Radon has been used to determine groundwater velocity and groundwater discharge into wetlands at the southern downstream boundary of the Crau aquifer, southeastern France. This aquifer constitutes an important high-quality freshwater resource exploited for agriculture, industry and human consumption. An increase in salinity occurs close to the sea, highlighting the need to investigate the water balance and groundwater behavior. Darcy velocity was estimated using radon activities in well waters according to the Hamada “single-well method” (involving comparison with radon in groundwater in the aquifer itself). Measurements done at three depths (7, 15 and 21 m) provided velocity ranging from a few mm/day to more than 20 cm/day, with highest velocities observed at the 15-m depth. Resulting hydraulic conductivities agree with the known geology. Waters showing high radon activity and high salinity were found near the presumed shoreline at 3,000 years BP, highlighting the presence of ancient saltwater. Radon activity has also been measured in canals, rivers and ponds, to trace groundwater discharges and evaluate water balance. A model of the radon spatial evolution explains the observed radon activities. Groundwater discharge to surface water is low in pond waters (4 % of total inputs) but significant in canals (55 l/m²/day).

The interactions between groundwater and surface water under a river island in South Korea were investigated. Sampling was focused on the western part of the island, where a groundwater heat pump system with groundwater monitoring wells has been installed. A riverside area was selected for the system because a high specific capacity was expected. However, surface-water intrusion remains a major concern, as the efficiency of the system can be impaired by groundwater temperature changes following surface-water infiltration. A combination of radon (²²²Rn) concentrations and microbial diversity was analyzed, along with hydraulic data. A process for estimating residence time based on ²²²Rn data and flow direction was developed to identify relationships with microbial diversity. The spatial distribution of ²²²Rn concentration was affected by fluctuations of river water levels caused by surrounding dam discharge and seasonal effects. Groundwater flowmeter data supported observations on these distributions. The estimated residence times indicated that river-water infiltration into the study site affected the groundwater flow direction. The microbial diversity, based on cluster analyses after pyrosequencing, showed significant variation with river flow direction and rainfall events, which was in accordance with the ²²²Rn tracer results. The combination of frequent ²²²Rn concentration measurements with microbial data allows better characterization of the dynamic interactions between groundwater and surface water.

Submarine groundwater discharge (SGD) is widely acknowledged as a key driver of environmental change in tropical island coral reefs. Previous work has addressed SGD and groundwater-reef interactions at isolated submarine springs; however, there are still many outstanding questions about the mechanisms and distribution of groundwater discharge to reefs. To understand how groundwater migrates to reefs, a series of offshore ²²²Rn (radon) and submarine electrical resistivity (ER) surveys were performed on the tropical volcanic island of Mo’orea, French Polynesia. These surveys suggest that fresher water underlies the fringing reef, apparently confined by a <1-m-thick low-permeability layer referred to as a reef flat plate. Reef flat plates have been documented elsewhere in tropical reefs as thin, laterally continuous limestone units that form through the super-saturation of calcium carbonate in the overlying marine waters. In other tropical reefs, the reef flat plate is underlain by a highly permeable karstic limestone formation, but the submarine reef geology on Mo’orea is still uncertain. Numerical modeling of two-dimensional reef transects and SGD quantifications, based on water budget and radon/salinity mass balance, support the confining nature of the reef flat plates and indicate important implications for SGD impacts to tropical reefs. Except where incised by streams or local springs, reef flat plates may route SGD to lagoons or to the reef crest 100s of meters offshore. Because groundwater can transport pollutants, nutrients, and low pH waters, the reef flat plate may play an important role in the spatial patterns of reef ecology and coastal acidification.

Submarine groundwater discharge (SGD) as a major source of alkalinity has rarely been studied in Jiaozhou Bay, China. The presented study used radon (²²²Rn) and radium isotopes to investigate SGD and its influence on alkalinity and nutrient inputs into the bay. Time-series observations of ²²²Rn were used to quantify groundwater dynamics over tidal time scales and the results showed that the SGD rates at point-scale were 0–67.2 (mean: 17.8) cm/day and 0–43.6 (mean: 12.3) cm/day in wet and dry seasons, respectively. Using radium mass balance models, the SGD in the whole bay was estimated to be (1.29–2.60) × 10⁷ m³/day in wet season and (5.81–6.83) × 10⁶ m³/day in dry season. Thus, both sets of results indicated higher SGD fluxes in wet season than in dry season. Such a seasonal variation pattern suggests a rapid response to local precipitation. The alkalinity fluxes associated with SGD were generally greater than those from the local rivers. Among the nutrient sources, SGD contributed about 63, 24 and 37% of total dissolved inorganic nitrogen, reactive phosphorus and silicate inputs, respectively. These results demonstrated that groundwater seepage is a major factor driving alkalinity and nutrients (especially dissolved inorganic nitrogen) into Jiaozhou Bay. SGD may have an important influence on the budgets of elements (C, N, P) and ecological environments in coastal waters.

Understanding the spatial distribution and variability of geochemical tracers is crucial for estimating groundwater influxes into a river and can contribute to better future water management strategies. Because of the much higher radon (²²²Rn) activities in groundwater compared to river water, ²²²Rn was used as the main tracer to estimate groundwater influxes to river discharge over a 323-km distance of the Big Sioux River, eastern South Dakota, USA; these influx estimates were compared to the estimates using Cl⁻ concentrations. In the reaches overall, groundwater influxes using the ²²²Rn activity approach ranged between 0.3 and 6.4 m³/m/day (mean 1.8 m³/m/day) and the cumulative groundwater influx estimated during the study period was 3,982–146,594 m³/day (mean 40,568 m³/day), accounting for 0.2–41.9% (mean 12.5%) of the total river flow rate. The mean groundwater influx derived using the ²²²Rn activity approach was lower than that calculated based on Cl⁻ concentration (35.6 m³/m/day) for most of the reaches. Based on the Cl⁻ approach, groundwater accounted for 37.3% of the total river flow rate. The difference between the method estimates may be associated with minimal differences between groundwater and river Cl⁻ concentrations. These assessments will provide a better understanding of estimates used for the allocation of water resources to sustain agricultural productivity in the basin. However, a more detailed sampling program is necessary for accurate influx estimation, and also to understand the influence of seasonal variation on groundwater influxes into the basin.

Poyang Lake is the largest freshwater lake in China and is well known for its ecological and economic importance. Understanding the contribution of groundwater to Poyang Lake is important for the lake’s protection and management. In this study, stable isotopes (δD and δ¹⁸O), ²²²Rn measurements, and corresponding models (²²²Rn and ¹⁸O mass balance models) were employed to evaluate the groundwater discharge and associated chemical inputs to Poyang Lake. The results showed that the distribution of δ¹⁸O in the lake water reflects the groundwater discharge into the lake. The groundwater discharge estimated using the ²²²Rn mass balance model was in reasonable agreement with the groundwater discharge derived from the ¹⁸O mass balance model. The ²²²Rn mass balance model showed that the groundwater discharge rate was 24.18 ± 6.85 mm/d with a groundwater discharge flux of (2.24 ± 0.63) × 10⁷ m³/d, which accounts for 6.52–11.14% of river-water input in the Poyang Lake area. The groundwater discharge flux estimated using the ¹⁸O mass balance model was 3.17 × 10⁷ m³/d, and the average groundwater discharge rate was 26.62 mm/d. The estimated groundwater discharge was used to estimate the associated chemical fluxes. It was found that groundwater-derived heavy metals such as iron and manganese are potential threats to the lake ecological system because of their large inputs from groundwater discharge.