Field (sampling, tracer experiment, monitoring)


Feb 22, 2014 (7 years and 10 months ago)


Quantification of parameters guiding mobility of trace elements

Developments of industry and fast growth of the City of Zagreb have considerably
affected groundwater quality of the Zagreb aquifer system. Numerous studies of lateral and
vertical potential
ly toxic metal distribution in soils show elevated concentration of some
potentially toxic metals in topsoils, as well as increase of some metals with depth. The
ultimate goal of the unsaturated zone research of project partner UNIZG is risk assessment of
groundwater metal contamination that has its source in or on unsaturated zone by 1D
numerical simulations of metal transport. Field and laboratory work were performed in order
to determine parameters for modeling and model calibration.

Field (sampling, t
racer experiment, monitoring)

Investigative profile is located about eight hundred meters from the right bank of Sava
river. Undisturbed and disturbed samples of investigated soil horizons and sediment layers
were taken for laboratory analyses (chemical,

mineralogical, pedological and
hydrogeological), as well as for batch and column sorption experiments. Coarse sediment
sampling, especially for analyses which require undisturbed samples was challenging.

Determination of longitudinal dispersivity and mo
lecular diffusion will be determined
in the field by tracer experiment. The plot area (cleaned of vegetation and leveled) for tracer
application is 6 m2. Over a period of 15 min a tracer cocktail of 1 L CaCl2
solution (70 g
CaCl2) will be spread on the soi
l surface followed by 50 L pure water for flushing.
Concentrations of tracer (CaCl2) in percolating water will be analyzed in leachate, after
occasional sampling by suction lysimeters, installed at four different depths. Installation of
suction lysimeters
in sandy soil layers has not been successful.

Monitoring of parameters such as water content, electrical conductivity, pressure
heads and concentrations of metals in percolating water is necessary for calibration of models.
Thitherto, field measurement i
nstruments are installed on the investigative profile. Due to
loose sediment in the lower part of the profile which cause collapse of the profile, and due to
insurance of the equipment, pedological burrow in the proximity of the profile was excavated.
instruments for each parameter (water content, electrical conductivity and pressure head)
are installed at two different depths. Water content (soil moisture) is being measured using
Time domain reflectrometry (TDR) equipment; electrical conductivity (EC)
using EC
and pressure head (h) using tensiometers.

Laboratoy analyses and sorption experiments

Undisturbed samples were analyzed for bulk density, porosity, water capacity, air
capacity, and sediment density while on disturbed samples grain size

distribution (wet sieving
and sedimentation), mineralogical analyses (using X
ray diffraction), chemical analyses
(using atomic absorption spectrometry), cation exchange capacity determination, as well as
hydraulic conductivity (by constant
head permeamet
er for coarse sediments and falling
permeameter for fine sediments) were conducted.

The sorption parameter (distribution coefficient, Kd) of Cd, Pb and Zn for different
soil layers taken from pedological burrow was studied by means of batch experime
Samples were air
dried, sieved, and treated with metal solution of different concentrations.
Soil suspensions were shaken 24 h to ensure sufficient time for solid
liquid equilibrium to be
reached. The initial and final filtrated solutions were analyzed

using flame atomic absorption
(instrument Perkin Elmer AAnalyst 700) to determine potentially toxic metal concentration
before and after the batch experiment. Desorption batch experiment was performed to
determine the reversibility of the sorption reactio
ns. The soils used in the sorption were
treated by 0.01M CaCl2 and shaken for 24 h. The soil and liquid were separated by
centrifugation. The metal concentrations in solution were analyzed by flame atomic
absorption (instrument Perkin Elmer AAnalyst 700).

Dispersion coefficient is being determined in the laboratory at the moment on the
column of soil. The soil column apparatus consists of the soil column itself, a column support
structure, a peristaltic pump unit connected to bottle with solution and outp
ut solution bottles.
Leaching columns made of plexiglas with an inner diameter of 5 cm and heights of 25 cm are
used for this test. At the top and bottom of the column, filter papers are being used to prevent
finest grains from flowing out of the column an
d to support the soil. Samples are uniformly
packed in soil columns and slowly saturated. The tracer solution (CaCl2) are beeing be
pumped into the column inlet at a constant flow rate. Percolated solution samples will be
collected from the beginning of th
e experiment in equal intervals and tracer concentration will
be analyzed using ion chromatography.

After chloride application soil columns will be irrigated by metal solutions prepared
from Pb(NO3)2, ZnCl2 and CdCl2 in order to determine sorption proper
ties. Samples of the
leachate with metals will be collected according to preliminary model results and analyzed
using atomic absorption spectrometry (AAS) for determination of lead, zinc and cadmium
concentrations. The experiment will be ended when concent
ration in the final sample reaches
the initial concentration. At the end of the test, the column samples will be cut into 10 mm
slices. Such soil samples will be treated by aqua regia solution and analyzed to reveal
concentrations and vertical distribution
s of selected trace elements which left bound to the soil

A list of parameters with short description of the procedure (reference or standard
number), equipment use (with detection limits and precision) is presented in Table 4 of the