LAUSR

Limbang River Basin (LRB), an equatorial tropical river basin which witnesses the effect of climate change and terrain modification, is hydro-statistically characterized in the present research. In order to characterize the seasonal dynamics of hydrochemical characteristics of surface water, a total of 72 surface water samples were collected from 24 locations in the Limbang River and its tributaries for three different sampling periods (November 2016 (S1), March 2017 (S2), and September 2017 (S3)). Physical parameters (pH, electrical conductivity (EC), total dissolved solids (TDS), dissolved oxygen (DO), turbidity), nutrients (sulfate, nitrate, ammonia, phosphate), major ions (Ca, Mg, Na, K, Cl, HCO3), and trace metals (Co, Ni, Cd, Fe, Mn, Pb, Zn, Cu) were analyzed using standard procedures and the results were statistically processed through Pearson correlation and factor analysis. Gibbs diagram, Piper plot, partial pressure of carbon dioxide, and saturation index of carbonate minerals were also employed to determine the geochemical characteristics and dominant processes operating in the river basin. Higher mean EC value of 129.28 μS/cm in S3 and higher mean pH (6.77) in S1 were observed. The results show spatial and temporal variations in analyzed parameters but are within tolerable limits of the Malaysian water quality and WHO standards. A decreasing order of HCO3- > Cl- > Na+ > Mg2+ > Ca2+ > K+ was noted in the mean concentration of major ions, whereas the mean concentration of trace metals follows the order of Fe > Ni > Pb > Mn > Zn > Co > Cu > Cd. Higher mean concentration of HCO3 (47.09 mg/L) in S2 and higher mean concentration of Fe (1.77 mg/L) in S1 were observed. The Pearson correlation test (r ≥ ± 0.70) shows varying relationships among the parameters and is different in three sampling periods. Piper diagram revealed two distinct chemical facies such as mixed Ca2+-Mg2+-Cl- and Ca2+-Mg2+-HCO3- irrespective of seasons. At the same time, Gibbs plots indicated dominant control of precipitation (rainfall) with an influence of mineral dissolution over the surface water chemistry. An increasing trend of log pCO2 values from upstream to downstream shows the direct recharge of rainwater with lower log pCO2 values in the upstream side and mixed water with higher log pCO2 in the downstream side. The saturation index indicates the under-saturated state of the carbonate minerals in the order of SIMagnesite > SICalcite > SIAragonite > SIDolomite and suggesting the effect of dilution through monsoon rainfall. Factor analysis revealed the dominance of high sediment load in response to severe rainfall, weathering, leaching and dissolution of bed rocks, hydrolysis of organic nitrogen through decomposition of microorganisms, and the aerobic decomposition of organic nitrogenous matter and atmospheric fallout as the major contributors of surface water geochemistry.