Quality of Stored Rainwater in Surface and Subsurface Tanks and Health Implications in Makoka, Dar es Salaam, Tanzania
A study in Makoka, Dar es Salaam, reveals that rainwater stored in underground tanks exhibits higher levels of iron and fecal coliform contamination compared to surface tanks, posing significant health risks and necessitating improved maintenance and design practices to ensure water safety.
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In many cities where the tap runs dry, people turn to the sky for their next drink. They catch rain as it falls from the clouds, guiding it off their roofs into tanks to hold for later use. This practice, known as rainwater harvesting, offers a lifeline when municipal supplies fail or when the cost of piped water is too high. However, the water that falls from the sky is not always the purest substance once it touches the ground. As it slides over rooftops, it picks up dust, leaves, and animal droppings. As it sits in a container, it can interact with the materials of that container or seep in from the surrounding soil. The safety of this water depends heavily on where the tank is placed and what it is made of. If the storage method is flawed, the very resource meant to save a household could become a source of illness, carrying invisible germs or dissolved metals that harm the body over time.
In the bustling, water-stressed neighborhoods of Makoka, a sub-ward in Dar es Salaam, Tanzania, a researcher set out to understand exactly how these storage choices affect water quality. Grace Msangi, working with Mzumbe University, focused on a community where water rationing is common and many families rely on their own harvested rain. The study compared two main ways of storing this water: tanks sitting above the ground and tanks buried beneath the surface. The goal was to see if the location of the tank or the material it was built from changed the chemical makeup or the cleanliness of the water inside. To do this, the researcher collected forty-eight samples of stored rainwater from different homes. These samples came from a variety of containers, including plastic tanks, metal tanks, and tanks made of cement and iron, known as ferro-cement. Some were buried underground, while others stood in the open air. The water was then tested in a laboratory to measure three specific things: how well the water conducted electricity, which indicates the presence of dissolved salts and minerals; the total amount of solid material dissolved in the water; and the concentration of iron. Finally, the water was examined under a microscope to look for fecal coliform, a type of bacteria that signals the presence of waste from humans or animals.
The results painted a clear picture of risk. The water stored in underground tanks was consistently more polluted than the water stored in tanks above ground. When the researcher measured the electrical conductivity, a sign of dissolved minerals, the underground samples showed higher levels than those from surface tanks. One sample taken from an underground ferro-cement tank recorded a reading of 364.5 microsiemens per centimeter, while the lowest reading came from a tank above ground. This difference suggests that the underground environment introduces more contaminants, likely through cracks or leaks that allow groundwater or floodwater to mix with the stored rain. Similarly, the amount of total dissolved solids was higher in the underground tanks. One sample from an underground tank contained 204.5 milligrams of dissolved solids per liter, compared to just 10.67 milligrams in a sample from an above-ground tank. While these levels were still within the safety limits set by the World Health Organization, they were significantly higher than the rainwater collected directly from the sky before it touched any surface, which contained only 5.51 milligrams per liter. This increase confirms that the process of storing the water, particularly underground, adds impurities to it.
The most concerning findings involved iron and bacteria. Iron is a common element in the earth's crust, but too much of it in drinking water can cause health problems, ranging from digestive issues to damage to internal organs. The study found that two specific samples exceeded the safe limit of 0.3 milligrams per liter recommended by the World Health Organization. One sample from an underground ferro-cement tank contained 0.464 milligrams per liter, and another from an above-ground metal tank contained 0.92 milligrams per liter. The high iron in the metal tank was likely caused by the water eating away at the tank's material over time, a process known as leaching, especially since rainwater is slightly acidic. The iron in the underground tank likely came from the surrounding soil seeping in through cracks. Even more alarming was the discovery of fecal coliform bacteria. This bacteria was found in two samples from underground tanks but was completely absent in all the samples from above-ground tanks. The presence of this bacteria in the underground tanks points to a serious hygiene issue, likely caused by leaks that allowed wastewater from nearby latrines or animal droppings from the ground to enter the storage system.
These findings suggest that while rainwater harvesting is a vital resource for families in Dar es Salaam, the method of storage matters deeply. The study indicates that burying tanks underground, perhaps to save space or keep the water cool, introduces a higher risk of contamination from the soil and nearby waste. The presence of fecal bacteria in these underground tanks is a direct link to water-borne diseases like diarrhea, cholera, and typhoid, which can spread quickly in crowded urban areas. The research does not claim that all underground tanks are unsafe, but it shows that the ones tested in this area were more prone to contamination than those above ground. To make this water safe, the study recommends that households regularly clean their tanks, fix any cracks or leaks immediately, and consider diverting the very first flow of rain, which carries the most dirt, away from the storage container. For those who must rely on underground tanks, the design must be careful to keep them far away from septic tanks and sewage pipes. Simple steps like boiling the water or adding chlorine before drinking can also protect families from the hidden dangers lurking in their stored supply.
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