Application of Sr isotope geochemistry to groundwater issues · Application of Sr isotope...
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Transcript of Application of Sr isotope geochemistry to groundwater issues · Application of Sr isotope...
Application of Sr isotope geochemistry to groundwater
issues
Case Study: Bénin, Africa
Collaboration with Dr. Steve Silliman
BACKGROUND
• Chemical composition of water present on the continents is determined largely by: – the congruent and incongruent dissolution of
rock-forming minerals; – sorption of ions to the surfaces of suspended
particles; – Uptake of nutrients by plants and animals;
BACKGROUND • Water (and sediment) also contain elements that
have radiogenic isotopes that formed by the decay of their long-lived radioactive parent nuclides in the rocks of the continental crust.
• Thus, the isotopic composition of these elements (e.g., Sr) in water depend on the age and parent-daughter ratios of the bedrock exposed to weathering in the drainage basins of the continents.
Sr isotope compositions – crustal surface waters
• Depends upon: – the 87Sr/86Sr ratios and Sr concentrations of each
rock type present in drainage basin;
– the area of surface exposure of the different rock types;
– the susceptibility to chemical weathering of the minerals contained within the rocks;
– mixing of water derived from different rock types within streams entering the basin
87Sr/86Sr compositions - Advantages
• Unlike chemical compositions, Sr isotope ratios are NOT fractionated/varied by:
– Changes in temperature, pH, etc;
– Biological activity
– However, Sr isotopes can monitor effectively mixing between different components
History of Pumping and Water Quality
1992 1994 1996 1998 2000 2002 2004 2006 20080
50
100
150
200
250
300
Year
Chlo
ride
mg/
l
300
Chl
orid
e –
mg/
l 1950 1960 1970 1980 1990 2000 20100
2
4
6
8
10
12
14
Year
Mea
n Ra
te o
f Dis
char
ge -
MG
D
Year
Year
Pum
ping
- M
GD
14
CONCLUSIONS
• Elemental abundances and Sr isotope compositions for samples from Lake Nokoue are consistent with binary mixing between crustal-derived surface waters and seawater;
• The results reported here suggest that seawater is not a major component of subsurface water within the Godomey Well Field;
• The combined elemental and Sr isotope data require the presence of a third component- marine-derived evaporites/brines?