ABSTRACT: Nitrate reduction by pyrite oxidation is abundant in the Netherlands subsoil and limits the breakthrough of nitrate into deeper aquifers in parts of the country. Regional and local examples from the Netherlands show that the downward movement of an agricultural pollution front with nitrate might mobilize sulphate, trace metals and arsenic when pyrite is encountered. Therefore, European groundwater protection policy should not only focus on nitrate, but also on these reaction products.