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Other studies have also observed similar trends for dissolved Mississippi River constituents, and such observations generally hold true for other turbid rivers of the world. An accurate assessment of the overall health of such a river is thus incomplete if we study only dissolved riverine constituents; we must also study the sedimentary constituents, which is just what an ongoing USGS project is doing. The Atchafalaya and Mississippi River Deltas Study is evaluating the delivery of sediment-associated contaminants to the Gulf of Mexico by the Mississippi River system and its important distributary, the Atchafalaya River.
Our hypothesis is twofold:
The Mississippi River system, which drains almost half of the conterminous United States, ranks seventh among rivers worldwide for water discharge (580 km3/yr) and sixth for suspended-sediment discharge (200 million metric tons per year). Together, the Mississippi and Atchafalaya Rivers provide almost all of the freshwater influx to the Gulf of Mexico.
Unlike most other large rivers of the world, the Mississippi River is highly controlled or regulated. About a third of the total flow to the Mississippi River is diverted at the Old River Control Structure to form the Atchafalaya River. The construction of dams and reservoirs all along the Mississippi River has resulted in substantial decreases in total suspended-matter concentrations. The recent Missouri River flood of 1993 may also have effectively decreased total suspended-matter concentrations in the lower river in subsequent years, and such large flood events probably serve to periodically scour and flush out recently deposited riverbed sediment.
Riverborne contaminants discharged into the Gulf of Mexico originate from a wide range of natural, industrial, municipal, and agricultural sources. For example, in the above-cited Circular, Meade reports that the combined discharge from industrial and municipal point sources amounts to as much as about 2 percent of the total discharge of the Mississippi River. This delivery of riverine constituents affects coastal ecosystem health and productivity and plays an important role in the formation and intensity of seasonal hypoxic events in the Mississippi Bight, during which the water has anomalously low concentrations of dissolved oxygen. To address the issue of present and historic hypoxia, we are comparing sediment from sites where the water column is chronically hypoxic, or oxygen poor, with sediment from sites that are chronically oxic, or oxygen rich.
We are also comparing results obtained from cores collected from the Mississippi and Atchafalaya River deltas during various river-discharge stages. Ongoing work by USGS scientists in Menlo Park, CA (Bob Rosenbauer, Keith Kvenvolden, Tom Lorenson), Reston, VA (Bill Orem, Dick Poore), and St. Petersburg, FL (Jack Kindinger, Pamela Campbell, Peter Swarzenski), is examining changes in the organic and inorganic composition of sediment samples collected across river-ocean salinity gradients, as well as recent deltaic sediments and their pore waters.
In 2002, USGS scientists Tom Lorenson and Jim Flocks participated in a cruise aboard the research vessel Marion Dufresne to Orca and Pigmy Basins in the Gulf of Mexico to assess offcontinent riverine fluxes during Holocene time (approximately the past 10,000 years). Such data are allowing us to reconstruct historical contaminant inventories from which we hope to develop historical perspectives on hypoxia and nutrient loading (delivery of excess nutrients, such as nitrogen, to the gulf). Ultimately, we intend to use such information to better understand how these sediment-hosted contaminants move either directly or indirectly through biota and affect ecosystem health.
in this issue:
Gulf of Mexico Contaminants
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