A pioneering rapid and direct measurement of dissolved 222Rn in the field has been used here to explore interaction between surface and groundwater in the source area of the Yellow River (SAYR). The results indicate average 222Rn activity of 2,371 Bq/m3 in surface water and 27,835 Bq/m3 in groundwater. The high 222Rn activity (up to 9,133 Bq/m3) found in the southeast part of the SAYR suggests possible influence of permafrost on the exchange between surface water and groundwater. The remarkable contrast among the different samples of a stream in the Shuangchagou basin, a typical basin in the SAYR, clearly indicates groundwater infiltration along the north tributary and occurrence of groundwater end-member in the south tributary. Considering no 222Rn decay and atmospheric evasion, the daily average fraction of groundwater input to the surface water through the end-member in a location (S1) is estimated at 19%. Despite the up to 40% uncertainty, this is the first estimate of a reference value for groundwater input in this basin and which can be improved in the future with more samples and measurements. 222Rn can be a rapid and easily measured tracer of surface water–groundwater interaction for future investigation in the Qinghai-Tibet Plateau.
INTRODUCTION
It is essential to understand the exchange of surface water and groundwater to ensure sustainable water resource utilization and protection, in terms of water quality and quantity. Under a warming climate and degrading permafrost condition, surface water and groundwater transformation may have changed greatly on the Qinghai-Tibet Plateau, including the source area of the Yellow River (SAYR). The SAYR has received increasing attention in the last decade due to regional socioeconomic development in the region (Jin et al. 2009; Luo et al. 2012). Several methods have been widely used in assessing surface–groundwater interaction including analytical and field measurements (Krause et al. 2007; Anibas et al. 2011; O'Connor & Moffett 2015). However, due to the harsh natural conditions in the SAYR, field work faces many difficulties. For example, numerical methods require significant amounts of detailed hydrological information and long time-series monitoring data of groundwater to identified unique interaction fingerprints.
Isotope techniques have been used widely to investigate the exchange between surface water and groundwater in terms of amounts and directions. Isotopes, such as 18O, 2H (Kumar et al. 2008; Herczeg & Leaney 2010; Petitta et al. 2010), and also dissolved noble gases and anthropogenic compounds, such as SF6 or chlorofluorocarbons (CFCs), have been commonly used (Cook et al. 2003, 2006). In addition to these tracers, the naturally occurring radon-222 (222Rn) constitutes a vital tool for quantifying groundwater outflows to surface water because it is greatly enriched in groundwater compared to surface water (by 2–4 orders of magnitude), relatively easy to measure, and chemically conservative (Cook et al. 2008; Chanyotha et al. 2014). Since 222Rn has a short half-life (3.82 d), it has great potential in the study of rapid mixing processes that occur on time scales of hours to several days. The obvious differences of 222Rn activity can be found in surface water if groundwater discharge exists (Dimova et al. 2013). In addition, the developed instrumentation (RAD H2O, Durridge Inc.) allows for easy and rapid direct measurement of 222Rn in water.
222Rn has been widely used to evaluate the exchange between groundwater and surface water (Wu et al. 2004; Cook et al. 2006; Oyarzún et al. 2014). The 222Rn method also has potential in harsh environments, such as the Qinghai-Tibet Plateau, in order to assess hydrological interactions. For example, Su et al. (2014) present a new mass balance method with the tracer 222Rn estimating gross surface water and groundwater exchange in Nalenggele River, in the southwestern part of the Qaidam basin, where conventional hydrologic data are sparse. The reliability and accuracy of the method have also caused some concerns during the last few decades due to uncertainties regarding variation of 222Rn activity in groundwater, heterogeneous distribution of 222Rn parent nuclide 226Ra and hyporheic exchange (Cook et al. 2006; Stellato et al. 2008).
Presently, obtaining hydrological data in general in the Qinghai-Tibet Plateau are hampered by the harsh environment and difficulties in performing long-lasting field work. Technological advances in the measurement of dissolved 222Rn have opened up the possibility for relatively rapid and direct assessment in the field, thereby making field campaigns profitable in terms of time and effort. Based on this, we have initiated a pioneering 222Rn investigation in water systems of the SAYR in the hope of making the method a potential spatial and temporal monitoring program. The investigation's main objective is to perform an assessment of the suitability of 222Rn as a hydrological tracer in plateau areas, such as the SAYR, exploring details of the exchange between surface water and groundwater for a larger scale investigation in the future.
