Graduate Thesis Or Dissertation
 

Water and anion movement in selected soils of western Oregon

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  • Experiments were carried out to study the movement of water and halide anions in perched water tables or saturated zones in the soil and upper rock mantle of eight different experimental sites located on low hills near the western border of the Willamette Valley. At each site the experimental procedure consisted of first injecting a solution of either KBr or KI into one of three line sources located at different depths in the soil-rock mantle. Ground water samples were then periodically collected from both saturated and unsaturated depths in the soilrock mantle at points both upslope and downslope from the line source. These samples were analyzed for either I⁻ or Br⁻ by specific ion electrodes. Tensiometers and piezometers were monitored at different depths in the soil-rock mantle at each site to determine water pressure potentials (P's) which were used to calculate hydraulic gradients both in the vertical and downslope directions. Saturated hydraulic conductivity measurements were made on intact cores of soil and weathered rock from different depths at each site. In situ conductivity measurements were made where possible at depths where intact cores could not be obtained. Conductivities of intact cores were found to be log normally distributed in the A horizons, B horizons, and also possibly in the C horizons. Further, an exponential relationship was found between the percent pore space occupied by pores drained by P = -50 cm of water and the conductivity of intact cores taken from different depths at all sites. Assuming that pores drained by P = -50 cm transmit most of the water flowing through the soil-rock mantle expressions were obtained for an average pore velocity (μ) of water flowing through pores drained by P = -50 cm, and for an average pore velocity (vi) of water flowing through different pore size increments (assuming a capillary bundle model) within this pore space. Plots of P's and rainfall vs. time indicated that impermeable regions of rock near or below the 110 cm depth are mainly responsible for the development of perched water tables at most sites. Similar plots indicated that subsurface flow contributed water to sites located on lower hill slope positions. Halide anions were found to move rapidly downslope by saturated flow at all experimental sites. These rapid rates of anion movement as well as other observed characteristics of anion movement suggested that water may move through the soil-rock mantle at these sites by laminar flow through interconnected regions of high conductivity and/or via turbulent flow through macroscopic voids. To attempt to describe the maximum rates of anion movement at several sites, pore water velocity vectors μ and Vi were calculated by resolving these vectors into vertical and downslope components. μ vectors failed to account for the maximum rates of anion movement even when conductivity coefficients were selected to maximize μ in the downslope direction. However, vi vectors when maximized in the downslope direction roughly approximated the maximum rates of anion movement. This together with some additional evidence concerning the apparent or effective diffusion of halide anions in the B horizon at site 8 suggested that the capillary bundle model of the soil-rock mantle may be a good approximation of the laminar flow paths in at least several sites. However, the possibility that turbulent flow occurs through macroscopic voids in the soil-rock mantle must also be considered as a possible flow mechanism in all experimental sites.
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