Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997

SEDIMENT SAMPLING IN THE NATIONAL STREAM QUALITY ACCOUNTING NETWORK

Richard P. Hooper
U.S. Geological Survey
Atlanta, GA

The National Stream Quality Accounting Network (NASQAN), a national water-quality monitoring program begun in 1973, was re-designed in 1995 to focus on four of the Nation's largest river basins-the Mississippi, Columbia, Colorado and Rio Grande. NASQAN stations are located on major tributaries in the four river basins, along the main stem of rivers where there is a large increase in flow, and upstream and downstream from reservoirs with greater th an one-year average residence time. Such reservoirs serve as sinks for nutrients and sediment. In some cases, sets of reservoirs are treated together. (For example, the outflow from Lake Sakakawea in North Dakota is measured, but the inflow to Lake Oahe, the next downstream reservoir, is not because there is little additional streamflow between the lakes.) The subbasins defined by the NASQAN stations average about 70,000 square miles in the Mississippi River Basin; 40,000 square miles in the Columbia River Basin; 30,000 square miles in the Colorado River Basin; and 23,000 square miles in the Rio Grande Basin. The smaller subbasins in the Colorado and Rio Grande result from the requirement to measure the inflows and outflows from the large reservoirs present on these rivers, such as Lake Mead and Amistad Reservoir.

Stations operated by NASQAN will be sampled frequently enough to characterize the variation in concentration that occurs during a year, particularly the variation that occurs between low and high flow, in different seasons of the year, and during different hydrologic regimes such as periods when snowmelt dominates the river discharge. By sampling the river under these different conditions, the mass flux of a constituent can be reliably determined. Constituent fluxes can be compared among stations-across spatial scales-enabling a comparison of data collected by both NASQAN and the National Water Quality Assessment (NAWQA). For example, the yields of contaminants can be compared between stations; whether a contaminant is gained or lost between any two adjacent stations can be determined; and the amount of material delivered to a reservoir or estuary can be calculated. The ability to determine these three values-the source, transport, and delivery of constituents-enables a broad range of scientific and policy issues to be addressed.

The suspended sediment concentration and the percentage of sand and silt is determined for all NASQAN samples. Except for four stations immediately below dams, a large-volume sample is collected for chemical analysis of the sediment. The sample, which can be as large as 200 liters for Columbia River stations, is shipped to the USGS Atlanta Sediment Chemistry laboratory for de-watering by flow-through centrifugation and freeze-drying. The solids are then subjected to a hard digestion for the analysis of the following trace elements: Ag, Al, As, Ba, Be, Cd, Cr,Cu, Fe, Li, Mn, Mo, Ni, P, Pb, Sb, Se, Sr, Ti, Tl, U, V, Zn. If there is sufficient mass available, total carbon, total organic carbon, total inorganic carbon, total sulfur, and Hg are also measured.

Once samples have been collected over a range of hydrologic conditions and the variability of the chemical content of sediment characterized, it is assumed that this variability will be small relative to variations in discharge and sediment concentration, the two other factors that determine the mass flux of sediment-associated constituents. Therefore, after three years of data collection, the routine collection of these large-volume samples will be stopped. Samples will be collected at a lower frequency to determine whether the chemical composition of the sediment has changed. To increase constituent coverage and sampling frequency, sampling at many previous NASQAN stations was discontinued. (At one time, more than 500 NASQAN stations were operated.) A particularly important gap in ambient water-quality monitoring is in the coastal zone. The USGS is working with EPA and NOAA to develop a coastal-zone monitoring strategy for presentation to Congress.

As a part of that design effort, a synoptic sample collection has been proposed (Art Horowitz, personal communication). Recently deposited bed sediment would be collected at 130 previous NASQAN sites for the analysis of trace elements and persistent organic pollutants, such as DDT. Samples would be split be particle size prior to analysis. Combined with suspended sediment concentrations collected over 10 to 20 years at these sites, an initial estimate of flux of these contaminants to the coastal zone can be made, with two key assumptions. First, the bed sediment is assumed to be chemically representative of the suspended sediment. Second, the concentration of the sediment is assumed to have not changed significantly over the previous decade. Clearly, neither of these assumptions are perfectly met, but, given the lack of data on the flux of these compounds, even an order-of-magnitude estimate would be of considerable value. Furthermore, the synoptic sample will help to identify "hot spots" that should be included in the new coastal monitoring program.

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