The use of Radon (Rn222) isotopes to detect groundwater discharge in streams draining Table Mountain Group (TMG) aquifers

COPYRIGHT © The Author(s) Published under a Creative Commons Attribution 4.0 International Licence (CC BY 4.0) Environmental isotopes have been used for decades as natural tracers in studies aimed at understanding complex hydrogeological processes such as groundwater and surface water interactions. Radon (Rn222) is a naturally occurring, radioactive isotope which is produced from radium (Ra226) during the radioactive decay series of uranium (U238). Since U238 is present in most geological substrates, Rn222 is produced in various lithological structures and subsequently transported with groundwater through fractures and pore spaces in an aquifer towards surface water discharge points in rivers and springs. This study aimed to determine (i) the concentration of Rn222 within both surface water and groundwater in Table Mountain Group (TMG) aquifer systems, and (ii) the feasibility of using Rn222 isotopes as a natural tracer in groundwatersurface water interaction studies. This study was conducted in a highly fractured TMG aquifer system near Rawsonville, South Africa. Surface water from two perennial rivers (i.e. Gevonden and Molenaars), together with groundwater from a nearby borehole, were sampled and their corresponding Rn222 concentrations measured. Our study found median Rn222 concentrations in the Gevonden River of 76.4 Bq∙L-1 and 67.2 Bq∙L-1 in the dry and wet seasons, respectively. Nearly 12% of surface water samples exceeded 100 Bq∙L-1. These abnormally high Rn222 concentrations can only be attributed to the influx of groundwater with extremely high Rn222 concentrations. Under ambient (no pumping) conditions, Rn222 concentrations in groundwater range between 130–270 Bq∙L-1. However, when the borehole was pumped, and inflowing water from the surrounding aquifer was sampled, even higher Rn222 concentrations (391–593 Bq∙L-1) were measured. These extremely high Rn222 concentrations in groundwater are believed to be attributed to the underlying granitic geology and the prevalence of faults. The use of Rn222 isotopes as an environmental tracer in groundwater–surface water interaction studies is therefore regarded as a feasible option in similar highly fractured aquifer systems. The use of Radon (Rn222) isotopes to detect groundwater discharge in streams draining Table Mountain Group (TMG) aquifers


INTRODUCTION
The interaction between surface water and groundwater is dynamic and complex (Sophocleous, 2002;Hunt et al., 2005). Understanding the linkages between groundwater and surface water bodies is critical for sustainable utilization and management of these complex systems. According to Wu et al. (2004), it is critical to establish the flow paths, patterns, water quantity and quality of the water flowing between surface water and groundwater, to develop and manage water resources efficiently. Since the properties of surface water and groundwater are usually chemically, physically and biologically different, the exchange of water between the two entities may have a significant impact on the water quality of either of these hydrological units (Kalbus et al., 2006).
Radioactive and stable isotopes have been applied as tracer techniques in many Earth systems studies world-wide (Thomas and Rose, 2003;Gibson et al., 2005;Kalbus et al., 2006;Praamsma et al., 2009). For roughly half a century, environmental isotopes have been used as natural tracers in studies aimed at understanding hydrogeological processes (Thomas and Rose, 2003). This field has developed in its scope and currently environmental isotopes are also used to study the exchange of water between groundwater and surface water sources (Midgley and Scott, 1994;Thomas and Rose, 2003;Wu et al., 2004). Other hydrological tracer studies include the application of stable isotopes such as 18 O and 2 H (e.g. Harrington et al., 2002;Hunt et al., 2005;Praamsma et al., 2009), as well as radioactive isotopes such as tritium (e.g. Scanlon, 2000), 14 C (e.g. Harrington et al., 2002) and radon (Rn 222 ) (e.g. Ellins et al., 1990;Kalbus et al., 2006;Moreno et al., 2014;Srinivasamoorthy et al., 2018).
Rn 222 is a naturally occurring, odourless, radioactive noble gas with a half-life of 3.83 days (Ellins et al., 1990;Wu et al., 2004). Rn 222 isotopes are daughters produced from radium (Ra 226 ) during the radioactive decay series of uranium (U 238 ) (Broecker et al., 1967;Schubert et al., 2008). Since U 238 is present in most rocks and has a half-life of 4.5 billion years, Ra 226 and Rn 222 are readily available in geological substrates such as granites, carbonaceous shale and metamorphic rocks (Hoehn and Von Gunten, 1989;Ellins et al., 1990;Schubert et al., 2008;Moreno et al., 2014). Ra 226 , and subsequently Rn 222 concentrations, tend to increase along faults and fractures, particularly when aquifers have been exposed to tectonic activities (Ellins et al., 1990;King et al., 1996;Vogiannis et al., 2004). Therefore, the lithology of the aquifer material influences the concentration of Rn 222 found in the aquifer. The radioactive decay of Ra 226 alongside and within the aquifer is the principal means for the accumulation of Rn 222 in groundwater, whereby the Rn 222 gases continuously diffuse into the pore spaces of the aquifer resulting in Rn 222 -enriched groundwater (Loomis, 1987;Hoehn and Von Gunten, 1989;Cecil and Green, 2000;Wu et al., 2004). Consequently, Rn 222 concentrations are significantly higher in groundwater than in surface water bodies, with rainfall containing no Rn 222 . High Rn 222 concentrations measured in surface water may therefore be indicative of groundwater influx into the surface water source.
The application of radioactive isotopes, such as Rn 222 , in hydrogeological studies has been mostly limited to studies in Asia, Europe, North and South America (e.g. Loomis, 1987;Hoehn and Von Gunten, 1989;Almeida et al., 2004;Moreno et al., 2014;Singh et al., 2018). Our study is one of the first in South Africa using Rn 222 as an environmental tracer in hydrogeological studies between groundwater and rivers. Other studies, such as Eilers et al. (2015), Masevhe et al. (2017) and Botha et al. (2019), quantified Rn 222 concentrations in either surface water or groundwater (not both) in different parts of South Africa. The objective of this study is to determine (i) Rn 222 concentrations within both surface water and groundwater, and (ii) the feasibility of using Rn 222 isotopes as a natural tracer in groundwater-surface water interaction studies in Table Mountain Group (TMG) aquifer systems.

