Lab-Scale Experiment for Assessing the Effect of Resuspension and Oxygenation on Sediment Oxygen Demand

This study examined the effects of resuspending lake sediment for different time periods in a lab-scale tank under both oxic and anoxic conditions on sediment oxygen demand (SOD) and other related sediment properties. This lab-scale study was conducted as a first step to determine if a proposed method for reducing SOD by treating sediments in lakes and reservoirs is feasible. SOD is a critical process responsible for the formation of anoxic hypolimnia in lakes and reservoirs. A reduction in SOD may delay or eliminate the onset of anoxia in the hypolimnia, preventing adverse ecosystem effects and improving water quality and ecosystem function. The proposed treatment method for lakes and reservoirs would resuspend sediment into the water column and mix the suspension under aerated conditions such that near saturated DO conditions are maintained during mixing. The sediment is resuspended such that it is fully exposed to dissolved oxygen, thereby allowing oxygen-mediated chemical and biological reactions to proceed more rapidly without being rate-limited by oxygen availability as occurs within intact sediment. By maximizing oxygen uptake rates of sediment processes over a period of time, the oxygen consuming processes responsible for SOD may be partially quenched, thereby reducing SOD once the treated sediment has resettled. Current methods for oxygenation of sediments rely on oxygenation of overlying water such that oxygen diffuses into sediment. This process is typically conducted over a period of years. The method proposed may have economic benefits when compared to hypolimnetic water oxygenation treatments if the increased operating cost of rapid treatment is offset by a shorter overall required treatment time. The first step for testing the rapid method was done in a laboratory to determine if SOD is reduced due to the treatment and if oxygenation of sediments results in a greater decrease in SOD than resuspension without oxygenation. Bottom sediments were collected from a local eutrophic reservoir, split into two samples, and then placed into 284 liter aquarium tanks. Sediment samples were resuspended for 3 hours, allowed to settle, resuspended for an additional 24 hours, allowed to settle, then resuspended for an additional 120 hours before being allowed to settle again. SOD, organic matter content, and sediment and water chemistry parameters were measured before and after each treatment period. Initial SOD in the lab experiments was greater than that measured in the lake. SOD was reduced an average of 32% over the course of the experiment, with no significant benefit from oxygenation during resuspension. Organic matter and other sediment quality parameters remained unchanged throughout the treatment. The concentration of all water quality parameters with the exception of Mn increased in the water column over the course of the treatment. Overall, the rapid oxygenation treatment method explored in this study did not appear to be an economically feasible alternative to existing longterm treatment methods of hypolimnetic oxygenation, but resuspension of sediments without oxygenation may have potential as a reservoir sediment remediation technique. However, due to the high degree of experimental error in this data, further studies must be conducted to determine if SOD reduction can be repeated under more closely controlled conditions. Further studies are also required to investigate the economic feasibility, potential benefits, and potential negative impacts of the resuspension treatment method.