Implications of maize fertigation with treated domestic wastewater on soil heavy metal contamination and health risks

In many parts of the world, fresh water is becoming harder to find, while the need to grow food continues to rise. One practical solution that farmers and city planners are exploring is the reuse of treated household wastewater for irrigation. This water, which has passed through treatment systems to remove harmful pathogens and organic waste, still contains nutrients like nitrogen and phosphorus that plants need to grow. However, this reuse comes with a hidden concern: household water often carries traces of heavy metals from everyday products like detergents, cosmetics, and plumbing. While these metals are present in tiny amounts, they do not break down. Over time, they can build up in the soil and move into the crops we eat. The central question for scientists is whether using this treated water on staple crops like maize creates a new danger, or if the risks are already present in the environment regardless of the water source.

A researcher at Ardhi University in Dar es Salaam, Tanzania, set out to answer this question by growing maize under controlled conditions. They wanted to see if irrigating with treated domestic wastewater would cause heavy metals to accumulate in the soil or the corn kernels more than using clean tap water would. To do this, they planted maize in a screenhouse to keep rain from interfering with their results. They divided the plots into two groups: one group received the treated wastewater, while the other received clean tap water. Both groups were watered using a drip system to ensure the plants received consistent amounts. The researcher then carefully measured the levels of six specific heavy metals—lead, chromium, cadmium, copper, zinc, and nickel—in the soil at different depths and in the harvested maize grains. They also calculated how much of these metals a person might consume by eating the corn and estimated the potential health risks for both adults and children.

The study revealed a complex picture where the source of the water was not the primary driver of contamination. When the researcher analyzed the soil, they found that the treated wastewater did not significantly increase the amount of heavy metals compared to the clean water. In fact, for some metals like cadmium and nickel, the soil irrigated with wastewater actually had lower concentrations than the control plots. However, the soil did show signs of moderate enrichment with copper, zinc, and chromium, and there was a surprising anomaly with lead. While lead levels were low near the surface, they spiked dramatically in the deeper soil layers of the wastewater plots, suggesting that the metal was moving downward through the soil profile in ways that are not yet fully understood. Despite these variations, the overall pollution levels in the treated plots were not statistically different from the clean water plots, indicating that the wastewater itself was not the main culprit adding new toxins to the ground.

The situation was different when the researcher looked at the maize grains themselves. Regardless of whether the corn was watered with treated wastewater or clean tap water, the grains contained dangerous levels of several heavy metals. The lead levels in the corn averaged 2.21 milligrams per kilogram, which is far above the safe limit of 0.2 milligrams per kilogram set by international health organizations. Chromium and zinc also exceeded safe thresholds in both groups. This finding suggests that the contamination was already present in the environment or the soil before the experiment began, rather than being introduced by the irrigation water. The plants were absorbing these metals from the soil regardless of the water source. The only metal that the corn did not absorb in significant amounts was cadmium, which remained undetectable in the grains.

The researcher also looked at how easily the plants took up these metals from the soil, a process known as bioaccumulation. They found that for most metals, the plants did not transfer much of the contamination from the soil into the edible grain. The exception was zinc, which the maize absorbed readily, showing a high potential for accumulation. This means that while the soil might hold onto many of the heavy metals, zinc moves easily into the food we eat. When the team calculated the health risks associated with eating this maize, the results pointed to a clear vulnerability: children. Because children have smaller bodies but often eat similar amounts of food relative to their size, they face a much higher daily intake of these metals than adults. The study found that lead and nickel were the biggest contributors to non-cancer health risks, while nickel and chromium posed the greatest threat for long-term cancer risks.

Perhaps the most striking conclusion of the study is that the maize grown with clean tap water carried equal or even higher health risks than the maize grown with treated wastewater. The total risk scores for children eating corn from the clean water plots were higher than those from the wastewater plots. This indicates that the background pollution in the soil and the general environment is the real problem, not the reuse of treated water. The treated wastewater did not make the situation worse; in some cases, it appeared to be slightly safer than the alternative. The study suggests that the heavy metals found in the food are coming from sources other than the irrigation water, possibly from the natural geology of the area or other forms of environmental pollution.

Ultimately, this research highlights that while reusing treated wastewater is a viable strategy for water conservation, it does not solve the broader issue of heavy metal contamination in the food supply. The presence of dangerous metals in staple crops like maize is a systemic issue that exists independently of the water used to grow them. The findings call for a shift in focus from blaming wastewater to addressing the underlying sources of pollution in the soil and water systems. Protecting public health, especially for the most vulnerable group, children, will require integrated monitoring of water and soil quality, along with targeted efforts to clean up the environment. The study confirms that the water source is not the sole determinant of safety, and that a comprehensive approach is needed to ensure that the food we eat remains safe for everyone.