Monitoring And Modeling Of Hydrology And Subsurface Nutrients Within Vegetative Treatment Areas

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Vegetative treatment areas (VTA) are commonly used as an alternative means of treating agricultural wastewater. Little information exists regarding the effectiveness of these VTAs at removing nutrients in the subsurface. Furthermore, current design methods and recommendations do not fully incorporate hydrological processes that govern likelihood of soil saturation and surface discharge. The first study utilized an applied tracer and a simple binary mixing model within two VTAs to characterize incoming wastewater movement following an event. Results demonstrated that concentrated surface flow paths existed within both VTAs. Rapid preferential flow to shallow monitoring wells was also observed. A shallow restrictive layer (i.e. fragipan) likely exacerbated surface flow but restricted runoff movement to deeper groundwater. A more comprehensive VTA design process is called for that accounts for shallow soils and antecedent moisture conditions. The importance of regular maintenance and design measures to prevent the formation of concentrated flow paths to prevent surface discharge was made apparent. The second study investigated subsurface nutrient removal within three VTAs (WNY, CNY-East, and CNY-West) receiving silage bunker runoff. This was one of the first studies performed on VTAs receiving this type of wastewater. Conservative tracer and nutrient data from a monitoring well network within each VTA were used to calculate mass balances. Mass removal of ammonium in all three VTAs was over 60%. Very little nitrate entered or exited any of the VTAs. Removal of soluble reactive phosphorus varied, and actually increased in one VTA where soluble reactive phosphorus loading was relatively low. Results also demonstrated that nutrient reduction mechanisms other than vegetative uptake can be significant within VTAs and that groundwater impairment from leaching of nitrate beneath the VTAs was not likely. Results highlighted the importance of capturing concentrated low-flows in VTA systems. The third study built upon the findings of the first study. An existing model was modified and adapted for VTA design and/or site evaluation. This model accounts for soil depth and cumulative rainfall. It was calibrated using continuous groundwater elevations collected within a VTA. It is available in an easy-to-use format and is a significant improvement over current design methods.

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