HOW EFFECTIVE ARE NATURAL RIPARIAN BUFFERS AT FILTERING SEDIMENTS AND CONTAMINANTS FROM AGRICULTURAL FIELDS?

Erosion from agricultural watersheds has been identified as the major process contributing sediment as well as sediment-bound nitrogen and phosphorous to the Chesapeake Bay. In recent decades, population growth and increased development of agricultural areas has accelerated the flux of sediments and contaminants to the nation’s largest estuary. One common method for protecting surface waters from agricultural sediment sources is a riparian buffer, which in Virginia, is typically a 100’ zone of native of vegetation forming a transitional boundary between the upland and aquatic environments. While this practice is thought to provide a natural filter strip to prevent sediments and other elements from entering surface waters, the effectiveness of natural riparian buffers in maintaining water quality in Virginia has not been thoroughly tested.

Due to the continued decline of water quality in the Chesapeake Bay and its major tributaries, we hypothesize that the standard 100’ forested riparian buffer is failing to completely trap sediments and contaminants derived from agricultural lands in Virginia. This study aims to quantify sediment, nitrogen, and phosphorous transport through riparian systems buffering tributaries to the lower James and York Rivers from agricultural fields. We analyze shallow and deep hydrologic transport pathways, by measuring sediment at the surface and dissolved nutrients in groundwater. In order to gauge the amount of sediment transport along hillslope transects, ¹³⁷Cs is used as a tracer to quantify sediment loss and storage. Transects have two lateral sampling points in the O-Horizon, upper mineral soil, and lower mineral soil; care was taken to establish continuity between transects with a sampling point in the agricultural field, at the entrance to the riparian buffer, multiple points through the buffer, and at one meter from the stream. Multiple transects were taken from agricultural fields, and from control sites that have limited anthropogenic impacts. Control sites in preserved areas are used to determine the base level of ¹³⁷Cs present in coastal plain soils, and to account for natural nutrient and sediment transport that occurs on forested hillslopes.