Article pubs.acs.org/est
Novel Instruments for in Situ Continuous Rn-222 Measurement in Groundwater and the Application to River Bank Infiltration B. S. Gilfedder,*,†,‡ H. Hofmann,†,‡ and I. Cartwright†,‡ †
School of Geosciences, Building 28, Room 124, Monash University, Clayton, Victoria, Australia 3800 National Centre for Groundwater Research and Training, GPO Box 2100, Flinders University, Adelaide SA 5001, Australia
‡
S Supporting Information *
ABSTRACT: There is little known about the short-term dynamics of groundwater−surface water exchange in losing rivers. This is partly due to the paucity of chemical techniques that can autonomously collect high-frequency data in groundwater bores. Here we present two new instruments for continuous in situ 222Rn measurement in bores for quantifying the surface water infiltration rate into an underlying or adjacent aquifer. These instruments are based on 222Rn diffusion through silicone tube membranes, either wrapped around a pole (MonoRad) or strung between two hollow end pieces (OctoRad). They are combined with novel, robust, low-cost Geiger counter 222Rn detectors which are ideal for long-term autonomous measurement. The down-hole instruments have a quantitative response time of about a day during low flow, but this decreases to 7 m. The other major benefit of the OctoRad is that there is little back pressure due to the eight separate flow paths and the springs, which hold the silicone open. Accurel tubing, which is a hydrophobic membrane containing micropores through which gases can diffuse, can also be easily strung between the two ends of the OctoRad. Due to its brittle nature it is not possible to curl the Accurel around the pole of the MonoRad without kinking. Detectors. Both the MonoRad and OctoRad are connected in closed circuit with a detector which measures 222Rn activities in air. We have used two types of detectors. The more sensitive of the two is the RAD7 radon monitor from Durridge Inc. that counts decay events of 222Rn daughter products 218Po and 214Po using a planar silicon−germanium solid state detector. This is a particularly sensitive instrument as nearly all noise is eliminated by sophisticated alpha spectrometry circuits. For the highest temporal resolution only 218Po is counted, with secular equilibrium between 222Rn and 218Po taking about 15 min (∼5 half-lives). We have also employed for the first time a Geiger counter + logger setup from AWARE Electronics as a low-cost, lowenergy and small-footprint instrument for detecting 222Rn activities in groundwater. The RM-80 Geiger counter has a Ø 44.4 mm pancake type Geiger−Müller tube that is sensitive to alpha, beta, and γ radiation from all isotopes in the 222Rn decay chain between 222Rn and 210Pb. In practice, it will be most sensitive to the high-energy emitters such as 222Rn, 218Po, 214Po, and 214Bi. 210Pb will contribute insignificantly to the counts as it has a half-life of 26 years and is a low-energy emitter. The detector is powered by an AWARE LCD-90 microcontroller, which also counts and logs the data (in counts per minute) to nonvolatile flash memory (max 110 000 measurements). The detector−logger setup can run for over a year on three AA batteries, making it ideal for long-term field deployment. Another major advantage is that multiple instruments can be
■
MATERIALS AND METHODS Sampling Instruments. The method for continuous 222Rn measurement presented here comprises two parts: a 222Rn extraction device that is suspended in the screened section of a bore, and a 222Rn detector which is located at the top of the bore. We have built two different extraction devices, although both rely on 222Rn diffusion through a silicone or Accurel membrane tube. The characteristics of the silicone membranes have been described in Hofmann et al.35 The first, and most simple of the two instruments, is simply a 10 m silicone tube (7 mm diameter, 1 mm wall thickness; Roth GmbH) wrapped around a 1.3 m pole. This instrument is dubbed MonoRad. At the bottom and top ends of the pole the membrane is connected to PVC tubing which runs to the top of the bore and is connected in closed circuit with a 222Rn detector (Figure 1A). The MonoRad is suspended in the screened section of the bore. Radon diffuses from the groundwater into the air loop which is circulated through a detector at the top of the bore. The second design is slightly more complex. It is comprised of eight 1.3 m silicone tubes strung between two hollow brass end pieces, with a hollow steel tube down the middle holding the two ends together and allowing air flow (Figure 1B, Supporting Information Figure 1). Air is pumped down the steel tube and back up through the silicone membranes, while 222Rn diffuses through the silicone membranes into the air loop. The top brass piece is connected in closed circuit to a detector at the top of the bore. This instrument is called the OctoRad. 1.3 m long springs were inserted inside the silicone tubing to stop 994
dx.doi.org/10.1021/es3034928 | Environ. Sci. Technol. 2013, 47, 993−1000
Environmental Science & Technology
Article
deployed at a site for the same setup cost as a single RAD7. The limitation with the Geiger counter is that it is sensitive to γ radiation from terrestrial and cosmic sources, and thus background counts (∼30 cpm) are considerably higher than with the RAD7 (
