Impact of measurement scale on infiltration and phosphorus leaching in Ozark floodplains

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. The significance of this problem has been highlighted by litigation, with one case even reaching the U.S. Supreme Court (Arkansas et al. v. Oklahoma et al., 503 U.S. 91) which required the upstream state to meet downstream water quality standards. The overarching objective of this line of research was to characterize phosphorus leaching to alluvial aquifers in the coarse gravel floodplains of the Ozark ecoregion, while the specific objective of this paper was to quantify infiltration and hydraulic conductivity across a range of scales (point to 100 m²) to evaluate the effect of the scale of measurement. It is hypothesized that hydrologic heterogeneities (e.g., macropores and gravel outcrops) in the subsurface play an integral role in impacting flow and contaminant transport between the soil surface and alluvial aquifers. Innovative field studies, including plot scale injection experiments, were performed across a range of soil types at each of three floodplain sites in the Ozark ecoregion. Solutes in the injection water included phosphorus, P (highly sorptive), Rhodamine WT (slightly sorptive), and chloride (conservative). Plots maintained a constant head of 2 to 9 cm for up to 52 hours. Effective saturated hydraulic conductivity (X_eff) data, based on plot scale infiltration rates, were high (0.6 to 68 cm hr^-1) and varied greatly, even within a single floodplain. The K_eff was also measured with a double ring infiltrometer and estimated at the point scale using Retention Curve (RETC) along with particle size distribution data. Point scale estimates were significantly lower than plot scale K_eff and also failed to capture the variability of K_eff within a field site. The estimated permeability of the limiting layer reported by the U.S. Natural Resources Conservation Service (NRCS) Soil Survey was consistent with point scale estimates of K_eff, but was lower than plot scale K _eff at most sites. Plot scale infiltration tests are recommended over double ring infiltrometer tests or point scale estimates, although only small plots (1 m by 1 m) are necessary.