Nutrient and Suspended-Sediment Trends in the Missouri River Basin, 1993–2003

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Abstract

Trends in streamflow and concentration of total nitrogen, nitrite plus nitrate, ammonia, total phosphorus, orthophosphorus, and suspended sediment were determined for the period from 1993 to 2003 at selected stream sites in the Missouri River Basin. Flow-adjusted trends in concentration (the trends that would have occurred in the absence of natural changes in streamflow) and non-flow-adjusted trends in concentration (the overall trends resulting from natural and human factors) were determined. In the analysis of flow-adjusted trends, the removal of streamflow as a variable affecting concentration allowed trends caused by other factors such as implementation of best management practices to be identified. In the analysis of non-flow-adjusted trends, the inclusion of any and all factors affecting concentration allowed trends affecting aquatic ecosystems and the status of streams relative to water-quality standards to be identified. Relations between the flow-adjusted and non-flow-adjusted trends and changes in streamflow, nutrient sources, ground-water inputs, and implementation of management practices also were examined to determine the major factors affecting the trends.

From 1993 to 2003, widespread downward trends in streamflow indicated that drought conditions from about 2000 to 2003 led to decreasing streamflow throughout much of the Missouri River Basin. Flow-adjusted trends in nitrite plus nitrate and ammonia concentrations were split nearly equally between nonsignificant and downward; at about one-half of the sites, management practices likely were contributing to measurable decreases in concentrations of nitrite plus nitrate and ammonia. Management practices had less of an effect on concentrations of total nitrogen; downward flow-adjusted trends in total nitrogen concentrations occurred at only 2 of 19 sites. The pattern of non-flow-adjusted trends in nitrite plus nitrate concentrations was similar to the pattern of flow-adjusted trends; non-flow-adjusted trends were split nearly equally between nonsignificant and downward. A substantial source of nitrite plus nitrate to these streams likely was ground water; because of the time required for ground water to travel to streams, there may have been a lag time between the implementation of some pollution-control strategies and improvement in stream quality, contributing to the nonsignificant trends in nitrite plus nitrate. There were more sites with downward non-flow-adjusted trends than flow-adjusted trends in both ammonia and total nitrogen concentrations, possibly a result of decreased surface runoff from nonpoint sources associated with the downward trends in streamflow. No strong relations between any of the nitrogen trends and changes in nutrient sources or landscape characteristics were identified.

Although there were very few upward trends in nitrogen from 1993 to 2003, there were upward flow-adjusted trends in total phosphorus concentrations at nearly one-half of the sites. At these sites, not only were pollution-control strategies not contributing to measurable decreases in total phosphorus concentrations, there was likely an increase in phosphorus loading on the land surface. There were fewer upward non-flow-adjusted than flow-adjusted trends in total phosphorus concentrations; at the majority of sites, overall total phosphorus concentrations did not change significantly during this period. The preponderance of upward flow-adjusted trends and nonsignificant non-flow-adjusted trends indicates that in some areas of the Missouri River Basin, overall concentrations of total phosphorus would have been higher without the decrease in streamflow and the associated decrease in surface runoff during the study period. During the study period, phosphorus loads from fertilizer generally increased at over one-half of the sites in the basin. Upward flow-adjusted trends were related to increasing fertilizer use in the upstream drainage area, particularly in the 10 percent of the drainage area closest to the monitoring site. This relation was not seen with the non-flow-adjusted trends in total phosphorus concentrations, indicating that decreasing streamflow and associated decreasing surface runoff in the basin during the study period may have offset the effects of increasing fertilizer use.

There were fewer sites with upward trends in suspended sediment than in total phosphorus. Although phosphorus can be transported by sorption to particulate material, the different trend patterns of the two constituents indicate that changes in suspended-sediment concentrations were not contributing to a concomitant change in total phosphorus concentrations in the Missouri River Basin. At some sites, pollution-control strategies or other human activities were contributing to a measurable decrease in suspended-sediment concentrations, but at the majority of sites, there were no measurable effects from pollution-control strategies. Spatial differences in stream density and overbank storage may have contributed to the spatial variability in flow-adjusted trends in suspended-sediment concentrations throughout the basin. Sediment loading probably was less affected by overbank storage at sites with higher stream densities, and consequently, pollution-control strategies may have contributed to measurable decreases in suspended-sediment concentrations at these sites. In contrast, at low stream-density sites where overbank storage was occurring, pollution-control strategies may not have contributed to measurable changes in suspended-sediment concentrations because the sediment loading prior to BMP implementation would have already been attenuated by overbank storage. There were more downward non-flow-adjusted trends than downward flow-adjusted trends in suspended-sediment concentrations, indicating that naturally decreasing streamflow over the study period was as or more influential in decreasing the concentrations of suspended sediment than were pollution-control strategies or other human activities. If streamflow had not decreased during the study period, it is unlikely that overall concentrations of suspended sediment would have decreased at many sites.

The streamflow and flow-adjusted trends in concentration for the period from 1993 to 2003 were placed in a longer context by comparing them to longer term, non-monotonic trends for the period from 1985 to 2003 at a subset of the sites. From 1985 to 2003, streamflow generally decreased from about 1985 to 1991, increased from about 1992 to 1996, and decreased from about 1997 to 2003. During the same period, many flow-adjusted trends in total nitrogen, nitrite plus nitrate, total phosphorus, orthophosphorus, and suspended-sediment concentrations occurred between 1985 and 1991 and between 1997 and 2003; unlike with streamflow, the direction of the flow-adjusted trends varied among sites and among time periods. These longer term, non-monotonic patterns indicated that consistent monotonic changes in streamflow and concentration may not have occurred through the entire period from 1993 to 2003, as indicated by the shorter term trend analysis.

The longer term patterns in streamflow also indicated that the decreasing steamflows observed from 1993 to 2003 in the Missouri River Basin likely will not continue indefinitely. In some parts of the basin, nutrient and suspended-sediment concentrations may have been higher without the decrease in streamflow and the associated decrease in surface runoff that occurred during the study period. Without additional steps to minimize surface runoff or nutrient loading on the land, it is possible that concentrations will increase when streamflow and runoff begin to increase once again. In addition, results from three case studies indicated that a substantial portion of the total flow and nitrate load in streams may consist of ground-water inflow in some parts of the basin. In these areas, nutrient loading to streams may be addressed by management practices focused not only on reducing surface runoff but also on maintaining and (or) improving ground-water quality.