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WATER RESOURCES RESEARCH, VOL. 41, W10414, doi:10.1029/2005WR004061, 2005

Percolation and transport in a sandy soil under a natural hydraulic gradient

Christopher T. Green

U.S. Geological Survey, Menlo Park, California, USA


David A. Stonestrom

U.S. Geological Survey, Menlo Park, California, USA


Barbara A. Bekins

U.S. Geological Survey, Menlo Park, California, USA


Katherine C. Akstin

U.S. Geological Survey, Menlo Park, California, USA


Marjorie S. Schulz

U.S. Geological Survey, Menlo Park, California, USA


Abstract

Unsaturated flow and transport under a natural hydraulic gradient in a Mediterranean climate were investigated with a field tracer experiment combined with laboratory analyses and numerical modeling. Bromide was applied to the surface of a sandy soil during the dry season. During the subsequent rainy season, repeated sediment sampling tracked the movement of bromide through the profile. Analysis of data on moisture content, matric pressure, unsaturated hydraulic conductivity, bulk density, and soil texture and structure provides insights into parameterization and use of the advective-dispersive modeling approach. Capturing the gross features of tracer and moisture movement with model simulations required an order-of-magnitude increase in laboratory-measured hydraulic conductivity. Wetting curve characteristics better represented field results, calling into question the routine estimation of hydraulic characteristics based only on drying conditions. Measured increases in profile moisture exceeded cumulative precipitation in early winter, indicating that gains from dew drip can exceed losses from evapotranspiration during periods of heavy (“Tule”) fog. A single-continuum advective-dispersive modeling approach could not reproduce a peak of bromide that was retained near the soil surface for over 3 years. Modeling of this feature required slow exchange of solute at a transfer rate of 0.5–1 × 10−4 d−1 with an immobile volume approaching the residual moisture content.

Received 24 February 2005; accepted 22 June 2005; published 22 October 2005.

Keywords: bromide tracer test; flow and transport; natural hydraulic gradient; unsaturated zone.

Index Terms: 1875 Hydrology: Vadose zone; 1832 Hydrology: Groundwater transport; 1838 Hydrology: Infiltration; 1847 Hydrology: Modeling.

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Citation: Green, C. T., D. A. Stonestrom, B. A. Bekins, K. C. Akstin, and M. S. Schulz (2005), Percolation and transport in a sandy soil under a natural hydraulic gradient, Water Resour. Res., 41, W10414, doi:10.1029/2005WR004061.