Water Quality and Stratified Flow: How Density Currents Mitigate the Effects of Point and Nonpoint Loadings

Inaugural Heinz G. Stefan Fellowship Award Ceremony

Keynote Speaker: Dennis Ford, President and Principal Hydrologist, FTN Associates (and one of Dr. Stefan's first graduate students!)

2016 Award Recipient: Maria Garcia-Serrana, PhD Candidate, Department of Civil, Environmental, and Geo-Engineering, University of Minnesota (advisor John Gulliver)

Lake Catherine is a run of the river hydropower reservoir located on the Quachita River southeast of Hot Springs, AR in central Arkansas.  It is located immediately downstream of Lakes Hamilton and Quachita which are also hydropower projects.  Lake Quachita is a large Corps of Engineers impoundment that provides flood control and storage for the system of three lakes and is characterized by excellent water quality.  Cold hypolimnetic releases from Lake Quachita pass through both Lake Hamilton and Lake Catherine as density currents. The surface waters of Lake Catherine are typically much cooler than the upstream lakes.  Since Lakes Catherine and Hamilton have no storage, they are normally operated in series so that releases from Lake Hamilton coincide with releases from Lake Catherine.  The flow regime in Lake Catherine is a complex, dynamic combination of riverine and lake water movements.  During the summer months, the peaking hydropower operations result in reverse currents that move surface water back upstream after hydropower shuts down.

The water quality of Lake Catherine is impacted by 18 point sources, which enter the Lake either directly or indirectly, and nonpoint source runoff from urban and industrial areas.  By any measure of eutrophication, Lake Catherine should be characterized by poor water quality but it’s not because of complex, stratified flow dynamics.  The City of Hot Springs WWTP discharges directly into Lake Catherine immediately below Lake Hamilton through a submerged outfall.  The buoyant plume is quickly mixed with the cold water releases from Lake Hamilton and, after mixing, passes through Lake Catherine as a density current with minimal exposure to light.  An industrial discharger located near the middle of the lake uses a submerged multi-port high rate diffuser to quickly mix the highly dense effluent with ambient lake water, which eventually falls back to lake bottom and moves as a density current.  To minimize water quality impacts, the diffuser is operated as a hydrograph controlled release with a unique design that aspirates upstream dilution water to maximize dilution and keep the diffuser operating efficiently.  Near the dam, an electric generating plant draws cool, hypolimnetic water for cooling and discharges it back to the surface as a buoyant thermal plume.

Over the years, there has always been public interest with the water quality of Lake Catherine because of the large number of point/non-point sources to the Lake. To address the public, the State Legislature asked the Arkansas Department of Environmental Quality (ADEQ) to conduct a water quality study in 2013.  ADEQ concluded that it was safe to drink the water, eat the fish, and enjoy primary contact recreation. Our 30 years of study indicate how complex stratified flow dynamics mitigate the many point and non-point sources to the lake.