Valerie J. Kelly

Long-term trends in alkalinity in large rivers of the conterminous US in relation to acidification, agriculture, and hydrologic modification

Alkalinity increases in large rivers of the conterminous US are well known, but less is understood about the processes leading to these trends as compared with headwater systems more intensively examined in conjunction with acid deposition studies. Nevertheless, large rivers are important conduits of inorganic carbon and other solutes to coastal areas and may have substantial influence on coastal calcium carbonate saturation dynamics. We examined long-term (mid-20th to early 21st century) trends in alkalinity and other weathering products in 23 rivers of the conterminous US. We used a rigorous flow-weighting technique which allowed greater focus on solute trends occurring independently of changes in flow. Increasing alkalinity concentrations and yield were widespread, occurring at 14 and 13 stations, respectively. Analysis of trends in other weathering products suggested that the causes of alkalinity trends were diverse, but at many stations alkalinity increases coincided with decreasing nitrate+sulfate and decreasing cation:alkalinity ratios, which is consistent with recovery from acidification. A positive correlation between the Sen-Thiel slopes of alkalinity increases and agricultural lime usage indicated that agricultural lime contributed to increasing solute concentration in some areas. However, several stations including the Altamaha, Upper Mississippi, and San Joaquin Rivers exhibited solute trends, such as increasing cation:alkalinity ratios and increasing nitrate+sulfate, more consistent with increasing acidity, emphasizing that multiple processes affect alkalinity trends in large rivers. This study was unique in its examination of alkalinity trends in large rivers covering a wide range of climate and land use types, but more detailed analyses will help to better elucidate temporal changes to river solutes and especially the effects they may have on coastal calcium carbonate saturation state.

Regional and temporal differences in nitrate trends discerned from long-term water quality monitoring data

Riverine nitrate (NO3) is a well-documented driver of eutrophication and hypoxia in coastal areas. The development of the elevated river NO3 concentration is linked to anthropogenic inputs from municipal, agricultural, and atmospheric sources. The intensity of these sources has varied regionally, through time, and in response to multiple causes such as economic drivers and policy responses. This study uses long-term water quality, land use, and other ancillary data to further describe the evolution of river NO3 concentrations at 22 monitoring stations in the United States (U.S.). The stations were selected for long-term data availability and to represent a range of climate and land-use conditions. We examined NO3 at the monitoring stations, using a flow-weighting scheme meant to account for interannual flow variability allowing greater focus on river chemical conditions. River NO3 concentration increased strongly during 1945-1980 at most of the stations and have remained elevated, but stopped increasing during 1981-2008. NO3 increased to a greater extent at monitoring stations in the Midwest U.S. and less so at those in the Eastern and Western U.S. We discuss 20th Century agricultural development in the U.S. and demonstrate that regional differences in NO3 concentration patterns were strongly related to an agricultural index developed using principal components analysis. This unique century-scale dataset adds to our understanding of long-term NO3 patterns in the U.S.

Century-scale perspective on water quality in selected river basins of the conterminous United States

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Monitoring the Water Quality of the Nation's Large Rivers: Columbia River Basin NASQAN Program

In 1995, the U.S. Geological Surveys (USGS) National Stream Quality Accounting Network (NASQAN) Program began monitoring the water quality of the Columbia River Basin, applying a basinwide approach in order to understand water quality on a regional scale. A primary objective of the Columbia NASQAN Program is to provide an ongoing characterization of the concentrations and mass flux (amount of material or load passing a location per unit time, generally expressed as tons per day) of sediment and chemicals at key locations in the basin. These data can then be used to determine regional source areas for these materials, and to assess the effect of human influences on observed concentrations and constituent loads. NASQAN complements the ongoing USGS National Water Quality Assessment (NAWQA) Program, which is performing a detailed assessment in three subbasins of the Columbia River Basin. NASQAN monitors the larger rivers in the basin, downstream of NAWQA study units.