UNDERSTANDING AND PREDICTING THE IMPACT OF AGRICULTURE ON THE ENVIRONMENTAL INTEGRITY OF MANAGED WATERSHEDS
Location: Water Quality and Ecology Research
Title: Evaluating the influence of wetland vegetation on chemical residence time in Mississippi Delta drainage ditches
Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 3, 2009
Publication Date: April 24, 2009
Citation: Kroger, R., Moore, M.T., Locke, M.A., Cullum, R.F., Steinriede Jr, R.W., Testa III, S., Bryant, C.T., Cooper, C.M. 2009. Evaluating the influence of wetland vegetation on chemical residence time in Mississippi Delta drainage ditches. Agricultural Water Management, 96: 1175-1179.
Interpretive Summary: Bare drainage ditches efficiently move water off the production landscape and into receiving aquatic systems. Unfortunately, this rapid transport can also result in significant amounts of sediment, nutrients, and pesticides which might harm plants and animals in rivers, lakes and streams. Using a salt tracer chemical, a comparative study was done to compare the water holding time of two ditches—one with and one without plants. The ditch with plants held water twice as long as the one without plants. This indicates that during storm runoff, the ditch will continue to carry water (preventing flooding) from the production landscape; however, it will slow the speed of water down in order to allow for the contaminants to be processed by the plants in the ditch.
The presence of emergent vegetation within channelized aquatic environments has the capacity to provide a number of biological functions as well as alter the hydrology of the system. Vegetation within the channel exerts roughness, drag and friction on flowing water, reducing flow rates, increasing water depths and increasing hydraulic retention time. By increasing the hydraulic retention time, chemical residence time (CRT) is increased, thus improving the potential of pollutant mitigation. The study compared two gemorphologically similar drainage ditches, one vegetated and one non-vegetated to evaluate the effect obligate, in stream wetland vegetation had on CRT. A fluoride (Fl-) tracer was amended to both ditches with nutrients and sediments to simulate stormwater runoff event. The measured CRT of the vegetated drainage ditch was at least twice that of the non-vegetated ditch. These results suggest that with the presence of vegetation increasing CRT, chemical removal rates will improve, and as a result increase the possibility of microbial transformation, adsorption, and macrophyte assimilation. By dredging or clear-scraping ditches and removing the vegetative component, farmers and managers alike will increase water flows, decrease CRT and potentially increase pollutant loads into aquatic receiving systems.