A42

Goldschmidt Conference Abstracts 2010

Environmental isotopes investigation
on groundwater residence time andrecharge processes in a coastal
aquifer, South-East Tanzania

Time-lapse Electrical Resistivity
Tomography (ERT) monitoring of insitu hydrogeochemical changes
associated with an emulsified
vegetable oil injection forbioreduction of Uranium(VI)

SAID SULEIMAN BAKARI1*, PER AAGAARD1,
ROLF D. VOGT1, FRIDTJOV RUDEN2, INGAR JOHANSEN3
4
AND STEINAR GULLIKSEN

University of Oslo P.O.Box 1047 Blindern, 0316 Oslo,
Norway (*correspondence: s.s.bakari@geo.uio.no)
2Ruden Aquifer Development Ltd, N-1628 Engelsviken
NORWAY
3IFE, NO-2027 Kjeller Norway
4NTNU Gløshaugen 7491 Trodheim Norway

1To determine the residence time and recharge processes ina coastal aquifer south-east Tanzania, we analyzed major
elements and environmental isotopes of surface-, rain- andground-water samples collected from the coastal watershed inApril 2009. The 87Sr/86Sr ratios of ground- and surface-waterof the study area reflect the lithological characteristics of thecoastal aquifer as a result of weathering processes and water–rock interactions [1]. The slope for rainwater data (%2H = 10.5
+ 7.2%18O, R2 = 0.97) is slightly lower than global meteoricwater lines (%2H = 10 + 8%18O, R2 = 1), which reflect
evaporation and preservation of seasonal fluctuations. The
wide range of the %18O and %2H values in the upper aquifer
indicate replenishment with modern meteoric water. In
contrast, the narrow range of depleted stable isotope values
combined with chemical signal in the lower aquifer indicatesthat deep groundwater is a part of the regional flow system.
The 14C data suggest that the residence time of deep
groundwater is in the order of several thousands of years andreflect past recharge during a colder climate than at present. Inaddition, the hydrochemical data characterized deep
groundwater as a high water quality of Na-HCO3 type andshallow groundwater as a low water quality of Na-Ca-Cl type.
[1] Faure G. (1986) 2nd ed. Springer, Berlin Heidelberg

GREGORY S. BAKER1, S.S. HUBBARD2, D.B. WATSON3,E. GASPERIKOVA2, Y. WU2 AND S. BROOKS3

University of Tennessee (gbaker@utk.edu)
Lawrence Berkeley National Laboratory
3Oak Ridge National Laboratory
12

Bioreduction of Uranium (VI) to Uranium (IV) with anelectron donor was tested in Area 2 at the DOE Oak Ridge
National Laboratory (ORNL) Integrated Field Research
Challenge (IFRC) site in 2008-2009. Low U concentration(below US EPA MCL of 0.03 mg/L) were achieved previously
by frequent injection of electron donor. To reduce the costs
and improve the sustainability for remediation and site
maintenance, the research team is exploring the effectivenessof a slowly degrading substrate (commercial emulsified
vegetable oil, SRS) as an electron donor.
Time lapse surface electrical (TLERT) data are expected
to be useful for understanding the distribution of amendments
and long-term reactivity in the presence of subsurface
heterogeneity. Our project effort focused on four tasks:(1) Laboratory Experiments. Flow-through laboratory
experiments were conducted using site material andgroundwater to understand the electrical response to the SRS
biostimulation. This study suggested that time-lapse electricalmethods should be useful for monitoring the replacement ofgroundwater with the SRS and subsequent changes due tobiodegradation. (2) Field Monitoring. Time-lapse surface
electrical datasets (eight in the first months, and 15 total overthe first year) were used to monitor the SRS field
biostimulation experiment along two key transects: one
parallel to the injection flowcell centerline and one
perpendicular to the injection well gallery. (3) Wellbore Data.
Co-analysis of the time-lapse fluid electrical logs and wellbore
aqueous geochemical data, both collected during the SRS field
experiment, has been performed. These data suggest that thefluid electrical conductivity responds significantly to the SRS
injection, but that subsequent decreases due to denitrificationand increases due to sulfate reduction partially cancel eachother out. (4) Synthesis. The three components are analyzed tointerpret the time-lapse surface ERT datasets in terms of
insights developed through laboratory experiments andthrough analysis of wellbore data.