Characteristics of Natural and Urban Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 129

carbonate accumulation is often found below 0.5-0.6 m depth. Hence it was hypoth-
esised that barium might behave in the same way in the form of a low-solubility
contaminant.
Chemical analysis indicated that the molar ratio of water soluble SO 4 2 − to
extracted barium was of the order in the range of 1-10 − 5 to 1-10 − 6 and there-
fore the barium would all be in inert form as barite (BaSO 4 ). Notwithstanding its
low solubility, over a very long geological period the barite had distributed itself
over the soil profile in the form of a natural chromatogram. It was concluded that
the barium was far too insoluble to present a risk to human health and the ecology
and as a result the development was able to go ahead.

2.7.2 Arsenic in Weathered Rock at the New Victorian Museum,
Melbourne

During excavations for underground car parking and an underground cinema for
the new Museum Victoria a number of samples returned slightly elevated total
arsenic concentrations in excess of the 20 mg

kg dw − 1 Soil Quality Standard for
“uncontaminated” soil. The deeply weathered folded and faulted strata of shale,
siltstone and sandstone of Silurian age were exposed in the excavation and showed
strongly kaolinised and ferruginised zones. These were sampled separately and sub-
jected to analysis of total iron and arsenic, citrate-dithionite extractable iron and
arsenic (Holmgren 1967 ), followed by determination of total arsenic in the residue.
Thus it could be proved that the bulk of the arsenic was generally liberated along
with the extractable iron, and that the kaolinised zone possessed very little arsenic,
although the high iron zone did not always have high arsenic. An extraction with a
buffered solution of acetic acid and sodium acetate at pH 6 according to the Toxicity
Characteristic Leaching Procedure 5 (TCLP), which is a routine extraction proce-
dure in Victoria to model availability to plants, could not extract any arsenic from
the samples.
Due to the sometimes extremely high iron contents in the rusted strata, the stan-
dard citrate-dithionite extraction was not able to liberate the bulk of the iron and
therefore, presumably, the bulk of the arsenic. These analyses were part of a com-
mercial environmental assessment and therefore could not be pursued with greater
scientific thoroughness.
Table 2.8 summarises the laboratory data. Note that the TCLP extraction employs
a mild extractant, a buffered sodium acetate - acetic acid solution at pH 6. Once
again, the evidence provided by a soil chemistry-based assessment lifted the concern

·

5 The TCLP, or Toxicity Characteristic Leaching (not Leachate) Procedure is designed to deter-
mine the mobility of both organic and inorganic analytes present in liquid, solid, and multiphasic
wastes. The TCLP analysis simulates landfill conditions. Over time, water and other liquids per-
colate through landfills. The percolating liquid often reacts with the solid waste in the landfill, and
may pose public and environmental health risks because of the contaminants it absorbs. The TCLP
analysis determines which of the contaminants identified by the United States Environmental
Protection Agency (EPA) are present in the leachate and their concentrations.