The influence of mechanical circulation on water column stability and dissolved oxygen in stratified lakes

Surface circulators have been used in wastewater and industrial lagoons for many years, but only recently have they been designed for lakes and reservoirs as a water quality management tool. Their relatively low capital investment and operational costs make them an attractive chemical free option for ameliorating the symptoms of eutrophication. Little research has been conducted on their effectiveness in impacting water quality. Today many lake managers are waiting for independent research on surface circulators' performance before installing such systems.;The Occoquan Reservoir was the primary site for a study conducted to examine the influence of surface circulation on water quality. Eight surface circulators were installed in the reservoir in 2006 in an effort to lower iron and manganese by increasing dissolved oxygen in the lower waters. Oxygen and temperature profiles were recorded at three main sampling sites from April through October 2007. These data were compared with historical data collected by the Occoquan Watershed Monitoring Laboratory (OWML) to determine the influence of circulator operation. Near field flow studies were conducted around one circulator using an acoustic doppler velocimeter (ADV).;It was found that the circulators had significantly less influence on the temperature and dissolved oxygen of Occoquan Reservoir. This appeared to be contributed by the low circulation flow rates that were measured.;A modeling effort was also conducted to simulate measured temperature and dissolved oxygen profiles through 2007. Since CE-QUAL-W2 does not explicitly contain a module to directly simulate surface circulation, the pump module was modified to transfer water from a discrete depth in the hypolimnion to the surface. After a satisfactory agreement between measured and simulated temperature and dissolved oxygen profiles was obtained, the unknown zero flow condition of the reservoir was studied. Next, a series of hypothetical lakes were simulated to explore the influence of various lake sizes, circulation flow rates, and intake depths on the temperature and dissolved oxygen distribution. It was found that the vertical temperature distribution was relatively easy to affect, and it responded to the physical mixing from the surface circulators in a conservative manner. The vertical dissolved oxygen distribution was harder to influence since it was controlled by many processes involving oxygen supply and demand in the water column and the sediments.;A new approach for evaluating the circulator performance was developed based on the reduction in sediment area subjected to anoxic overlaying water. This reduction in anoxic sediment area is considered proportional to the reduction in iron and manganese to the lake, and would therefore represent a benefit to the water quality.;Key Words: Surface circulators, temperature and dissolved oxygen profiles, ADV, and CE-QUAL-W2.