Home News Search for toxic flow and how we collected samples

Search for toxic flow and how we collected samples


Finding the mouth of Sabaki River was tough. We also had some trouble. One of our drones crashed into a bridge and fell in the thickest part of the polluted river.

Then there were difficult policemen guarding the Sh82 billion Thwake dam, who gave us a headache.  Having locked us out, we had to find an alternative access point to the river,  but they still did not allow us any peace. At the end of it all, reason prevailed and we took our samples.

By the time we picked our first samples, we had done nearly half the work back in the office. We started by mapping the entire river. We then broke it into several blocks, depending on the intensity of human and industry activity.

We spent days researching the river basin, all its tributaries, and its 390-kilometre journey as it courses through several counties to the coast. But the main study area was the Nairobi River basin, where most of the pollution happens.

To get the complete picture of what was going on, we had to follow the river all the way from Kiambu County where it starts, through Nairobi County to  Machakos, Makueni, Taita Taveta and Kilifi till it enters the Indian ocean at Sabaki Bridge.


The river changes its name as it goes along. From Nairobi, river empties itself into Athi River, then Galana, finally ending up as Sabaki River.

The scientists helped narrow down our investigation by selecting 10 sampling points on the river. From these points, we collected 49 samples comprising of water, sediments, fish, vegetables, crabs and other aquatic animals.

The sampling points were purposely selected to represent the major anthropogenic activities along the river, which were mainly agricultural and industrial. Our sampling points were Ondiri springs in Kikuyu, Ndwaru or Satellite near Kawangware estate, and Chiromo at the highway interchange near the Nairobi museum.

The other points were Gikomba, Dandora just a few metres from the dumpsite, and the bridge on Outer Ring Road.

The next sample was picked at Thwake in Makueni County, from where the river is being diverted to harvest water for the Sh82 billion dam. The last two points were in Malindi near Sabaki Bridge, where the river ends.

As we picked the samples, we recorded the exact GPS coordinates of every sampling point using a smart phone.

We picked four water samples at each of the 10 sampling points to make the sample collection watertight. The samples were aseptically obtained in pre-sterilised water-sampling containers for microbial quality. We also collected an extra sample in a plastic container for physiochemical analysis.

At every sampling point, the pH and temperature of the water were obtained.

We then transported our samples in cool boxes to the University of Nairobi’s Public Health, Pharmacology and Toxicology (PHPT) laboratories for analysis. The samples in Malindi were flown to Nairobi to make sure they arrived within eight hours of collection and testing as with the other samples.

The project’s lead researcher was Prof James Mbaria, the chairman of the PHPT department. Dr Nduhiu Gitahi, the Principal Technologist in charge of laboratories at the department, assisted him.

The microbial water quality was analysed within six hours of sampling and results recorded.

Physiochemical analysis was carried out after all the samples were obtained. Sediments and fish, where available, were also sampled for chemical analysis.

We tested for the presence of 12 different metals of toxicological importance using the East African standard known as procedure 67.

These were mercury, chromium, arsenic, cadmium, lead, selenium, copper, zinc, manganese, aluminium and barium.

The testing method used is known as the Atomic Absorption Spectrophotometric.

We also analysed the metals according to the National Environment Management Authority standards, using the spectrophotometry methods.

We tested the water for bacteria using Kenyan standards. The parameters used were the total coliforms, total fecalstreptococcus and Escherichia coli.

Our study did not stop there. To know how bad the water is, we did a third round of tests, this time checking for physical and chemical properties.

The tests included temperature, pH, total dissolved solids, nitrates, ammonia, total suspended solids, fluorine, sulphates, sodium, chloride, calcium, phenolics and cyanide.

PH expresses the acidity or alkalinity of a solution on a scale on which seven is neutral. Lower values are more acidic and higher values more alkaline.

It took another month for complete test results to be out.


Please enter your comment!
Please enter your name here