Study site
This study was conducted in the Rawsonville area (33° 42' 55" S; 19° 14' 48" E) of the Western Cape, which is situated in the winter rainfall region of South Africa (Fig. 1). The area is characterized by a Mediterranean climate with cold, wet winters and warm, dry summers. The mean annual precipitation (MAP) for the area is approximately 800 mm, most of which occurs between the months of June and August (Conradie, 1995). The vegetation type may be described as mountainous fynbos (Mucina and Rutherford, 2006). The surrounding geology in this area comprises mainly of highly fractured TMG sandstone forming part of the Cape Supergroup, with scattered granitic outcrops (Thamm and Johnson, 2006).
The Gevonden and Molenaars rivers were studied to determine the interaction between groundwater and these surface water bodies using Rn 222 isotopes. The Gevonden River is a small, perennial headwater stream which flows along the Waterkloof Fault before its confluence with the Molenaars River. The Molenaars River is a major perennial river with its headwaters situated in the Klein Drakenstein Mountains. The Molenaars River eventually joins the Breede River just northeast of Rawsonville. During the dry summer months, the Gevonden River is characterized by a low stream stage but often floods its banks during the wet season, which is a common characteristic of many fynbos mountain rivers (Brown et al., 2004). A borehole (BH3) situated within 100 m of the Gevonden River was sampled to determine groundwater Rn 222 concentrations under both ambient (no pumping) and pumping conditions between both wet and dry seasons (Fig. 2).

METHODS
Surface water samples (n = 32) were collected from the Gevonden and Molenaars rivers during both dry and wet seasons. Sampling points were randomly selected but also confined by accessibility constraints. Water samples were collected using 250 mL sample bottles, which were sealed under water to prevent Rn 222 gases from escaping (Weaver et al., 2007). Samples were collected 15 cm below the water surface as to avoid the air-water interface where gaseous exchange would influence Rn 222 concentrations (Ellins et al., 1990).
Groundwater samples were collected to represent various depths down the borehole profile (average of 25 m intervals) under both ambient conditions (i.e., no pumping) and pumping conditions (at 2 L/s pumping rate), with the use of a depth-specific sampler. This device is connected to a power source, and when lowered to a specific depth in the borehole a small pump displaces the water between two non-return valves and the sample is secured when the pump is switched off. Water was then carefully transferred into the plastic sample bottles.
The Rn 222 concentrations were then analysed in a controlled laboratory environment using a RAD-7 electronic radon detector and RAD-H2O accessory (Durridge Company, USA). This instrument is known to be very precise and capable of measuring concentrations as low as 1 mBq (Masevhe et al., 2017). Tests were conducted at normal room temperature with sufficient desiccant in the air sample path of the equipment (see Botha et al., 2019 for detailed protocol).
In order to correct for the radioactive decay between the time the sample was collected and the time at which Rn 222 concentration was measured, the results were corrected using a decay correction factor (DCF) which is a simple exponential function (Eq. 1) adapted from Durridge RADH20 Owner's Manual (2012). Initial Rn 222 concentrations at time of sampling can be calculated based on the following formula: (1) Where C Rn is the corrected Rn 222 concentration (Bq•L -1 ), C M is the Rn 222 concentration measured by the RAD-7 detector (Bq•m -3 ), exp refers to the exponential function and t is the time since the sample was collected (hours).

