Geogenic versus Anthropogenic Metals and Metalloids

Transcription

1 Geogenic versus Anthropogenic Metals and Metalloids Geochemical methods for evaluating whether metals and metalloids are from geogenic versus anthropogenic sources 1

2 Definitions Geogenic from natural geological processes Anthropogenic from the influence of human activity 2

3 Outline 1 Traditional Environmental Investigative Approach 2 Geochemical Investigative Approach 3 Means and Methods 4 Case Study 3

4 Traditional Environmental Investigative Approach 4

5 Traditional Investigative Approach Metals (and metalloids) are assumed anthropogenic A risk assessment determines if concentrations present potentially pose a risk to human health or the environment Metals potentially posing a risk are removed, immobilized or isolated 5

6 Traditional Investigative Approach Problems with this approach: o The assumption that metals are anthropogenic is not always valid. Large portions of the earth are naturally enriched in metals. o Traditional investigative approaches are generally inadequate to evaluate whether metals are geogenic or anthropogenic. These approaches: Generally ignore complex geologic conditions Sampling typically based on current and past site operations Generally emphasize statistical background concentrations 6

7 Traditional Investigative Approach o Problems with the traditional environmental analytical techniques: Incomplete and inefficient digestion Interferences caused by moderate to high iron content Inability to identify minerals and their constituent elements Inability to evaluate natural geological and geochemical influence 7

8 Geochemical Investigative Approach 8

9 Geochemical Investigative Approach Designed to evaluate the source(s) of metals: o Geogenic Are the metals from natural processes? o Anthropogenic Are the metals from human activities? If so: Are the metals from current and/or historic site activities? Are the metals from human activities unrelated to site activities (e.g., historic mining activities)? 9

10 Geochemical Investigative Approach Clear data quality objectives (DQOs) can help identify key decisions: o Are minerals present that contain the metals of interest? o Are these or similar metal-bearing minerals present both onsite and offsite? o Are geochemical signatures consistent with a geogenic source? o Are geochemical signatures similar both onsite and offsite? 10

11 Geochemical Investigative Approach Geological / Geochemical methods employed: o Field / Portable Methods Direct observation of outcrops or drill core by professional geologists experienced in examining mineralized rock Field screening and quantitative analysis using x-ray fluorescence (XRF) for metal concentrations Qualitative analysis of powdered samples using x-ray diffraction (XRD) for mineral identification Analysis of thin sections by polarized light microscopy (PLM) for rock and mineral identification 11

12 Geochemical Investigative Approach Geological / Geochemical methods employed: o Laboratory Methods Analysis of powdered samples using x-ray diffraction (XRD) for mineral identification Evaluation of thin sections by scanning electron microscope (SEM) and/or electron microprobe (EMP) for chemical composition of individual mineral grains o Specialized Laboratory Methods Isotopic analysis to help differentiate multiple sources of metals X-ray absorption spectroscopy (XAS) to determine valence state Among others being developed 12

13 Means and Methods 13

14 Visual Examination of Outcrops, Drill Cores and Hand Samples Rock and soil samples are visually examined to determine classification and mineralogy Rock and/or soil types associated with locations with elevated metals are identified Samples are identified for further screening 14

15 Field Screening and Full Suite Analysis by Field-Portable X-Ray Fluorescence (XRF) Samples are screened using hand-held XRF to determine overall content and variability of target metals Selected samples are further analyzed for a full suite of metals using a benchtop XRF 15

16 Field Screening and Full Suite Analysis by X-Ray Fluorescence (XRF) Representative results from benchtop XRF: Element Result (ppm) Element Result (ppm) Element Result (ppm) Element Result (ppm) Element Result (ppm) EPA Method 6200 Elements As 213 Cd 3.6 Mn 1,310 Se 1.6 V 75.7 Cr 2,940 Cl 337 Mo 10.3 Sn 6.5 W 13.3 Ni 693 Cu 23 P 9,360 Sr 83.7 Zn 294 Ag 2.7 Fe 87,400 Pb 13.7 Th 18.7 Zr 1,740 Bi 22 Hg 13.9 S 1,170 Ti 3,900 Ba 168 Ca 70,600 K 18,300 Sb 7 U 11 Nb 233 Element Result (ppm) Element Result (ppm) Element Result (ppm) Element Result (ppm) Platinum Group Pt 34 Pd 90.7 Rare Earth Ce 97.3 Nd 100 Sm 311 La 68.3 Pr 61.3 Y

