3.3.1 Sampling methods for hot-spring fluids
Overview:Sample collection is probably the most important step in fluid analysis (Hamilton, 1976).
It is therefore important to minimize all possible source of contamination during sampling to achieve precise and accurate analysis. In the field, complicated treatments for sampling apparatuses (filter, bottles, and devices) are difficult and requirement of special equipments, which are also not available.
Thus, sample preservation and storage must be took into account as in situ analyses of hot-spring fluid is impossible. To solve these issues, I have been developed sample strategies to collect hot-spring fluids in the field for determination of abundances of cations, anions, and gas.
Fluid collection for cation determinations: To collect hot-spring samples, a 125-ml and wide-mouth polyethylene bottle (Thermo Fisher Scientific Inc., USA) were used. The bottle was consec-utively rinsed twice with 6 mol L−1HCl, thrice with distilled water, twice with 0.5 mol L−1HNO3, thrice with distilled water, and finally with de-ionized H2O before using. In addition to minimize contamination from fluid-delivering pipe, fluid was only collected from the tap after pumping for five minutes. In a case of sampling at bath-tub where the hot-spring fluid migrates up from the base, fluid was directly scooped from bath-tubs.
To filtrate hot-spring fluid, nalgene reusable filter (Thermo Fisher Scientific Inc., USA) is used.
To separate a suspended particle, fluid was filtrated through 0.4µm-pore size and 47 mm diameter polycarbonate membrane filter (Toyo Roshi Kaisha, USA). The washing procedure of the filter was followed as that of bottle. In the field site, vacuum was applied to basal container for extraction wrinkling of fluid in the upper chamber. To maintain temperature and pH during filtering process, the hot-spring fluid was directly poured into upper chamber. This step is important step to minimize changing in element concentrations as a function of temperature and pH. After each ∼2 minutes filtering, about 100 mL of fluid was collected in the bottle, which is located inside vacuum gasket. A schematic illustration of fluid-filtering system is shown in Figure 3.2. To minimize losses of elements because of absorption and precipitation of elements on the wall of bottle, the fluid was immediately acidified to 0.05 mol L−1HNO3by addition of 16 mol L−1 HNO3 (Batley and Gardner, 1977). At each location, 6 filtered and 1 unfiltered aliquots were collected. Note that cold-spring fluid was collected the same procedure as the hot-spring fluid.
Fluid collection for anion determinations:To determine abundances of anions, 125ml-size and narrow-mouth polypropylene bottle (Thermo Scientific, USA) is used. To reduce contamination, the bottle is rinsed with distilled water and hot-spring fluid by 3 and 2 times before collection of fluid.
In the field site, about 100 mL of hot-spring fluid was directly collected in the bottle from pipe after pumping for five minutes. Notes that the hot-spring fluid is not acidified.
Fluid collection for Rn determination: To collect Rn in hot-spring fluids, a glass vial with foil-lined cap (20 ml, GV1, Meridian Biotechnologies Ltd., UK) was used. The glass vial with a bottle top dispenser was weighed before and after addition of 10 ml of water-immiscible scintillation cocktail (Ecoscinti O, National Diagnostics, Inc., USA). To prevent the oil-water emulsion, 0.1 g of 50
NaCl was added to the scintillation vial. Then the vial with water-immiscible scintillation cocktail and NaCl was weighed. The weights of the vial and water-immiscible scintillation cocktail and NaCl were determined. A schematic illustration of Rn collection from hot-spring fluid was shown in Figure 3.3.
At each hot-spring sites, hot-spring fluid was slowly accumulated in and overflow out a funnel with hose, which is connected with a tap. After temperature of hot-spring fluid was stabilized, 10 ml of hot-spring fluid was taken by a glass syringe from funnel. The syringe is inverted, gently pushed, and tapped with finger to remove to remove an excess volume and bubbles from sample. To prevent turbulence, the 10 ml of hot-spring fluid was then slowly injected below the scintillation cocktail through syringe’s needle. The vial was closed with the cap and sampling time was recorded. To dissolve Rn in the scintillation cocktail, the vial was vigorously shaken for 30 seconds. At each hot-spring site, three aliquots were collected with intervals within 3 minutes to account for the effect of a sampling condition. Sample weight was determined the sample during the sampling day. Blank was determined using 10 ml of water as the same the sample.
Measurements of physical- and chemical hot-spring properties: Measurements of tempera-ture, pH, electrical conductivity, and dissolved oxygen were in situ conducted at sampling sites by using a digital thermometer IT-2000, pH meter AS700, multi-measure dissolved oxygen and conduc-tivity AS810. The three devices are from AS ONE Corporation, Japan. The temperature and pH of total 57 hot-springs in Misasa area were determined. Among the total of 57 hot-springs, the electrical conductivity, and dissolved oxygen 10 selected hot-springs were determined.
Sampling locations:The latitudinal and longitudinal coordinates of sample site is determined by a global positioning system device (GPS, Garmin etrex 10J). The GPS is from Garmin Ltd., (Japan).
The locations of 57 of hot-springs and 2 cold springs in Misasa area were determined and shown in Table 3.1.
