Date of this Version
Submitted to the Faculty of the Graduate College of the Oklahoma State University (2012)
Increased nutrient loads have resulted in several adverse impacts on surface water quality, including excessive algal growth, fish kills, and drinking water taste and odor issues across the United States and especially in the Ozark ecoregion of northeastern Oklahoma and northwestern Arkansas. Nitrogen is a concern, but phosphorus (P) is generally considered the limiting nutrient in most surface water systems. Scientists and engineers need to identify critical nutrient source areas and transport mechanisms within a catchment in order to cost effectively protect and enhance drinking water systems, recreation activities, and aquatic ecosystems. While surface runoff is considered to be the primary transport mechanism for P, leaching through the vadose zone and subsurface transport through coarse gravel subsoils to gravel bed streams may be significant and represents a source of P not alleviated by current conservation practices (e.g., riparian buffers). Therefore, the overarching objective of this research was to characterize subsurface phosphorus transport in the coarse gravel floodplains of the Ozark ecoregion.
An injection test was performed which showed preferential flow paths and physical non-equilibrium in the coarse gravel vadose and phreatic zones. Preferential flow paths were interpreted to be buried gravel bars. Long-term flow and transport monitoring was performed at two floodplain sites, showing aquifer heterogeneity and large scale bank storage of stream water, as well as large scale, stage-dependent transient storage of P in the alluvial aquifer. Subsurface P transport rates in the alluvial aquifers were quantified and found to be significant compared to surface runoff P transport rates on well managed pastures. In order to quantify P leaching through the vadose zone, a berm method was developed and utilized for plot-scale infiltration experiments. The surface soil type (ranging from silt loam to clean gravel) and macroporosity were found to have a significant impact on P leaching capacity. This research highlighted the difference between the conceptual model of uniform piston infiltration and actual infiltration in field conditions. Since floodplains are well-connected to alluvial aquifers and streams in gravelly watersheds, a higher level of agricultural stewardship may be required for floodplains than upland areas. This has implications for the development of best management practices specifically for floodplains in the Ozark ecoregion due to their close proximately and connectedness to streams.