Solute transport in gravelly, loess soils of the Rathdrum Prairie, Idaho

Gravelly, loess soils of the Rathdrum Prairie region of northern Idaho have low water holding capacities and high permeabilities, making an underlying aquifer susceptible to contamination from surface-applied agrichemicals and municipal wastes. The water-quality of this regional drinking water supply is protected by a "sole-source" designation which requires heightened management of potentially harmful land use. The purpose of this study is to quantify the hydraulic properties of a representative Rathdrum Prairie soil and to determine its solute transport characteristics. Specific objectives are: (1) to determine water-release and hydraulic conductivity functions, (2) to examine the effects of gravel and transient infiltration on solute transport, and (3) to ascertain the mechanisms of solute dispersion. A 0.15 ha experimental site was established on a representative Andic Xerumbrept. The site was equipped with a micro-irrigation system and instrumented with TDR probes and tensiometers to measure soil-water content and matric potential at the 0- to 30-, 30- to 60-, and 60- to 90-cm depths. Fifty-four soil-solution samplers were installed at 30-, 60-, and, 90-cm depths to measure tracer concentrations. A quasi-steady infiltration experiment was conducted by introducing a pulse of tracer solution when irrigation water was being applied every 8 h at 1.2 cm d{dollar}sp{lcub}-1{rcub}{dollar}. A transient infiltration tracer experiment was conducted by introducing a pulse of tracer solution when irrigation water was being applied every 48 h at 1.5 cm d{dollar}sp{lcub}-1{rcub}{dollar}.; Water release and hydraulic conductivity functions, obtained using the instantaneous profile method, depended on gravel content. Generalized hydraulic functions were obtained for the Ap and AB horizons by scaling soil-water content by the relative volume increase in gravel. Success of this scaling method indicates that the hydraulic properties of the {dollar}<{dollar}2-mm fraction are similar in both surface horizons. Scaling was not useful for extending the generalized hydraulic functions to the Bw horizon. This result reflects the substantial differences in texture of the {dollar}<{dollar}2-mm fraction between the surface and subsurface horizons.; Field-averaged breakthrough curves (BTCs) from the tracer experiments showed that approximately 1.4 pore volumes of water were required to move the solute pulse through the soil profile under steady infiltration and that 2.2 pore volumes were required under transient infiltration. The greater infiltration requirement of the transient experiment was attributed to bypass of the active transport volume by approximately 50% of the transient water flux. Solute transport was flux-dependent unless cumulative infiltration was corrected for bypass flow. Distributions of local BTC parameters, obtained using moment analysis, were represented best by normal functions. Analysis of scale dependence of these distributions showed that local-scale variability accounted for only 50 to 70% of the spreading of the field-averaged BTCs, illustrating that solute transport models must be carefully tailored to the scale of the intended application.; Process modeling of bypass-corrected BTCs revealed that solute transport in the upper 60 cm of the soil profile was a convective-dispersive process under both infiltration regimes. This finding implies that large quantities of gravel encouraged rapid lateral mixing of solutes and that a convection-dispersion transport model is appropriate in this region of the soil. An increase in bypass-corrected BTC variance between 60 and 90 cm indicated that solute transport across the horizon boundary was influenced by localized water flow and that solute movement in this region was not a convective-dispersive process.