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

Abstract

[1] Natural geochemical tracers ( 222 Rn, 3 H, 3 He, and 4 He) 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 222 Rn 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 226 Ra in the sediments; and (2) groundwater activities measured in monitoring wells. Calculated 222 Rn SGD rates based on sediment supported activities ranged from 0.04 to 0.14 cm d À1 , and estimates based on monitoring well activities ranged from 0.01 to 0.07 cm d À1 and corresponds to 1.41 km 3 yr À1 of discharged water over our study area, equivalent to <1% of the Mississippi River during the same time frame. 3He and 4 He, 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.

Hydrogeological modeling of submarine groundwater discharge on the continental shelf of Louisiana

“…Rather the near-shore coastal zone serves as a groundwater recharge area of saline water that then forms the seawater recirculation system beneath the coastal plain. The modeling results are consistent with interpretations of the rates of submarine groundwater discharge derived from geochemical tracers, presented in a companion paper by McCoy et al (2007). …”

“…[41] Results of the geochemical tracer study reported by McCoy et al [2007] yield interpretations consistent with the estimates derived from hydrogeological modeling. Both approaches indicate no SGD immediately offshore of coastal Louisiana.…”

“…The fluid flux estimates on the shelf should be interpreted with these approximations in mind. Fluid discharge that may be related to other geologic processes on the Louisiana shelf, such as episodic fluid expulsion along faults or mud volcanos [McCoy et al, 2007] is not considered in the calculations presented here. [16] Two cross-sectional models were constructed to derive an estimate of SGD for the study region (see Figure 2).…”

“…While the model is run in a transient mode to compute density-dependent flows, the boundary conditions are held constant. The modeling component of this integrated investigation of SGD does not investigate issues related to the seasonality of flow in the Mississippi River, or seasonality in the rates of SGD, as discussed in the companion paper based on geochemical tracers [McCoy et al, 2007]. Three- Figure 6; the upper plot shows the magnitude of the groundwater flux across the upper surface of the model domain, and the lower plot illustrates the fluid density distribution and groundwater flow patterns.…”

“…The zone starts at the southern limit of the onshore system, located at approximately X = 130 km, and extends to the coastline. Boreholes completed in Permeable Zone A in this region of the coastal plain are often flowing artesian wells [McCoy et al, 2007], indicative of upward groundwater flow. The assignment of the specified freshwater concentration boundary at the model surface has allowed a thin lens of low salinity groundwater to be present at the land surface.…”