Abstract:
Geochemical variations in stream sediments (n = 54) from the Mahaweli River of Sri Lanka have been
evaluated from the viewpoints of lithological control, sorting, heavy mineral concentration, influence
of climatic zonation (wet, intermediate, and dry zones), weathering, and downstream transport. Com positions of soils (n = 22) and basement rocks (n = 38) of the catchment and those of <180 m and
180–2000 m fractions of the stream sediments were also examined. The sediments, fractions, soils
and basement rocks were analyzed by X-ray fluorescence to determine their As, Pb, Zn, Cu, Ni, Cr, V, Sr,
Y, Nb, Zr, Th, Sc, Fe2O3, TiO2, MnO, CaO, P2O5 and total sulfur contents. Abundances of high field strength
and ferromagnesian elements in the sediments indicate concentration of durable heavy minerals includ ing zircon, tourmaline, rutile, monazite, garnet, pyriboles, and titanite, especially in <180 m fractions.
The sediments show strong correlation between Ti and Fe, further suggesting presence of heavy mineral
phases containing both elements, such as ilmenite and magnetite. The basement rocks range from mafic
through to felsic compositions, as do the soils. The river sediments lack ultrabasic components, and over all have intermediate to felsic compositions. Elemental spikes in the confluences of tributary rivers and
high values in the <180 m fractions indicate sporadic inputs of mafic detritus and/or heavy minerals to
the main channel. Al2O3/(K2O + Na2O) and K2O/Na2O ratios of the sediments and LOI values of the soils
correlate well with the climatic zones, suggesting intense weathering in the wet zone, lesser weathering
in the intermediate zone, and least weathering in the dry zone. Low Sr and CaO contents and Cr/V ratios
in stream sediments in the wet zone also suggest climatic influence. Fe-normalized enrichment factors
(EFs) for As, Pb, Zn, Cu, Ni and Cr in stream sediments in the main channel are nearly all <1.5, indicating
there is no significant environmental contamination. The chemistry of the sediments, rocks and the soils
in the Mahaweli River are thus mainly controlled by source lithotype, weathering, sorting, and heavy
mineral accumulation.
