An
economic professor from Bates College frequently asked his
students, “What is wrong with water? What makes the
management of water resources so complex?” The answer
was simple. “It moves!”
Rivers have no nation. It is polluted in one country and enters
another country that can bring conflict among countries. Viewing
such possible conflict in South Asian countries sharing water
bodies viz Bangladesh, India, Nepal and Pakistan, the South
Asian Water Analysis Network (SAWAN) programme was initiated
by The Cooperative Monitoring Center (CMC) of Sandia National
Laboratories, USA in 2001. Under this programme the partner
countries monitor transboundary river water quality and share
data and information. For Nepal, ENPHO is monitoring Bagmati
and Narayani rivers, the two major rivers entering India and
finally merging with the Ganges (Photo 1)
Photo 1: ENPHO monitoring water quality of Narayani
at Amaltarighat near Nepal-India border
Three
years of continuous monitoring shows that Bagmati and Narayani
have lower concentrations of pollution parameters tested when
the rivers reach Nepal-India border. It is fact that Bagmati
is severly polluted in Kathmandu Valley and Narayani is little
bit polluted in Narayangadh area. But, when the rivers reach
Nepal-India border through relatively human settlement free
areas, they self purify. (Shrestha et. al, 2004)
Figure 1: Example showing the self-improvement of
dissolved oxygen in Bagmati river in Gaur which was terribly
low in Kathmandu valley. (Shrestha et al. 2004)
Do rivers self purify? Yes, rivers have the natural capacity
to self purify from polluting substances and has been proved
by different studies. River self purifies through combination
of several physical, biological and chemical processes. In
this article, various river self-purification mechanisms are
discussed.
Dilution
Dilution is a well-known physical process through which rivers
self purify. When a polluted river is joined by less polluted
tributaries or during the rainy seasons, volume of water in
the polluted river is increased thus reducing the pollutants
concentrations by dilution process.
Sedimentation
Sedimentation is another physical mechanism through which
pollutants concentrations in a river is reduced. In this mechanism
sand in the river bed acts as a sink for the pollutants. When
river reaches flat lands from the hills and gorges it spreads,
velocity reduces and suspended pollutants settle on the sand
bed.
Aeration
River water quality is also improved by the process of aeration.
When river runs through gorges and hills, turbulence mixes
air into water increasing the dissolved oxygen (DO). The increased
oxygen concentration facilitates many chemical and microbiological
processes in water required to reduce the pollutants concentrations.
Adsorption
In this process, pollutants are adsorbed onto rocks, pebbles,
sand particles, logs and plant surfaces thereby reducing the
concentration in the river water.
Phyto-remediation
Aquatic plants and vegetation on the riverbanks absorb nutrients
such as nitrate and phosphate from water. In addition, studies
have shown that plants are also capable of removing pesticides
and heavy metals from water. Especially, when a segment of
a river turns into a marsh on its way, the river quality is
largely improved by the vegetation grown there.
Floatation
Recently, the floatation mechanism has been put forward as
one of the processes in river self-purification. After rapid
mixing of water in falls, gorges or rapids, air bubbles act
as vehicles to lift many pollutants to the water surface.
The top layer looks like a layer of foam or froth in which
most of the pollutants, especially organic pollutants are
concentrated. The top layer, which is exposed to atmosphere,
facilitates oxidation of the pollutants converting them into
less harmful forms. Since the upper layer is also exposed
to direct sunlight, volatile organic compounds are removed
from that layer either due to increased temperature or by
photochemical reactions. If this foam can be removed by construction
of artificial traps at different segments of the river, high
concentrations of many pollutants can be removed from the
river. For example, in Boyne River 120 km north from Toronto
foam was trapped laying tree logs across the river. In that
experiment, a complete process of self-purification took place
within a distance of 35-40m. Chemical analysis of water upstream
and downstream of the tree logs showed 66% and 93% removal
of ammonia and phosphate respectively. The floatation mechanism
and froth collection was also studied in Etobicoke creek for
1 year. That study also proved floatation as a major mechanism
of river self-purification.
Photo 2: Formation of foam after splash on rocks, a self-purification
step of Bagmati at Chovar.
Microbial
degradation
Another important process is microbial degradation of pollutants
which is facilitated by the physical processes. The shallow
and turbulent water results in high aeration of water, which
is necessary condition for growth of bacteria and other macro-organism
that consume pollutants.
Water
ecology is a complex chain of life supporting system. Each
and every segment of a river should be free of pollution for
the betterment of ecosystem and human health. It is a fact
that rivers do self purify. But, does that mean we should
continue polluting rivers and leave them to self purify?
Reference:
Bennett, L.L. (2004). Complexities With Transboundary Water
Resource Management: Progress and Stumbling Blocks. Department
of Economics, Bates College [Online].
Available:http://www.google.com.np/search?qcache:HLU6FI4JuNkJ:www.ucowr.siu.edu/updates/
pdf/V118_A1.pdf+bates+college+answer+water+moves&hl=en
Jan, 2005.
Fisenko, A.I. & Mikhailovskii, V. (1998). The Flotational
Mechanism of Etobicoke Creek's Self-Purification. The EJLB
Foundation [Toronto].
Shrestha, R.R., Shrestha, J. & Manandhar, A. (2004). Monitoring
of Transboundary Rivers - Bagmati and Narayani (May 2002-April
2004). Environment and Public Health Organization (ENPHO).
