Characterization of Nutrient and Sediment Transport in a Restored Agricultural Stream

Abstract

The Purlear Creek restoration project is a restored agricultural stream located in Wilkes County, North Carolina. The project site consists of 3,755 linear meters of restored stream located in a pastured beef operation. Restoration activities included the return of dimension, pattern and profile as well as reestablishing a reliable floodplain connection. Permanent fencing was installed to protect a 7.6 m vegetated buffer on either side of the stream and prevent cattle from damaging the riparian area.

Load calculations indicate a 36% (0.16 kg N⁄day⁄100m) decrease in mass of total nitrogen (TN) between the inflow and outflow stations located at the boundaries of the restored reach. Nitrate nitrogen (NO3-N) increased by 29%, but the corresponding decrease in total nitrogen indicates the presence of transformation processes within the project site. The mean TKN concentrations throughout the site were higher than other streams within the same Ecoregion as reported by the EPA.

The loading calculations indicate a 19% reduction in both TP and O-PO4-P as the load from the inflow tributaries pass through the restored site. The mean TP concentration for all the sampling stations throughout the study site were well above the TP concentration observed at other streams in the Ecoregion.

The TSS concentration exhibited a significant reduction in both concentration and load as the inflow surface water passed through the restored site. The inflow load was reduced by 82% (6.74 kg⁄day⁄100m) over the course of the study period. Permanent cross sections along the tributary below the unrestricted cattle access confirm the deposition of material along the channel and indicate a decrease in cross sectional area over the course of the study. Of the inflow tributaries, the tributary flowing from a pasture with unrestricted cattle access to the stream repeatedly returns markedly higher concentrations of constituents such as TN, Organic N, total ammonium nitrogen (NH3-N), total phosphorus (TP), orthophosphate phosphorus (O-PO4-P), total suspended solids (TSS) and Escherichia coli.

The Nitrogen Loss Estimation Worksheet (NLEW) and Phosphorus Loss Assessment Tool (PLAT) programs were used to quantify the anticipated nitrogen and phosphorus losses from within the pasture. These tools indicate an estimated field loss of 4,394 kg N⁄yr and 214 kg of P⁄yr when fertilizer application rates similar to the actual application rates were used for the study period.

Overall, the combination of the stream restoration and the implementation of a vegetated riparian buffer indicate an improvement in water quality within this agricultural headwater restoration. Conditions favor nutrient cycling which decreases the quantity of N. Sedimentation within the restored site decreases the quantity of TSS and TP transported through the site. Benthic macroinvertebrate collected post restoration does not indicate a favorable ecological response to the restoration efforts.