Controls on the geochemistry of rare earth elements in sediments and groundwaters of the Aquia aquifer, Maryland, USA

摘要

Groundwater samples were collected along two flow paths from the Aquia aquifer in Maryland, USA to investigate the rare earth element (REE) geochemistry of the flow system as a function of evolving groundwater composition. Neodymium (Nd) concentrations of Aquia aquifer groundwater range from 56.6 to 71.4pmolkg~(-1) along the Eastern Shore flow path (i.e., Eastern Shore of the Chesapeake Bay), and from 41.5 to 77.9pmolkg~(-1) on the Western Shore flow paths, and consequently fall within the range reported for other circumneutral to alkaline pH groundwaters. Shale-normalized REE patterns of Eastern and Western Shore Aquia groundwaters are similar (i.e., relatively flat) and do not vary substantially along either flow path. For example, (Yb/Nd)_(NASC) for Aquia aquifer groundwaters exhibit a limited range between 0.7 and 1.1. Aquia aquifer sediments were analyzed for REEs following a sequential extraction procedure. Neodymium concentrations for the bulk aquifer sediment range from 26 to 70μmolkg~(-1) (3.7 to 10.1ppm), or a factor of 2.5 to 7 lower than the Nd content of the upper continental crust. The sequential extractions reveal that the majority of the REEs in the aquifer sediments occur associated with the abundant carbonate shell fragments that are ubiquitous within the Aquia aquifer sediments. Furthermore, the sequential extraction data indicate that the light REEs (LREE) are more readily exchangeable (i.e., outer sphere surface complexed) on aquifer mineral surfaces than the heavy REEs (HREE), and the HREEs and middle REEs (MREE) exhibit a greater association with Fe/Mn oxides/oxyhydroxides within the sediments. Analysis of the abundant glauconite pellets within Aquia aquifer sediments indicates an overall enrichment in the HREEs relative to the LREEs for this mineral, compared to shale. Our data suggest that the chief source of REEs to Aquia aquifer groundwater is dissolution of carbonate biominerals (shell fragments) that occurs in the recharge zone and before groundwaters become saturated with respect to calcite and aragonite. Additionally, weakly sorbed REEs (outer sphere surface complexed) contribute LREEs and MREEs to Aquia aquifer groundwater as pH increases above 8, leading to the relatively flat, shale-normalized REE patterns that characterize Aquia aquifer groundwaters. Once released to the solution, REE form strong aqueous complexes with carbonate ions, LnCO_3~+ and Ln(CO_3)_2~-, with the dicarbonato complexes (i.e., Ln(CO_3)_2~-) predominating. The REE are subsequently transported with flowing groundwater in the Aquia aquifer as the negatively charged dicarbonato complexes exhibit a low affinity to sorb to negatively charged mineral surfaces in the aquifer sediments.