Impact of macropores and gravel outcrops on phosphorus leaching at the plot scale in silt loam soils

Authors: HEEREN, DEREK - University Of Nebraska; FOX, GAREY - North Carolina State University; Penn, Chad; HALIHAN, TODD - Oklahoma State University; STORM, DANIEL - Oklahoma State University; HAGGARD, BRIAN - University Of Arkansas

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2017
Publication Date: 2/1/2017
Citation: Heeren, D.J., Fox, G., Penn, C.J., Halihan, T., Storm, D.E., Haggard, B.E. 2017. Impact of macropores and gravel outcrops on phosphorus leaching at the plot scale in silt loam soils. Transactions of the ASABE. 60(3):823-835.

Interpretive Summary: Excess phosphorus (P) loss to surface waters result in negative water quality. Thus, there is a need to determine the source of P to surface waters, and learn more about how P is transported from soils to surface waters. The objective was to measure P leaching through silt loam soils into underlying gravel layers in gravel aquifers adjacent to streams. The surface soil type had a major impact on P leaching. Soils where the gravel layer was located closer to the surface resulted in higher infiltration rates and P transport. Water moved very quickly in these systems, up to 810 cm/hr. Dissolved P concentrations in the nearby observation wells ranged from 0.54 to 1.3 mg/L, demonstrating that soil P can be easily leached and transported in these gravelly subsoil systems. Understanding the source and transport mechanism is necessary for those who plan to design efficient control practices.

Technical Abstract: In response to increased nutrient loads in surface waters, scientists and engineers need to identify critical nutrient source areas and transport mechanisms within a catchment to protect beneficial uses of aquatic systems in a cost effective manner. It was hypothesized that hydrologic heterogeneities (e.g., macropores and gravel outcrops) in the vadose zone play an integral role in affecting flow and solute transport between the soil surface and shallow alluvial aquifers. The objective of this research was to characterize phosphorus (P) leaching through silt loam soils to alluvial gravel aquifers in the floodplains of the Ozark ecoregion at the plot scale. Solute injection experiments used plots (1 m ' 1 m, 3 m ' 3 m, and 10 m ' 10 m) that maintained a constant head for up to 52 h. Solutes in the injection water included P (highly sorptive), Rhodamine WT (slightly sorptive), and chloride (conservative). Electrical resistivity imaging identified zones of preferential flow. Fluid samples from observation wells indicated nonuniform subsurface flow and transport. The surface soil type, ranging from silt loam to clean gravel outcrops, had a significant impact on P leaching capacity, with gravel outcrops resulting in high infiltration rates and rapid solute detection in wells (e.g., 4 min). Even in silt loam soils without gravel outcrops, macropore flow resulted in rapid transport of P. Maximum transport velocity for soluble reactive P in one silt loam plot was 810 cm h-1, compared with a mean pore water velocity in the range of 25 to 130 cm h-1. Soluble reactive P concentrations in observation wells reached up to 0.54 mg L-1 in silt loam plots and 1.3 mg L-1 in gravel outcrop plots, demonstrating that a highly sorbing solute can be mobile.