Arsenic and Other Trace Elements in Groundwater in the Southern San Joaquin Valley of California Dirk Baron Geological Sciences California State University, Bakersfield
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
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.
ICP/MS Analysis Very low detection limits Multi element analysis Small sample size Isotope analysis Direct analysis of solid samples with Laser Ablation
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.
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.
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
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) 39.19 64.31 Mg (K) 1.29 1.39 Al (K) 4.07 3.95 Si (K) 9.54 8.92 K (K) 0.19 0.12 Ca (K) 1.24 0.81 Fe (K) 42.20 19.84 Zn (K) 0.29 0.12 As (L) 1.25 0.44 Ti (M) 0.73 0.09 Totals 100 100
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
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 2006. 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.
Regions with Elevated Arsenic in Groundwater
Occurrence of Arsenic in Groundwater in California From ACWA Arsenic Study, 2000
pe ph Predominance Diagram In natural waters As(III) occurs as a neutral species while As(V) is negatively charged.
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 0.6 50 ppm Arsenic minerals including arsenopyrite (FeAsS), realgar (FeAs), and orpiment (As 2 S 3 ) from Smedley and Kinniburgh, 2002
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.
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)
Total Arsenic in Sediments 0 200 KWB 23H01 KWB 23H01 KWB 24K01 KWB 24K01 As concentrations in groundwater: Depth (ft) 400 600 23H01 63 ppb 24K01 1.9 ppb 800 1000 0 5 10 15 20 Concentration (ppm)
30 % As III 70% As V
Spherules of framboidal pyrite (white) in shale (polished thin section). SEM backscattered electron image. Well 23H, depth = 550 ft.
Spherules of framboidal pyrite in shale (polished thin section). SEM backscattered electron image. Well 23H, depth = 690 ft.
WDS spectrum of pyrite in framboidal spherule showing distinct La peak for arsenic. Depth = 90 ft.
Electron microprobe analysis of pyrites Wt% Wt% Wt% Wt% Wt% Wt% SAMPLE DESCRIPTION S Fe As Al Si Total Standard 53.5243 46.5209-0.0012-0.0009 0.0123 100.0575 Standard 53.6546 46.2842 0.012 0.0058 0.0223 99.9788 23H-780 framboid D 52.0127 44.6197 0.1186 0.0725 0.0917 96.9153 23H-690 framboid c point 1 51.4348 44.8694 0.3652 0.0029 0.0304 96.7026 23H-690 framboid c point 2 51.6776 45.0822 0.3516 0.0068 0.0129 97.131 23H-690 spongy texture pyrite d point 1 51.8233 44.0877 0.0929 0.0058 0.0721 96.0818 23H-690 spongy texture pyrite d point 2 48.9509 43.4345 0.0974 0.0857 0.0776 92.6461 23H-690 framboid e point 1 51.675 45.4565 0.1185 0.0149 0.0012 97.2661 23H-690 framboid e point 2 47.6734 45.2968 0.1779 0.0183 0.0517 93.2181 23H-690 edge of big grain 51.189 44.0642 0.0743 0.0206 0.0225 95.3706 23H-690 framboid f 51.7888 44.6561 0.1552 0.0202 0.0301 96.6504 23H-690 framboid h 49.1612 44.6257 0.0579 0.1211 0.2381 94.204 Pyrites contain up to 0.37% of Arsenic
Authigenic pyrite crystals in shale (grain mount). SEM backscattered electron image. Well 23H, depth = 550 ft.
Close up view of authigenic pyrite crystals in shale showing dissolution textures (grain mount). SEM backscattered electron image. Well 23H, depth = 550 ft.
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
Eh ph Predominance Diagram Arsenic in a System with Iron and Sulfur Iron oxide & Iron hydroxide Pyrite
Acknowledgements CSUB California Energy Research Center US Department of Defense National Science Foundation US Department of Agriculture State of California
Typical Kern Water Bank Well