Application of high doses of organic amendments in a Mediterranean agricultural soil: An approach for assessing the risk of groundwater contamination

• Amendments applied to soil produce a high amount of soluble nitrogen pollutants.
• Sewage sludge provided a higher proportion of soluble forms of nitrogen than MSWC.
• Natural rainfall patterns do not produce effective aquifer recharge.
• Pollutants can accumulate in the vadose zone and constitute a “time bomb”.

Groundwater contamination by nitrate is one of the several environmental problems that may be caused by the application of organic amendments to agricultural soils, and is a key issue in the European Union's environmental policy. The use of predictive tools such as flow and transport models for soluble forms could be helpful for preventing the risk of contamination after the application of organic amendments. We investigated the dynamics of leachable inorganic nitrogen forms (NH4+-N and NO3−-N) produced by a single high-dose application of three different organic amendments (a municipal solid waste compost (MSWC), and aerobically (AES) and anaerobically (ANS) digested sewage sludge) in an agricultural soil over a two-year period and how solutes migrate through the vadose zone under a Mediterranean climate. Although the mineralization process was determined by the type of amendment, seasonal variations led to the release and transformation of leachable inorganic nitrogen forms. Sewage sludge (AES and ANS) treatments provide a higher proportion of soluble nitrogen forms than MSWC treatment, which produces a more stable organic matter. Under our field conditions, pollutants can accumulate in the vadose zone, constituting a time bomb which may lead to aquifer contamination further down the line. Hence an increase in water input would accelerate the migration of pollutants and increase the risk of groundwater contamination.