1. Trac (e) ing geochemical processes and pollution in groundwater M.J.M. Vissers P.F.M. van Gaans S.P. Vriend

2. Multilevel wells have advantages over single level GWQ networks when studying trace elements Many geochemical processes + The dynamic behavior of groundwater + Changes in input (anthropogenic influence) i.e. no steady state + (Analytical / sampling errors )

3. I will show this by presenting: Study area and processes that (may) occur Two example elements –Rubidium –Uranium

4. Study area and processes Map of the study area Sandy, unconsolidated aquifer, with ice-pushed ridge in the east Mainly Agricultural land use, eastern part cultivated in the 1920’s 10 Borings, total of 244 mini screens

5. Boring with mini well screens Calcite saturated waters NO3/Fe redox boundary SO4 redox boundary Groundwater level Streamlines Pine / deciduous forest Arable land (mostly corn) 2 km Clay A5 A10 Study area and processes Cross-section of the study area Filtrated over 0.45μm, analyzed on ICP-MS Sampled in 1989 (no trace elements), 1996 (½), and 2002 (all) Randomly analyzed on > 70 inorganic components and DOC

6. Study area and processes Processes and number of observed boundaries  > 60  11  9  4  5  Pollution / changes in input Iron reduction Mn reduction Sulphate reduction pH changes / carbonate buffering Mineral Dissolution / Precipitation Coprecipitation / Codissolution Adsorption / Desorption Kinetics Analytical problems In major elements

7.  Rubidium and Uranium  Two example elements Rubidium: “No” mineral phases, input from either recharge or sediment, and adsorption processes are expected to play role Uranium: Many saturation phases, depending on redox conditions. What is needed for interpretation? Concentration – depth profiles of trace element Knowledge derived from macro-chemistry Geochemical knowledge

8. Rubidium Concentration (μg/l) - depth profiles of all borings “Noisy profiles” Base level

9. Rubidium Input and adsorption, and influence of pH and redox in boring A7 Rubidium 0.3 μg/l in pristene water Adsorption plays a role (retention): boring A5 and A8 Input by recharge (up to 100 μg/l) No (direct) influence of redox and pH boundaries “Ox” Red Acid Buff

10. Uranium SI – Eh dependence of a 6 ppb groundwater Log Saturation index Eh (mv)

11. Uranium Concentration (μg/l) – depth profiles of all borings Low concentrations as complete boring is reduced: Uraninite U (µg/l) 

12. Uranium Oxic waters: Undersaturation, concentrations determined by recharge U (µg/l) 

13. Uranium High concentrations, not related to input U (µg/l) 

14. Uranium Concentration – depth profiles of boring A7 in μg/l μ

15. Uranium Iron reduced waters have concentrations of 0.001 – 0.05 μg/l (uraninite saturation) Input in recently recharged water: 0.1μg/l In deeper oxic water lower concentrations are found At reduction boundary (manganese reduced) concentrations reach 1 – 8 μg/l Source is the sediment

16. Conclusions In the examples, multilevel wells give possibility to: –Determine background concentration for Rb –Exclude redox and pH as important process for Rb –Show input and retention are important for Rb –Accuratly determine redox zone of high U –Exclude pollution as potential U-source –Estimate input of U from recharge and from sediment m.vissers@geog.uu.nl

17. Conclusions II Even with the help of multilevel wells, it is hard to determine trace element systematics m.vissers@geog.uu.nl