Characteristics of Natural and Urban Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 127

kg dw − 1 (Kabata Pendias 2001 ). World average natural abundance of bar-
ium in basaltic rocks is 250 mg

1,200 mg ·

kg dw − 1 , but some basalts have much higher barium
concentrations, e.g., tholeite basalt in north-east Ireland contains 1,350 mg

·

kg dw − 1
of barium (Krauskopf 1967 ). As far as is known, there is no systematic survey
of Victorian basalts that provide data on barium concentrations in these rocks and
hence it is unknown what barium concentrations may be expected in the rocks from
which these soils have developed.
In soils in the natural environment, barium will generally occur as barium sul-
phate (BaSO 4 ) due to the ubiquitous presence of sulphate ions. Some paint pigments
contain barium salts and thus barium concentrations can be elevated on derelict land
(Bridges 1987 ). Barium is also associated with radio-active fall-out and nuclear
waste. Barium sulphate is widely used as a safe tracer in medical practice due to
the insolubility of the compound.
Barium molecules released from weathering are not very mobile and readily pre-
cipitate as sulphates and carbonates as long as sulphates and carbonates are available
in the soil profile or can be applied as amendments. It must also be remembered that,
as other rock constituents are lost by leaching in the soil formation process, those
constituents that are less likely to be lost become proportionally more abundant in
the residue. The soil barium concentrations derived from laboratory tests, although
exceeding the Soil Quality Standard (NEPC 1999 ) were within the typical range
measured from soils in their natural state around the world. The elevated concen-
trations recorded at this site are associated with depths of between 0.5 and 1.0 m
in well-defined soil profiles that possess a rock basement with little colonisation
of plant roots beyond 450 mm due to the presence of a calcareous clay-dominated
B-horizon. The barium accumulations also occur in the profile where free lime is
omnipresent and the pH high, thereby limiting plant availability.
Barium precipitates in the presence of sulphate (SO 4 2 − ) to form barite (BaSO 4 ),
which is extremely insoluble (Ksp

·

10 − 10 ). Thus sulphate can be
employed to limit the actual level of risk to health and control mobility (Lehr
et al. 2002 ), so that gypsum can be used as an amendment to limit barium
availability.
Basic soil science would predict that the barium in the soil would be in an insol-
uble form, because all soils that have a living biosphere must contain sulphate from
decomposed proteins and rainfall accessions. However, like lime, the barium con-
centration could be a function of depth with its main accumulation somewhere down
the profile. Two pits were dug with an excavator for detailed sampling of the profile
in layers of 10 cm thickness. The soil profile exposed by the first pit was between
1 m and 1.2 m deep, the excavation limited by weathered rock floaters of vesicular
basalt. Layering in the profile was well-defined, with 600 mm of red-brown clayey-
loam (A-horizon), overlying a calcareous grey heavy clay (B-horizon). The latter
was strongly structured and showed “slickensides”, providing abundant evidence of
shrinking and swelling with fluctuations in soil moisture content. The B-horizon
was flecked with lime and underlain by an extensive calcareous layer about 100 mm
thick. The profile exposed by the second pit had better internal drainage, with more
pronounced layering and more duplex in character. Additionally, the rock was closer
to the natural surface and the B-horizon thinner.

= 1.3 ×