Water Chemistry of the Nilambe Oya, a Tributary in the Central Mahaweli Valley of Sri Lanka 1. Some Physicochemical Characteristics




Monthly and spatial variations of temperature, pH, electric conductivity, suspended solids, major cations (Na+, K+, Ca2+ & Mg2+) and anions (HCO3-, SO42- & Cl-)of the Nilambe Oya, draining a sub-catchment (6127 ha) of the Mahaweli basin woe determined from Sep. 1989 to Aug 1990 using standard methods. Temperature ranged from 18.0 °C to 32.2 0C with a significant seasonal variation. The pH varied from 5.40 to 8.10 with significant seasonal and spatial variations. Electric conductivity changed from 9 µS to 110 µS with only a significant inter-site variability. The stream water carried suspended solids ranging from 0.9 mg l-1     to 31.7 mg l-1 during the study period with significant seasonal and spatial variations.

Na+ and K+ concentrations ranged from 1.07 mg l-1   to 4.24 mg l-1   and from 0.25 mg l-1   to 1.77 mg l-1 respectively significant inter-site variations. Ca2+ and Mg2+ concentrations ranged between 0.11 mg l-1 and 7.16 mg l-1   and between 0.11 mg l-1 and 4.43 mg l-1 respectively also with significant spatial variations. The ranges of HCO3- (1.8 – 61.5 mg l-1) and Cl- (1.50 – 9.50 mg l-1) also showed significant monthly variations whereas the SO42- concentration ranged from 0.25 mg l-1 to 4.98 mg l-1 also with a significant monthly variation.

Concentrations of major ion species and ranges of temperature, electric conductivity, suspended solids and pH of the Nilambe Oya result due to a cumulative product of the geographical location, weather and climate, geochemical processes and anthropogenic activities of the watershed. Land use patterns play a significant role with respect to inter-site variability in physicochemical characteristics of the Nilambe Oya resulting in marked deviations from pristine values so far reported for tropical headwater streams.


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