RESULTS AND DISCUSSION
The results from this study suggest that Rn 222 enriched groundwater discharges into both the Gevonden and Molenaars rivers. Less than 15% of the surface water sampled from the Gevonden and Molenaars rivers had Rn 222 ranging between 1 and 20 Bq•L -1 (Fig. 3). Studies conducted in other rivers in various geologies around the world have found relatively low Rn 222 concentrations typically ranging between 0.3 and 20 Bq•L -1 (Hall et al., 1985;Ellins et al., 1990;Wu et al., 2004;Schubert et al., 2008). During both wet and dry seasons, Rn 222 ranged between 1 and 118.8 Bq•L -1 ( When the borehole was pumped and purged, the water entering the pump from the surrounding aquifer was sampled, and even higher Rn 222 concentrations (391-593 Bq•L -1 ) were measured (Fig. 4). Generally, Rn 222 concentrations in the pumped groundwater further increased during the wet season, probably due to active movement of water within the aquifer after being recharged and thus transporting Rn 222 gases through the aquifer. It is likely that the Waterkloof Fault, on which the Gevonden and Molenaars rivers intersect, is the primary transfer mechanism for Rn 222 gases to be mobilized by groundwater moving between U 238 and Ra 226 deposits within the TMG and released into the rivers via groundwater    2019) found that 78% of their groundwater samples had Rn 222 concentrations below 60 Bq•L -1 . Both these studies found considerably lower Rn 222 concentrations than we had measured in our study in the TMG.
Since the majority of the river samples had high Rn 222 concentrations, with roughly 12% of samples exceeding 100 Bq•L -1 , it is suggested that Rn 222 -enriched groundwater discharged into the Gevonden and Molenaars rivers, leading to an increase in Rn 222 detected. Ellins et al. (1990) and Cook et al. (2003) found similar results which indicated an instant increase in Rn 222 concentrations in the immediate areas surrounding groundwater discharge points into rivers. Therefore, this study suggests that Rn 222 may be applied as a useful, natural radioactive isotope to determine groundwater discharge into surface water bodies in highly fractured aquifer systems such as the TMG. Furthermore, there is potential to use the Rn 222 concentration measurements to estimate groundwater flow volumes, both within the borehole as well as rate of groundwater discharge into surface water, improving our understanding of groundwater-surface water interaction in complex systems such as the TMG (Schubert et al., 2011;Kafri, 2001).

CONCLUSIONS
Very high Rn 222 concentrations were measured in the Gevonden and Molenaars rivers due to Rn 222 -enriched groundwater discharging into the rivers. Groundwater in this region has extremely high Rn 222 concentrations (well above the global average) due to the underlying geology and proximity of nearby faults. This case study illustrates and verifies the application of Rn 222 isotopes as an environmental tracer in the assessment of groundwater-surface water interaction. It is particularly useful to apply in similar highly fractured aquifer systems such as the TMG. These radioactive isotopes have many of the required characteristics of an ideal environmental tracer, i.e., easily detectable in trace concentrations, chemically stable or inert for the required period of time, not typically found in large concentrations in surface water, present in most geological substrates and more importantly not filtered, absorbed nor adsorbed by the medium through which the water travels ( Future studies could use Rn 222 isotopes in order to quantify groundwater influx into surface water, or vice versa, in order to promote the sustainable utilization of these water resources within the TMG region, which has gained particular focus in recent years due to the 2015-2017 drought in the Western Cape. However, we suggest that future studies invest in a high-intensity sampling campaign with increased sampling, both in groundwater and the river, in order to improve estimates and analytical rigour as well as provide greater detail on spatial variation of Rn 222 concentrations along specific river reaches.