Nitrate Mediated Mobilization of Naturally Occurring Uranium in Groundwater

Transcription

1 Nitrate Mediated Mobilization of Naturally Occurring Uranium in Groundwater Karrie A. Weber (PI) University of Nebraska Lincoln School of Biological Sciences Department of Earth and Atmospheric Sciences Collaborators: Daniel Snow, SNR, UNL Kate Campbell, USGS NRP, Boulder, CO John Bargar, SSRL, Stanford, CA (Ad Hoc Collaborator) Funding Total: $467,500 Federal: $230,839 Non-Federal Match: $236,661 Duration: 9/1/2014-8/31/2018 Nitrate (ppm) Uranium (ppb) Water Center Advisory Board August 29, 2017 Journal Cover, Geology, August > 10 MCL=10 ppm > 30 MCL=30 ppb Kilometers Nolan and Weber Environ. Sci. Technol. Letters. 2:

2 Nolan and Weber Environ. Sci. Technol. Letters. Objective: determine the mechanisms governing the oxidative dissolution of U(IV) leading to U mobility, and quantify the rate and extent of these reactions in order to develop a reactive transport model to predict U mobility in ground water. Drinking Water U MCL : 30 ppb (µg/l)

3 Hypothesis (Tasks): 1.Microbial U reduction rates are slower than rates of oxidative dissolution of U(IV) by O 2, NO 3-, and Fe(III)) thus resulting in U mobilization. 2.Oxidation of U coupled to nitrate reduction can be traced using 18 O-labeled NO 3-.

4 Juergens et al Ground Water. Uranium Geochemistry Oxidized Uranium species, U(VI) soluble = mobile U(VI) 1. Carbonates desorp U(VI) from the surface of sediment particles. Reduced species, U(IV) insoluble = immobile

5 Weber et al Appl. Environ. Microbiol. Uranium Biogeochemistry Oxidation of the U(IV) minerals results in mobilization. 1. Carbonates desorp U(VI) from the surface of sediment particles. 2. Oxidation of U(IV) minerals.

6 Abiotic and biological reactions control U mobility 1 Nitrate-dependent U(IV) oxidation 2 Nitrate-dependent or oxygendependent Fe(II) Oxidation and the abiotic oxidation of U(IV) coupled to Fe(III) reduction 3 Uranium (and Fe(III)) reduction) 1 2 3

7 Hypothesis and Approach: 1.Microbial U reduction rates are slower than rates of oxidative dissolution of U(IV) by O 2, NO 3-, and Fe(III)) thus resulting in U mobilization. Subsurface samples will be collected via GeoProbe coring and geochemically characterized to identify zones of U(IV) bearing minerals as indicators of U reduction. Packed column experiments will be prepared from core sections containing U(IV) bearing minerals. Here we will experimentally determine the following: U(VI) reduction rates, U(IV) oxidation rates in the presence of nitrate and oxygen U(IV) oxidation rates by abiotically and biogenically precipitated Fe(III) oxides, Fe(III) reduction rates, Fe(II) oxidation rate by oxygen and nitrate.

8 Hypothesis and Approach: 2. Oxidation of U coupled to nitrate reduction can be traced using 18 O-labeled NO 3-. Subsurface samples testing positive for microbial U(IV) oxidation coupled to nitrate reduction will be placed in column experiments and amended with 18 O labeled NO O in the U(VI) species will be quantified.

9 Nolan and Weber Environ. Sci. Technol. Letters. Significant Findings: 1. Co-occurrence of groundwater nitrate and uranium concentrations in the High Plains And Central Valley Aquifers High Plains Aquifer Central Valley Aquifer

10 Nolan and Weber Environ. Sci. Technol. Letters. Significant Findings: 2. Uranium concentration in groundwater is spatially correlated with nitrate concentration High Plains Aquifer Central Valley Aquifer

11 Significant Findings: 3. Shallow wells are particularly sensitive to nitrate and uranium contamination High Plains 4 ρ=0.19 (p<0.0001) 4 Central Valley ρ=0.27 (p<0.0001) Log Noramlized Uranium (ug/kg) Log Noramlized Uranium (ug/kg) Log normalized Nitrate (mg/kg) 4 Log normalized Nitrate (mg/kg) Log Noramlized Uranium (ug/kg) Nolan and Weber Environ. Sci. Technol. Letters. 2: Log normalized Nitrate (mg/kg) Log Noramlized Uranium (ug/kg) < 100ft: ρ=0.25 (p<0.0001) < 100ft: ρ=0.61 (p<0.0001) Log normalized Nitrate (mg/kg)

12 Significant Findings: 4. Naturally occurring U exists as U(VI) and U(IV) in the subsurface and an alternative mechanism of U sequestration. U(VI) 50% +/-10% U(VI) 85% +/-10% Hastings Alda

13 Significant Findings: 5. Naturally occurring U(IV) in the clay is mobilized by nitrate. Bicarbonate alone had no significant effect after the addition of nitrate.

14 Significant Findings: 6. Microbial metabolism is responsible for nitrate mediated mobilization of naturally occurring uranium.

15 Future Funding? DOE Metal mobility at redox interfaces (in review) NSF Oxidative dissolution of naturally occurring U(IV)