Groundwater is a vital natural resource for Estonia, supporting ecosystems and agriculture and serving as a source of drinking water for a large share of the population. The best way to preserve groundwater quality is to protect it before contamination occurs. For this reason, groundwater vulnerability must be carefully assessed before any land use planning takes place. Magdaleena Männik, a doctoral student in geology at the University of Tartu, recently defended a dissertation in which she developed a new method to assess groundwater vulnerability across different regions of Estonia.

Some 13,000 years ago, Estonia was covered by a massive continental glacier. As it melted, the glacier left behind a layer of surface deposits with varying thickness and composition, including till, clay, sand and gravel. According to Männik, the thickness and type of these surface deposits are the most important factors determining why groundwater in some areas is at higher risk of contamination, while in others it is naturally well-protected.

"There are areas where pollution can seep into groundwater very quickly. However, if there is upward pressure from the groundwater that prevents water from moving downward, the area is very well protected from contamination," she explained. Thick layers of clay and till provide good natural protection for groundwater, Männik said, while thin or sandy layers allow pollutants to travel faster and deeper.

A tool to consider Estonian peculiarities

The term "groundwater vulnerability" refers to how sensitive groundwater is to contamination originating from the surface. In Estonia, it frequently appears in legislation and plays a significant role in shaping agricultural activity and land-use planning. "Given that groundwater is the source of drinking water for more than half of Estonia's population, protecting its quality is critically important," said Magdaleena Männik. Once groundwater becomes contaminated, cleaning it is technologically complex, time-consuming and costly.

However, Estonia's current groundwater vulnerability assessment guidelines date back to the 1980s and rely heavily on manually drawn maps. "To ensure a more transparent assessment and a more realistic picture of where our groundwater is well protected and which areas need more serious attention, we must adopt modern methods," Männik said. The method developed in her dissertation is a customized version of the DRASTIC method, which is based on a geographic information system (GIS) and allows for rapid vulnerability assessments using existing data.

The internationally used DRASTIC method assumes that the deeper the groundwater level, the better the natural protection. This approach makes sense in areas that primarily use shallow, unconfined aquifers, since greater depth generally reduces the risk of contamination reaching the groundwater. "In Estonia, however, hydrogeological conditions are much more complex. The main aquifers in use are often confined and groundwater vulnerability is shaped by the interaction between surface deposits and bedrock," Männik explained.

In her dissertation, she describes situations in former glaciated areas where the groundwater level is very close to the surface but still under strong pressure. "In these areas, pollution cannot seep downward because the groundwater layer pushes back, so to speak, and prevents contaminants from reaching deeper layers," Männik said. According to her, the customized DRASTIC method helps distinguish such naturally well-protected areas from those where the protection is weaker.

Intensive farming breaking through natural barriers

Groundwater vulnerability, however, is not determined solely by natural conditions. Magdaleena Männik incorporated the impact of land use into her methodology to assess the actual risk of contamination, taking human activity in a given area into account. One of the most significant threats to groundwater quality, she noted, is diffuse pollution from agriculture. When fertilizers are used, nitrates can seep through the soil into groundwater layers. Other sources of pollution, such as former industrial contamination sites, mining and inadequate wastewater systems, also affect groundwater quality.

Her dissertation found that areas that are naturally well-protected can still face a high risk of contamination if subjected to intensive agricultural use. In contrast, areas with thinner surface cover but less intensive land use are less vulnerable. The most at-risk zones are those where agriculture is practiced on land with thin surface deposits.

In Estonia, such sensitive areas include the Pandivere and Adavere–Põltsamaa regions. According to Männik's findings, groundwater is well protected in places like the Pärnu area, where thick layers of varved clay cover bedrock aquifers. Groundwater is also well protected in areas where the main aquifer is overlain by a regional confining layer, such as the Lontova clays or the confining layer of the Narva stage.

Groundwater cares naught for national borders

According to Estonia's Water Act, groundwater protection is based on various preventive measures that depend on the level of groundwater vulnerability. In areas with thin surface deposits and intensive agriculture, stricter regulations apply to fertilizer use. Groundwater protection is also enforced through the establishment of sanitary protection zones around drinking water sources and through water permit requirements. Additional measures may be introduced through water management plans, if needed, to ensure groundwater remains in good condition.

However, pollution, overuse or poor decision-making can affect both the quality and quantity of groundwater on either side of the national border. For this reason, Magdaleena Männik tested the effectiveness of her new method in a cross-border region between Estonia and Latvia. "Geological conditions don't change at the state border — both Estonia and Latvia have similar systems where the confined aquifers used primarily for drinking water are covered by surface deposits that largely determine groundwater vulnerability," she explained. According to Männik, the customized DRASTIC method is well-suited for use in Latvian conditions as well.

She noted that the new method made it possible to assess groundwater vulnerability in the Estonian-Latvian cross-border area using a unified and comparable framework. Previously, each country used different approaches to assess groundwater vulnerability, making it difficult to interpret results and coordinate joint water management planning.

Now, harmonized and science-based mapping of groundwater vulnerability is helping both countries assess groundwater conditions. It also informs decisions about where monitoring, protection or closer cross-border cooperation is needed to reduce risks. In recent years, Estonia and Latvia have launched an increasing number of joint projects aimed at improving groundwater assessment, monitoring and protection, as well as ensuring the sustainable use of drinking water resources throughout the cross-border region.

Magdaleena Männik defended her doctoral thesis, "Groundwater vulnerability assessment in confined aquifers: modifying the DRASTIC method for aquifers covered by Quaternary deposits" at the University of Tartu on November 21.

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