Rare Earth Element Geochemistry of Aquia Aquifer Groundwater and Sediments, Maryland, USA

Groundwater and sediment samples were collected along a groundwater flow path in the Aquia aquifer in Maryland, USA. Groundwater parameters measured in the field included pH, temperature, oxidation-reduction potential (Eh), dissolved oxygen, alkalinity, iron speciation, and the concentrations of sulfide, silica, manganese, ammonia, and nitrate. These measurements were used to determine the oxidation-reduction conditions and solution chemistry along the flow path. Rare earth element (REE) concentrations in both the groundwater and aquifer sediment samples were determined by inductively coupled mass spectroscopy. Nd concentrations of the groundwater range from ~8.12 to 10.29 ng/kg on the Eastern shore of the Chesapeake Bay and from ~5.97 to 11.24 ng/kg on the Western shore of the Chesapeake Bay. The Upper Continental Crust normalized (Yb/Nd)UCC ratios are relatively similar along the groundwater flow path with a slight increase toward the end of the flow path for both the Eastern and Western shore wells, ranging from ~ 0.7 to 1.1. The (Yb/Nd)UCC ratios illustrate the relatively flat UCC-normalized REE patterns of Aquia aquifer groundwater. Sediment samples were collected from an archived drill core sample (from Kent County, Maryland on the Eastern shore) stored at the Maryland Geological Survey and analyzed for REEs using a sequential extraction procedure. Our initial analysis indicates that Nd concentrations for the bulk aquifer sediment range from ~0.5 to 25 ppb. The sequential extractions reveal that much of the REEs in the aquifer sediments occur bound to carbonates. Shell fragments are abundant within parts of the Aquia sediments. Furthermore, the sequential extraction data indicate that the LREEs are more readily exchangeable (i.e., outer-sphere complexed) on aquifer sediment surfaces than the HREEs, whereas the HREEs and MREEs are associated with Fe/Mn oxides/oxyhydroxides. Detailed laboratory investigations (adsorption/desorption experiments) are currently underway to develop a surface complexation model for REEs in the Aquia aquifer.