In humid regions, river water naturally percolates through the ground into aquifers (which are layers of sand and gravel that contain water underground) during high-flow conditions. In arid regions, most rivers lose flow, and the percolating water passes through soil and aquifer material until it reaches the water table. During these percolation processes, potential contaminants present in river water are filtered and attenuated. If there are no other contaminants present in the aquifer or if the respective contaminants are present at lower concentrations, the quality of water in the aquifer can be of higher quality than that found in the river. In RBF, production wells —which are placed near the banks of rivers—pump large quantities ofwater. The pumping action creates a pressure “head” difference between the river and aquifer, which induces the water from the river to flow downward through the porous media into pumping wells.