Contribution of Heavy Metal Leaching from Agricultural Soils to Surface Water Loads

Point sources for surface water contamination have been reduced by 50 to 90% during the past decades in The Netherlands. However, quality guidelines for heavy metals are still exceeded in many surface waters. It has been suggested that leaching of heavy metals from (diffusively polluted) soils can be a major, and so far underestimated, source of metals in surface waters. To verify this, model calculations have been performed to quantify heavy metal leaching from agricultural and nature soils to surface waters. To do so, The Netherlands has been divided into 6,405 unique units that differ in soil type, metal contents, and hydrology. Metal concentrations in the leachate were calculated with scaled Freundlich equations that describe a relationship between soil properties, metal contents in the soil solid phase, and metal concentrations in the soil solution. Total leaching was calculated by combining hydrological fluxes to surface water with soil solution concentrations. Results showed that leaching of Pb and Zn accounted for approximately 40% of the total load to surface waters. In many areas, surface waters guidelines can be exceeded due to leaching of heavy metals only. Because current inputs of heavy metals to soil still exceed leaching, heavy metals will continue to accumulate in the soil, which indicates that leaching is likely to increase further.