Proceedings of the 2019 Mississippi Water Resources Conference

Year: 2019 Authors: Ho P., Shiller A.

A multi-year (2007-2011) chemical time series of eight stations in the western Mississippi Sound and northwestern Mississippi Bight was undertaken to examine the factors affecting the distributions of trace elements in this estuarine-coastal system. Key findings include the frequent development of bottom water hypoxia in this part of the Bight during late spring and summer, the likely contribution of submarine groundwater discharge (SGD) to the material flows (i.e., Ba, V and nutrients), and the effects of episodic events (i.e., tropical storms, cold fronts, the opening of the Bonnet Carr Spillway) on trace element distributions. A variety of trace metals (i.e., Ba, Mn, V and Cs) in Mississippi Sound surface waters were largely regulated by the temporal and spatial variations of riverine sources. For instance, in fall and winter, high concentrations of dissolved Mn at the most nearshore stations followed by a sharp decline in concentrations offshore, are indicative of Pearl River influence in the Mississippi Sound. Cs-rich water coming from St. Louis Bay is evident in the Mississippi Sound, but not observable in Mississippi Bight.

In hypoxic bottom waters, enriched Mn and Ba as well as depleted V were commonly observed. Consideration of the mass balances of dissolved Ba and V suggests that SGD can be a significant contributor to the chemical mass balance in this region, not just for certain trace elements, but for nutrients, too. Interestingly, a seasonal change in the direction of the V flux from the sediments suggests that the chemical conditions (i.e., pH, E_H, and/or DOC) of the groundwater are changing. During the study period, the Bonnet Carr Spillway was opened in April 2008 and May 2011. The spillway discharges Mississippi River water to Lake Pontchartrain and ultimately to our study area, which is supported by the observation of an extended freshwater signal and low Cs across the Mississippi Sound and Bight during these two spillway openings. Important remaining questions from this study include the extent to which SGD and/or Mississippi River water are necessary for establishment and maintenance of hypoxia in the Mississippi Bight and the reasons for the seasonal change in the direction of the sedimentary V flux.