MATERIALS AND METHODS
Study area
Schematic diagram of permafrost and groundwater situation in the Shuangchagou basin.
Schematic diagram of permafrost and groundwater situation in the Shuangchagou basin.
Sampling
Location of samples and the range of 222Rn activity in the SAYR (a) and Shuangchagou stream (b). N, S, and D represent the north tributary, south tributary, and the main stream in the Shuangchagou basin, respectively.
Location of samples and the range of 222Rn activity in the SAYR (a) and Shuangchagou stream (b). N, S, and D represent the north tributary, south tributary, and the main stream in the Shuangchagou basin, respectively.
Illustration of RAD H2O configuration (after WAT-250 system; Durridge Company, USA).
Illustration of RAD H2O configuration (after WAT-250 system; Durridge Company, USA).
is the concentration of 222Rn in the stream; is the concentration of 222Rn in the groundwater; is the concentration of 222Rn from the upstream, based on estimated loss of 222Rn resulting from the gas exchange and radioactive decay; is the stream discharge and is the groundwater discharge.
RESULTS
The result of dissolved 222Rn during the pilot sampling indicates a notable contrast of activity levels (at least by one order of magnitude) between groundwater and surface water in the SAYR (Figure 3(a)). The average 222Rn activity in surface water is 2,371 Bq/m3, with the minimum and maximum of 103 Bq/m3 and 9,133 Bq/m3, respectively. The groundwater contains much higher 222Rn activity than the surface water regarding a range from 10,398 to 41,583 Bq/m3 with a mean value of 27,835 Bq/m3.
222Rn activity measured during 26th and 27th September at an interval of 2 hours, in location S1 and D2.
222Rn activity measured during 26th and 27th September at an interval of 2 hours, in location S1 and D2.
DISCUSSION
222Rn distribution in SAYR
A comparison of the 222Rn activity in the SAYR with some worldwide published data is shown in Table 1. Cartwright et al. (2014) used 222Rn as a tracer to quantify groundwater inflow to the King River, South Australia and found up to 10 m3/m/day. In the arid Limari basin, North Chile, a low 222Rn level (less than 1,000 Bq/m3) was found in the reservoir waters compared to groundwater (around 20,000 Bq/m3), indicating that 222Rn is easily lost in surface water (Oyarzún et al. 2014). Similarly, a large difference of activity of 222Rn between surface water and groundwater was also observed in China (Wang 2002). Most 222Rn activity measured in surface water of the English Lake District was found to be low, with a few exceptions where samples taken from a limestone area or faulted area indicated direct influence of bedrock and groundwater discharge (Al-Masri & Blackburn 1999). A case study in central Italy, presenting different 222Rn activity in wells, indicates infiltration from the river to the aquifer (Stellato et al. 2013). These examples suggest that the difference in activity of 222Rn between surface water and groundwater makes 222Rn a potential tracer of interaction between these systems.
222Rn activity (Bq/m3) in the groundwater and surface water studied here compared with some selected data from the published literature
| Surface water | Groundwater | ||||
|---|---|---|---|---|---|
| Study | Range | Average | Range | Average | Reference |
| South Australia | 122–2,085 | 735 | 17,250–76,170 | 46,629 | Cartwright et al. (2014) |
| Central Italy | 100–1,300 | 400 | 7,000–19,000 | 14,700 | Stellato et al. (2013) |
| North Chile | 100–2,500 | 908 | 800–21,500 | 8,494 | Oyarzún et al. (2014) |
| England | 54–1,288 | 378 | – | – | Al-Masri & Blackburn (1999) |
| China | 10–8,940 | 1,260 | 200–3,274,000 | 81,096 | Wang (2002) |
| SAYR, China | 102–9,132 | 2,562 | 10,398–41,583 | 27,809 | This case |
| Surface water | Groundwater | ||||
|---|---|---|---|---|---|
| Study | Range | Average | Range | Average | Reference |
| South Australia | 122–2,085 | 735 | 17,250–76,170 | 46,629 | Cartwright et al. (2014) |
| Central Italy | 100–1,300 | 400 | 7,000–19,000 | 14,700 | Stellato et al. (2013) |
| North Chile | 100–2,500 | 908 | 800–21,500 | 8,494 | Oyarzún et al. (2014) |
| England | 54–1,288 | 378 | – | – | Al-Masri & Blackburn (1999) |
| China | 10–8,940 | 1,260 | 200–3,274,000 | 81,096 | Wang (2002) |
| SAYR, China | 102–9,132 | 2,562 | 10,398–41,583 | 27,809 | This case |
