This article, written by Leonie Joubert, is part of the SANBI Ecological Infrastructure Series. This case study focusses on our very own uMthinzima Stream, examining how a healthy river system cleans up effluent and improves water quality to the economically critical Midmar Dam.
CALL TO ACTION: Keep municipal waste water infrastructure, like sewage systems and storm water drains, properly maintained. This supports healthy rivers and wetlands, and improves water quality, thereby boosting human health.
A Stream Ran Through it
mThinzima Stream, may not be a large watercourse, but together with all the other veins that feed into the Midmar Dam near Pietermaritzburg, it is one of the critical small tributaries that meander through the KwaZulu-Natal Midlands and feed into a dam that supplies water to one of the largest economic hubs in the country.
At its headwaters, mThinzima’s water quality is excellent, seeping into the streambed from a relatively pristine catchment. But, as it meanders down towards the dam, it picks up large volumes of raw sewage which trickle down into the stream from some of the poorly serviced townships that have been built on the surrounding hillsides.
The quality of the water downstream of here, according to local WESSA (Wildlife and Environment Society of South Africa) environmental educator Dr Jim Taylor, is ‘appalling’. The bacterial load in the water is obviously a grave concern, since it can contribute to the kind of diarrheal disease that is the leading cause of death amongst children under the age of five globally.
Another worry here is the potentially toxic build-up of nutrients in the water that will eventually suffocate the dam and render its water prohibitively expensive to treat and recover.
When large volumes of excess nutrients flush into a water course – be it from overfertilisation of farmlands, or raw sewage leaking into the system, for instance – it’s like putting too much fertiliser on your lawn, says local environmental consultant Dr Mark Graham.
‘The more you put on, the more the grass grows, the more you have to mow the lawn,’ he explains.
In a river system, it means that the influx of phosphates and nitrates from the sewage give massive amounts of food for algae to grow on. As this aquatic ‘lawn’ blooms, it strips oxygen from the water, effectively suffocating the microscopic animal life (the zooplankton) in the water, and bigger animals like fish.
These algal blooms can also flush the water with potent toxins that have been known to kill animals that drink the water.
Meanwhile the dense algae can clog up pumps and water filters in treatment plants. This kind of eutrophication has made parts of the Hartbeespoort Dam in the North West Province unusable, something which has called for a clean-up operation which is expected to run up to hundreds of millions.
According to Taylor and Graham, eutrophication in the Midmar Dam is expected within just 15 years, if the current rate of nutrient pollution continues.
But something remarkable is also happening here – a sign of nature’s resilience, the importance of the services which a working natural system would offer us, and a warning of the need to help watercourses like the mThinzima stream stay healthy and functioning.
Graham and colleagues at the consulting firm, Ground Truth, have been working with WESSA and local communities over several months to test the quality of the water in the mThinzima stream.
Starting at the point where it meets the Midmar Dam, and working steadily up towards the headwaters, various teams sampled the water and tested it to see how abundant the small animal life was, these ‘micro-invertebrates’ or little insects. If there’s abundant life, the river is healthy; if not, it’s taken a hammering.
All this information was loaded into a new tool available to such researchers, called the Stream Assessment Scoring System, or MiniSASS, which then overlays the information onto an aerial photograph of the stream system, using the graphic of a crab to flag where the stream was sampled.
If the sample shows the water is healthy, the site is flagged with a blue crab. If it’s beginning to deteriorate, the crab colour changes to orange. If it’s very poor quality – meaning there is very little micro-animal life in the water – the crab will show up as red.
Too much raw sewage puts terrible strain on the river’s clean up systems and there is only so much a river can be expected to carry before it collapses, affecting people, livestock and crops. This is where the astonishing thing comes in. Looking at the aerial photograph of the mThinzima stream, the graphic of the crab at its headwaters is blue – healthy.
As the water runs down through mPophomeni township outside Howick, the crab flagging the testing site lights up with an angry red, indicating insect life in the river is collapsing. But a bit further downstream, the crab colour softens to a less aggressive yellow, meaning the river is recovering, and animal life responsible for processing the sewage is bouncing back again.
Here, according to Dr Mark Graham, is what’s going on at a scale too small for the naked eye to see: river systems have their own ecological pyramid. It’s just like in the Kruger National Park, you will have grass and trees; then you’ll have impala and other buck grazing; lions killing the buck; and vultures picking apart the remains.
Similarly, in a healthy river system you have algae growing on the rocks; minute grazing organisms eating the algae and reeds and other plant-based sediments; there are mayflies and dragonflies, the latter of which are predators, which eat these creatures, all the way up to the fish at the top of the food chain; then you have the crabs and shrimps which are the vultures and scavengers of this aquatic ‘veld’.
When raw sewage leaks into the river, the aquatic insects at their larval stage that are present in the water start to eat up the nutrients in the sewage, reducing this solid waste to their very basic components, namely nitrates and phosphates. Some insects shred the solid matter, some suck it up and pass it on to fish, as they themselves are eaten.
This suite of micro-animals are cleaning and processing and filtering the water. Some of the reeds and other water plants will use these nutrients to grow, banking them away in their stems and leaves.
These plants may be eaten by grazing animals or fall into the river and get trapped in the mud, meaning they are locked away for a period of time. Some insects might metamorphose into the adult stage and fly away from the river, ‘exporting’ those nutrients from the system. It’s a vast, dynamic and often invisible process.
But when the system is overloaded with nutrients, it’s over to the algae which, just like an over-fertilised lawn, grow in abundance. In the process, they strip the water of oxygen, leaving the micro-invertebrates to suffocate. As they die, the miniSASS crab turns to red.
However, further downstream, because the crab lights up as yellow, this indicates that in this particular case, animal life in the stream has somehow been able to recover, in spite of the water being overloaded with nutrients from the sewage.
But by the time the water reaches the Midmar, the miniSASS crab turns back to red because of the additional build-up of pollution in the water.
Two lessons emerge from this story. Firstly, a tool like miniSASS is a‘game-changer’, says Graham. Previously, water quality testing was usually done by municipalities, and the resulting water management decisions remained with the authorities. (This created ‘a power gradient’ between people on the ground, and government officials responsible for managing their water, maintains Taylor.)
‘In all my years in aquatic science, this is the first time we’ve been able to represent water health in such a simple way, so that you can show a municipal manager or a politician, and they can see immediately what’s going on in the water system,’ explains Graham.
MiniSASS is a way of allowing citizen scientists to feed trustworthy data into a mapping system that everyone can see. A visual representation of changing water quality, in this way, makes it immediately obvious to the viewer what’s going on in the river. Here, citizen science helps democratise water management – the citizens know what’s going on in the water system, and they can hold their water managers accountable for decisions being made on their behalf.
Secondly, though, is that the clean-up services offered free to us by healthy functioning rivers are the very basis for keeping water safe and clean.
But there’s only so much workload any river can be expected to carry before it collapses under the strain.
If the tributaries of the Midmar Dam continue to be the default sluice for under-serviced communities in the surrounding catchments, the entire eThekwini water system will be compromised in the very near future. Stormwater drains and sewage infrastructure must be maintained so they don’t fail and spill out their contents into nearby streams and rivers.
Municipalities around the country need to take urgent action to ensure that sanitation and service delivery to historically neglected communities is prioritised; that stormwater drains and sewerage infrastructure don’t fail and spill out their contents into nearby streams and rivers, and that the tributaries feeding important water sources are conserved.
The situation, as it is, is a ‘time bomb’, according to Graham.
Dr Mark Graham
Phone: 033 343 2229; 082 377 7089
Dr Jim Taylor
Phone: 033 330 3931; 082 458 0976