Chris Gammons, Montana Tech. Rio Tinto, Spain

Transcription

1 Diel processes in low-ph streams Chris Gammons, Montana Tech Rio Tinto, Spain

2 Diel processes in different ph regimes low-ph streams Small diel changes in ph Fe redox cycles high-ph streams Large diel changes in ph (DIC system) Sorption of cationic metals (Zn 2+, Mn 2+ ) Transition: low ph high ph Kinetic effects Faster rates of Fe(II) oxidation and hydrolysis during day Co-precipitation or sorption of As and trace metals onto fresh HFO particles

3 Field sites Coal mine drainage, MT (submitted) Fisher Creek, Montana Rio Tinto, Spain (Gammons et al 2008) Rio Agrio, Argentina (Parker et al 2008)

4 Fe(III) photoreduction FeOH 2+ + h Fe 2+ + OH Fe 3+ -DOC + h Fe 2+ + DOC + H + Fe(OH) 3 (s) + h Fe 2+ Light can reduce Fe(III) in both dissolved and solid forms DOC catalyzes this reaction Rio Tinto, Spain Less important at ph > 6

5 Fe cycling below mine adit discharges Fe, mg/l Fe 2+ Total Fe photoreduction, oxidation, hydrolysis, sorption Fe 3+ Giffen Spring, MT Distance, time Giffen Spring, MT Diurnal processes Rate of Fe 2+ oxidation is faster in warm water Solubility of ferric-hydroxide (HFO) is higher in cold vs warm water Sorption of solutes onto HFO is ph- and T-dependent

6 Example: Coal mine drainage, Giffen Spring Temp ph ~ Temperature, o C dissolved Fe, mg l Zn dissolved Zn, mg l Fe :00 16:00 20:00 00:00 04:00 08:00 12:00 16:00 20:00 00:00 Time of day Gammons et al., submitted Faster rate of Fe(II) oxidation and hydrolysis results in lower dissolved Fe, Zn during the day in downstream waters.

7 F1: ph 3.3 Fisher Creek New World Cu-Au Mining District F2: ph 5.5 F3: ph 6.8 (Gammons et al 2005a)

8 Fisher Creek: Temp/pH Temperature, C F1 F3 F2 10:00 14:00 18:00 22:00 02:00 06:00 10:00 X Data All stations had a large diel change in temperature ( 11 C) All stations had very small diel ph changes (< 0.15 units) ph ph ph F F F :00 14:00 18:00 22:00 02:00 06:00 10:00 time (Gammons et al 2005a)

9 Fisher Creek F1 site: ph ~ F1 Total Fe Total dissolved Fe Fe, mg/l 6 4 Dissolved Fe(III) 2 0 Particulate Fe Dissolved Fe(II) 10:00 14:00 18:00 22:00 02:00 06:00 10:00 time Daytime drop in total Fe (solubility of HFO as T ) Daytime photoreduction of Fe(III) to Fe(II)

10 Fisher Creek F2 site: ph ~ F2 Fe, mg/l Fe(II) Fe(III) Total Fe Total dissolved Fe suspended particles 10:00 14:00 18:00 22:00 02:00 06:00 10:00 time Photoreduction of HFO causes daytime increase in Fe(II) and total dissolved Fe Fe mainly dissolved in day, particulate at night

11 Fisher Creek F3 site: ph ~ Total Fe 0.1 F3 Particulate Fe Fe, mg/l 0.01 Total dissolved Fe Dissolved Fe(II) Dissolved Fe(III) :00 14:00 18:00 22:00 02:00 06:00 10:00 No evidence of Fe(III) photoreduction Night-time increase in Fe(II) and total dissolved Fe Slower rate of oxidation and hydrolysis at night More sorption of Fe(II) onto HFO during day

12 Diel cycling of copper at F Temp Total Cu 18 Cu, mg/l Dissolved Cu Temperature, C :00 14:00 18:00 22:00 02:00 06:00 10:00 Time Particulate Cu (Gammons et al 2005a)

13 Last example: Rio Tinto, Spain One of the largest VMS deposits on Earth Mined for over 4000 years Severe AMD large % of total metal load to world s oceans Extensively published > 70 journal articles in past 10 years Life in extreme environments Analog for life on Mars? Iberian Pyrite Belt

14 Results: Fe speciation at Rio Odiel 1.5 Upper Odiel dissolved Fe, mg/l Fe(III) Fe(total) Fe(II) Gammons et al., 2008

15 Fe speciation, Rio Tinto Fe(III) Rio Tinto 7 6 dissolved Fe(III), mg/l dissolved Fe(II), mg/l 900 Fe(II) Gammons et al., 2008

16 Applications to life in extreme environments and astrobiology Most abundant microbes are Fe-oxidizers Photoreduction Amils et al., Env. Sci. Bio/Technology, 1 (2002)

17 Summary Acid streams tend to have small diel changes in ph, DO ph is strongly buffered Low primary productivity, so small DO swings Large cycles in Fe redox speciation and concentration Changes in rate of oxidation of Fe 2+ Photoreduction T- and ph- dependence of HFO solubility Diel cycles in Fe may or may not impart cycles in trace metals & arsenic

18 Sources of data Gammons C. H., Nimick D. A., Parker S. R., Cleasby T. E. and McCleskey, R. B. (2005a) Diel behavior of Fe and other heavy metals in a mountain stream with acidic to neutral ph: Fisher Creek, Montana, USA. Geochim. Cosmochim. Acta 69, Gammons C. H., Wood S. A. and Nimick D. A. (2005b) Diel behavior of rare earth elements in a mountain stream with acidic to neutral ph. Geochim. Cosmochim. Acta 69, Gammons C. H., Duaime T. E., Poulson S. R., Parker S. R. (in review) Geochemistry and stable isotope investigation of acid mine drainage associated with abandoned coal mines in central Montana, USA. Submitted to Chemical Geology, Nov. 14, Gammons C. H., Nimick D. A., Parker S.R., Snyder D.M., McCleskey R.B., Amils R., Poulson S.R. (2008) Photoreduction fuels biogeochemical cycling of iron in Spain s acid rivers. Chem. Geol. 252, Parker S.R., Gammons C.H., Pedrozo F., Wood S.A. (2008) Diel changes in metal concentrations in a geogenically acidic river: Rio Agrio, Argentina. J. Volcanology Geothermal Res. 178,