Anthropogenic tritium as indicator for groundwater inflow into major rivers - Potentials and challenges of a tracer application.

The inflow of groundwater into a large river can contribute significantly to the total discharge of the river. Especially at low river water level, the inflowing groundwater can strongly influence the river water quality. Therefore, both the localisation and quantification of groundwater inflow into rivers is essential for their sustainable management. Corresponding field investigations usually rely on tracer applications based on stable water isotopes (δH/δO), naturally occurring radioisotopes (e.g. Rn) and standard water parameters (T, pH, EC). However, an extension of this tracer toolbox is desirable. In recent studies, the idea of using anthropogenic tritium that is released from a nuclear power plant (NPP) at an upstream location as an additional tracer has been pursued. The concept assumes that the inflow of (practically tritium-free) groundwater and tributary water dilutes the anthropogenic tritium inventory of the river and thus enables quantification of the total groundwater/tributary inflow ("tritium dilution concept"). Since the inflow of tributaries is usually easy to quantify, it is suggested that the inflow of groundwater can be derived from the tritium data. In the presented pilot study, we investigated the applicability of this "tritium dilution concept" and evaluated the general challenges associated to the uncertainty of input parameters. The study was executed exemplarily at an extended river section affected by the release of NPP process water (Vltava/Elbe River, Czech Republic). The evaluation of two 1-month tritium timeseries recorded at two monitoring/gauging stations 168 km apart allowed quantifying the total groundwater/tributary inflow into the river within this section based on the tritium data with a precision that was close to the gauged increase in river discharge (+72 % and +77 %, respectively). On the other hand, the groundwater inflow that was derived after subtracting the gauged tributary inflow from the quantified total water inflow was comparably low and therefore within the approaches' range of uncertainty. From this it can be concluded that the "tritium dilution concept" is applicable if the required input parameters, namely the natural tritium groundwater/tributary endmember and the tributary discharge, are known with sufficient precision. This outcome is promising for studies that focus on less extended river sections with input parameters that are easier to define.