Coupled use of 36Cl and radiogenic 4He in deep groundwater to deduce the influence of past freshwater-seawater mixing: Examples from a coastal sedimentary basin and a crystalline rock area in Japan

Long-term sea level fluctuations induced by climate change should have caused large impacts on groundwater flow regime in the Japanese islands. Evidence of such influence can be traced from the deep groundwater of coastal areas, which has been formed through the repeated mixing of meteoric groundwater (freshwater) and seawater. This study utilizes ³⁶Cl coupled with radiogenic ⁴He in an attempt to obtain insights into the influence of climate-induced sea level fluctuations on deep groundwater systems. Given a seawater component brought into the subsurface, both of the production of ³⁶Cl and the accumulation of radiogenic ⁴He can theoretically be described with residence time or age in a closed system. If a seawater component is mixed with meteoric groundwater, the ⁴He concentration would be enriched or diluted depending on the relative difference in their ages, and the ³⁶Cl/Cl ratio would change accordingly with Cl concentration. Therefore, any deviation of measured data from an equal age line (or a "growth curve") in a ⁴He-³⁶Cl plot may be interpreted as a result of freshwater-seawater mixing. This approach was adopted in a coastal sedimentary basin and a crystalline rock area in Japan. The sedimentary basin in Aomori Prefecture is mainly filled with Neogene to Quaternary sediments including "Green Tuff" formations of lower to middle Miocene ages. Deep groundwater samples were more likely to show radiogenic ⁴He concentrations comparable to or less than the growth curve, which suggests the mixing of older seawater and younger meteoric groundwater. Assuming a young meteoric groundwater end-member of ³⁶Cl/Cl = 100 × 10^-15 (Cl = 5 mg/L), ³⁶Cl/Cl ratios of seawater fractions (Cl = 19,000 mg/L) were calculated to estimate their ages. Overall, calculated seawater ages become older from coastal to inland areas. Relatively old ages of seawater fraction were encountered in the west of Lake Ogawara, corresponding to the deepest part of the basin as indicated by the negative gravity anomaly. At the same time, in the central Kamikita Plain, relatively young seawater ages range inland from the coastal area. These results may imply hydrogeological controls on the flow of old seawater and on the intrusion of young seawater during transgression in a sedimentary rock area. Conversely, the Seto Inland Sea area is characterized by granitic rocks, which can constitute fractured-rock aquifer systems. Majority of saline deep groundwater samples exhibited extremely low ³⁶Cl/Cl ratios (~1 × 10^-15 or less) and greater radiogenic ⁴He concentrations than the growth curve, clearly indicating the mixing of younger seawater and older meteoric groundwater. The ages of seawater and meteoric groundwater fractions were separately calculated from the ³⁶Cl/Cl ratios of seawater end-members and the ⁴He concentrations of meteoric groundwater end-members, respectively. Seawater ages in saline deep groundwaters were mostly less than a few tens of thousands years, while meteoric groundwater fractions showed older ages. This may indicate that the seawater intruded into the aquifer during the Holocene transgression period. The results in turn suggest that the groundwater in a fractured granitic aquifer is relatively easily replaceable during a transgression-regression cycle.