Quantifying subsurface hydrology effects on chemical transport in agriculture drainage ditches using a 20 meter flume.

摘要

Agriculture drainage ditches serve as the veins of the Midwestern agricultural landscapes. The transport of chemical fertilizers and pesticides in these ditches affects both 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 Center for Research and Education 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. A series of short term injection studies, using phosphorus and bromide, were performed under three subsurface hydrologic conditions representing a losing stream (water table set below the sediment bed), saturation of the sediment bed and a seepage condition (water table set higher than sediment bed). Five treatments were compared: two seepage rates, two drainage rates and one saturation treatment.;Surface water quality samples were collected by automatic samplers located at 5, 10, 15, and 20 meters on one minute time intervals for the designated experimental timeframe. Drainage water quality samples were also collected every 2.5 meters during the drainage treatment. It was found that under drainage conditions, there was no influence from the sediments on phosphorus mass transport to the surface water; i.e. surface water that entered the subsurface sediments did not return to the surface water column. The removal rate of total mass phosphorus during drainage conditions was directly related to the hydraulic flux of the treatment. During seepage and saturation conditions, the bromide and phosphorus analysis indicated possible storage in dead pool zones of the flume that were immeasurable, indicating that dead pool zones could be more influential during certain hydrologic conditions.