Water quality at the Buffalo National River, Arkansas, 1991-2001

The Buffalo National River (BNR) is a relatively unpolluted, free-flowing river with riffle-pool geomorphology, located in north-central Arkansas. The specific objectives of this study were to: (1) evaluate differences between physicochemical properties and concentrations in water quality samples representing base flow and surface runoff conditions, (2) determine trends in physicochemical properties and concentrations using three datasets (all data, data representing base flow conditions, and data representing surface runoff conditions), and (3) compare flow-weighted constituent concentrations and yields at the BNR to relatively undisturbed catchments and a relatively developed catchment. Water quality trends were evaluated (alpha = 0.1) using constituent data from 1991 through 2001 and using subsets of these data representing water quality samples collected during either base flow or surface runoff conditions. Trends were assessed by (1) appropriately transforming water quality data and daily discharge, (2) flow-weighting water quality data using a smoothing technique, and (3) evaluating residuals from smoothing versus time for trends. Trend analyses suggested that only nitrogen, sediment, and E. coli concentrations increased from 1991 through 2001 in the BNR, particularly during surface runoff conditions. Most temporal changes in constituents occurred during surface runoff conditions, and these changes were not necessarily reflected during base flow conditions. Flow-weighted nutrient concentrations and yields were greater at the BNR compared to median values for relatively undeveloped basins across the U.S. Nutrient concentrations and yields at the BNR were only slightly greater than or equal to the values representing the 75th percentile of reference streams. However nutrient concentrations and yields at the BNR were less than at relatively developed basins within the same ecoregion. By evaluating base flow and surface runoff water quality samples separately for trends, we gained additional insight into the particular flow conditions exhibiting significant trends. Identification of flow conditions associated with trends aids in determining constituent sources and hence appropriate constituent abatement.