Chemical Identification of Source Waters in a Rural Sub Estuary of Chesapeake Bay (Chester River)

Abstract:

Complimentary trace metal and water quality surveys were conducted along a 45-km longitudinal transect of the Chester River, a tidal tributary of Chesapeake Bay, to identify potential chemical differences associated with input from different water sources. The Chester River serves as the receiving waters of a largely agricultural watershed on Maryland’s Eastern Shore. Delineating water sources within the system can inform management and nutrient reduction strategies. Surface water samples were collected and syringe- filtered in the field. Samples were analyzed to determine the concentrations of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Cd, Sn, Ba, W, Pb and U. Dissolved nitrate and orthophosphate, Chlorophyll a and total suspended matter were also determined for each sample. Hydrocasts of salinity, temperature, pH, and relative fluorescence of chlorophyll were used to reconstruct water column characteristics and provide insight into mixing with Chesapeake Bay. Results from a complete transect survey conducted over a single tidal maximum (+/- 2 hours) show that Ba is elevated in the Chester River (621 nMol/L) relative to Chesapeake Bay, but exhibits no distinguishable trend with salinity in the river. The Ba results suggest complex input from groundwater along the transect and potential dissociation from riverine sediments and suspended particles. Arsenic, Cr, and V demonstrate marked conservative mixing along the salinity gradient. Arsenic ranged from 23 nMol/L at the lowest salinity and the highest concentration (218 nMol/L) at the mouth where the Chester River enters Chesapeake Bay. This apparent conservative behavior is likely the result of simple mixing between fresh- and saltwater over what is assumed to be a relatively short residence time (days) for water in the river system. The plots of U and W with salinity show curvilinear distributions with positive slopes and may reflect the importance of uptake by sediments and vegetation in the upper reaches of the river. Manganese concentrations are also curvilinear but decrease with increased salinity, potentially due to changes in redox conditions as freshwater mixes with more saline Bay water.