1. Arsenic in de Dutch subsoil...Introduction... Tony Appelo Welcome! Concentration patterns Introduction of the speakers

2. Arsenic concentrations in groundwater D.J. Huisman, TNO

3. Pleistocene, low sealevel In groundwater: about 5000 ppb Fe 2+, 5 ppb As

4. Iron ore (’oer’) in Salland, NL Ratio Fe / As in FeOOH is about 1000

5. Holocene, sealevel rise FeOOH transforms with S from seawater in pyrite, FeS 2 Ratio Fe / As in FeS 2 is about 1000

6. Vegetation grows, peat develops...

7. Peat is exploited

8. FeS 2 aerated, oxidizes FeS 2 retransforms in solid FeOOH, FeCO 3 and SO 4 2-, Fe 2+, As Ratio Fe / As in water is ???

9. What is the relation among As and SO 4 2-, Fe 2+,.... ??

10. PROGRAM of the morning 11.00 dra Sophie Vermooten (TNO, Utrecht) : Regional occurrence of As in the Netherlands 11.30 Prof. dr Pieter Stuyfzand (VU, Amsterdam / KIWA, Nieuwegein): As in the Dutch drinkingwater production 12.00 drs Slavek Vaşak (TNO, Utrecht) : IGRAC's inventory of hazardous substances in groundwater on global scale 12.15 discussion 12.30 Lunch