Trend Analysis of Water Quality in Northwest Arkansas Streams Reflects Changes in the Watershed

Watershed export of nutrients, sediments, and chemicals impacts receiving waters. Changes within the watershed (e.g., anthropogenic or climatic) can alter the transport of constituents in streams. Stream monitoring is crucial for understanding these effects. This study developed a potential improvement to flow-adjusting constituent concentrations in streams, an important step of analyzing monitoring data in lotic systems for trends. The method incorporates a K-fold cross-validation procedure to optimize a model explaining the relationship between the concentration and streamflow, thus providing a valuable tool to researchers in water quality. Additionally, two case studies were conducted on watersheds located in northwest Arkansas using monitoring data collected from 2009 to 2015. The first case study focused on phosphorus concentrations in the Illinois River watershed and illustrated significant decreases in soluble reactive phosphorus following reductions of effluent phosphorus from upstream wastewater treatment plants. However, no significant trends were found in total phosphorus at the most downstream site on the Illinois River, suggesting that there are legacy sources of phosphorus remaining in the watershed. The second case study focused on nitrogen and phosphorus in the three main inflows to Beaver Lake, where primary productivity will likely cause the lake to violate its water quality standard for chlorophyll-a concentration. Data collected at two sites in Beaver Lake showed elevated chlorophyll-a concentrations and one site near Lowell, Arkansas, the location of a major drinking water supply intake, showed increasing trends from 2001 to 2015 for total nitrogen as well as chlorophyll-a. Monitoring data of the inflows illustrated the variability in hydrological and climatic factors (e.g., drought), which affects nutrient delivery to Beaver Lake. Long-term monitoring of streams in both watersheds will be crucial for understanding the processes that affect water quality and will better inform watershed management