Measurement of infiltration in small field plots by a portable rainfall simulator:: Application to trace-element mobility

Elevated concentrations of trace metals in soil can increase the risk of pollution to ecosystems and human health. This cannot be predicted solely from the total and/or extracted concentration of metals from soil samples, as movement of trace elements to the groundwater is also a result of the flow solution through the vadose zone. The rate at which trace elements move are not usually directly measurable, and thus it must be estimated taking into account water transport through the soil. Therefore, a field portable drop-former rainfall simulator has been designed and used to study trace-element mobility in small field plots. The rainfall simulator permits a wide range of variation in rainfall intensities and provides a homogeneous distribution of the simulated rain in a 0.25 m(2) plot with low cost per data collected and short time. Performance of the rainfall simulator has been evaluated and a preliminary assessment of the amount of pollutants present in the soil (As, Cu and Zn) that can reach groundwater via soil drainage is made by combining rainfall-simulation experiments with infiltration estimates based on a stochastic model of the local climate. The study was conducted in soils affected by the Aznalcollar toxic spill in the Guadiamar river basin (Spain). Infiltration experiments reveal that the trace elements could be classified according to their mobility as As < Cu < Zn. The presence of high gravel content below this depth increased the amount of drainage and therefore the risk of groundwater pollution, especially with Zn, which was found below 50 cm depth.