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 Home | Atchafalaya & Mississippi River Deltas | Reports | Sediment Hosted Contaminants
Summary | Objectives & Strategy | Impacts & Products | Collaborators | References | Contact Information

U.S. Department of the Interior
U.S. Geological Survey
Open-File Report 01-215

Project Objectives and Strategy:

The Mississippi River tracks an ancient mid-continental rift valley as it drains 41% of the 48 conterminous United States. Suspended-sediment concentrations have been decreasing in the mainstem Mississippi (Fig. 1) from the 1950s to present due to dam/reservoir construction and erosion-sensitive agricultural practices. The yearly mean discharge of ~ 580 km3 of water and 200 million metric tons of suspended sediment to the Gulf of Mexico (Fig. 2) place this river system well within the top 10 largest rivers of the world. The Mississippi River receives a significant proportion of its water budget from the Ohio River, but the Missouri River contributes the majority of suspended particles (Meade, 1996). This bi-modal influx of water and sediment yields only a loose relationship between discharge and suspended-sediment concentration (Fig. 3A,B) in the mainstem river. A three-year record at Tarbert Landing, MS, indicates that usually much more than 70% of the suspended load consists of fines - particles that are less than 0.062 mm in size (Fig. 4B).

Graph showing 50 years of suspended sediment concentrations in the mainstream Mississippi River at Tarbert Landing, MS (USACE data). Two graphs - one showing 40 years (1960 to present) of mainstem Mississippi River discharge measured at Fort Tarbert Landing, MS (USACE data) and one showing discharge from 1/1/98 to present.
Figure 1 Figure 2
Figure 1. 50 years of suspended sediment concentrations in the mainstem Mississippi River at Tarbert Landing, MS (USACE data). [View Enlargement]

Figure 2. A: (top graph) 40 years (1960 to present) of mainstem Mississippi River discharge measured at Fort Tarbert Landing, MS (USACE data). B: (bottom graph) Discharge from 1/1/98 to present. [View Enlargement]

Almost all (> 92%) of the trace-element load in the Mississippi River is transported downstream in particulate form, either as suspended sediment (including colloids) or bed load (Trefry et al., 1986; Taylor et al., 1990; Swarzenski and McKee, 1999). Seasonal sediment storage and remobilization in the Atchafalaya/Mississippi River system, as well as physico-chemical shelf effects therefore control the ultimate delivery of reactive pollutants to the Gulf of Mexico (Shiller and Boyle, 1993; Swarzenski and McKee, 1999; Mossa, 1996; Allison et al., in press). In the lower reaches of the Mississippi River, sediments are typically stored (Fig 2B) during low discharge (Q < 14,000 m3 sec-1) and remobilized during high discharge stages (Q > 20,000 m3 sec-1) (Demas and Curwick, 1988; Mossa, 1996). During such periods of storage, key diagenetic reactions such as remineralization and the reduction of Fe/Mn carrier phases can significantly change the solubility of certain scavenged contaminants (Trefry and Presley, 1986, Santschi et al., 2001). Similar geochemical reactions also occur in floodplain, deltaic or shelf sediments, and thus the fate of sediment-hosted contaminants during down-stream and across-shelf transport collectively define pollutant loading to the Gulf of Mexico (Goñi et al., 1997).

Two grahs - one showing dishcharge versus suspended sediment concentration and the other showing percent fines versus suspended concentration at tarbert Landing, MS. (data compiled by USGS). Two graphs - one showing auspended sediment concentrations and the other showing percent fines in the mainstem Mississippi River from 1/1/98 to present (compiled by the USGS).
Figure 3 Figure 4
Figure 3. A: (top graph) Dishcharge versus suspended sediment concentration and B: (bottom graph) Percent fines versus suspended concentration at Tarbert Landing, MS. (data compiled by USGS). [View Enlargement]

Figure 4. A: (top graph) Suspended sediment concentrations and B: (bottom graph) Percent fines in the mainstem Mississippi River from 1/1/98 to present (compiled by the USGS). [View Enlargement]

Our working hypothesis for this project is that differential processing of terrigenous, sediment-hosted contaminants in the lower Atchafalaya and Mississippi Rivers and their respective receiving basins (shelf versus shelf break) and flood plains can significantly alter the environmental fate and ultimate availability of these constituents. This differential processing is primarily due to the discharge of Atchafalaya River waters into a broad, shallow inner shelf environment, while Mississippi River waters discharge into the Gulf of Mexico, beyond the shelf break (Fig. 5). We are addressing this differential processing by collecting water, suspended sediment and bottom sediment samples in these two river systems (i.e., lower river, floodplain and shelf).

Map showing location of study.
Figure 5. Site location map.

We can thus evaluate the effect of seasonal sediment storage, early diagenetic reactions, remobilization and eventual deposition on the following subset of sediment-hosted pollutants: (a) pesticides, PAHs and other organic pollutants, (b) trace metals that exhibit variable particle affinities or bioavailabilities (e.g., U, V, Mo, Ba, Sr, Fe and Mn), and (c) organic carbon and nutrients. The following specific objectives are addressed for each river system under variable discharge regimes:

  1. examine the extent of early diagenesis by looking at sediment/pore water profiles of such constituents as Fe, Mn, U, Mo, V, nutrients and organics;
  2. examine the biogeochemical transformations that occur as a result of sediment remobilization (resetting geochemical gradients);
  3. assess the historical inventory of pollutants and their transformation reactions;
  4. evaluate the overall delivery of sediment-hosted pollutants that eventually reach the Gulf of Mexico.

Results from (a-d) will be used to develop predictive models of pollutant transport, environmental fate and overall ecosystem health.


U.S. Department of the Interior, U.S. Geological Survey, Gulf of Mexico Integrated Science
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