In the SAYR data, at least one order of magnitude difference of 222Rn activity is observed between surface water and groundwater (Figure 3(a)). Although there is no explicit range of typical 222Rn activity in surface water and groundwater, a maximum reference value of about 1,000 Bq/m3 can be considered for surface water (Bennett 1994; Wang 2002). Green & Stewart (2008) also indicated that 222Rn values greater than 1,000 Bq/m3 in surface water can be indicative of a recent addition (i.e., groundwater discharge to the surface flow). 222Rn activity in surface water of up to 9,133 Bq/m3 observed in the SAYR, clearly suggests the influence of groundwater. The SAYR is located in the mosaic transition zones of seasonally frozen ground, and discontinuous and continuous permafrost, which can play a critical role in water exchange process. Distribution and thermal regimes of permafrost and seasonal freeze–thaw processes are closely related to groundwater dynamics. Most samples in the SAYR are in the continuous permafrost zone, except for samples P8 and P9 (Figure 1). The discontinuous permafrost zone with limited areal extents and thicknesses, is closely linked to groundwater effects from the nearby free of permafrost area. The conditions for groundwater recharge, flow, and discharge are generally good which promote frequent exchanges with the surface water, and the consequent high 222Rn activity in P8 and P9 (Figure 3(a)).
The 222Rn activity in the northwest of SAYR shows values of less than 1,000 Bq/m3, and thus is consistent with the suggested reference value for surface water. However, 222Rn activity in the southeast of the SAYR is found to be up to around 4,068 Bq/m3 on average, indicating possible groundwater influence in these places. The continuous permafrost is generally thick and serves as a relatively stable regional aquitard with poor vertical water percolation. As a result, exchange with deep groundwater (e.g., sub-permafrost water) in this zone is meager due to limitations exerted by the permafrost layer. Instead, the shallow groundwater (e.g., supra-permafrost water) is dynamically exchanged with the surface water (Cheng & Jin 2012). The hydrogeology in the continuous permafrost zone, therefore, is characterized by extensive occurrence of aquifer and linear (point or strip) enrichment (accumulation) of groundwater. Another possible explanation of high 222Rn activity in the southeast of the SAYR is the possible degradation of the permafrost. Owing to the warming climate and increasing human activities, the permafrost degradation has been more striking around the Sisters Lakes and in the east of the SAYR, causing the reduction in areal extent from continuous and discontinuous to sporadic and patchy permafrost, thinning of permafrost, and expansion of taliks (Jin et al. 2009). The conditions of permafrost may change from aquitard to an aquifer upon thawing or warming to the melting point of water, and talik channels can be formed or enlarged. Accordingly, the high 222Rn activity in the southeast of the SAYR can also be detected as a result of the degradation of permafrost which can also enhance the frequent exchanges of groundwater and surface water.
222Rn in Shuangchagou basin
Contrast in the 222Rn activity is found between the two stream tributaries (Figure 3(b)) in the Shuangchagou basin. Generally, the direct contact of the water with the atmosphere and turbulence effects allow for escape of 222Rn, if present. This situation produces low 222Rn activity in the surface water unless there is a 222Rn source from the sediment decay or groundwater discharge along the stream. However, high 222Rn occurs in S5 (5,880 Bq/m3) and S1 (4,580 Bq/m3), while 222Rn activity consistent with the reference value in surface water (less than 1,000 Bq/m3) is observed in the south tributary (S2, S3, and S4). The sampling area is around 5 km2, and sample locations are close to each other sharing the same Quaternary clastic aquifer conditions. Possible groundwater end-member (discharging water) would be expected near S1 and S5 in the south tributary. All the samples in the north tributary stream show high 222Rn activity (on average 8,705 Bq/m3), thus suggesting active groundwater infiltration along the north tributary. From the geomorphological point of view, there may exist a flow path or talik channel that may enhance groundwater flow along the north tributary pathway. The high 222Rn activity in the north tributary is rather in agreement with groundwater discharge along the tributary rather than discharge through the end-member. The highest 222Rn (up to 19,740 Bq/m3) is observed in D2 which was sampled in the trunk stream near the confluence of two tributaries. 222Rn activity in D2 is much higher than the total contribution from N1 and S1, indicating also possible groundwater discharge near D2.
