Using 222Rn to quantify wetlands interflow volume and quality discharging to headwater streams

Abstract

Headwater streams are highly dependent on groundwater discharge to maintain low flows during dry periods and to dilute pollutants. Groundwater discharge to streams can have different flow paths, either from groundwater flowing directly to the river through the hyporheic zone or groundwater that emerges at the contact with a riparian wetland and flows mainly on the wetland surface. Differentiating these flows could be useful to assess the contribution of riparian wetlands in protecting stream water quality. The objective of this research was to expand the use of 222Rn as a groundwater tracer for small streams in headwater catchments to distinguish flows received directly from the aquifer and through riparian wetlands. 222Rn activities, phosphate (PO43−) and nitrate (NO3−) concentrations, along with stream flows were used in a mass balance model to establish the proportions of groundwater flow that discharge to a small stream located southwest of the Paris Basin (France). This watershed is typical of headwater catchments in this region because it receives a wastewater treatment plant (WWTP) effluent at its source and its banks are occupied by many small riparian wetlands. To obtain the best accuracy of groundwater flow assessment, the field work was done during low flow conditions, where the stream flow was only 0.079 m3/s at the outlet. The model gives a good estimation of each flow path with 83 % of the stream baseflow originating from riparian wetlands. The large contrast in 222Rn activity between groundwater inflow from the aquifer (mean of 21 200 Bq/m3) and interflows from wetlands (mean of 2310 Bq/m3) renders the mass balance model sensitive to the separation of these two types of groundwater flow paths. At the head of the stream, water is characterized by high concentrations of PO43− and NO3− due to the WWTP effluent into the stream (13 and 21 mg/L respectively). All groundwater flows are PO43− free and contribute to the improvement of stream water quality. The NO3− cycle is more difficult to constrain because of the spatial heterogeneity in groundwater concentrations. Nevertheless, the results of the modeling approach showed that the main part of the evolution of NO3− concentrations along the river can be explained by the dilution of stream flow with interflows. The method developed is considered sufficiently accurate to quantify groundwater inflows for different flow paths in headwater catchments and to estimate the impact of groundwater flow paths on stream water quality.