Geochemical behaviour of trace elements in a tropical river, Northwest Borneo

What is it about?

The Baram river has no distributary and exhibits seasonally varying salinity gradient from its mouth at Kuala Baram in the South China Sea to Marudi, a town on the banks of this river around 110 km upstream. This river water column remains turbid throughout the year and has a large number of suspended particles. These suspended particles are the sites of interactions of dissolved trace elements due to iron-manganese oxyhydroxide coatings found on their surface that adsorb and form complexes. Moreover, the organics found in the river water interact with the surface of these suspended particles and form metal-organic complexes. Thus, a considerable quantity of trace elements might be associated with the particulates along with their dissolved load. As the water column parameters are not stable during its flow through the estuarine region, a continuous exchange of the trace elements occurs between their dissolved and their particulate forms. There exists a dynamic equilibrium between these two states of the trace elements which is controlled by the instantaneous local environmental conditions. The equilibrium constant of these interactions is widely known as the partition coefficient (Kd). By correlating the water column parameters with the partition coefficient, it should be possible to discern the geochemical processes prevailing in the estuarine region of this river segment - the Lower Baram.

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Why is it important?

In the downstream, the Baram has a single deep channel and devoid of distributaries and delivers its freshwater and sediments to the South China Sea. Due to tides and waves, the seawater gets into the lower part of the Baram river and influences its water column characteristics up to Marudi. A change in the geochemical environment in Lower Baram drives the exchange of the trace elements between water column, sediments and suspended particulates. The particulate bound trace metals are highly reactive, and that is in continuous exchange with the dissolved form. This continuous exchange is governed by the equilibrium constant Kd, otherwise, known as the "partition coefficient". Using a redefinition of the partition coefficient, we have attempted to elucidate the factors that exert control over the behaviour of the trace elements in the Lower Baram river. We have identified desorption, ion-exchange, dissolution, authigenic clay formation and remobilisation of orthophosphate as some of the primary geochemical processes. These results have ramifications for the understanding of estuarine processes, biogeochemical cycling of the major and trace elements, and affect the form in which the trace elements occur in the Lower Baram, and thereby controls their bioavailability and toxicity.

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