THE NATURE OF WATER REPELLENCY IN URBAN SOIL SAMPLES: A HYPOTHESIS
Gabriele Ellen Schaumann and Dörte Diehl
Technical University Berlin
Department of Environmental Chemistry, Institute of Environmental Technology
Sekr. KF 3, Straße des 17. Juni 135
D-10623 Berlin
Gabi.Schaumann@TU-Berlin.DE
Under field conditions, soil is subjected to temperature and moisture dynamics. These induce swelling
and shrinking as well as changes in the surface characteristics of soil organic matter, which are not
fully reversible. Although the wettability is an important factor for sorption and transport processes in
soils, the knowledge about the reasons for water repellency and its effects on other soil properties is
still insufficient.
Water binding and matrix properties were characterized with 1H-NMR-Relaxometry and Differential
Scanning Calorimetry, both of which represent young methods in the field of soil chemistry. We
characterized the thermal behavior as well as physicochemical aging of soil organic matter and their
interplay with water, moisture conditions and temperature (Schaumann (2005; Schaumann et al.
(2005a; Schaumann and LeBoeuf (2005)). Physicochemical aging is a slow process and lasts for days
to months and years; it most probably involves the formation of water bridges between individual
hydrophilic functional groups (Schaumann and LeBoeuf (2005)) and increases matrix rigidity
(Schaumann et al. (2005b)). Fluctuations and changes of moisture conditions may accelerate or
reverse the aging process (Schaumann (2005)). We observed hysteresis for water absorption
(Schaumann (2005)) as well as matrix and surface properties (Hurraß and Schaumann (2006)).
In complementary experiments, we investigated changes in wettability upon drying, remoistening and
sample storage (Bayer and Schaumann (2006; Hurraß and Schaumann (2006)) and studied
temperature-dependent wetting kinetics (Diehl and Schaumann (2006)). Sample storage lead to a
partial approach of wetting characteristics between samples which initially differed in soil water
repellency (Hurraß and Schaumann (2006)), but differences in wettability persisted in one of the two
investigated urban locations.
The nature of the wetting process may differ between individual locations. It may be controlled by
chemical or physical reactions. Chemical reactions most probably involve the disruption of ester
linkages and other condensation products (Diehl and Schaumann (2006)), while the physical process
is probably mainly dominated by the formation of a water film on the surface (Bayer and Schaumann
(2006; Diehl and Schaumann (2006)).
Comparison of both investigations lead us to the hypothesis that the physical aging mechanism found
by Differential Scanning Calorimetry may represent one process controlling time and moisture
dependent changes in soil water wettability. Our hypothesis –which will be demonstrated and
presented in this contribution- describes soil organic matter from a new viewpoint considering it as
amorphous matrix. The rate of change in surface characteristics would then be controlled by matrix
rigidity, water content, dynamics of moisture conditions and aging time.
Keywords:
soil water repellency, origin, physicochemical effects, wetting kinetics, activation energy, aging
References
Bayer, J. and G. E. Schaumann (2006): Development of soil water repellency in the course of
isothermal drying and upon pH changes in two urban soils. Hydrol. Process.(special issue on water
repellency), subm.
Diehl, D. and G. E. Schaumann (2006): The nature of wetting on urban soil samples: Wetting kinetics
and evaporation assessed from sessile drop shape. Hydrol. Process., subm.
Hurraß, J. and G. E. Schaumann (2006): Properties of soil organic matter and aqueous extracts of
actually water repellent and wettable soil samples. Geoderma, in press, DOI:
10.1016/j.geoderma.2005.05.012.
Schaumann, G. E. (2005): Matrix relaxation and change of water state during hydration of peat.
Colloids Surfaces A: Physicochem. Eng. Aspects 265(1-3), 163-170.
Schaumann, G. E.; E. Hobley; J. Hurraß and W. Rotard (2005a): H-NMR Relaxometry to monitor
wetting and swelling kinetics in high organic matter soils. Plant Soil 275(1-2), 1-20.
Schaumann, G. E. and E. J. LeBoeuf (2005): Glass transitions in peat - their relevance and the impact
of water. Environ. Sci. Technol. 39(3), 800-806.
Schaumann, G. E.; E. J. LeBoeuf; R. C. DeLapp and J. Hurraß (2005b): Thermomechanical Analysis
of air-dried whole soil samples. Thermochim. Acta 436(1-2), 83-89.