Identification of He sources and estimation of He ages in groundwater of the North China Plain

- The terrigenic 4He concentrations vary over three orders of magnitude in NCP.
- A typical radiogenic He isotopic ratio was reported for the first time in NCP.
- A mantle-derived helium was certified in the groundwater in the coastal plain.
- The estimated 4He ages are comparable to the 14C ages under suitable conditions.

Dissolved helium concentrations and 3He/4He ratios were measured for 18 groundwater samples collected from the Quaternary confined aquifers in the North China Plain (NCP). The dissolved helium concentrations ranged from 1 × 10−7 to 1 × 10−6 cm3STP·g−1 in the 14 samples from the central plain, but was approximately two orders of magnitude higher, between 6 × 10−6 and 9 × 10−5 cm3STP·g−1, in 4 samples from the coastal plain. Based on these concentrations and the corresponding 3He/4He ratios varying from 0.09 to 0.55 Ra (where Ra is the 3He/4He ratio of air), the dissolved helium in groundwater in the central plain was identified to be primarily a mixture of atmospheric helium with radiogenic helium and a representative radiogenic helium ratio was estimated to be 0.035 Ra. Despite the high fraction of terrigenic 4He in the samples from the coastal plain, their 3He/4He ratios were found to be significantly above this radiogenic value, ranging between 0.20 and 0.37 Ra, indicating the presence of a mantle-derived He component in this area. About 2-4% mantle helium was estimated to be present in the groundwater of the coastal plain, which probably is associated with the regional Cangdong fault and tectonic activities. Based on the radiogenic He component, 4He ages of the groundwater in the central plain were calculated by assuming either pure in situ production or an external helium flux J0 of 4.7 × 10−8 cm3STPcm−2a−1. The estimated 4He ages fall between 9.5 and 51.4 ka and are comparable to the 14C ages, suggesting that the results of 4He dating are reasonable and can be an effective tool to estimate groundwater residence times under suitable conditions.