华北平原高氟地下水中稀土元素分布和分异特征

高氟地下水是世界各国研究者广泛关注的重大环境问题。尽管对高氟地下水的化学特征、形成机理和扩散机制等已有不少研究,但其稀土元素(REE)的含量和分异特征以及这些特征能否反映高氟地下水的形成和分布尚不清楚,这在一定程度上限制了REE在高氟地下水中的运用。本研究以地下水氟离子异常严重地区——华北平原为研究区,沿地下水流向采集浅层和深层地下水样,研究分析了水中氟离子和REE的地球化学特征。浓度分析结果表明地下水氟离子浓度介于0.28 mg/L和9.33 mg/L之间,其中55%超出我国饮用水标准规定值1.0 mg/L;PHREEQC计算结果反映地下水中氟以NaF、CaF⁺、MgF⁺和自由态F^-形式存在,其中自由态F^-含量占主导(85.42%99.39%);高氟地下水主要分布于中部冲积湖积平原以及东部冲积海积平原,60%高氟地下水样分布在180 m深度以下;水化学图件分析结果指示浅层高氟地下水的形成主要受蒸发浓缩作用的控制,而深层高氟地下水是水岩相互作用下的矿物溶解和离子竞争吸附共同作用的结果。研究区地下水REE含量处于pmol/L至nmol/L级别,PHREEQC模拟计算结果表明REE主要以碳酸络合物( {\mathrm{REECO}}_3^{+}和$REE(CO_{3})_{2}^{-})$的形式存在,与氟离子络合的稀土元素(REEF²⁺和 {\mathrm{REEF}}_2^{+})占01.18%;上陆壳(UCC)标准化结果显示,所有地下水均呈重REE(HREE)和中REE(MREE)相对于轻REE(LREE)富集的模式,且具有显著Ce负异常(0.11* = Ce_UCC/(La_UCC×Pr_UCC)^0.5<2.29)特性;地下水富HREE主要归因于HREE比LREE优先与碳酸根络合,并且形成更加稳定的碳酸络合物。沿地下水流向,深层地下水中总REE含量与地下水中氟浓度均呈现不断上升的变化趋势,同时高氟地下水比低氟地下水更易富集重稀土元素,说明稀土元素对深层含水层富氟行为具有一定的指示作用。

Distribution and fractionation of rare earth elements in high fluoride groundwater from the North China Plain

High-fluoride groundwater is a major environmental problem that has caused global concerns. Although numerous studies have been done on the chemical characteristics, formation and migration mechanisms of high-fluoride groundwater, little is known about the concentrations and fractionation characteristics of rare earth elements (REE) in high-fluoride groundwater or if REE can be used as a tracer for studying the formation and distribution of high-fluoride groundwater. These uncertainties have limited REE application in high-fluoride groundwater research. In this paper, we investigated the fluoride and REE concentrations and distributions in groundwater collected along a flow path in the North China Plain (NCP) where strong fluoride anomalies in groundwater were observed. We found the groundwater fluoride concentrations ranged from 0.28 to 9.33 mg/L, with 55% groundwater exceeding the China drinking water standard (1 mg/L). According to PHREEQC calculation, fluorine occurs in groundwater as NaF, CaF⁺, MgF⁺ and, predominantly, F^-(85.42%-99.39%). High-fluoride groundwater is mainly distributed in the central alluvial lacustrine plain and the eastern alluvial marine plain, with 60% at depths below 180 m. Hydrochemical analysis indicated the formation of shallow high-fluoride groundwater is mainly controlled by evaporation and concentration, while deep high-fluoride groundwater is a result of mineral dissolution and competitive ion adsorption through water-rock interactions. The groundwater REE concentration is at pico to nanomolar level, and, according to PHREEQC calculation, REE species are mainly carbonate complexes ( {\mathrm{REECO}}_3^{+} and $REE(CO_{3})_{2}^{-})$, with 0-1.18% REE in complex with F^-(REEF²⁺ and {\mathrm{REEF}}_2^{+}). The upper continental crust (UCC)-normalized REE patterns are characterized by enrichments of heavy REE (HREE) and middle REE (MREE) over light REE (LREE) and have significant negative Ce anomalies (0.11*=Ce_UCC/(La_UCC×Pr_UCC)^0.5<2.29). The HREE enrichment in groundwater is mainly attributed to the preferential complexation of HREE over LREE or HREE with carbonate in forming more stable carbonate complexes. Along a groundwater flow path, both REE and fluoride concentrations in deep aquifers generally increase following similar trends. Besides, high-fluoride groundwater is more prone to HREE enrichment. These findings suggest that REE can potentially be used as a fluoride indicator for studying fluoride enrichment in natural aquifer systems.