Quantifying Subsurface Hydrology Effects On Chemical Transport in Drainage Ditches Using a 20-Meter Flume.

Abstract

      Agriculture drainage ditches serve as the veins of the Midwestern agricultural landscapes.   The transport of chemical fertilizers and pesticides in these ditches affect the local and downstream ecosystems.  Although much research has already been conducted on chemical transport in streams and drainage ditches as well as through drainage tiles; there has not been sufficient research on the effects of subsurface hydrology on nutrient storage and interactions between the stream water and the hyporheic zone.  In this study, a 20-meter flume was filled with ditch sediment from Marshall Ditch at Purdue University’s Agronomy Farm in West Lafayette, IN to serve as an artificial drainage ditch.    A water table control was built such that the stream bed can be set to either drainage, saturated or seepage conditions.   The flume can also be adjusted for different surface flow hydraulics, i.e., flow depth and flow rate.   An injection study is designed to quantify nutrient transport under three subsurface hydrologic conditions representing a losing stream (water table set below the sediment bed), saturation of the sediment bed and the seepage condition (water table set higher than sediment bed).    Surface water quality samples will be collected by automatic samplers located at 0, 5, 10, and 20 meters every minute for one hour.  Sediment samples will be taken before and after each injection.  Drainage water quality samples will be also collected every 2.5 meters in the losing stream treatment.   The results will be used to calculate nutrient uptake length; uptake velocity and uptake flux as the injection plume travels the length of the simulated stream reach.  We anticipate the changing subsurface hydrologic condition will have a significant impact on these nutrient exchange parameters between the stream water and bed sediment and this information will improve the understanding of nutrient fate and transport in agricultural drainage ditches.