ASSESSING THE SPATIAL REPRESENTATIVENESS OF A WATER QUALITY MONITORING STATION IN AN AGRICULTURAL RESERVOIR

Reservoirs are valuable tools for managing riverine water resources. Monitoring the quality of water in these systems in a spatiotemporally representative way is thus critical, particularly when they are used for water supply purposes. To minimize the cost and labor while enhancing the efficiency of a monitoring program needed to assess reservoir water quality, any selected monitoring site must optimally characterize the overall hydrogeochemistry of the system. We therefore assessed how effectively a single, continuous monitoring station at Carlyle Lake, an impoundment of the intensively managed Kaskaskia River in central Illinois, represented the water quality of the entire reservoir. This monitoring effort is part of the Great Rivers Ecological Observation Network (GREON), an initiative to advance the science and understanding of river and watershed ecology by collecting and sharing high resolution data on key water quality parameters. As part of the GREON project, a monitoring buoy was deployed near Carlyle Lake’s dam from 2015 through 2020. The buoy was equipped with multiple water quality sensors, including standard parameters (e.g., temperature) as well as agriculturally relevant variables (e.g., turbidity, nitrate, chlorophyll, blue-green algae (BGA)) that can be proxies for eutrophication. From 2016 to 2017, ten additional locations across the reservoir were sampled eight separate times to determine the extent to which their water quality measurements deviated from those collected by the GREON buoy. We found that temperature, turbidity, and nitrate data collected by the GREON buoy were representative of sites up to ~12 km away, but the measurements collected at the most upstream sites could deviate up to nearly two orders of magnitude from the GREON buoy’s recorded values. Other agriculturally relevant water quality parameters, like chlorophyll and BGA, had high spatiotemporal variations across the reservoir and were thus not well represented by the GREON buoy (even for a sample site only a few meters away from the station). These results show the GREON program’s capacity to continuously monitor certain agricultural pollutants (e.g., nitrate) near reservoir outlets that can subsequently be delivered to downstream ecosystems, thereby informing stakeholders on reservoir water quality dynamics.