In situ supplies of radiogenic He in residual soils of shallow granite aquifers: Spatial distribution of dissolved He throughout the Korean Peninsula

Abstract

In Korea, where shallow weathered granitic aquifers are ubiquitous, several previous studies have reported strong radioactivity in shallow groundwater, which eventually led to the closure of production wells; however, no integrated study has analyzed factors controlling the spatial distribution of radionuclides in groundwater. Herein, we compiled the existing measurements of He, Ne, Ar, Kr, and Xe from 122 shallow wells (<200 m in depth) to differentiate and analyze the distribution of non-atmospheric (excess) 4He as a proxy for radionuclide accumulation in groundwater. The closed-system equilibration (CE) model accurately predicted the measured 4He in samples (root-mean-square deviation = 0.486), indicating its application in discerning the physical conditions of groundwater recharge in shallow granitic aquifers. Fifty-five samples were selected to identify the origin of strong radioactivity in shallow aquifers; low-quality samples were screened based on the (1) goodness of fit (>1%), (2) plausibility of modeled excess air concentrations (<0.05 cm3 STP/g), and (3) recharge temperature (5–20 °C). The distribution of excess He in the aquifers was largely dependent on the rock type, as defined by three distinct trends in He isotope excesses. Specifically, we observed (1) a large contribution of a mantle source of He isotopes, with values as high as ∼6.7 RA (RA = air = 1.39 × 10−6), on Jeju Island (quaternary basalt); (2) a small but significant contribution of mantle in Yanggu (Jurassic hornblende granite), with values up to 2.3 RA; and (3) a purely radiogenic source of 4He (i.e., no mantle contribution) throughout the remaining five granitic or sedimentary aquifers. Moreover, the shallow aquifer in Wonju (Jurassic biotite granite) exhibited a measurable amount of excess He, without large faults. The radiogenic sources scattered in the shallow granite aquifer caused localized 4He concentration peaks, even in young groundwaters (1.6–19.9 y). The concentration of radioelements peaked at a relatively shallow depth of 18.5 m, refuting the diffusive supply of radionuclides from the external deep reservoir. This was attributed to the high range of 4He accumulation rates (3.51 × 10−9 to 1.17 × 10−8 cm3STP/g/y), which originated from the high in-situ production of residual soils weathered from U-abundant protolith. The present study demonstrated that excess He quantified using the CE model can help identify the origin of strong radioactivity in shallow groundwater.