CONTROLS ON GROUNDWATER CHEMISTRY IN THE CAPE COD AQUIFER, MASSACHUSETTS: THE IMPACT OF ACCESSORY MINERAL PHASES ON SOLUTE CONCENTRATIONS, 87Sr/86Sr, AND RARE EARTH ELEMENT DISTRIBUTIONS

Open Access
Author:
Alexander, Brian W.
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Susan Louise Brantley, Thesis Advisor
Keywords:
  • rare earth elements
  • groundwater
  • Cape Cod
  • strontium isotopes
Abstract:
To quantify mineral dissolution and relative reaction rates in a mineralogically simple aquifer, we examined controls on solute concentrations, strontium isotopes, and rare earth element and ytrrium (REY) behavior within the Cape Cod aquifer, Massachusetts. The aquifer is dominated by carbonate-free Pleistocene glacial sediment and is primarily comprised of quartz (~90% wt.) and feldspar (5%), with plagioclase being the most abundant phase within the feldspar fraction (plagioclase/K-feldspar ~1.5). The remaining aquifer sediment consists of accessory mineral phases which are dominated by authigenic glauconite and Fe-oxides. Silica concentrations and pH increase systematically with depth within the aquifer while Sr isotope ratios (87Sr/86Sr) systematically decrease. Sr concentrations and 87Sr/86Sr are not well correlated, and groundwater 87Sr/86Sr values are consistently lower than 87Sr/86Sr of the bulk aquifer sediment or the 87Sr/86Sr of a K-feldspar component separated from the bulk aquifer sediment. The Sr isotope ratios of the groundwater are similar to a plagioclase-rich separate obtained from the bulk aquifer sediment, and Si-87Sr/86Sr-depth relationships are consistent with the dissolution of accessory plagioclase. Potassium-Sr-87Sr/86Sr relationships indicate dissolution of a K-rich aquifer component in addition to plagioclase, which is inferred to be glauconite. Though rich in Rb, authigenic glauconite contains Sr with relatively low 87Sr/8bSr due to the young Pleistocene age of the aquifer sediments, and will have a Sr isotopic signature reflecting geologically modern seawater. The impact of glauconite on solute behavior as a function of depth is expected to be non-uniform due to its variable abundance in the aquifer sediments. REY concentration trends as a function of depth in the Cape Cod groundwater are similar to K and Rb, but differ from those of Sr and Ca. REY patterns (shale-normalized) in the groundwater are light rare earth element (LREE) depleted, and contain negative Ce anomalies and positive Y anomalies (and super-chondritic Y/Ho ratios). No positive Eu anomalies that would be consistent with feldspar dissolution are observed in the groundwater REY patterns. REY behavior in the groundwater is therefore controlled by a Rb-K-rich phase, inferred to be glauconite. Interpretation of groundwater chemistry within the Cape Cod aquifer is heavily influenced by the impact of relatively minor aqui-fer components such as glauconite, and indicates that even within this relatively simple aquifer, only careful consideration of solute behavior from multiple sources fully characterizes mineral-water interactions.