Solute transport in the vadose zone and groundwater during flash floods

Water flow and solute transport in the vadose zone and groundwater during flood events were investigated in the lower reach of the Kuiseb River, Namibia, and in controlled column experiments. Simultaneous measurements of water level and electrical conductivity of the flood water in the stream channel and in the groundwater together with variations in the vadose zone water content, temperature, and pressure profiles allowed a detailed analysis of the various mechanisms governing solute transport in the subsurface during flash floods. The results indicated that on the land surface, flash floods emit, at their wetting fronts, instantaneous compression waves that propagate downward through the unsaturated zone to the water table. These compression waves generate abrupt solute-displacement events in the groundwater immediately after the arrival of the flood on the land surface, long before deep percolation and groundwater recharge begin. Each flood event launches into the vadose zone a wetting front that propagates down through the vadose zone and recharges the groundwater upon arrival at the water table. The first wetting front of each flood season leaches out soluble salts that have accumulated in the vadose zone during the dry season. However, water percolation through the unsaturated zone does not leach out the entire soluble salt capacity of the sediment, even if percolation takes place under high water-head flooding conditions for long periods. The incomplete leaching of the unsaturated zone by the percolating water releases soluble salts into the groundwater during every recharge event as a result of the rise of the water table into the vadose zone; this process results in a temporal increase of the groundwater electrical conductivity (EC).