Arsenic and Other Trace Elements in Groundwater in the Southern San Joaquin Valley of California

Transcription

1 Arsenic and Other Trace Elements in Groundwater in the Southern San Joaquin Valley of California Dirk Baron Geological Sciences California State University, Bakersfield

2 Trace Element Maximum Contaminant Level Arsenic 10 µg/l (as of 2006) Uranium 30 µg/l (as of 2003) Chromium (total) Selenium Boron Nitrate/Nitrite 100 µg/l 50 µg/l Toxic to crops from <0.5 mg/l 10/1 mg/l

3 CSUB Geology Research Labs Laser Ablation ICP/MS Perkin Elmer Elan 6100, Cetac LSX 200: Trace element analysis of liquid and solid samples Funding: US Department of Defense Infrastructure Support Program $282,000, 2000.

4 ICP/MS Analysis Very low detection limits Multi element analysis Small sample size Isotope analysis Direct analysis of solid samples with Laser Ablation

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6 CSUB Geology Research Labs X Ray Diffraction PANalytical Empyrean XRD: Identification, characterization, and quantification of minerals and other crystalline solids. Funding: National Science Foundation Major Research Instrumentation Program $200,000, 2014.

7 CSUB Geology Research Labs Scanning Electron Microscopy Hitachi S 3400, Oxford EDS and WDS, Gitan CL: High resolution imaging Elemental Analysis Cathode Luminescence imaging Funding: US Department of Defense Infrastructure Support Program $200,000, 2005.

8 SEM BSE image showing pore spaces filled with Fe rich biotite (B) that contain grains of pyrite (Py). Plagioclase grain (P) bottom right showing pre dissolution features. Potassium feldspar (K) and quartz (Q) framework grains also present. Vedder Formation, Mandell 3 well, Rio Bravo oil field, Sec. 27, T28S, R25E Depth 11,423 feet

9 Fe-oxide clay coating Quartz Grain SEM BSE image, sample AC4 4a and EDS spectrum showing distinct As peak above background level. Dust, Nellis Dunes, Nevada Element Weight % Atomic % O (k) Mg (K) Al (K) Si (K) K (K) Ca (K) Fe (K) Zn (K) As (L) Ti (M) Totals

10 CSUB Geology Research Labs Coming Soon: Micro CT Scanner Non destructive 3 D imaging of Cores: Mineral distribution Grain size Porosity Funding: US Department of Defense, Army Research Office $500,000, 2016

11 Arsenic Arsenic (As) is toxic and carcinogenic and the most common inorganic groundwater contaminant. Drinking water standard lowered to 10 ppb (from 50 ppb) effective In natural waters, As occurs primarily in two oxidation states As(III) (arsenite), and As(V) (arsenate). Unlike other heavy metals which usually occur in water as positively charged cations, As occurs as a neutral species or negatively charged oxyanions. As is a common trace constituent in rocks and minerals and can adsorb to mineral surfaces.

12 Regions with Elevated Arsenic in Groundwater

13 Occurrence of Arsenic in Groundwater in California From ACWA Arsenic Study, 2000

14 pe ph Predominance Diagram In natural waters As(III) occurs as a neutral species while As(V) is negatively charged.

15 Natural Sources of Arsenic Average As concentration in the Earth s crust is approximately 2 ppm Acidic rock (granite) 1.3 ppm Basic rock (basalt) 1.9 ppm Volcanic Glass 5.9 ppm Marine Shale 3 15 ppm Unconsolidated sediments ppm Arsenic minerals including arsenopyrite (FeAsS), realgar (FeAs), and orpiment (As 2 S 3 ) from Smedley and Kinniburgh, 2002

16 Geochemical Control of Arsenic Concentrations in Groundwater Rocks and minerals contain a few ppm of arsenic but this remains generally bound to minerals and is not released into groundwater Oxidizing environments: As bound to Fe and Mn oxides and hydroxides Reducing environments: As bound to sulfide minerals Changing redox conditions can lead to dissolution of oxides, hydroxides, or sulfides and release of As.

17 modern Kern River Case Study: Arsenic in SJV Groundwater A' A' Q Tc A approximate limits of Kern River Fan 0 10 km (from Page, 1986)

18 Total Arsenic in Sediments KWB 23H01 KWB 23H01 KWB 24K01 KWB 24K01 As concentrations in groundwater: Depth (ft) H01 63 ppb 24K ppb Concentration (ppm)

19 30 % As III 70% As V

20 Spherules of framboidal pyrite (white) in shale (polished thin section). SEM backscattered electron image. Well 23H, depth = 550 ft.

21 Spherules of framboidal pyrite in shale (polished thin section). SEM backscattered electron image. Well 23H, depth = 690 ft.

22 WDS spectrum of pyrite in framboidal spherule showing distinct La peak for arsenic. Depth = 90 ft.

23 Electron microprobe analysis of pyrites Wt% Wt% Wt% Wt% Wt% Wt% SAMPLE DESCRIPTION S Fe As Al Si Total Standard Standard H-780 framboid D H-690 framboid c point H-690 framboid c point H-690 spongy texture pyrite d point H-690 spongy texture pyrite d point H-690 framboid e point H-690 framboid e point H-690 edge of big grain H-690 framboid f H-690 framboid h Pyrites contain up to 0.37% of Arsenic

24 Authigenic pyrite crystals in shale (grain mount). SEM backscattered electron image. Well 23H, depth = 550 ft.

25 Close up view of authigenic pyrite crystals in shale showing dissolution textures (grain mount). SEM backscattered electron image. Well 23H, depth = 550 ft.

26 Hypothesis for Elevated Arsenic Concentrations in Groundwater Arsenic is bound to pyrite in organic matter rich deep aquifer with reducing conditions. Natural processes and possibly water banking operations introduce oxygenated water into the deep reduced aquifer Pyrite starts dissolving, releasing As into the groundwater Eventually, fully oxidizing conditions will lead to the formation of iron oxides and hydroxides which will adsorb dissolved As

27 Eh ph Predominance Diagram Arsenic in a System with Iron and Sulfur Iron oxide & Iron hydroxide Pyrite

28 Acknowledgements CSUB California Energy Research Center US Department of Defense National Science Foundation US Department of Agriculture State of California

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31 Typical Kern Water Bank Well