Abstract
Submarine groundwater discharge (SGD) is often an important, yet understudied, process in coastal estuarine systems. In the western Mississippi Sound, samples were taken of surface waters for chemical species typically enriched in SGD (i.e., Ra quartet, Ba, methane, 222Rn, nutrients). Mass balance approaches were used on the radon and radium isotopes to determine an estimated water flux from SGD to the Sound. The radon and radium estimated SGD fluxes were 7.0×106 m3/d (0.04 m/d) and 9.1×106 m3/d (0.05 m/d), respectively. The local river input from the Pearl, Wolf, and Jourdan Rivers combined to this area has an average of 3.2×107 m3/d flux to the western Sound. Based on this average, the SGD flux was between 22% and 28% of river input. While SGD is only a fraction of riverine water flux, estimates of SGD nutrient fluxes (DIN, DIP, DSi) indicate that SGD is an equivalent or greater source of nutrients to the Sound as compared with local rivers. This could mean that SGD is the main source of nutrients to the Sound when river fluxes are lower than normal. A time series along the coastline of the Sound where there were elevated radon signatures during several surveys was also conducted. Monthly, five sites were sampled over the course of 17 months (June 2018 - November 2019), including during the opening of the Bonnet Carré Spillway (BCS), a diversion that keeps flood waters of the Mississippi River from reaching New Orleans. The average seepage measured during the time series was 0.08 m/d, but when the local rivers experienced extremely low discharge, groundwater influence increased, and during August 2018, the seepage was up to 0.19 m/d. However, with increasing river discharge in the winter months, along with BCS influence, the SGD input became negligible in the winter/spring of 2018/19.
