干旱区生态环境知识资源中心

Water shortage in Iran and most countries in arid and semiarid zones is approaching the critical stage. As about 60% of the water used in Iran is supplied by the underground resources, and the natural recharge cannot replace the exploitation, the artificial recharge of groundwater (ARG) is actively pursued in this country. Desiltation of turbid floodwaters in sedimentation basins (SBs) is a prerequisite for the ARG, executed close to the heads of alluvial fans on an extensive area in Iran. As the sediment removal efficiency of the SBs is not 100%, downward migration of some clay-sized particles, both in the SBs and infiltration ponds, causes the gradual impermeability and eventual clogging of the vadose zone (VZ), and decreases the economic life of the recharge systems. Moreover, groundwater contamination by pollutants adsorbed by soil colloids is another serious matter. Systematic sampling of the coarse grained alluvium beneath two SBs, one in operation for 13 years and another for 10 years, were carried down to a depth of 7.5m. This was repeated in another area outside, but adjacent to the ARG system, as the control. XRD patterns and SEM revealed that size and shape of clay particles, pore-size-distribution of the VZ and the amount of water passing through the basins determined clay species transportability as follows: Smectite congruent to playgorskite > chlorite > illite. Possible transformation of smectite into spiolite will be discussed.