiently than As under oxidizing conditions. This might partly explain the predominance of Sb over As in most water samples.