17 Mineralogical Analysis by Field-Portable X-Ray Diffraction (XRD) Samples are powdered and analyzed for major mineral phases by XRD Diffraction pattern shows the presence of: o quartz o feldspar (albite) o chlorite o spinel o pyroxene 17

18 Petrographic Analysis by Polarized Light Microscopy (PLM) Polished thin sections are examined using a polarized light microscope (PLM) under transmitted and reflected light Using transmitted light, most minerals exhibit specific optical properties Using reflected light, opaque minerals exhibit specific reflectance properties 18

19 Mineralogical and Petrographic Analysis by Electron Microprobe (EMP) Polished thin sections are carbon coated and then examined o backscatter imaging Heavy elements backscatter electrons stronger than light elements and thus appear brighter BSE images are used to detect mineral phases of differing chemical composition o qualitative analysis by energydispersive x-ray spectrophotometry (EDS) 19

20 Mineralogical and Petrographic Analysis by Electron Microprobe (EMP) o quantitative analysis by wavelength-dispersive x-ray spectrophotometry (WDS) o x-ray mapping of specific area using WDS to determine the distribution of target elements o two discrete minerals present Sample Point As Co Fe Ni S

21 Isotopic Analysis of Metals Isotopes are relatively easy to analyze in a fixed laboratory, and are typically reported as ratios Isotope analysis can be useful for fingerprinting possible sources Routinely analyzed for lead o Lead has multiple stable isotopes: Pb-204, Pb-206, Pb-207 and Pb-208. All but Pb-204 are produced through the radioactive decay of uranium and thorium More labs are now performing analyses for elements such as copper, iron, mercury and zinc 21

22 Elemental Analysis by Microscopic X-Ray Fluorescence (µ-xrf) General term for µ-xrf spectrometry methods utilizing a synchrotron radiation (SR) source, including: o x-ray absorption near-edge spectroscopy (XANES) o microscopic XRF (µ-xrf) o microscopic XRD (µ-xrd) Evaluate element distribution (µ-xrf), oxidation state (XANES) and crystalline structure (µ-xrd) Specialized analysis generally requiring access to a synchrotron 22

23 Case Study 23

24 Case Study Site is located on the shore of a reservoir in the San Gabriel Mountains of Southern California. An excavation removed the regolith (crushed rock) containing perchlorate, petroleum hydrocarbons and metals. High concentrations of arsenic (As), chromium (Cr) and nickel (Ni) were encountered in both rocks and welldeveloped iron oxide coatings in underlying bedrock. 24

25 Case Study Onsite geology consists of felsic, mafic and ultramafic rock, numerous dikes, and significant evidence of faulting and fractures. Visible evidence of hydrothermal mineralization. Gold and silver, and to a lesser extent cobalt, copper, lead and tungsten, were mined in area from 1860 s through 1930 s, with hard rock mining operations located within a mile of the site. 25

26 Case Study A geochemical background study identified the presence of As, Cr and Ni in rocks from onsite and offsite Minerals containing percent levels of As, Cr and/or Ni were identified Hydrous iron oxides were found to contain As concentrations up to 58,900 ppm in mineral coatings and fracture fillings. 26

27 Case Study Arsenic, Chromium and Nickel-Bearing Minerals Arsenic Minerals Formula Weight % Arsenic Nickeline NiAs 56% Cobaltite CoAsS 44% Gersdorffite NiAsS 45% Arsenopyrite FeAsS 46% Chromium Minerals Formula Weight % Chromium Chromite FeCr 2 O 4 54% Chrome-bearing Magnetite Fe[Fe,Cr] 2 O 4 15% Nickel Minerals Formula Weight % Nickel Pentlandite [Fe,Ni] 9 S 8 34% Nickeline NiAs 44% Gersdorffite NiAsS 35% 27

28 Case Study 28

29 Case Study High-temperature ore minerals containing As, Cr and Ni were identified in rocks from onsite and offsite locations. Results showed similarities in chemical signatures in rocks and oxide and hydroxide coatings. Ore minerals are consistent with minerals recovered economically from nearby mines operated in the 1870 s. The geochemical methods proved effective in identifying the presence of ore minerals that can fully account for the concentrations of As, Cr and Ni at the site. 29

30 Questions? Contact Information: Jeffrey Hess, Chief Scientist Gilbane Environmental 1655 Grant Street, Suite 1200 Concord, California