Preliminary Sediment Accumulation and Phosphorus Retention Behind Low Grade Weirs in the Mississippi Delta

Author(s): Usborne, E.; Kröger, R.

Agricultural phosphorus (P) loads, carried by surface runoff through ditches, contribute to eutrophication. Low grade rip rap weirs potentially reduce P from entering primary aquatic systems by slowing water flow through drainage ditches, allowing sediments to settle out and P to sorb to soil. Due to the relatively new application of weirs in Lower Mississippi Alluvial Valley drainage ditches, their effects over time are not understood. Weirs have the potential to alter the pH and hydraulic residence time within the ditch system, thereby affecting P retention by ditch soils. Three weirs and a control have been monitored monthly from the time of installation for one year in order to track changes in P bioavailability ratios and sediment deposition rates. Sediment depth was recorded using a permanent reference marker located behind the weir and P fractions were measured on collected sediment samples. Loosely bound P and iron phosphate were considered bioavailable while reductant-soluble P and aluminum phosphate were considered non-bioavailable. Locations that are losing sediment coincide with increasing bioavailability ratios and locations that are accumulating sediment have declining bioavailability ratios. Predicting time periods between soil P saturation and sediment accumulation limits will allow us to inform farm managers of optimal times to dredge ditches for greatest phosphorus retention efficiency.
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1 Best Management Practices #1
2 Delta Water Assessment
3 Flood Assessment & Mgmt.
4 Wetlands
5 Watershed Mgmt. #1
6 Non-Point Source Assessment
7 Modeling
8 Water Quality
9 Best Mgt. Practices #2
10 Delta Water Conservation
11 Sedimentation
12 Storm Water
13 Watershed Mgt. #2
14 Public Water Systems
15 Surface Water Assessment & Evaluation

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