Interpreting kinetics of groundwater-mineral interaction using major element, trace element, and isotopic tracers

摘要

To determine the residence time of groundwater, hydrologists use environmental tracers or numerical modelling. If mineral reactions within an aquifer could be used to determine the age of packets of water, such calculations would be extremely useful. Such calculations assume plug flow in the aquifer, and assume that laboratory mineral-water reaction kinetics are applicable in the field system. Application of such a model to an extraordinarily "simple" aquifer ― the Cape Cod aquifer in Massachusetts, USA ― reveals several puzzles related to interpretation of mineral reactivity. Despite a mineralogy consisting of more than 90% quartz and alkali feldspar, dissolution of quartz in the aquifer is not expected due to concentrations of Si higher than saturation with respect to quartz. However, flow rates inferred from Si gradients in the aquifer and alkali feldspar dissolution rates measured in the laboratory are too fast by a factor of about 50 as compared to observations and hydrologic models developed for the site. Furthermore, analysis of solute concentrations and Sr isotopes are consistent with only minor dissolution of alkali feldspar. In contrast, observed concentrations are consistent with dissolution of plagioclase (present at very low concentrations) and ion exchange between the aquifer fluid and accessory phases such as glauconite. This conclusion documents that chemistry of accessory minerals, when more reactive than the host minerals, can dominate groundwater solute compositions. While this observation is not necessarily unexpected, it emphasizes that further investigations of in situ reactions as tools for dating groundwater must rely upon thorough characterization of aquifer mineralogy combined with the use of major and trace element as well as isotopic analysis. Without such characterization, misinterpretation of reaction stoichiometries and kinetics in natural systems will be unavoidable.