4.5.1 Tritium, Deuterium and Oxygen Isotopes
The 18O and 2H isotope content ratios of collected spring and groundwater samples were determined using Gas Source Isotope Ratio Mass Spectrometry (IRMS) with detection limits of 0.15 ‰ for 18O and 1.5 ‰ for 2H. Oxygen isotope ratios were measured by the Carbon Dioxide Equilibration Syringe method proposed by Matsui (1980), in which carbon dioxide is equilibrated with CO2 and then isolated from water vapour and other trace gases prior to injection into the mass spectrometer. Gehre et al. (1996) method was used to prepare samples for deuterium analysis in the mass spectrometer. This method involves reducing sample water into elemental hydrogen through use of a Heated Chromium Furnace packed with oxidizable material at 850°C. The resulted H was measured in Isotope Ratio Mass Spectrometer (IRMS) of delta S type (Fa. Finnigan MAT, USA). Data quality was checked through an internal laboratory calibration of standards (Coplen et al., 1991).
The 3H was analyzed using the electrolytic enrichment with a detection limit of 0.2 TU.
Distillation of samples under N2-atmosphaere, then NaO2 in electrolysis cells was added as batch process electrolytically decomposes (Taylor, 1982; Rozanski & Groening, 2004). PbCl2
was added on the produced NaOH. A distillation process was adopted until drying , water after addition of Ultima gold radiometric detection over 1000 minute in liquid scintillation spectrometer Quantulus 1220 Canberra luggage pool of broadcasting corporations 2770 TR/SL (EG&G Wallac, Finland). By this method, Tritium emits beta decay electrons, which excites the solvent. The solvent transfers its energy to the solute, which emits light photon pulses which are detected and counted.
4.5.2 Radium Isotopes (Rn222, Ra223, Ra224 and 226Ra) 4.5.2.1 DURRIDGE RAD 7 (Rn 222, Ra226)
Radon-222 gas was extracted and counted using RAD 7 device manufactured by (Durridge Co. Inc) (figure 4.4). The sample was purged by air pumped in by the RAD 7 internal pump.
The out flowing gas was dried with a drier before entering the RAD 7 device in order to keep the humidity into the device always less than 10 %, which is required to obtain an optimum Rn-222 concentration result, then the radon start for counting.
The starting counting time was written down for each sample. This time was used latter for the decay-corrected back to the time of sampling. Each sample was purging for about 40 minutes to account all the radon gas in the samples.
Then the reading of Rn-222, obtained from the Device and the volume of water in the radon bubbler were written down. The Rn-222 concentration values were decay corrected back to the time of sampling in order to assess the in situ radon concentrations. They were calculated for all the samples by using the following formula:
F= 1/exp (-2.097 *10-6 * Δt).
Where F is the radon decay corrected-back factor, Δt is the difference of time between the analysis time and sampling time (in second).
The factor of volume was calculated for each sample by using the formula:
VF = 900/ VI.
Where VF is the volume correction factor, VI (in ml) is the initial volume of the sample in the gas bubbler (between 30 and 35 ml), and 900 ml is the constant number storage in the RAD7 device.
The actual radon concentration for each sample was calculated by using the following equation:
Rn 222 = (Rn (RAD7)* F * VF)/1000.
Where Rn (RAD7) is the radon reading got direct from RAD7 device.
The 222Rn concentrations were measured and calculated for each sample taken using the same procedures and equations that mentioned above.
a) b)
Figure 4.4: DURRIDGE RAD 7 device a) Photo of the RAD7 device during Rn 222 measurements. b) Schematic technical circuit for Rn 222 for water samples measurements.
For Radium (226Ra) sampling and analysis, polyethylene containers of 25 L were used.
Radium was extracted by passing the water sample through a 5 cm diameter, 30 cm long exchange column packed with 10 grams of MnO2-coated acrylic fiber (Moore and Reid, 1973) at a flow rate of 5-10 ml/minute for radium pre-concentration as described in figure 4.3. The MnO2 fibers after the radium pre-concentration transferred to glass tube and barged for 2-3 minutes using He gas, then these tubes were incubated for 30 days for the equilibration of radium daughters. The recovery of Radium-226 activity (dpm/L) on the MnO2 fiber was determined by the radon emanation technique analyzed on a Durridge RAD7 electronic radon monitor (figure 4.5); activities were decay-corrected to the time of collection.
a) b)
Figure 4.5: DURRIDGE RAD 7 device a) Photo of the RAD7 device during Ra-226 measurements. b) Schematic technical circuit for Ra-226 for water samples measurements.
4.5.2.2 Delayed Coincidence Counter (Ra223, Ra224)
The same sampling and filtration procedures were applied for Ra-223 and Ra-224 collection as described in figure 4.3. The measurement procedure is based on the observation that radon ejected from the MnO2 fiber (Rama et. al., 1987). The partially dried MnO2 fibers were analyzed for total dissolved 223Ra and 224Ra activity using a delayed coincidence counter system (Moore and Arnold, 1996) with RaDeCC version 1.15 software (Scientific Computer Instruments). The average detector efficiencies (average of 2 to 4 detectors used for each isotope), for 223Ra and 224Ra were 0.4 and 0.5, respectively, with a typical precision of ±2–5%
for each individual detector. Each sample was counted from three to six times with an average statistical counting error of ±16% for 223Ra and ±5% for 224Ra. All the measurement procedures for analyzing 223Ra and 224Ra using the Delayed Coincidence Counter were passed on the methodologies adapted from Moore and Arnold (1996), while the calculations and corrections between the initial and final counts were based on Giffin calculations and studies (Giffin et al., 1963).