3.3.2 Determination of major- and trace-element abundances
Determination of abundances of major- and trace-elements: To determine abundances of trace elements including Li, Be, Rb, Sr, Y, Cd, In, Cs, Ba, REEs, Tl, Pb, Bi, Th, and U, 18 selected hot-spring fluids collected in Misasa are analyzed by the FDC-ID-IS technique. The hot-spring fluids were pre-concentrated by reducing the weight of the samples from∼125 g to 3.5 g, resulting into pre-concentration factor ofC∼35. Abundances of major elements (Na, Mg, K, and Ca) and transition elements (Sc, V, Mn, Fe, Co, Ni, Cu, Zn, and Ga), and Al and P are determined by the FDC-ID-IS technique after determination of abundances of trace elements. To avoid matrix-effected signal suppression, major- and transition-elements are determined by C<1 after re-dilution from C∼35. The analyses of the selected 18 hot-spring fluids and two cold-spring fluids were duplicated.
Determination of abundances of F, Cl, Br, N, and S:To determine abundances of F, Cl, Br, N, and S in hot-spring fluids, the sample were calibrated by 5 working standard solution including F−, Cl−, NO−2, Br−, NO−3, PO3−4 and SO2−4 (No.810H1527, Kanto chemical Co., INC, Japan), which were analyzed in the sequence of sample measurement. Abundances of F, Cl, Br, N, and S are determined in the form of abundances of F−, Cl−, Br−, NO−3, and SO2−4 by ion chromatography using a compact IC 761 (Metrohn Switzerland). The operation conditions of compact IC 761 was referenced to Wang et al. (2010).
Detection limit is estimated as threeσ of background signal obtained by compact IC 761 from purified water (n=4), typically expressed in unit of microsiemens per centimeter per second. Using element abundance and signal intensity of standard solution, the detection limit is converted into a sample detection limit, which is expressed in unit of milligram per liter. The detection limits are 0.04, 0.11, 0.24, 0.28, and 0.28 mg L−1for F−, Cl−, Br−, NO−3, and SO2−4 , respectively. Total blank procedure is less than 0.03, 0.08, 0.07, 0.21, 0.27 mg L−1, for F−, Cl−, Br−, NO−3, and SO2−4 , respectively. Reproducibility is defined as a standard deviation of repeated analyses (n=5) on standard solution. The reproducibilities are less than 5% for all 5 element studies. Accuracy is defined as deviation of determined values from those of reference values on the standard solution.
The accuracies are less than 1.2% for all 5 elements. By considering low blank, small detection limit, better reproducibility, and accuracy, the technique is applied to determination of abundances of F, Cl, Br, N, and S in hot-spring fluids. To determine abundances of anions with various, hot-spring fluids are determined with original solution and diluted one. Higher abundances such as Cl−and SO2−4 are determined with dilution of approximately 40 times, while lower abundances of NO−3, F−, and Br− are determined with original solution.
3.3.3 Determination of Radon abundance
To determine Rn abundance, the fluid sampled from a cold well in Kobe Pharmaceutical University was used as standard solution. The222Rn abundance (referred as Rn abundance in this study) of well fluid is 230 Bq L−1, which is determined by Ishikawa et al. (2004). Five aliquots of well fluid for this experiment were collected on February 27 in 2017, and delivered with a high-concentration standard to Misasa on February 28 in 2017. After experiment at Misasa, the remaining fluid was returned to Kobe Pharmaceutical University (March 1) for third measurement.
At Kobe Pharmaceutical University, the analysis was conducted by liquid scintillation counting using a PerkinElmer Tri-Carb 2300TR (Ishikawa et al., 2004). At Misasa, the analysis was conducted by liquid scintillation counting using a Hidex Triathler Type 425-034 according to the procedure described in Yasuoka et al. (2009). The Rn abundance (CRn) is given as radioactivity Bq kg−1, and calculated as
CRn=N/60
f V exp 0.693T 3.824
!
(3.1) whereNis a net count,T is an elapsed time after sampling (in days), f is a conversion factor from count to radioactivity, andV is a volume of fluid (in kg). The f of 4.5 is used for this analysis, according to Tanaka et al. (2013).
Reproducibility is defined as a standard deviation of mean (n=5). The reproducibility is<3%
for all 3 analyses at Kobe Pharmaceutical University, Misasa, and Kobe Pharmaceutical University.
Accuracy is defined as a deviation of determined value at Misasa and the second determined value at Kobe Pharmaceutical University from the fist determined value at Kobe Pharmaceutical University.
The accuracy is<1% for all analyses. The accurate and precise analysis of standard solution allows applying for determination of abundances of Rn in hot-spring fluids in Misasa.
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3.3.4 Determination of Sr isotopic composition
The Sr isotopic composition was determined using 0.1−0.8 mL of the remained aliquot after deter-mination of major- and trace-element abundances using the FDC-ID-IS. The Sr isotopic analyses were conducted by the Finnigan MAT262. The procedures of Sr isotopic analyses according to of Yoshikawa and Nakamura (1993). The measured ratios of standard materials during analysis were
87Sr/86Sr=0.71022±21 (2σ, n=4) for NIST987. The blank level is<31 pg. The Sr isotopic ratio is per-formed without correction on data because contribution of blank on the sample is negligible when the amount of Sr is>200 ng. All data are adjusted for instrumental discrimination to87Sr/86Sr=0.71024 for NIST987 (Makishima and Masuda, 1994).