Trac(e)ing geochemical processes and pollution in groundwater

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 1920s
• 10 Borings, total of 244 mini screens

5
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 gt 70 inorganic components
  and DOC

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Study area and processes Processes and number of
observed boundaries

• gt 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
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? 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
RubidiumConcentration (µg/l) - depth profiles of
all borings
9
RubidiumInput 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

10
UraniumSI Eh dependence of a 6 ppb groundwater
Log Saturation index
Eh (mv)
11
UraniumConcentration (µg/l) depth profiles of
all borings
U (µg/l) ?
12
Uranium
U (µg/l) ?
13
Uranium
U (µg/l) ?
14
UraniumConcentration 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

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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_at_geog.uu.nl
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Conclusions II

• Even with the help of multilevel wells, it is
  hard to determine trace element systematics

m.vissers_at_geog.uu.nl