The Geochemical Evolution of Riparian Ground Water in a Forested Piedmont Catchment
The Geochemical Evolution of Riparian Ground Water in a Forested Piedmont Catchment
复制标题
DOI：
10.1111/j.1745-6584.2003.tb02434.x
复制
发表时间：
2003-12-01
期刊：
影响因子：
2.6
通讯作者：
Schlosser, Peter
中科院分区：
文献类型：
--
作者：
Burns, Douglas A.;Plummer, L. Niel;Schlosser, Peter
在佐治亚州亚特兰大附近的帕诺拉山研究流域（PMRW）测定了河岸地下水中的主要风化反应及其速率。在河岸（腐岩）含水层中完成的19口浅井和在花岗岩中完成的1口井中，测量了地下水样品中主要溶质的浓度，并根据氯氟烃和氚/氦-3数据计算了每个样品的表观年龄。SiO2、Na+和Ca2+的浓度总体呈下降趋势，在靠近流域出口的钻孔中最高。这些溶质的浓度与地下水的表观年龄之间存在着很强的正相关关系。地下水的表观年龄在源头处是现代的（0到1年），在山谷中间是6到7年，在钻孔处是26到27年，位于源头下游500米处。化学演化的质量平衡模型表明，地下水化学的下游变化在很大程度上可以由斜长石到高岭石的风化作用来解释，钾长石、黑云母、角闪石和方解石的风化作用可能也有贡献。将地下水年龄与地球化学物质平衡模拟结果相结合，估算了风化反应的原位速率。斜长石的风化速率最高（约为6.4 μ mol/L/年），但由于地球化学演化过程中地下水暴露的矿物表面面积无法估计，因此无法与PMRW和其他地方发表的大多数长石风化结果进行比较。然而，对与地下水接触的矿物表面面积的初步估计提供了观察到的溶质浓度，表明本研究中计算的斜长石风化速率与PMRW先前研究中计算的速率相似，比以前发表的长石风化实验室研究慢三到四个数量级。河岸地下水地球化学演化的精确模型是准确模拟PMRW流域河流水地球化学演化的必要条件。
The principal weathering reactions and their rates in riparian ground water were determined at the Panola Mountain Research Watershed (PMRW) near Atlanta, Georgia. Concentrations of major solutes were measured in ground water samples from 19 shallow wells completed in the riparian (saprolite) aquifer and in one borehole completed in granite, and the apparent age of each sample was calculated from chloroflourocarbons and tritium/helium-3 data. Concentrations of SiO2, Na+, and Ca2+ generally increased downvalley and were highest in the borehole near the watershed outlet. Strong positive correlations were found between the concentrations of these solutes and the apparent age of ground water that was modern (zero to one year) in the headwaters, six to seven years midway down the valley, and 26 to 27 years in the borehole, located similar to 500 m downstream from the headwaters. Mass-balance modeling of chemical evolution showed that the downstream changes in ground water chemistry could be largely explained by weathering of plagioclase to kaolinite, with possible contributions from weathering of K-feldspar, biotite, hornblende, and calcite. The in situ rates of weathering reactions were estimated by combining the ground water age dates with geochemical mass-balance modeling results. The weathering rate was highest for plagioclase (similar to 6.4 mu mol/L/year), but could not be easily compared with most other published results for feldspar weathering at PMRW and elsewhere because the mineral-surface area to which ground water was exposed during geochemical evolution could not be estimated. However, a preliminary estimate of the mineral-surface area that would have contacted the ground water to provide the observed solute concentrations suggests that the plagioclase weathering rate calculated in this study is similar to the rate calculated in a previous study at PMRW, and three to four orders of magnitude slower than those published in previous laboratory studies of feldspar weathering. An accurate model of the geochemical evolution of riparian ground water is necessary to accurately model the geochemical evolution of stream water at PMRW.