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Organic matter is one of the most important and fundamental constituent in soil, which usually
present in transported sediments. Therefore a wide spectrum of physically and chemically
different organic matter can be expected in alluvial type sedimentary gem deposits.
Composition of soil organic matter is very important, because it is tightly link with
environmental issues such as mine gas poisoning, carbon budget and bioavailability of toxic
elements in the soils.
This study was conducted to identify the composition, distribution and impacts of the soil
organic matter in the sedimentary profile of an alluvial gem deposit in Pelmadulla, Sri Lanka.
Seventy seven soil samples were collected along the sedimentary profile in three different
locations using auger drilling method. Chemical bonding structure of the each soil sample was
analyzed in the mid IR region of FTIR spectroscopy. Total organic matter (TOM) and the total
organic carbon (TOC) contents of the of each soil sample were measured using loss on ignition
and the Walkley-Black wet oxidation methods respectively. In addition, selected basic physical
and chemical parameters of the soil such as pH, oxidation-reduction potential, moisture
content, and elution conductivity were also tested.
Results indicate pH is in slightly acidic in nature and soil elution conductivity is higher in the
peat layer of the bottom alluvial deposit. FTIR spectroscopic analysis proved the presence of
several organic functional groups/bonds such as saturated aliphatic (2950-2970 cm-1
, 2860-
2880 cm-1
), unsaturated aliphatic (2915-2935 cm-1
, 2845-2865 cm-1
), alcoholic (3200-3645 cm1
), carbonyl (1680-1820 cm-1
), aryl (1450-1615 cm-1
), disulfides (570-705 cm-1
) and ether
(2815-2850 cm-1
). Some of those functional groups such as carbonyl and disulfide in organic
matter may cause for the poisonous gas emissions. The ratio of TOC/TOM is slightly higher in
the top and the bottom of the alluvial deposit. This indicates the high-carbon organic matters
are present in the top and the bottom most layers. Conversely, carbon content of the middle
layers in the deposit is less due to the intermediate level of organic matter degradation. In
addition, middle region of the alluvial deposit is consisting of sandy nature, which reflects the
dry climatic formation in the past thus the organic matter content is low. High soil elution
conductivity in the bottom of the deposit indicates that the soluble ions were leached to the
bottom and accumulate in the peat layer.
