228 Ra / 226 Ra activity ratio in groundwater around Mount Fuji , Japan

We estimated the groundwater age from Ra/Ra ratios in young groundwater and relevant rocks in the volcanic area of the Kakitagawa River around Mount Fuji, Japan, and compared our results with those from H/He age determination. The groundwater residence time estimated from the Ra/Ra activity ratio in groundwater and relevant rocks agreed well with the H/He age, suggesting that the Ra/Ra ratio of groundwater can be used to estimate residence time of young groundwater in volcanic areas.


Introduction
Groundwater younger than 100 years can be dated from environmental radioactivity of isotopes with short half-lives ( 3 H/ 3 He [1, 2], 85 Kr [3,4]), or chemicals like CFC-12 [4].The 3 H/ 3 He dating method accurately estimates the residence time of shallow groundwater.One of the advantages of this method is small sample volume: only several tens of milliliters of groundwater.The 3 He in groundwater originates from 3 sources: tritiogenic He, mantle He, and radiogenic He produced in the rock.Especially as the contribution of mantle He is greater than those from radiogenic and tritiogenic sources, when groundwater is dated with 3 H/ 3 He in volcanic areas, we have to determine the ratio of the three sources of 3 He.In contrast, as 85 Kr originates only from the atmosphere, it is an excellent tracer for dating groundwater in volcanic areas.However, as the concentration of 85 Kr in groundwater is ultra low, for measurement, 85 Kr must be separated from about 10 4 l of ground water.
Groundwater dating by these methods has both advantages and disadvantages, but the disadvantages of the individual methods can be offset by using multiple tracers.Therefore, development of additional groundwater dating techniques is desired.
Ra-228 and Ra-226 are progenies of Th and U, respectively.The 228 Ra/ 226 Ra ratio in groundwater depends on the Th/U in the relevant rocks.As the 228 Ra and 226 Ra in shallow groundwater in volcanic areas originates only from rock, and collecting radium isotopes from groundwater is much easier than collecting 85 Kr, Ra may be a good tracer of groundwater age in volcanic areas.In this study, we evaluated the application of radium isotopes to groundwater dating in a volcanic area.As the 228 Ra half-life is short (5.8 y) and the 226 Ra half-life is long (1600 y), the 228 Ra/ 226 Ra ratio in natural waters is used to trace water movement [5][6][7] or estimate water origin [8].We estimated groundwater age from 228 Ra/ 226 Ra ratios in groundwater and relevant rocks in a volcanic area.We compared the 228 Ra/ 226 Ra groundwater ages with those from 3 H/ 3 He dating for the Kakitagawa River near Mount Fuji, Japan.The water of the Kakitagawa River originates from Mount Fuji groundwater.

Kakitagawa River observation area
The observation area is in the northeast of the Kanto Mountains.The Kakitagawa River is in the eastern district of Shizuoka Prefecture.Sampling points for 228 Ra/ 226 Ra ratio in the Kakitagawa River were located along a basaltic lava flow (Mishima lava flow) area of Mount Fuji (Figure 1).The geological detail for the Kakitagawa area is described by Tsuya [9].The basaltic rock samples for Kakitagawa were collected from the Mishima lava flow.

Preparation of Mn-impregnated acrylic fiber and groundwater sampling
Mn-impregnated acrylic fiber was prepared after the procedure of Moore [10]: 100 g of acrylic fiber was put into 1 l of 0.3 mol • l −1 KMnO 4 solution and kept for 12 h at 50 • C to form Mn-impregnated acrylic fiber; the product was then rinsed well with flowing deionized water and dried completely in an electric oven at 50 • C. A detailed description on the Mn-impregnated acrylic fiber is given by Ohta et al. [11].
226 Ra and 228 Ra were collected from groundwater flowing through this Mn-impregnated acrylic fiber.Groundwater samples for measuring radium isotopes were collected three times a year from the Kakitagawa River from 2005 to 2006.

Investigation of ( 228 Ra/ 226 Ra activity ratio) T =0
We measured the ratio ( 228 Ra/ 226 Ra) T =0 with the following methods: 1) Leaching test from rock samples,  Rock samples (granite) were sliced and placed in a vessel with stainless steel utensils.The vessel was filled with pure water that had been de-aerated with a vacuum pump.The samples were then stored with the water for about 1 month, after which the water was separated from rock and the radium isotopes were collected from the water onto 20 g of a cation exchange resin (H form).Ra isotopes in mountain water were collected from the Okutama area in the western part of Tokyo and from the island of Izu-Oshima, Japan.Mn-impregnated acrylic fiber was put at the bottom of the cliff after rain, and radium isotopes supplied from rock with a minimal residence time to water were collected on the fiber.

