Chromium Rich Particles Found in Groundwater Monitoring Wells

1
Chromium Rich Particles Found in Groundwater
Monitoring Wells Dane W. Scott, Allen W. Apblett,
Sara Sayler and Nicholas F. Materer Department
of Chemistry, Oklahoma State UniversityStillwater
, Oklahoma 74078-3071

• Inspection of Well Pumps for Corrosion
• Since corrosion of the chromium containing
  stainless steel construction of the well is a
  suspected source, the pumps were examined for
  corrosion and photographed.
• Above, are photos of pumps from wells with high
  and low levels of Cr.
• Little corrosion is observed despite the varying
  concentrations of Cr present. Thus, well
  corrosion is eliminated as a main source.
• .

Abstract At a military installation in
Oklahoma, elevated chromium concentrations were
found in monitoring wells located away from
obvious industrial sources. Two reports indicate
that chromium is present in suspended particles
found in the well water. To identify the source,
well water was collected from 12 monitoring wells
with and without chromium contamination. The
suspended particles were separated from the water
by submicron filtration. The particles were
studied using X-ray photoelectron spectroscopy,
X-ray powder diffraction spectroscopy and atomic
absorption spectroscopy. Elemental composition
of these particles is inconsistent with bio-well
corrosion of the well head and inspection of the
dedicated well pumps indicates little corrosion.
Although X-ray photoelectron spectroscopy found
Cr(VI) on the surfaces of these particles, the
bulk located within the particles is Cr(III). No
exchangeable chromium was found adsorbed on the
surface of the particles. X-ray diffraction
measurements indicate the presence of goethite,
an iron oxide hydroxide. Stepwise chemical
dissolution of the particles supports this
result. The most probable explanation for the
observed chromium is the dissolution of the clay
soil to form goethite. Analysis of the core soil
obtained from four drill sites contain chromium
which is expected to be concentrated in the
goethite particles by the dissolution process.

• Study Site and Well Locations
• Sampling Procedure
• After collecting three to four 1L samples of
  water from 12 wells, the pump was removed and
  inspected for evidence of corrosion.
• Soil from four new well locations, Sites 1-4, was
  collected as a function of depth during drilling.

• Problem
• Chromium is a known carcinogen and concentrations
  exceeding the EPA limit of 100 ppb have been
  found in groundwater monitoring wells not near
  known anthropogenic sources.
• Objective Identify the Source
• Well water analysis was performed in 2000 and
  20031,2 which included measurements of chromium
  concentration vs. volume of water pumped.
• The source was suggested to be a result of
• Corrosion of the Cr containing stainless steel
  well resulting in Cr adsorption to suspended
  solids in the well water.
• No single source or plume was identified
  localizing the source to within the well.
• These studies were used in order to select wells
  with high and low levels of Cr located away from
  known industrial sources for analysis.

Well 1-84B High Cr concentration
Well 1-83B Low Cr concentration

• Composition of the Suspended Solids
• Concentrations of Al, Cr, Fe, Mn and Ni by
  mass
• The amount of chromium in the suspended solids
  accounts for the ppb levels found in the
  unfiltered well water samples.

• Elemental Composition of the Soil
• Al and Fe ( by mass
• Cr, Ni and Mn (ppm)

• Elemental Analysis of Well Water
• After submicron filtration, a Varian GTA 120
  graphite furnace atomic absorption spectrometer
  was used to analyze well water samples for Cr,
  Al, Ni, Fe, and Mn.
• Atomic Absorption Results
• 3 - 13 ppb Cr 3 - 10 ppb
  Fe 7 - 19
  ppb Al 24 - 402 ppb Ni 0.7 - 22
  ppb Mn
• The extremely low levels of Cr in the filtered
  well water implies that the Cr must be contained
  in the suspended solids.

• Characterization of Solid Samples
• Approximately 60 mg of dried and homogenized soil
  and suspended solid sample was totally dissolved
  by acid digestion using EPA method 3050B.
• Flame atomic absorption spectroscopy was then
  performed to determine Al, Fe, Cr, Mn and Ni
  concentrations.
• Additionally, a sequential acid extraction
  reported by Tessler and Campbell was used to
  quantify chromium present in the following
  fractions3
• Exchangeable- trace elements in which changes in
  ionic strength affect the adsorption process of
  trace elements
• Carbonates- chromium release upon acidification
• Mn/Fe Oxides- chromium bound to oxides which is
  released under anoxic conditions
• Organic- oxidation of organic material releasing
  bound chromium
• Residual- chromium contained within the crystal
  structure of minerals

• X-Ray Diffraction Measurements
• The X-ray diffraction pattern indicates the
  presence of goethite, an iron oxide hydroxide.
• We suspect that goethite is formed by the
  dissolution of the iron rich clays in the well
  water.
• Chromium is present bound to goethite in the
  suspended solids as evident by the amount of
  chromium extracted which is bound to Fe/Mn
  oxides.

• Conclusions
• Based on visual inspection of the pumps, well
  corrosion was eliminated as a source of the
  elevated chromium.
• Elemental analysis of the well water and
  suspended solids in combination with X-ray
  photoelectron spectroscopy and X-ray diffraction
  measurements show that dissolution of silicates
  in the basic well water accounts for elevated Cr
  levels in unfiltered water.
• Thus, the presence of naturally occurring
  chromium in the soil is suspected as the source
  for the elevated chromium concentrations.
• Funding
• Center for Aircraft and Systems/Support
  Infrastructure
• References
• 1 IT Corporation, Final Report Identification
  of the Sources of Elevated
• Chromium and Nickel Concentration ion
  Groundwater at Tinker Air Force Base,
• Austin, Texas, 2000.
• 2 IT Corporation, Final Identification of the
  Sources of Elevated Chromium and
• Nickel Concentration in Groundwater at
  Monitoring Wells 1-82A, 1-83B, 1-84A
• and 1-84B Tinker Air Force Base, Oklahoma,
  Austin, Texas, 2003.
• 3 Tessler, A. Campbell, P. G. C. Bisson, M.
  Anal. Chem. 1979, 51, 844-850.

• Fractional Extraction of Chromium from Top Soil
  and the Suspended Solids
• Top soil chromium is present mostly in the
  residual fraction while contained mostly in the
  Fe/Mn oxides in the suspended solids.
• Dissolution of the soil in the basic well water
  leads to the presence of ppb levels of trace
  metals in the filtered well water.

X-ray Photoelectron Spectroscopy of the Particle
Surface

• Ion Beam Sputtering was used to examine layers
  below the surface

Aragonite(obscures other peaks)
Goethite

• Cr 2p peaks in the X-ray Photoelectron
  Spectroscopy of the goethite layer.