Applying <scp> ²²⁴ Ra </scp> and <scp> ²²³ Ra </scp> to Trace Lateral Groundwater Discharge into Lake Qinghai, China

Quantifying lacustrine groundwater discharge (LGD) is important for understanding the dynamics of lake ecosystems and their expansion. This study focuses on Lake Qinghai, employing radium isotope models to evaluate the contributions of both shallow and deep groundwater. The data indicate that the activity of ²²³Ra and ²²⁴Ra demonstrates a pronounced gradient, decreasing from the shoreline to the center of Lake Qinghai. Additionally, vertical stratification characteristics were observed. The spatial distribution of radium isotope activity suggests that there is discharge of both shallow and deep groundwater into the lake. Deep groundwater migrates slowly and its apparent age reflects the time elapsed since the water became enriched in Ra and was isolated from the source, in the study system this age is estimated to be 10.1 d. In contrast, shallow groundwater displayed varied apparent ages in different regions: 7.9 d in the north, 13.1 d in the south, and 7.4 d in the southeastern area of the lake. The LGDs of shallow groundwater discharge in the north, south, and southeast areas of Lake Qinghai were estimated by ²²⁴Ra as 1.89 × 10⁶ to 2.69 × 10⁶ m³/d, 3.25 × 10⁶ to 3.99 × 10⁶ m³/d, and 4.51 × 10⁶ to 6.33 × 10⁶ m³/d, respectively. For deep groundwater, the LGD was 0.16 × 10⁶ to 0.29 × 10⁶ m³/d. Annually, the total LGD fluxes of shallow and deep groundwater are 27.86 × 10⁸ to 37.59 × 10⁸ m³/year and 0.58 × 10⁸ to 1.06 × 10⁸ m³/year, respectively. This study is the first to evaluate shallow and deep groundwater discharge around the lake. Understanding these discharge dynamics is essential for developing effective management strategies to preserve lake environments.