Improving water quantity and quality in a mountain hard-rock watershed (La Vera, Spain) through Ancestral Nature-Based Solution for Managed Aquifer Recharge

Water is a living resource, but in the high, rocky mountains of Spain, it often arrives as a fleeting gift. In these steep, granite landscapes, rain and snowmelt rush down slopes so quickly that they rarely have time to soak into the ground. Instead, they run off into rivers, leaving the underlying aquifers—hidden reservoirs of fresh water—starved and the water that does remain dangerously pure, lacking the essential minerals that make it safe and pleasant for human consumption. For centuries, communities in these regions have faced a dual challenge: how to capture enough water to survive dry summers, and how to ensure that the water they drink is chemically balanced.

Nature-based solutions offer a way forward. These are strategies that work with natural processes rather than against them, using the land itself to store and treat water. One such approach involves managed aquifer recharge, where humans intentionally guide surface water into the ground to refill underground reservoirs. While modern engineering often relies on pumps and concrete tanks, ancient communities developed their own sophisticated methods. They built simple, unlined channels along hillsides to slow down rushing streams, allowing the water to seep into the soil and rock. This paper explores one of these ancient systems, known as pesqueras, in the La Vera region of western Spain. It investigates not just whether these channels successfully refill the groundwater, but also what happens to the water's chemistry as it travels underground. The study reveals that these ancestral structures do more than just store water; they act as natural filters and mineralizers, transforming thin, acidic mountain runoff into water that is better suited for drinking.

In the San Gregorio watershed, a rugged area on the southern slopes of the Sierra de Gredos, a local community called the Ocho Caños Irrigation Community has been restoring these ancient channels. For decades, rural depopulation had allowed the pesqueras to fall into disrepair, but the community has since rehabilitated over 12 kilometers of them. These channels divert water from streams and spread it across the hillsides, where it infiltrates a thick layer of weathered granite. This weathered rock acts as a sponge, holding the water and allowing it to percolate slowly downward. Previous studies had already shown that these restored channels contribute significantly to the region's water supply, accounting for nearly 58 percent of the total groundwater recharge in the area, or about 2 million cubic meters of water per year. However, scientists did not know how this process affected the quality of the water or the specific chemical changes that occurred as it moved through the rock.

To answer these questions, researchers from the Instituto Geológico y Minero de España and other institutions conducted a detailed investigation during two distinct seasons: a wet spring and a dry autumn. They collected water samples from the streams, the recharge channels themselves, and the natural springs that feed the local towns. By analyzing the chemical composition of these samples, including the levels of dissolved minerals and the unique signatures of oxygen and hydrogen atoms within the water molecules, the team could trace the journey of the water from the surface to the deep aquifer. They looked for signs of how the water interacted with the granite bedrock, which is rich in minerals like feldspar and mica.

The findings confirmed that the restored pesqueras are functioning exactly as a sophisticated natural treatment system. The water entering the channels starts as very dilute, acidic runoff with almost no dissolved minerals. As it sinks into the weathered granite, it begins a slow chemical conversation with the rock. The water dissolves minerals from the granite, picking up essential elements like sodium, potassium, calcium, and magnesium, as well as bicarbonate and silica. This process, known as water-rock interaction, gradually increases the mineral content of the water. The result is a transformation from water that is too pure and acidic to be ideal for consumption into water that is well-balanced and safe to drink. The study showed that the water emerging from the springs downstream had significantly higher mineral levels than the water at the top of the watershed, proving that the underground journey through the granite had enriched it.

Crucially, the researchers used stable isotopes—slight variations in the weight of water molecules—to track the movement of the water. These isotopes act as a fingerprint, allowing scientists to distinguish between water that fell as rain directly on the ground and water that was diverted through the channels. The data showed that the springs feeding the town of Aldeanueva de la Vera contained water that retained the isotopic signature of the high-elevation streams diverted by the pesqueras. This confirmed that the water was indeed traveling from the recharge channels, through the deep underground rock layers, and emerging miles away at the springs. The isotopic evidence also revealed that the water spent more time underground than it would have without the channels, allowing for a longer period of interaction with the rock and a more complete chemical conditioning.

The study also highlighted how the geology of the area influences the final taste and composition of the water. As the water moved from the upper parts of the watershed, where the rock is a type of granite called granodiorite, to the lower areas, where it meets a different rock type called leucogranite, the chemical balance shifted. In the upper zones, the water picked up more sodium and potassium. As it flowed further down into the leucogranite zones, it began to absorb more calcium and magnesium. This natural progression means that the water arriving at the town is a complex mixture, refined by the specific minerals of the rock it passed through. The researchers found that despite these changes, the water remained well within the safety limits set by the World Health Organization for drinking water, proving that this ancient method of management produces a high-quality resource.

The success of the pesqueras in La Vera offers a powerful lesson for water management in a changing climate. The restoration of these channels, driven entirely by local knowledge and community effort, has created a system that is both resilient and low-cost. It does not require electricity or complex machinery; it relies on the natural slope of the land and the chemical properties of the rock. By slowing the water down and letting it soak in, the community has secured a reliable supply of groundwater that is less vulnerable to drought. Furthermore, the process naturally improves the water's quality, turning a potentially problematic, ultra-pure runoff into a wholesome drinking supply.

This research validates the idea that traditional ecological knowledge can provide scientifically sound solutions to modern environmental challenges. The pesqueras are not just historical artifacts; they are active, functioning infrastructure that enhances both the quantity and quality of water. The study demonstrates that when communities work with the natural processes of their landscape, they can create water systems that are robust enough to withstand the pressures of climate change. The water that emerges from the springs in La Vera carries the story of its journey: a journey from the sky, guided by human hands, conditioned by the earth, and delivered as a vital resource for the community. This approach suggests that in other mountainous regions with similar geology, restoring such ancestral systems could be a key strategy for securing water for the future.