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Evaluating Phosphorus Removal Filters At The Agricultural Field Scale
Nadkarni, Nisha
Nadkarni, Nisha
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Excess phosphorus (P) contributes to the eutrophication of freshwater bodies and subsequent harmful algal blooms. This is an increasingly urgent problem in freshwater systems globally, including the Lake Champlain Basin. Agriculture, including dairy farming, is a prominent part of the landscape within the Lake Champlain Basin and contributes a significant portion of the P load entering Lake Champlain. In-field agricultural management practices such as no-till management and cover cropping support reductions in total P (TP), but do not effectively address dissolved P. There are promising edge-of-field practices that may reduce dissolved and total P loads from agricultural surface and subsurface runoff, but their efficacy remains untested. This study investigates the performance of two surface and two subsurface edge-of-field (EOF) iron-based filters for P removal installed as part of a Conservation Effects Assessment Project (CEAP) Stacked Practices and Innovative Phosphorus Removal project. The field-scale study took place on a dairy farm in Bridport, Vermont and was initiated in 2021. The study evaluated the ‘stacking’ of multiple conservation practices at the field-scale including manure injection, no-till management, cover cropping, and edge-of-field phosphorus removal technologies, including surface ditch filters and subsurface tile drain filters. Both filter systems were comprised of metal iron shavings mixed with pea gravel with the goal of reducing dissolved and total P loads.
This thesis documents the monitoring of stormwater runoff at the inlet and outlet of EOF filters from 2021 to 2025. The results of the EOF filter monitoring from 2021 to 2023 were collected and analyzed in Chapter 3 of the study conducted by G. H. Myers (Myers, 2023). This thesis combines the results from Chapter 3 in Myers (2023) with additional new results from the EOF filter monitoring during 2023-2025. One subsurface filter proved very effective for soluble reactive phosphorus (SRP) and TP removal, removing 87% of cumulative SRP load and 69% of cumulative TP load from monitored events. The second subsurface filter removed 51% of cumulative SRP load and 54% of cumulative TP load during monitored events. The nutrient removal performance of the surface filters could not be monitored due to the occurrence of severe sediment accumulation and clogging that caused both filters to hydraulically malfunction following a flood event early in the two-year study period. Surface runoff volumes and nutrient loads were still monitored and quantified prior to entering surface filters to support a larger watershed-scale study that encompasses the fields in this study. It is recommended to optimize filter sizing and hydraulic design to reduce the impact of sediment loading and preferential flow pathways on surface and subsurface EOF filter performance. The study provides statistically significant results that demonstrate tile drain filters can be an effective management strategy for reducing SRP and TP losses from agricultural fields.
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2026
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