Sources and Distribution of Arsenic in Groundwater

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1 Sources and Distribution of Arsenic in Overview of the geochemical properties of arsenic and how they control its concentration in groundwater and affect treatment options. Natural and anthropogenic sources of arsenic in groundwater. Distribution of arsenic in groundwater in California and the Southern San Joaquin Valley in particular. Factors that lead to high As concentrations in groundwater Case study from a well in the Kern Water Bank 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 As is a heavy metalloid element with properties of both metals and non-metals. 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 neutral or negatively charged oxyanions. Occurrence of Arsenic in in the United States Occurrence of Arsenic in in California From USGS, 2000 From ACWA Arsenic Study,

2 pe-ph Predominance Diagram Adsorption of Arsenite (As(III)) on Iron Oxyhydroxides In natural waters As(III) occurs as a neutral species while As(V) is negatively charged. From Dzombak and Morel, 1990 Adsorption of Arsenate (As(V)) on Iron Oxyhydroxides Anthropogenic Sources of Arsenic Wood preservatives Agricultural applications (pesticide, herbicides, feed supplements) Glassware Mining (for example Iron Mountain Superfund site with As concentrations up to 850 ppm in acid mine drainage waters) From Dzombak and Morel, 1990 Arsenic Use in the United States From Welsh et al., 2000 Natural Sources of Arsenic (from Smedley and Kinniburgh, 2002) 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 ) 2

3 Q Tc Geochemical Control of Arsenic Concentrations in Eh-pH Predominance Diagram For a System with Iron and Sulfur 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, and sulfides and release of As. Pyrite Iron oxide & Iron hydroxide Factors contributing to Naturally Elevated Arsenic Concentrations in Closed basins in arid or semi-arid areas Reducing aquifers derived from alluvium Changing redox conditions lead to dissolution of oxides, hydroxides, or sulfides and release of As bound to them. Hydrothermal waters (for example Hot Creek up to 160 ppb) Evaporative waters (for example Mono Lake ~20 ppm) Case Study Sources of Arsenic in a Well from the Kern Water Bank modern Kern River A' A' A 63 ppb 1.9 ppb approximate limits of Kern River Fan 0 10 km (from Page, 1986) 3

4 Total Arsenic in Sediments from Two Wells in the Kern Water Bank As concentrations in groundwater: Depth (ft) % As III 70% As V 63 ppb 1.9 ppb Concentration (ppm) 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 backscatteredelectron image. Well 23H, depth = 690 ft. 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 WDS spectrum of pyrite in framboidal spherule showing distinct La peak for arsenic. Depth = 690 ft. 4

5 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 = Hypothesis for Elevated Arsenic Concentrations in some KWB 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 For a System with Iron and Sulfur Pyrite Iron oxide & Iron hydroxide Typical Kern Water Bank Well 5