Sediment and Nutrient Retention by Wetlands Receiving Inflows from a Mississippi River Diversion: A Mass Balance Approach

Science Center Objects

Diversions are currently used in the Mississippi River to stimulate delta growth via increased sediment supply. This technique may also help to reduce nutrient loads before its discharged into the ocean. Scientists at USGS assess how wetlands retain the sediment and nutrients that come from these diversions. 

Map of discharging sediments data into subsiding deltaic wetlands
Map of discharging sediments data into subsiding deltaic wetlands

Science Issue and Relevance: Mississippi River diversions are being evaluated as tools to stimulate delta growth through increased sediment supply. Though discharging sediments into subsiding deltaic wetlands is widely accepted as a means to offset subsidence, river diversions have also been proposed as a means to reduce nutrient loads in river water before it is discharged to the continental shelf. Previous studies examining the impact of river diversions on sediment deposition have been inconclusive, largely because many other processes occurring over short time scales (tides, meteorological forcing, hurricane landfalls) can also influence sediment deposition, which is generally assessed over much longer timescales (months to years). Examinations of nutrient removal in diversion receiving basins have been similarly inconclusive, largely because it can be difficult to quantify constituent export from diversion receiving basins with numerous export pathways.

Methodology for Addressing the Issue: This study will use two approaches to more clearly assess variation in retention of fluvial constituents and deposition responses to short-term fluctuations in river inputs through the Davis Pond freshwater diversion, which discharges Mississippi River water into upper Barataria basin, Louisiana. First, short-term (~monthly) deposition will be assessed with 7Be, a radioisotope with a short (53 d) half-life. Second, we will take advantage of the limited number of export routes to use a mass balance approach to calculate sediment and nutrient budgets for the receiving area under different flow regimes. Short (~10 cm) soil cores will be obtained from the marsh surface at 20 sites in the ponding area to the Davis Pond freshwater diversion (29.8864°N, 90.2847°W) under different operational flow (~15, 30, 60, 150 m3/s) and river conditions (discharge, temperature, sediment load). Cores will be sectioned into 1-cm increments and analyzed for 7Be activity, loss-on-ignition, bulk density, and grain size. Rates of recent (month) sediment deposition will be inferred from 7Be activity profiles.

Illustration of sediment import and export
Illustration of sediment import and export

Concurrent with core collection, constituent (total suspended solids, NO3, PO4) concentrations and water flux will be measured at the inflow channel upstream of the outfall area and also at each of the seven discrete export channels at the downstream end of the outfall area. Mass balance calculations will be performed on the materials flux data to determine the degree to which the wetlands in the outfall area are sinks for river materials entering the area. Correlations between sediment removal and deposition rates will be sought.

Future Steps: Thus far data have been collected under low (30 m3/s) discharge conditions. We intend to repeat sampling at other inflow rates during 2016.

Location of the Study: Throughout the Louisiana coastal zone, Latitude: 29.678°, Longitude: -91.552°