Hydrochemistry of two headwater peatlands affected by deep gullies in the Maloti Mountains of Lesotho

High in the mountains of southern Africa, where the air is thin and the landscape is defined by rolling grasslands, lie hidden reservoirs of water that sustain life far beyond their borders. These are peatlands, vast, spongy wetlands built from layers of dead plant matter that have accumulated over thousands of years. They act as nature's sponges, soaking up rain and snowmelt and releasing it slowly to feed the rivers below. This slow release is vital for keeping water flowing during dry spells, supporting local communities, and even providing water to neighboring countries. However, these delicate systems are under threat. Deep, narrow trenches known as gullies are cutting through the peat, draining the water and drying out the soil. To understand how these scars affect the water's journey and chemistry, researchers traveled to the Maloti Mountains of Lesotho to study two specific peatlands. They wanted to see if the gullies were simply draining water away or if they were fundamentally changing the chemical makeup of the water itself, which in turn could alter the plants that grow there and the health of the entire ecosystem.

The researchers focused on two peatlands named Motete and Matsoku, located at elevations above 3,000 meters. These areas are prone to deep gullies, some reaching depths of up to six meters, which have been widening since the 1980s. To investigate the hidden movements of water, the team installed a network of pipes and wells at various depths and distances from these gullies. They collected water samples during both the wet summer months and the dry winter season, capturing rain, snow, groundwater, and stream water. By analyzing the chemical ingredients in these samples—such as calcium, magnesium, and iron—and measuring the specific types of hydrogen and oxygen atoms within the water molecules, the scientists could trace where the water came from and how it moved through the landscape. This method allowed them to see the invisible pathways of water flow without needing to dig up the ground or rely on complex machinery that might be damaged in such remote terrain.

The study revealed that the gullies have drastically altered how water moves through these peatlands. In a healthy, undisturbed peatland, groundwater rich in minerals naturally pushes up from below, feeding the surface and keeping the soil wet and fertile. The researchers found that in areas far from the gullies, this upward flow of mineral-rich water still occurs, supporting the lush vegetation typical of these wetlands. However, near the gullies, the story is different. The deep cuts in the earth act as drains, pulling the water table down and allowing rainwater to sink directly into the ground. This rainwater is chemically different; it is poor in minerals and, as it seeps through the soil, it triggers chemical reactions that release iron and create acidity. In the winter, when the ground is dry, the water near the gullies becomes concentrated with salts and minerals due to evaporation, but during the wet season, it is diluted by fresh rain. This seasonal swing creates a harsh environment where the soil chemistry shifts from the stable, mineral-rich conditions needed by native sedges and mosses to a more acidic, nutrient-poor state.

The impact of this chemical shift is visible in the plants. Where the water remains mineral-rich and stable, the peatland supports a diverse mix of grasses, sedges, and mosses. But near the gullies, where the water table has dropped and the chemistry has changed, the vegetation is being replaced by hardy, drought-tolerant shrubs and upland plants that would not normally survive in such a wet environment. The researchers observed that the deeper the gully, the more severe the change. In the Matsoku peatland, where a massive gully cuts the landscape in half, the drainage is so strong that large sections of the peatland are shifting from being a source of water to a place where water is lost. This transformation threatens the peatland's ability to hold water and release it slowly to the rivers downstream. The study suggests that without intervention, these gullies will continue to deepen, drying out the peat and permanently changing the landscape from a water-retaining sponge into a drained, eroded slope.

The findings point to a clear need for action to restore these vital ecosystems. The researchers noted that simply blocking the gullies with small dams made of stone and timber has worked in other parts of the world to restore water levels and vegetation. Applying such methods in Lesotho could help stop the drainage, allowing the water table to rise and the mineral-rich groundwater to once again reach the surface. This would not only protect the local biodiversity but also safeguard the water resources that flow from these mountains to communities and countries downstream. The study underscores that the health of these high-altitude wetlands is not just a local concern but a critical component of the region's water security, and that understanding the hidden chemistry of the water is the first step toward saving them.