Measurement of 228 Ra and 226 Ra
The Mn-impregnated acrylic fiber with the collected Ra isotopes was dried and packed into an air-tight tin canister (76 mm φ × 24 mm H) for γ-ray spectrometry and stored for one month to allow 214 Pb and 228 Ac to reach radioactive equilibrium with 226 Ra and 228 Ra, respectively.Radioactivity of 214 Pb and 228 Ac was determined from the 351 keV and 911 keV γ-rays, respectively.Details of the methods for sample collection and determination of radioactivity are available elsewhere [12,13].and water exposed to the rock samples were 1.0 ± 0.1 (n = 2) and 1.8 ± 0.4 (n = 4), respectively.The average 228 Ra/ 226 Ra ratio of the water exposed to the rock was 1.8 times higher than that of the relevant rock.

Investigation of (
Table 2 shows the 228 Ra/ 226 Ra activity ratio of rock and mountain water from two sedimentary rock areas and a basaltic area.The average 228 Ra/ 226 Ra activity ratios in mountain water from two sedimentary rock areas of Okutama area were 2.5 and 2.8 (n = 2).The average activity ratio for the sedimentary rock from both of these areas was 1.5 ± 0.1 (n = 3).The ratio in water was 1.7 or 1.9 times those in the relevant sedimentary rock.In the basaltic area, Izu-Oshima, the 228 Ra/ 226 Ra activity ratio in the water and the rock were 1.2 and 0.5, respectively, and the 228 Ra/ 226 Ra ratio in the water was 2.4 ± 0.8 times that in the basaltic rock.As shown in Tables 1 and 2, the 228 Ra/ 226 Ra ratio of the water supplied from the relevant rocks was approximately double that of the rock samples.Thus, the ( 228 Ra/ 226 Ra) T =0 in groundwater may be nearly double that in the relevant rock.Furthermore, the 228 Ra/ 226 Ra ratios in water and those in local rocks are almost the same for basaltic, sedimentary rock, and granite areas.

Summary and conclusions
The 228 Ra/ 226 Ra ratios in groundwater and rock samples suggest that: (1) The average activity ratio of 228 Ra/ 226 Ra in water supplied from rock was about double that of the rock samples.Therefore, the ( 228 Ra/ 226 Ra) T =0 ratio when the radium isotopes were supplied into groundwater from the relevant rock might have been nearly the double of that of the relevant rock.
(2) The activity ratio in the groundwater of the shortest residence time of Kakitagawa was estimated from the relevant rocks (Mishima lava flow) to be 1.

Table 1 :
The activity ratio of the supplied radium from rock sample to water. 228Ra/ 226 Ra (rock) : activity ratio of 228 Ra/ 226 Ra in rock, 228 Ra/ 226 Ra (water) : activity ratio of 228 Ra/ 226 Ra in water supplied from rock sample, AR: activity ratio between water and rock of 228 Ra/ 226 Ra.Rock type 228 Ra/ 226 Ra (rock) 228 Ra/ 226 Ra (water) AR (water) / AR (rock)

228 Ra/ 226 Ra) T =0Table 1
lists the average 228 Ra/ 226 Ra ratio in the granite and the water supplied from the rock samples, and the 228 Ra/ 226 Ra activity ratio of water to rock.The average 228 Ra/ 226 Ra activity ratios in the granite rock sample

Table 3
shows the 228 Ra/ 226 Ra activity ratio in groundwater of the Kakitagawa River and Mishima lava flow.The 228 Ra/ 226 Ra activity ratio of 4 basaltic lava-flows in the Kakitagawa area ranged from 0.8 to 1.0 (average

Table 3 :
[2] activity ratio in groundwater.228Ra/ 226(rock)avg : average of 228 Ra/ 226 Ra in rock, 228 Ra/ 226 Ra (water) : range of 228 Ra/ 226 Ra in Kakitagawa River water, AR: activity ratio of 228 Ra/ 226 Ra, 228 Ra/ 226 Ra (T =0) : initial value of 228 Ra/ 226 Ra in water estimated from the rock.Assuming that the Ra isotopes in groundwater were supplied from the overlying rock in the aquifer with constant ratio, the 228 Ra/ 226 Ra activity ratio may decrease with increase in the residence time of the groundwater in the aquifer.Supposing that the ( 228 Ra/ 226 Ra) T =0 activity ratio in the groundwater is twice that of the relevant rocks, the activity ratio for the youngest groundwater of the Mishima lava flow may be 1.8, and ( 228 Ra/ 226 Ra) T =0 may be assumed to be 1.8.Based on this assumption, from the initial 228 Ra/ 226 Ra activity ratio of groundwater (from 0.8 to 1.1), we estimate the residence time of the Kakitagawa groundwater to be from 9 to 16 years.In fact, the residence time of the groundwater of the Kakitagawa River was 10 years determined by 3 H/ 3 He groundwater dating[2].The residence time of the Kakitagawa estimated by the present Ra in the groundwater and the relevant rock agrees well with that determined by 3 H/ 3 He groundwater dating, suggesting that the Ra isotope ratios of groundwater can be used to estimate the residence time of young groundwater in volcanic areas.
8, resulting in a residence time of Kakitagawa river waters from 9 to 16 years.The residence time of Kakitagawa estimated from the 228 Ra/ 226 Ra ratios agrees well with the 3 H/ 3 He age.Our results suggest that the 228 Ra/ 226 Ra activity ratio can be used to date young groundwater in volcanic areas.