How groundwater influences Europe’s surface waters

Searching for groundwater on the Springendalse Beek, Netherlands. Image: Vince Kaandorp

This week we have a guest post by Vince Kaandorp of Deltares, a water research institute based in the Netherlands. Vince writes on the often-overlooked importance of groundwater in shaping and supporting life in rivers and lakes.

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A large portion of the water on Earth is hidden from sight, stored below our feet as groundwater. About 30% of the freshwater globally is believed to be stored as groundwater: 25 times the amount of fresh surface water. This groundwater has an influence on the aquatic ecology in our surface waters. While a proportion of discharge in streams originates from overland flow or direct precipitation, another big part comes from groundwater: either through local springs, diffuse seepage (seepage over bigger areas), or drainage pipes in agricultural regions. Groundwater influences not only small streams, but also rivers and even lakes.

All streams are not created equal: some have a higher contribution of groundwater than others as a result of differences in geology and topography. Because precipitation needs time to travel through the soil, groundwater is a delayed form of discharge compared to overland flow and direct precipitation. As such groundwater is a relatively stable source of water throughout the year and can prevent streams from ceasing flow during dry periods.

Stream water is often heavily ‘topped up’ by groundwater inputs during dry weather. Image: Vince Kaandorp

This groundwater characteristic forms the basis of the Baseflow Index (BFI) which gives an indication of the size of the groundwater contribution and can be calculated from stream discharge measurements. This metric is often used in studies to get an idea of the importance of groundwater for streams. For instance, in the Regge and Dinkel catchment in the Netherlands, the tributaries of the Dinkel river have very different BFI values as some have more groundwater input than others. The streams with less groundwater are known to fall dry during summer, while the ones with more groundwater flow even in the driest periods of the year.

Golden saxifrage growing along the banks of a stream is a good indicator of groundwater inputs. Image: Vince Kaandorp

The influence of groundwater can be seen in the field, that is, if you know what you’re looking for. Springs are a clear direct indicator of groundwater but vegetation can also give a good idea about groundwater. Some species, such as the Golden saxifrage plant often grow on stream banks at locations with significant groundwater inputs.

Have you ever seen orange depositions or slime on a stream bank? Or an oily sheen floating on the water? You might have located a seepage zone too! Groundwater is often anoxic and contains dissolved iron. As a result, as soon as this water comes to the surface certain bacteria start oxidizing the iron, which results in these orange and oily phenomena.

Apart from providing a stable supply of water to streams, groundwater also influences water chemistry and temperature. Groundwater has a different chemical composition to surface water, can contain iron, and is often unpolluted. In addition, the temperature of groundwater is generally around the yearly average temperature (about 12°C in the Netherlands), and is thus a cold-water input during summer and a warm-water input during winter. In this way, groundwater can provide stable temperature habitats for aquatic ecology in streams, and help prevent the water from freezing during cold winters!

Groundwater and surface waters are jointly affected by multiple stressors. Image: Vince Kaandorp

Due to its stable discharge, stable temperature and often unpolluted chemistry, groundwater can mitigate the harmful effect of stressors. For example, a stream with a large groundwater input is likely to be less prone to the effects of climate change. On the other hand, groundwater also functions as a connecting flow path between the catchment and stream, and can thus connect agricultural fields with a stream. This means that chemicals used by farmers, such as nitrate from manure or herbicides and pesticides, flow through the ground and eventually appear in the stream.

There can be a large time lag in this process, because the travel time of the groundwater can be 10s or even 100s of years. This also means that the effect of management practices such as the removal of agricultural fields upstream can take multiple decades to manifest in the stream itself!

The River Elsbeek flowing through agricultural areas where it has been channelised. Image: Vince Kaandorp

In practice there are diverse linkages between groundwater and streams, because a complicated system of groundwater-surface water interactions exists in which groundwater input and output is variable both in time and space.

Because of its importance for many streams and its link with management practises, further research on the groundwater contribution to streams is done by Deltares within the MARS Regge and Dinkel Case Study. This study will help us gain better understanding how groundwater transports and influences stressors, how groundwater is linked with aquatic ecology and how groundwater can be conserved and protected through European management practices.