What beryllium-7 could tell us about functionality of sub-surface agricultural drainage (tile drains)

Abstract:

Multiple lines of evidence indicate that increased sub-surface agricultural drainage (tile drains) has shifted hydrologic flow patterns in the Upper Midwest, USA. The placement and maintenance of individual tile-drain systems at the field scale is less understood. Beryllium-7 (⁷Be), a short-term cosmogenic radionuclide with a 54-day half-life, is delivered to the Earth’s surface primarily in precipitation. Due to its short half-life and affinity for sorption, ⁷Be has been used as an erosion tracer on the event to seasonal timescale. High levels of ⁷Be have been used to indicate recent surface erosion due to precipitation. More recently, high levels of ⁷Be in bulk runoff and surface water samples have been used to indicate fast-flow hydrologic pathways through small watersheds. We present novel data comparing ⁷Be in bulk water samples from paired surface and tile-drain discharge from edge-of-field monitoring sites, managed by the same producer, draining to western Lake Erie, Great Lakes, USA. Samples were collected at the event scale in conjunction with other water-quality parameters. Event-based samples indicate a larger presence of direct surface connections to rainfall and detachment of surface soil in the traditionally entrenched, topographic-soil tile-drain system in one field relative to the adjacent field which has a higher-density system. This is evidenced by tile-drain discharge, that would be expected to drain soil water, exhibiting similar ⁷Be activities as that measured in surface runoff, that would consist of newly precipitated water and soil particles exposed during these events. While this relation between what is likely an older system, with a higher potential for unmanaged, direct surface connections (and thus, decreased water quality) is not surprising, this type of information is rarely known about individual tile-drain systems. In practice, usually only the outlet into the stream channel is known and there is little to no information about the functionality of the tile-drain system. This exploratory study indicates the potential for ⁷Be to enable a comparative way to characterize tile-drain functionality with minimal ⁷Be background sampling, thus identifying tile-drain systems that disproportionately contribute to water-quality concerns.