Er bestaat onduidelijkheid over de effecten van sulfaat op de waterkwaliteit en over het relatieve belang van verschillende bronnen van sulfaat in het Hollands-Utrechtse veenlandschap. Deze studie laat zien dat de oxidatie van het gebiedseigen veen meestal de belangrijkste bron is en dat de veenpolders netto exporteurs zijn van sulfaat. Over het algemeen zijn de sulfaatconcentraties in het oppervlaktewater van het laagveengebied vrij hoog. Dit heeft een verarmende invloed op de watervegetatie. _ Annual sulfate mass balances have been constructed for low-lying peat polders in the Netherlands. These peat polders are generally used for dairy farming or nature conservation, and can have large areas of shallow ditches and ponds (mean 16%, range 6-43%). As a consequence of continuous drainage, the peat (organic matter and sulfides) in these polders mineralizes and the soil subsides at rates between 2 and 10 mm y". This mineralization is generally the most important internal source of sulfate, providing some 96 kg SO 4 ha -1 mm -1 subsidence y 1 . During summer, when evapotranspiration exceeds rainfall, sulfate accumulates in the unsaturated zone, to be washed away and drained off during the wet autumn and winter. In some polders, upward seepage of early Holocene, brackish water can be a source of sulfate. Aquatic sediments are an important temporary sulfate sink, which is emptied every 6 to 10 years when the ditches are dredged and these sediments are put back on the fields. External sources are precipitation and water supplied during summer to compensate for water shortage, but these are minor compared to internal release. Peat polders export sulfate to the regional water system and the sea during winter drainage. Most surface waters have high sulfate concentrations; 76% exceed a threshold of 0.3-0.5 mmol SO s4 1 -1 which may lead to impoverished aquatic communities.