Sediment and Phosphorus Accumulation Dynamics behind Newly Installed Low-Grade Weirs in Agricultural Drainage Ditches.

Low-grade weirs are controlled surface drainage management structures that are being advocated for improving water quality of downstream ecosystems within agricultural landscapes. Low-grade weirs are small controlled drainage structures that are placed in series in an agricultural drainage ditch to create biogeochemical conditions for enhanced nutrient retention but also to slow runoff velocities and encourage sedimentation. This study evaluated eight low-grade weirs for sediment and P accumulation in agricultural drainage ditches 1 yr after installation. On average, sediment (weir: 54 ± 25cm; reference: 13 ± 7 cm; ≤ 0.001) and water (weir: 9 ± 7cm; reference: 1.16 ± 2 cm; ≤ 0.05) depth was greater upstream of weirs when compared within ditch references sites. There were no significant differences in total P concentrations of sediments or between any P fractions between 1-yr-old weirs and their respective references. Bioavailability ratios of P (i.e., the ratio of potentially bioavailable to nonbioavailable P fractions) were also found to be similar between weir and reference sites. Based on these results, weirs increase the hydrological capacity of drainage ditches and significantly retain more sediment and P within the drainage ditch within 1 yr of construction. Future research will examine temporal changes in weir sediments and associated P concentrations to aid our understanding of how maintenance of weirs should occur to maximize physical and chemical characteristics for greatest sediment and P retention.

Continuous, short-interval redox data loggers: verification and setup considerations.

Reduction-oxidation or redox potential is typically collected by measuring redox at a single time interval and returning to the electrode to collect subsequent intervals to generate a temporal gradient of changes in redox. Typically, intervals between sampling are on the scale of hours, days, and weeks, rather than one, five, or 20 minutes due to logistical constraints of collection. These constraints are labor (i.e., constant measurements 24/7) and technology driven (i.e., construction of a unit that is capable of accurately and precisely measuring redox at fine temporal scales). This study describes a continuous, short interval redox data logger that is capable of measuring ±10 mV at minute time intervals. To ensure quality assured and quality controlled data, the redox unit was subjected to tiered verification procedures that documented hardware and probe sensitivity to changes in voltage. Furthermore, the setup was laboratory tested against known mV redox solutions (Zobel, 225 mV), flooded in soil medium over 48 h, and subjected to drying over 48 h. Results highlight and verify the accuracy and precision of the redox probes and hardware for measuring stability and changes in redox. Future research will investigate field operations of redox probes and create spatially and temporally detailed investigations to changes in redox as a result of vegetation, flooding, and management.