WATER-MOVEMENT THROUGH AN AGGREGATED, GRAVELLY OXISOL FROM CAMEROON

Increasing population pressures have caused increased utilization of the gravelly (stone line) soils common to the hilly landscapes of equatorial Africa. This study was conducted to determine if macropore water flow and immobile-water regions should be considered in describing solute movement while developing nutrient-management strategies. Break-through curves (BTC's) from miscible displacement experiments with tritiated water were measured from 70-cm long by 9.6 cm internal diameter, water-saturated, undisturbed soil columns. Simulations produced by theoretical transport models were fitted to the BTC's to determine the magnitude of dispersivity and immobile-water regions. Gravel separates composed of kaolinite, gibbsite, goethite and manganese oxides had porosities ranging from 0.13 to 0.32 m3/m3, with a composite sample porosity of 0.20 m3/m3. Adsorption coefficients of tritium ranged from 0.031 to 0.052 ml/g for the three horizons in the soil columns. Columns containing gravel (30% by volume and 62% by weight) gave asymmetrical BTC's. A convective-dispersioe (CD) transport model was unable to simulate observed BTC's accurately. The mobile/immobile (MIM) water model provided close agreement to BTC's obtained at flow rates ranging from 2.71 to 111 cm/d. The water-saturated soil columns had about 50% of all water in immobile regions. Soil water dispersivity was 3.3 cm2-n dn-1 (with empirical constant n=1.3) from a curvilinear plot of the dispersion coefficient and the mobile pore-water velocity. Parameters estimated from one column were applied to the BTC's of a similar soil column. The MIM model showed close agreement between the measured and the independently estimated BTC's. These soil characteristics can contribute to the rapid deep transport of a limited quantity of solute and to the storage and/or slow diffusive mass transport of the remaining solute from within immobile regions. © 1990.