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Groundwater, the hidden underground cousin of aboveground water occurring in rivers and dams, is out of sight, so usually out of mind too. It is only once the above-ground rivers run dry, such as during drought, that people start exploring below their feet. This was particularly true in the Western Cape during the 2016-2018 drought, when drilling into aquifers to augment water supplies became hotly contested. The problem with groundwater is that because it’s invisible, it’s hard to know where it is, how much there is and how it can be used sustainably. People also often forget that groundwater is part of the water cycle and linked to many other groundwater dependent ecosystems above-ground.

Our work published in WaterSA in 2021, tested the idea that a specific environmental isotope, which is a naturally-occurring gas, could be used to trace the presence of groundwater in above-ground river systems. This radioactive isotope, known as radon (Rn222), occurs in high concentrations in groundwater. However, when groundwater is exposed to air, radon gas escapes into the atmosphere which results in increasingly lower concentrations in rivers as you move away from the groundwater source. Radon isotopes are derived from the radioactive decay of uranium (U238), which occurs in high concentrations in geologies, such as granites, shales and certain metamorphic substrates. The highly-fractured Table Mountain Group, forms part of the Cape Supergroup which extends across the Western and Eastern Cape provinces, and includes many formations high in uranium and, ultimately, radon. We predicted that the groundwater from aquifers within these formations should be radon-enriched and where this enriched groundwater discharges into rivers, there would be a sudden increase in radon within the stream.

Gevonden River in Rawsonville, Western Cape, where we showed, using a natural occurring isotope, that groundwater contributes towards surface water flows within the river. Image: Jaco Nel

To examine if this was the case, water samples were collected from groundwater as well as the Gevonden and Molenaars rivers in Rawsonville in the Western Cape (Fig. 1). Radon concentrations were determined in the laboratory using a RAD-7 electronic radon detector. Compared to other studies around the globe where rivers had radon concentrations below 20 Bq.L-1, 85% of samples collected in the Gevonden and Molenaars rivers had radon concentrations exceeding 20 Bq.L-1, with some as high as 119 Bq.L-1 (Fig. 2). Dry season groundwater samples collected from a borehole alongside the Gevonden River, had radon concentrations between 130 and 264 Bq.L-1 (Table 1). These measurements were collected under ambient conditions, which means that the borehole water has limited interaction with the aquifer and is open to react with the atmosphere. To access water inside of the aquifer that had no exposure to the atmosphere, we pumped the borehole and sampled fresh water discharged from the aquifer. These fresh groundwater samples had substantially higher radon concentrations (391-593 Bq.L-1).

Figure 1. Radon concentrations in the Gevonden and Molenaars rivers

Figure 2. Ambient radon concentrations in groundwater samples

Since geological material is the only source of radon in the environment, such high radon concentrations in rivers can only be attributed to the flow of groundwater into these rivers. Many rivers are considered to be groundwater-dependent ecosystems but it is not always easy to confirm the interaction between groundwater and surface water bodies such as rivers, wetlands, and dams. Our work has shown that radon can be reliably used as a tracer to determine how dependent rivers are on groundwater inflow to sustain their flow, especially during dry seasons. It could even be used to quantify the amount of groundwater discharge.

Reference

Strydom, T., Nel, J.M., Nel, M., Petersen, R.M. and Ramjukadh, C.L., 2021. The use of Radon (Rn222) isotopes to detect groundwater discharge in streams draining Table Mountain Group (TMG) aquifers. Water SA, 47(2), pp.194-199.

This article was originally published in the 2021/2022 Research Report.