Researchers of HUN-REN CSFK, together with Portuguese experts, have analysed the efficiency of bank filtration of micropollutants, and the research has produced some important results

It is well known that aquifers along rivers and lakes play an important role in providing drinking water for the population, as vast quantities of water can be treated efficiently by the porous sediments from various contaminants. In many countries, a remarkable part of the water supply is provided by bank-filtered water (e.g., more than 40 percent in Hungary; Figures 1-3). In many overpopulated African and South Asian countries with water scarcity or highly polluted surface water (e.g., Egypt, Nigeria, India, Pakistan, etc.), increasing bank-filtered water abstraction is the only realistic option to provide the population with potable drinking water.

The monitoring of aquifers with shallow depths (usually 10-20 m), exposed to surface pollution is of utmost importance worldwide. Researchers from HUN-REN CSFK and the University of Pannonia, together with Portuguese experts, have investigated the efficiency of bank filtration for substances with high risks, based on international experience. They also sought to identify which environmental or anthropogenic factors are the most important for the efficiency of filtration, according to the current state of science. The researchers analysed thousands of results and information published in the literature and found that although bank filtration is efficient for pharmaceutical residues in general, there are more conflicting results for certain antibiotics (e.g., sulfamethoxazole), hormone derivatives (e.g., ethinylestradiol) or the anti-inflammatory drug diclofenac. In the case of pesticides and industrial pollutants, many studies report "negative" efficiency, meaning that lower concentrations are detected in surface water than in bank filtrates. This may be due to seasonal patterns of chemical use on the one hand, but also to groundwater inflow to the aquifer on the other.

An outstanding result of the analysis of some of the factors influencing the efficiency of bank filtration is the identification of similarities and differences between different types of studies (e.g., laboratory, technological, or field studies) (Figure 4). The field studies most often refer distance from surface water, seasonality, microbiology, and redox potential as the main reasons for the different removal of pharmaceutical residues, while for pesticides and industrial pollutants, the contaminated groundwater infiltration from agricultural fields or industrial sites is assumed to be the main factor in the change in concentrations. This also raises the question whether the efficiency of bank filtration for pharmaceuticals is not in many cases due to the fact that groundwater, sometimes contaminated with pesticides and industrial pollutants but containing hardly any pharmaceutical residues, dilutes bank filtrates to such an extent that the concentration of pharmaceuticals falls below detection limits.

The data provided by the Hungarian and Portuguese researchers show that groundwater inflows account for more than half of the water extracted by bank-filtered wells, especially along smaller rivers. Because it is difficult to measure, the role of pumping rate, and abstraction volume is not usually investigated, even though some models suggest that they have a crucial impact on the “tapping” of regional groundwater and that the infiltration of groundwater further from rivers and lakes increases exponentially with increasing abstraction.

The research has also highlighted that laboratory studies often publish completely different results from field studies due to a lack of environmental complexity and that researchers highlight the factors that are easier to measure in the laboratory to be important in filtration efficiency. These factors can also differ between types of substances in laboratory studies. For example, while the role of temperature and adsorption is considered to be of particular importance for pesticides, these influencing factors are less frequently mentioned for pharmaceuticals. This is presumably due to the fact that the analysis of pesticide removal has a longer history. As some of them have been subject to mandatory monitoring for decades because of their hazardous effects on human health, many more complex and longer-term experiments have been carried out.

In addition to summarising the results of previous studies, the Hungarian and Portuguese researchers have demonstrated that methodological differences between field and laboratory studies, determined by different geographical or environmental conditions, lead to different results. They highlight the high efficiency of bank filtration, especially along large rivers such as the Danube. However, the results can be affected by agricultural and industrial contaminants in groundwater infiltrating from the background, especially during increasing the pumping, along smaller rivers or in wells further from the surface water body. Therefore, it is not sufficient to protect aquifers along the rivers and lakes, because agricultural and industrial pollution can easily infiltrate into the shallow aquifers and impair drinking water quality, especially when the volume of water abstraction needs to be increased.

This research was supported by the National Research, Development, and Innovation Office (NRDI Office), Hungary (project identification number: 2021-1.2.4-TÉT-2021-00029; 2020–1.1.2-PIACI-KFI-2021-00309) and the National Multidisciplinary Laboratory for Climate Change (RRF-2.3.1-21-2022-00014).

Publication:

Attila Csaba Kondor, Anna Viktória Vancsik, László Bauer, Lili Szabó, Zoltán Szalai, Gergely Jakab, Gábor Maász, Marta Pedrosa, Maria José Sampaio, Ana Rita Lado Ribeiro, 2024. Efficiency of the bank filtration for removing organic priority substances and contaminants of emerging concern: A critical review. Environmental Pollution, Volume 340, Part 2. https://doi.org/10.1016/j.envpol.2023.122795; https://www.sciencedirect.com/science/article/pii/S0269749123017979