The variation of 222Rn activity in S1 and D2, observed in Figure 5, may be explained by the variation of daily temperature and the presence of permafrost. The study period (the end of September) is at the transition when ground thawing approaches the end and the freezing period starts. The variation of ground temperature may influence the distribution of the permafrost, which in turn affects porosity and permeability of the aquifer (Luo et al. 2014b). The increasing temperature at noon will also cause more ground ice to melt and recharge to the stream while the low night–morning temperature will reduce the recharge. The increasing trend of 222Rn in S1 in the late afternoon suggests incorporation of permafrost partial melting in the groundwater discharge which is most likely due to diurnal higher temperature in the morning and afternoon. Compared to location S1, the larger 222Rn activity variation in D2 is likely due to influences of both the groundwater discharge and input of the two tributaries.
Here we ignore the gas exchange with the atmosphere and decay loss as the low activity in the upstream and at the sampling locations is comparable. Also, 222Rn activity in the groundwater can barely be monitored as no springs or wells exist nearby. Therefore, assumption of the average 222Rn input from the upstream of 297 Bq/m3 (the measured value in S2), 222Rn activity in groundwater of 27,835 Bq/m3 (the average 222Rn activity of groundwater measured in the SAYR), and the mean daily 222Rn activity monitored during the 2 days of sampling were considered. The faction of groundwater discharged in the possible groundwater end-member (location S1) can be calculated by using Equation (2). This calculation provides a minimum estimate assuming that 222Rn is not subjected to any atmospheric evasion or decay losses through downstream transit. The average contribution of groundwater to the surface water is 19% on the first day, while a similar value (18.9%) is also estimated for the second day. This is the minimum groundwater input value, as larger inputs would be expected if correction for atmospheric evasion and 222Rn decay is included. The result is also consistent with the data in other places of the Qinghai-Tibetan Plateau (20–48% groundwater input; Yao & Yao 2010) and in other permafrost areas of China (12–15%; Liao et al. 2008). Although we could not estimate the groundwater contribution to the whole water budget in Shuangchagou basin because of limited samples, we emphasize that the estimation given here could act as a reference value for groundwater input to surface water.
The 222Rn activity in groundwater end-member is crucial for the application and may be a source of uncertainty (Burnett et al. 2007). For example, Santos & Eyre (2011) estimated an uncertainty of 28% in the final discharge rate for the variability in groundwater end-member. The groundwater 222Rn activity calculated here is estimated by the mean value of 27,835 ± 10,869 Bq/m3 (n = 7) in the SAYR. This value when combined with temporal variability, may result in 40% uncertainty of 222Rn activity in the estimates of groundwater discharge. A larger number of groundwater samples both in the Shuangchagou basin and the SAYR should be collected in future work in order to reduce the uncertainties.
The 222Rn activity approach presented here has proved to be an efficient and inexpensive method to assess groundwater input even when detailed hydrogeological information is not available. Especially for the harsh environment of the Qinghai-Tibet Plateau, where hydrological experiments are difficult to perform, the method becomes even more valuable. However, further investigation including seasonal 222Rn measurements coupled with 226Ra are needed to comprehensively explore the interaction between groundwater and surface water in the Shuangchagou basin.
CONCLUSION
Measurement of dissolved 222Rn in the SAYR provided a simple and effective tool for assessing the surface water and groundwater connectivity in the Qinghai-Tibet Plateau area. Distinctive 222Rn activity values are observed in the surface water and groundwater indicating specific characterization of each water type. Applying a simple 222Rn mass balance model, the fraction of groundwater contributes around 19% to the surface water in location S1. Future continuous measurement and expansion of the base line data in the region will facilitate tracing of exchanges between surface and groundwater and thereby provide a monitoring system for changes in the water supply to the Yellow River. The data will also provide an interesting monitor of the permafrost conditions once detailed data of 222Rn activity distribution are acquired.
ACKNOWLEDGEMENTS
This work is supported by the Chinese Academy of Sciences Key Strategic Program ‘Hydrological impacts from a degrading permafrost in the SAYR’ (Grant No. KZZD-EW-13) and NSF China Program ‘Study on changes in hydraulic connections in the SAYR using isotope tracing techniques’ (Grant No. 41472229), and partially supported by Open Foundation of State Key Laboratory of frozen soil engineering (SKLFSE201301) and National Natural Science Foundation of China (Grant No. 51539003).
