OCC-Commissioned Study on Subsurface Phosphorus Transport

Riparian Buffers, Floodplain Management and Subsurface Phosphorus in Alluvial Floodplains in the Ozark Plateau.

To protect and enhance the status of drinking water, aquatic ecosystems, and recreation activities, water managers need to identify critical nutrient sources and transport mechanisms within a watershed to develop strategies for reducing contaminant loads. For phosphorus, the primary transport mechanism from source areas into surface water systems is usually considered to be surface runoff, with subsurface transport considered to be negligible. However, unique local or regional conditions in some areas call for consideration of subsurface transport as a contributor to pollutant loading.

With funding from the U.S. Environmental Protection Agency under Section 319 of the Clean Water Act, OCC has been implementing conservation practices in the Illinois River and Honey Creek basins in northeast Oklahoma to decrease phosphorus reaching the streams. Conservation practices of specific focus include the use of riparian buffers to filter surface runoff and fences to keep cattle out of the streams. Both of these have proven effective in decreasing phosphorus in surface runoff, but may not help with subsurface phosphorus transport.

The Ozark ecoregion includes caves, sinkholes and streams that lose water to ground saturation. Erosion of limestone has left an abundance of chert gravel, producing gravel-bed streams and aquifers overlain with gravelly loam or silt loam soils. Previous work conducted by OSU researchers at a site in eastern Oklahoma found that areas of gravel subsoil actually serve as subsurface “veins” or preferential flow paths (PFPs) of water to and from streams with estimated water movement of 140 to 230 meters per day.  It was then hypothesized that these PFPs, if not blocked from the soil surface, could provide a rapid phosphorus transport mechanism through the groundwater system to the stream.

OCC funded a new study conducted by researchers at OSU’s Department of Biosystems and Agricultural Engineering. The study, Riparian Buffers, Floodplain Management and Subsurface Phosphorus in Alluvial Floodplains in the Ozark Plateau, specifically looked into the issue of transport of phosphorus at sites in the Illinois River and Honey Creek watersheds. The primary study objectives were to (1) document occurrence of PFPs from several floodplain sites of differing size and location in the watersheds, (2) determine the activity of the PFPs and their relation to instream flow dynamics and water quality, and finally, (3) determine the transport capacity of these PFPs to help quantify phosphorus loading to streams.

Using subsurface geophysics to map PFPs at four floodplain sites, researchers found zones of high water movement within each floodplain. Water table height data showed that PFPs act as divergent zones, allowing stream water to quickly enter the groundwater system, or convergent zones, draining a larger groundwater area, depending on stream stage. Active PFPs connected to the stream showed phosphorus levels approaching that found in the stream itself during flood events, proving the speed and completeness of interaction between groundwater and surface water. Modeling scenarios showed that alluvial aquifers have a capacity for subsurface phosphorus transport, substantiating the possibility of bypassing riparian buffers. Also, where phosphorus leaching through topsoil is significant, subsurface flow paths may be a rapid means of transporting phosphorus from fertilizer at the soil surface into the stream, bypassing riparian buffers.

The findings of the study could have a significant impact on how riparian floodplains are managed in the Ozark and similar ecoregions, and may provide guidance on recommended buffer widths to prevent phosphorus bypassing buffers through subsurface flow paths. 

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