Using Radon-in-Water Concentrations as Indicator for
Groundwater Discharge into Surface Water Bodies – Enhanced Data Processing
Eric Petermann & Michael Schubert
Helmholtz Centre for Environmental Research, Department Groundwater Remediation
Radon - a Tracer for Groundwater Discharge
The radionuclide radon (222Rn) serves as an excellent tracer for groundwater discharge into surface water bodies since radon is
highly enriched (three to four magnitudes) in groundwater relative to surface waters. Hence, positive anomalies in the spatial
distribution of radon in surface water are an indicator for groundwater discharge.
However, the detected radon signal is influenced by two processes that can lead to serious misinterpretation if they are not
considered. First, radon-in-water concentrations are usually derived from a radon-in-air detector (see fig. 1) and show hence a
distinct response delay between radon-in-water concentration and related radon-in-air records. Second, groundwater
discharge and consequently radon concentration in the surface water is not steady state, but rather influenced by a variety of
processes (e.g. Santos et al 2012). Measurements that are conducted in a coastal marine environment over a period of a few
hours are for instance highly influenced by tidally induced water level fluctuations.
Here, we present methods for quantifying both (1) the detectors’ response delay as well as (2) the tidal influence on radon data.
Applied Detection Set-up
Figure 1: Sketch of the applied detection set-up
Radon detector
RAD7 (Durridge inc., USA), passivated
implanted planar silicon (PIPS) alpha detector
Type of extraction
Radon counting cycle
Water flow rate
Teflon hollow fibre degassing cartridge
(MiniModule®,Membrana GmbH, Germany)
5 min
~ 1 l/min
Air flow rate
Air filled Volume
Water/air interface area
~ 1 l/min
1.6 l
0.92 m²
Table 1: Technical specifications of the applied detection set-up.
Response Delay of mobile Radon Detectors
The response delay is caused by the combined effect of two
simultaneously occurring processes (fig. 2):
(1) Kinetic delay: radon degassing from the water into the
closed air loop is delayed due to the kinetics of the
water/air phase transition, and
(2) Decay delay: most mobile radon monitors rely on
detection of the decay of the short-lived alpha-decaying
radon progeny polonium (218Po), thus entailing a further
response delay due to the delayed decay equilibration of
the two radionuclides.
The magnitude of the kinetic delay depends on the design of
the selected detection setup parameters (e.g. type of
extraction, water pump rate; table 1). The decay delay on
the other hand is solely defined by the 218Po decay constant.
Quantification of Response Delay
We designed a laboratory experiment with a defined
radon-in-water input function, recorded the radon-in-air
response signal and analysed the two time-series (fig. 3).
Radon-in-air peak concentrations showed a distinct delay of
10 min and a smoothed shape relative to the radon-inwater peak concentrations. However, for reconstructing the
original radon-in-water signal based on the detected radonin-air time-series we developed a physical model
considering the delay causing parameters. It was shown
that the model allows reconstruction of the input signal
Contact:
without
anyEric
timePetermann
delay and with correct concentrations for all
concentration
fluctuations +49
lasting
, [email protected],
341 longer
2351674than ~10 min.
(Petermann & Schubert 2015)
Tidal Effect on Radon Concentration
A change of the tidal state causes fluctuations of the water level
and the related hydraulic gradient between groundwater and
seawater. This affects the magnitude of groundwater discharge
in coastal waters. If radon concentration mapping is performed
over a period of several hours significant tidally induced water level
fluctuations occur. Consequently, radon data has to be normalised
to an average water level to achieve comparability.
Sensitivity Analysis
For testing the sensitivity of radon concentration in relation to water
level fluctuations for a specific study site, a time-series
measurement of radon at a fixed location (fig. 5) over a period of
at least one tidal cycle has to be conducted (fig. 4). During low tide
radon concentrations are considerably elevated compared to high
tide concentrations. A simple exponential regression model can
be used to make corrections for this effect.
Figure 4: Radon concentration depending on tidal fluctuations.
Rnnormalised= Rnobserved / e (-1.02 * tide)
ra
Effect of data processing
Fig. 5 presents a comparison of raw field data (left) and processed data (right) for a marine radon mapping survey. In the
processed data the response delay as well as the tidal influence is removed. The raw radon data shows a rather diffuse
anomaly as a consequence of the smoothed response of the radon detector (cf. fig. 2, fig. 3). After applying this correction,
the spatial radon distribution shows a distinct anomaly in the innermost part of the bay (“A”) and another less prominent
anomaly (“B”) further north. This indication is supported by the occurrence of fresh groundwater in shallow (< 2 m below
ground level) coastal sediments in the vicinity of the radon anomalies.
raw data
processed data
B
Groundwater
discharge
A
Figure 2: Kinetic and decay delay of mobile radon detectors cause an overall
response delay (water pump flow rate: 1 l/min)
Figure 5a and 5b: Raw (5a, left) and processed (5b, right) radon concentration of a marine radon mapping survey.
Conclusion
We present a quantitative approach for calculating radon-in-water concentrations from recorded radon-in-air records for a given
detection set-up (incl. a set water pump rate). Additionally, a simple sensitivity analysis was performed to remove the effect of
tide induced water level fluctuations. These two data processing steps yield a significant improvement concerning the
accuracy of radon mapping and, consequently of localisation of groundwater discharge.
References
Figure 3: Modeled radon-in-water concentrations (blue line) calculated from radon-in-air
records (red line). The black line shows the to radon-in-water input signal.
Contact: Eric Petermann
Helmholtz Centre for Environmental Research, Department Groundwater Remediation
Permoserstraße 15, 04318 Leipzig, Germany
[email protected], +49 341 2351674
Petermann, E. & M. Schubert (2015): Quantification of the Response Delay of Mobile Radon-in-Air Detectors Applied for Detecting Short-Term Fluctuations of Radon-in-Water
Concentrations. European Physical Journal – Special Topic. Accepted.
Santos, I., Eyre, B. & M. Huettel (2012): The driving forces of porewater and groundwater flow in permeable coastal sediments: A review. Estuarine, Coastal and Shelf Science 98,
1-15.