An evaluation of submarine groundwater discharge along the continental shelf of Louisiana using a multiple tracer approach

Abstract

Natural geochemical tracers (222Rn, 3H, 3He, and 4He) were used to assess submarine groundwater discharge (SGD) along the continental shelf west of the Mississippi River. In order to assess SGD, groundwater, surface water, and sediment samples were collected on land and during six 4‐day cruises aboard the R/V Pelican between March 2003 and May 2004. A box model approach was used to quantify sources and sinks of 222Rn in the study area and to calculate SGD rates. SGD estimates were we based on two end member values for the potential advecting fluids: (1) that supported by 226Ra in the sediments; and (2) groundwater activities measured in monitoring wells. Calculated 222Rn SGD rates based on sediment supported activities ranged from 0.04 to 0.14 cm d−1, and estimates based on monitoring well activities ranged from 0.01 to 0.07 cm d−1 and corresponds to 1.41 km3 yr−1 of discharged water over our study area, equivalent to <1% of the Mississippi River during the same time frame. 3He and 4He, longer‐lived tracers, exhibited significantly greater anomalies in the eastern portion of the study area which corresponds with greater oil and gas extraction and the release of formation water into the water column in this region. While the total SGD was relatively minor, potential sources of SGD are many and we suggest formation water associated with oil and gas extraction, geothermal convection, and seawater recirculation are the primary sources with a minimal contribution from terrestrially derived topography driven flow.