Diffuse pollution occurs throughout
t h e l e n g t h o f a r i v e r ’s c o u r s e ,
whether it be rural communities
emptying raw sewage into the flow,
D’Arcy explains: “Diffuse pollution
is measured by the impact on the
watercourse, and you can quantify the
point sources, or the major discharges
that are licensed and controlled.
For example, a major city or town’s
discharge of sewage is treated,
licensed, and controlled; we therefore
know the quantity of it. Any industrial
effluence is measurable. Subtract
that from the catchment loads – you
quantify the rivers – and the rest is
diffuse pollution.” He adds that you
can sub-divide the measurements
and do land-use modelling, detailing
different aspects, including pesticide
use, nitrates, and traffic pollution.
In urban areas, where surface run-
off is not connected to treatment
works, pollutants deposited onto
impervious surfaces (such as
roads or pavements) are washed
into nearby surface waters. Such
pollutants include metals, pesticides,
hydrocarbons, and solvents, and
derive from various sources, including
the atmosphere and the abrasion of
roads, tyres, and brakes.
“You need to know what pollutant you
are looking for in the analysis,” D’Arcy
says, adding, “you only know what you
measure. So, if you don’t know that
traffic is a source of lead pollution and
copper from brake pads, for example, you
wouldn’t know to look for it.”
Water Sewage & Effluent November/December 2017
39
The transboundary problem
How is it measured?
Another consideration is the extensive
tracery of rivers that criss-cross
the country and the SADC region in
general: “There are transboundary
water issues with countries such as
South Africa, where you’ve got rivers
flowing out of the borders, into and
through other neighbouring countries.
If you had to take all the water from
the river to grow sugarcane, for
example, it impacts the neighbours.
Also, if you put a lot of herbicide
or nutrients onto the crop to help it
grow, then you can create nitrification
problems for the people in countries
downstream – as well as in your own
country, when the rains wash the
additive off the cane,” he adds.
In high nutrient waters with sufficient
sunlight, algal slimes can cover stream
beds, plants can choke channels
and blooms of plankton can turn the
water murky green. “Reservoirs are
becoming clogged with algae; for
example, in South East Asia, in the
or farmers using pesticides on their
crops and letting the rain wash it into
river, industrial waste being dumped
into it, or the effluent run-off of a coal
mine further upstream. In a water-
scarce country such as South Africa,
this resource is under daily assault
from unknowing perpetrators, both
large and small. Everybody wants and
needs water, and the international
implication is the transference of
pollution across the boundary by
intensive farming along waterways.
Pesticides used in agriculture
are transported to both surface and
groundwaters. Not only do they
threaten both wildlife and human
health, the excessive sediment run-off
from agricultural land results in turbid
waters and the clogging of spawning
areas. This, in turn, leads to loss of
aquatic habitats along the river’s
length.
contributor
Love thy neighbour
Mekong River,” D’Arcy adds. “Even
the Mississippi, in the Gulf of Mexico,
where tonnes of nitrate are used on
the bordering farmlands. It drains down
into the Gulf of Mexico and there it is
enriched and degrades as oxygen is
removed, and the result is so evident
that it is mappable on a dissolved
oxygen concentration map – maps of
zero concentration oxygen, or very low
oxygen,” he expands.
The biodiversity of freshwater
ecosystems has been degraded
more than any other ecosystem, and
more than 400 dead zones have been
identified in the world’s oceans. Pollution
is a major driver of this damage.
“On one hand, the farms are trying
to grow crops to feed the world, but in
tandem, are reducing the potential of
the sea to produce food. This is probably
affecting the fisheries, although I don’t
have the figures at hand; but it’s just
an example of transboundary issues,”
D’Arcy adds with concern.
field, you plant your crops, you treat
the pests with pesticide. However, the
more intense the activity, the greater
the risk of diffuse pollution.”