EFFECT OF SOIL DISTURBANCE ON AQUIFERRECHARGE: CASE STUDY ON THE SNAKERIVER PLAIN, IDAHO NATIONALLABORATORY, USA

摘要

Soil structure and heterogeneity influence the amount of water that percolates below the root zone to become aquiferrnrecharge. Flow through preferential channels can be extremely rapid. Layer contrasts inhibit downward flow. Soil disturbance byrnagriculture, waste disposal, or land reclamation can severely alter these and other features in ways that affect recharge. We conductedrnidentical experiments in an undisturbed silt-loam soil and in an adjacent simulated waste trench composed of the same soil disturbedrnby excavation and backfilling to a depth of 4 m. We obtained (1) lab- and field-measured unsaturated hydraulic properties and (2)rnfield-measured transient water content profiles through 24 hr of ponded infiltration and 75 d of redistribution. To explore sensitivitiesrnto particular features and processes, we simulated the water content profiles using the VS2DT numerical code. In the undisturbed soil,rnwetting fronts were highly diffuse above 2 m depth, with clear evidence of preferential flow but not layering. Below 2 m, layerrncontrasts severely limited downward flow. Darcian analysis of measurements below 2 m depth suggests that long-term average naturalrnrecharge is less than 2 mm/yr. In the disturbed soil, wetting fronts were sharp and initial infiltration slower. Without layer contrasts,rnhowever, water moved freely but slowly below the root zone. Numerical simulation starting from pre-test water contents predictedrnsharp wetting fronts, agreeing with measurements in disturbed but not undisturbed soil. Simulation starting from the macropore-flowaffectedrnmoisture profile of 3 hr after the start of infiltration, however, gave good predictions of undisturbed-soil infiltration andrnredistribution for the entire 75 d of data. Overall results suggest that soil disturbance, although it reduces preferential flow, mayrnincrease aquifer recharge if it neither leaves intact nor creates flow-impeding layers.