13:45 〜 15:15
[HTT15-P10] Tracing anthropogenic nitrate sources with 234U/238U isotopic ratios in groundwater in Kumamoto Area
キーワード：地下水、ウラン、同位体、硝酸イオン
Nitrate ion (NO3-) is one of the anthropogenic constituents dissolved in groundwater, and the high NO3- concentrations in groundwater require water quality improvement because of impact on human health and the environment. It is known that NO3- is derived from a variety of sources, such as ammonium chemical fertilizers, manure and sewage. Stable isotope ratio of nitrogen in nitrate (δ15NNO3) has been used to specify the sources of NO3-, but denitrification processes to N2 in reducing groundwater causes isotopic fractionation and overprint the original values. In this study, we focused on 234U/238U isotope ratios as a new proxy to groundwater hydrological research, and attempted to assume the sources of NO3- in the Kumamoto groundwater area. Since uranium in groundwater behaves as a conservative element dissolved as U(VI) except in extremely reducing conditions, 234U/238U isotope ratio is expected to specify the sources of NO3- in reducing groundwater even after denitrification. In the Kumamoto groundwater area, there is concern about elevated NO3- concentrations, mainly from ammonium chemical fertilizers and manure. There are two major aquifers composed of Aso pyroclastic flow deposits in this area, known as the first aquifer and the second aquifer. The first aquifer is basically shallow and unconfined, while the second aquifer is deeper and confined. This groundwater has a main flow from Mt. Aso to the Ariake Sea, flowing through the agricultural and livestock production areas in the northern region. We analyzed 234U/238U isotope ratios of groundwater and compared with their δ15NNO3.
234U is produced in trace amounts during the chain decay of 238U. Because of the difference in half-lives (T1/2(234U)≪T1/2(238U)), a state of radiative equilibrium is observed that the radiative decay rates of both are equal in a closed system for more than a few million years. However, the 234U/238U isotope ratios of uranium dissolved in groundwater are generally different from the radiative equilibrium depend on the type of host rock of the aquifer and the groundwater age. The activity ratio (AR), ratio of the radiative decay rates, is usually greater than 1 in groundwater.
About 30 groundwater samples of 50 mL-2 L each were treated by Fe-coprecipitation and anion exchange separation method to remove major dissolved components. The concentration of uranium in the solutions was measured by ICPMS (Agilent 8800), and the 234U/238U isotope ratios were analyzed by a multiple-collector ICPMS (NEPTUNE).
As a result, uranium concentrations were higher in agricultural and livestock production areas (>0.2 ppb) than in the other area. The AR showed ~1.0 in the first aquifer and ~1.2 and ~1.5 in the second aquifer in agricultural and livestock production areas, while the AR was around 1.3 in the other areas. Mixing plots using AR and the inverse of uranium concentration suggested multiple groundwater mixing. It was suggested that AR of the groundwater reflected the AR of phosphate ore (AR=1), a raw material for phosphorous fertilizers applied with ammonium chemical fertilizers in the agricultural areas, and the AR of manure (from grass) in livestock production areas respectively. The conservative behavior of uranium isotope composition in some post-denitrification reducing groundwaters was also confirmed.
234U is produced in trace amounts during the chain decay of 238U. Because of the difference in half-lives (T1/2(234U)≪T1/2(238U)), a state of radiative equilibrium is observed that the radiative decay rates of both are equal in a closed system for more than a few million years. However, the 234U/238U isotope ratios of uranium dissolved in groundwater are generally different from the radiative equilibrium depend on the type of host rock of the aquifer and the groundwater age. The activity ratio (AR), ratio of the radiative decay rates, is usually greater than 1 in groundwater.
About 30 groundwater samples of 50 mL-2 L each were treated by Fe-coprecipitation and anion exchange separation method to remove major dissolved components. The concentration of uranium in the solutions was measured by ICPMS (Agilent 8800), and the 234U/238U isotope ratios were analyzed by a multiple-collector ICPMS (NEPTUNE).
As a result, uranium concentrations were higher in agricultural and livestock production areas (>0.2 ppb) than in the other area. The AR showed ~1.0 in the first aquifer and ~1.2 and ~1.5 in the second aquifer in agricultural and livestock production areas, while the AR was around 1.3 in the other areas. Mixing plots using AR and the inverse of uranium concentration suggested multiple groundwater mixing. It was suggested that AR of the groundwater reflected the AR of phosphate ore (AR=1), a raw material for phosphorous fertilizers applied with ammonium chemical fertilizers in the agricultural areas, and the AR of manure (from grass) in livestock production areas respectively. The conservative behavior of uranium isotope composition in some post-denitrification reducing groundwaters was also confirmed.