Spatial variability of hydraulic and transport properties for coarse porous media.

摘要

Understanding flow and solute transport phenomena in the vadose zone is an essential step in predicting, controlling and eventually preventing groundwater contamination. The goal of this current study was to contribute to our understanding of transport of water and chemicals through coarse heterogeneous soils. Specific objectives included the compilation of a comprehensive spatial database of hydraulic and transport properties for reactive and non reactive tracers within coarse sand and gravel, and the determination of the spatial variability and covariance between these properties. A total of 544 steady state flow experiments and 177 solute transport experiments, involving Cl - and Sr2+ tracers were conducted on 84 undisturbed cores from the U.S. Geological Survey (USGS) research site at Cape Cod, Massachusetts. The experimental design was equivalent to a 2-dimensional cross-section with a sampling grid of 8 and 25 cm in the vertical and horizontal directions, respectively. This permitted the determination of any small-scale variability of the hydraulic and transport properties. Saturated volumetric water content ($q$sat) ranged from 0.211 to 0.488 with a mean value of 0.333 and standard deviation of 0.044, with vertical correlation length scales, $lv$, ranging from 44 to 73 cm, and horizontal correlation length scales, $lh$, ranging from 49 to 68 cm. Saturated hydraulic conductivity, Ksat, ranged from 32.4 to 2325.6 cm h-1 with a geometric mean of 219.6 cm h-1, and had $lv$ values ranging from 21 to 61 cm, and $lh$ values of 52 to 69 cm. As mean unsaturated water content, $q$, increased, there was an increase in the standard deviations. For $q$, $lv$ ranged from 34 to 64 cm, and $lh$ from 60 to 160 cm. Weak positive linear correlations were observed for ln (Ksat) and the following parameters from soil water retention (SWR) and conductivity models: $aVG$ - inverse air entry pressure head; n - pore size distribution index; and, $aG$ - inverse macroscopic capillary length scale. Existing stochastic theory was applicable for describing the influence of correlation between ln (Ksat) and $aG$ on pressure head variance and mean pressure head relationships. By comparing the ratios of the velocity of the Cl- to that of the pore water velocity, the spatial variability of immobile and mobile flow was investigated. For immobile water during unsaturated conditions, $lh$ (105 cm) was almost twice the value obtained for saturated conditions (57 cm). The mean effective dispersivity of the chloride ion, ranged from 4.08 to 5.33 cm during unsaturated conditions and from 3.19 to 6.51 cm during saturated conditions. Retardation coefficients for strontium, R, were log normally distributed and ranged from a mean of 1.39 to 1.90 during unsaturated conditions and from 1.10 to 1.26 during saturated conditions. With the exception of a 26-cm layer, where $lh$ ranged from 127 to 90 cm, R was randomly distributed. Generally, no significant correlations were detected between R and the hydraulic parameters from either the SWR or conductivity models. A direct linear relationship was observed between the relative change in mean volume weighted pore sizes with the relative change in retardation. However, there was an inverse relationship between the relative change in