RARE EARTH ELEMENTS AS GEOCHEMICAL TRACERS OF REGIONAL GROUNDWATER MIXING
RARE EARTH ELEMENTS AS GEOCHEMICAL TRACERS OF REGIONAL GROUNDWATER MIXING
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DOI：
10.1016/s0016-7037(97)00177-4
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发表时间：
1997-09
影响因子：
5
通讯作者：
K. Johannesson;K. Stetzenbach;V. Hodge
中科院分区：
文献类型：
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作者：
K. Johannesson;K. Stetzenbach;V. Hodge
本文分析了内华达州中南部（即Ash Meadows国家野生动物保护区、Spring Mountains、Pahranagat国家野生动物保护区和内华达州试验场）地下水系统中的稀土元素（REE），以探讨其作为区域地下水流动示踪剂的潜在用途。先前使用保守示踪剂（如氘和铀同位素）的研究发现，当地山区的补给是区域排放带（即灰草甸）泉水的主要来源（占总流量的60-70%），其余贡献来自东北的盆地间流动。使用页岩归一化稀土模式对这些地下水进行的初始混合计算与先前的研究结果一致；然而，由于预计稀土元素在天然水中不会表现出保守的行为，因此在随后的混合计算中，研究了溶液络合作用（增强稀土元素在溶液中的稳定性）和表面络合作用（导致稀土元素的颗粒反应行为）的影响。为了评估溶液和表面络合的作用，通过评估离子强度校正后的co3β 1ree、co3β2REE和[CO32−]f与稀土元素第一水解结合常数的比值，估算了各地下水中稀土元素的相对分配系数。然后使用相对分配系数来计算由于溶液和表面络合作用而在溶液中形成和持续存在的REE模式。计算的稀土元素值与实际测量的稀土元素浓度非常接近，表明稀土元素实际上受地下水溶液和表面络合作用的控制。计算出的稀土浓度随后用于确定混合比，其结果与最初的计算结果以及先前的研究结果相吻合。研究结果表明，稀土元素在溶液中的络合作用足以在一定程度上克服其吸附在含水层表层的亲和力，从而在不同含水层的地下水溶液中形成并持续存在不同的稀土元素特征。溶液络合克服表面络合的能力可能与带负电荷的二碳酸配合物的形成有关[即Ln(CO3)2−，其中Ln是任何稀土]，在这些地下水中每种稀土元素中都占很大比例。
The rare earth elements (REE) were analyzed in a groundwater system from south-central Nevada (i.e., Ash Meadows National Wildlife Refuge, the Spring Mountains, Pahranagat National Wildlife Refuge, and the Nevada Test Site) in order to investigate their potential use as tracers of regional groundwater flow. Previous investigations using conservative tracers (e.g., deuterium and uranium isotopes) identified recharge in local mountains as the primary source (60–70% of total discharge) for the springs in the regional discharge zone (i.e., Ash Meadows) with the remaining contribution being interbasin flow from the northeast. Initial mixing calculations for these groundwaters using shale-normalized REE patterns agreed well with the previous studies; however, because the REEs are not expected to behave conservatively in natural waters, the effect of both solution complexation, which acts to enhance the stability of the REES in solution, as well as surface complexation, responsible for the particle reactive behavior of the REES, were examined in subsequent mixing calculations. In order to assess the roles of solution and surface complexation, relative partitioning coefficients were estimated for each REE in each groundwater by evaluating the ratio of the ionic strength correctedco3β1REE,co3β2REE, and [CO32−]Fto the first hydrolysis binding constants for the REES. The relative partitioning coefficients were then used to calculate REE patterns expected to develop and persist in solution as a consequence of solution and surface complexation. The calculated REE values closely resembled the actual measured REE concentrations, suggesting that the REEs are, in fact, controlled by solution and surface complexation in these groundwaters. The calculated REE concentrations were subsequently used to determine mixing ratios, the results of which coincided with the initial calculations as well as the previous studies. The results of this study suggest that solution complexation of the REEs is sufficient to overcome, to a certain degree, the affinity of the REEs to be adsorbed onto surface sites in the aquifers such that distinctive REE signatures develop and persist in solution in groundwaters from different aquifers. The ability of solution complexation to overcome surface complexation is likely related to the formation of the negatively charged dicarbonato complex [i.e., Ln(CO3)2−, where Ln is any REE], which accounts for significant fractions of each REE in these groundwaters.