To understand groundwater processes, the invisible is needed to be made visible. Perhaps we should start with the fact that the water that comes from precipitation does not stop in the ground, but a considerable amount of water continues to move towards the groundwater table, a very significant surface under the ground. Above this, the pores are only partially saturated with water down to the groundwater level, the residual space being filled with air. The thickness of this unsaturated zone can range from one or two metres to tens of metres, and in our limestone mountains up to hundreds of metres. Rainfall or water from rivers and lakes must pass through this zone to become groundwater.
Water infiltrating from the surface reaches the groundwater table by passing through the unsaturated zone, thus ensuring the recharge of water resources.
This is reflected in the increase in water levels. But the interesting part happens below the water table, where every pore is already saturated with water. Formerly, it was thought that aquifers were separated by ideally impermeable rocks below the surface. Science has recognised that aquicludes do not exist in the natural world. Aquicludes can be penetrated, they just need time and drive to force the water through them. Surprising at first but, underground as well, energy differences move water between pores and fractures, just like in rivers. What is very different is that we are not talking about a riverbed, but about the movement of water in a reservoirs/bedrocks hundreds or even several kilometres thick; another difference is that the movement of groundwater is very slow.
The difference in groundwater level creates a subsurface flow that follows the topography. Water droplets reach different depths along the flow lines and spend different amounts of time below the surface. Water is observed to penetrate all layers, with no rock being able to completely obstruct the water's path. (original source: USGS)
Surely very few people consider that it is the differences in groundwater levels that keep groundwater moving from higher to lower elevations. The flows are organised in different flow systems depending on whether they connect near or distant surface recharge and discharge areas.
Groundwater flows transport groundwater from the reef areas towards the river valleys. If river levels drop, this can also lead to water table subsidence in higher areas in the distance.
The journey of these flow systems can last from a few years to millions of years, depending on the length and depth of penetration. But when we install producing wells, we are intervening in this complex system, shortening the residence time of water below the surface. Of course, we can't see any of this, so we can only rely on our measurements, observations and models.
Although the complexity of groundwater is beyond debate, we cannot forget the fact that 98% of the freshwater resources that mankind can mobilise are under our feet and available on any continent.
NaBa-MAR
While surface waters and ecosystems are highly sensitive to hydroclimatic extremes, the groundwater storage system operates in a different time scale and is able to compensate for these extremities. To exploit this buffering effect, managed water storage in shallow aquifers has been used as a technology worldwide for several decades. This method is suitable for using excess water from inland waters, floods, precipitation and treated wastewater, subject of course to legal requirements.
Managed aquifer recharge (MAR): bringing excess water collected at the surface to the subsurface to raise water levels. In contrast, a canal cut into the water table can drain groundwater from the site, causing subsidence.
Reservoirs can be used to mitigate drought problems, raise groundwater levels, restore ecosystems that also depend on groundwater, and meet various water needs. The planning process is a long and complex one that requires serious collaboration.
Managed aquifer recharge can also work at a regional scale. Groundwater recharged from higher groundwater recharge areas can follow flow paths to supply water to wells, help maintain shallow groundwater levels in agricultural areas and provide natural water to wetlands established in the drainage area.
Solutions for natural water retention on the surface are well known, inspired by the characteristics of the landscape. In our approach, we combine this with the subsurface. Here we use solutions based on the functioning of nature. In the subsurface, although we store water in aquifers, we change the distribution of the forces that move the groundwater by raising the water levels.
Although groundwater is invisible, it plays a key role in shaping the landscape and its habitats. The image above shows a view of a water-rich and water-scarce state. The lack of groundwater recharge when drought strikes is transforming flow regimes, with the whole region facing severe environmental changes.
This will not only have an impact locally, but also further away, through subsurface hydraulic connections. In other words, in the NaBa-MAR® process, we combine the knowledge of natural groundwater flows with local MAR technology to approach nature-based landscape-scale solutions. This concept has since been registered as a European trademark by ELTE as NaBa-MAR®. Our model visualises the implementation of the concept.
VIDEOS
In our experimental demonstration video, you can see the model in action. On the one hand, the installation brings to life the processes that affect the groundwater beneath our feet. On the other hand, it shows how the NaBa-MAR ® works. In this way, you can learn how we can use invisible processes to store water for drier periods in times of plenty.
The NaBa-MAR® model, a physical model that aims to demonstrate the connections between water infiltration, groundwater flow and drainage through canals in relation to the NaBa-MAR® process. This model is used to illustrate the effectiveness and impact of infiltration basins in aquifer storage and recharge; and the risk that infiltrated water can be drained if the canals are not managed appropriately.
