Geochemical Aspects of River System

Rivers and their corridors are important components of the terrestrial ecosystem. Rivers represents an important mode of transport for terrestrial material to the ocean in both dissolved and suspended forms. The chemistry of the water depends upon the physico-chemical and biological interactions between the water, rock and soil in the drainage system. The anthropogenic input of materials and atmospheric inputs considerably alters the river water chemistry. Several studies have focussed on the transport of material within the rivers and streams in order to establish biogeochemical budget of weathering and to estimate the denudation rates of catchment areas. Most of the work in India was done on large rivers like Ganges Brahmaputra. But little is known about the transport of terrestrial material by smaller rivers, which are having homogenous lithology with smaller drainage basin area. So the biogeochemical studies of smaller rivers are more appealing and important to exactly estimate the material transport and to develop any meaningful model for effective and reliable budgeting of material transport to ocean. Recent estimates show that the small rivers with basin area <10,000 sq.kms, could be a major source of terrestrial material to the ocean and the estimates of river inputs to the oceans based on data from the major rivers alone may lead to serious underestimation.

Among the various types of inland freshwater bodies, the riverine system is a unique type of ecosystem that generally covers different types of climatic zones, landscapes and bio-geographic regions. Rivers are complex lotic systems of flowing water draining specific land surfaces known as watersheds or catchment area. The hydrodynamic characteristics of a river are dependent primarily on the size of the river and the climatic and drainage conditions within the catchment area. The major advantages of lotic systems are that there is no gravity and there are no waste disposal problems. However, cleanliness of rivers is vital to the aquatic life and much of the cleanliness is dependent on land use activities in the river basin or catchment area.

With an effective drainage area of about 5,80,000 km² and an average annual discharge of 510.45 km³ the Brahmaputra is the largest river system in the subcontinent. Unlike other river systems of India, this river system is relatively free from pollution as no industrial belt is located alongside the riverbank area. Of late, there has been report of massive fish kill in the Brahmaputra basin especially in the tea belt. The present communication, therefore, deals with the water quality of the upper stretches of the Brahmaputra river in relation to fisheries.

The water quality is most effectively expressed by its oxygen concentration. Although many contaminants both organic and inorganic enter into the river each day, no more measurement is more all encompassing or more generally accepted and used than biochemical oxygen demand (BOD) and dissolved oxygen (DO). The concentration of dissolved oxygen (DO) is an indicator of the general health of a river, which is a function of various parameters that describe the natural and physical processes taking place in the river. The impact of waste input on the dissolved oxygen of a river remains a main concern for pollution control agencies. At low flow, organic waste input coming from various drains and sewage treatment plants are depleting dissolved oxygen concentration of the river, resulting DO deficit and violation of water quality standards prescribed by the pollution control agencies.

The tightening of these water quality standards in recent years due to rapid industrialization and development need to apply a reliable and accurate predictive method of the BOD and DO in the river. To predict changes in stream conditions that will follow proposed reductions in plant wastes or to predict the effect of new industries or treatment plants discharging to streams, the assessment of water quality parameters such as DO concentration are necessary.