Nature’s water purifier: Surface water-groundwater interactions

Water is essential for life. It is a critical resource to be preserved and protected. However, high-quality water is susceptible to contamination from various anthropogenic activities (e.g., mining, agriculture, chemical manufacturing, road salting). Today, there are also emerging contaminants such as micro-plastics and pharmaceuticals, whose long-term environmental impacts are unknown and areas of intensive scientific research.

Purifying water through engineering approaches is extremely costly but there are natural processes than can help. For instance, water that travels through the ground has the potential to react with the subsurface material and thus undergo natural processes of contaminant removal. Early hydrologists viewed surface water that entered a catchment more as a closed-pipe system assuming little to no interactions with the groundwater (Figure 1a). We now know that there is the potential for surface water to mix with the groundwater and that water that undergoes these interactions can undergo biogeochemical reactions that could be beneficial to restoring water quality (Figure 1b). In other words, watersheds can act as chemical reactors or natural filters of pollutants to some extent.

Figure 1 – Early paradigms in hydrology (Bencala 1993)

Furthermore, understanding surface water – groundwater interactions are important because it’s known that various aquatic life relies on these interactions to provide the nutrients and temperatures essential for a healthy habitat. Environmental engineers are often tasked to rehabilitate streams and rivers that may have been altered by anthropogenic activities (e.g., mining, urbanization) and to restore their previous ecological functions. However, proper ecological function of natural rivers and streams is a difficult term to define, thus hydrologists and hydrogeologist are partnering to advance this scientific frontier.

The process in which river water is driven into the subsurface and subsequently returns to the river is called hyporheic exchange. Hyporheic exchange can occur at various scales within the river, from the large river-corridor-scale (Figure 2a) the intermediate meander-scale (Figure 2b), and the small bedform-scales (Figure 2c). Understanding hyporheic exchange processes are important because the associated water is believed to undergo biogeochemical reactions that are attributed to the retention and degradation of pollutants within watersheds. The distribution of hyporheic residence times (the time taken for water to move through the subsurface) is an important factor when determining the magnitude of potential biogeochemical reactions.

Figure 2 – Surface water-groundwater interactions (hyporheic exchange) at different scales (Chow et al. 2018)

Researchers from the University of Tübingen’s Center for Applied Geosciences are providing new insights into these complex environmental processes. Since 2010, they have been investigating surface water – groundwater water interactions within a section of the Steinlach River, a tributary of the Neckar River in southwest Germany (Figure 3). They named their experimental test site the Steinlach River Test Site, which was chosen because it contained a well-defined meander with a relatively steep gradient making it ideal to monitor surface water-groundwater interactions.

Figure 3 – Steinlach River Test Site (Osenbrück et al. 2013)

Recently, our team has published new research in the journal Groundwater, investigating the effects of riverbed elevation on the surface water-groundwater interactions at the Steinlach River Test Site. Hydrologists refer to riverbed elevation as river bathymetry. Through the development of a 3D surface water-groundwater model of the Steinlach River Test Site we have found that having a highly-resolved bathymetry, which includes the detailed undulations of small-scale bedforms, leads to multiple scales of hyporheic exchange that are nested within one another. Figure 2b shows this nested behaviour of hyporheic exchange, where the small-scale purple arrows are nested within the larger-scale yellow arrows.

Conversely, lacking small-scale bathymetry contrasts can lead to the underestimation of bedform-scale hyporheic exchange, biasing hyporheic exchange towards larger meander-scale exchange. This can lead to overestimates of hyporheic residence times. This bias in the scale of hyporheic exchange can result in gross biases when calculating the catchment’s capacity to act as a reactor to attenuate pollutants.

Our study advances our understanding about the relationship between surface water-groundwater interactions and river bathymetry. With our new understanding we hope to equip river ecologists and environmental engineers with the mental tools required to preserve and maintain high-quality water within our rivers. Hydrologists and hydrogeologists are continuously learning more about the Earth’s complex environmental processes, with the hope that our newfound understanding will create a sustainable planet for all.

[Photo on Homepage: For illlustration. Meandering river by KAP Jasa]

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