Contrasting lacustrine groundwater discharge and associated nutrient loads in different geological conditions

The present study aims to investigate the lake water balance and the nutrient load balance in the East Dongting Lake (EDL) with a special attention given to the contribution from groundwater. The study was carried out first by taking water samples from the lake and the surrounding aquifers, then using lab analysis to determine the content of stable isotopes in the water samples, especially 222 Rn, which can be seen as an indicator of groundwater inflow. Subsequently, 222 Rn mass-balance model was applied for lake water constituents. The paper was written in a clear and concise way, the language was easy to understand, and the method used and the conclusion drawn was scientifically sound. Based on my assessment, would recommend that the paper is returned to the authors for minor to intermediate revision before it can be accepted for publication by HESS.

The present study aims to investigate the lake water balance and the nutrient load balance in the East Dongting Lake (EDL) with a special attention given to the contribution from groundwater. The study was carried out first by taking water samples from the lake and the surrounding aquifers, then using lab analysis to determine the content of stable isotopes in the water samples, especially 222 Rn, which can be seen as an indicator of groundwater inflow. Subsequently, 222 Rn mass-balance model was applied for lake water constituents. The paper was written in a clear and concise way, the language was easy to understand, and the method used and the conclusion drawn was scientifically sound. Based on my assessment, I would recommend that the paper is returned to the authors for minor to intermediate revision before it can be accepted for publication by HESS.
Major comments.
1, There is no clear indication of research novelty in the manuscript. It seems to me that the method used in the study is very mature and the conclusion is somewhat expected. So, I would suggest adding a few sentences clearly stating the current research gap in comparison to previous studies, and the innovative aspects of the present study in the Introduction section.
2, Based on the results shown in the study, the 222 Rn concentration was much higher (larger than 5000 Bq/m3) in the groundwater at the east side of the lake where karst aquifer dominates, and much lower in the west bank (2000-5000 Bq/m3) where porous aquifer dominates. This makes sense since karst groundwater usually has higher radon content, and the groundwater discharge rate calculated is thus 38.66 mm/d to the west and 92.82 mm/d to the east. However, contrary to our expectations, the 222 Rn concentration in the lake water in the West EDL is much higher than it is in the East EDL. The paper tried to find possible explanations to such phenomenon observed, and it was suggested that preferential pathways in the west may have existed which increased the inflow from groundwater. But this claim is less convincing because there is no evidence that the preferential flow pathways only exist in the west, and in principle, the fractured rock in the east is more prone to have preferential pathways.
My question is, could there be another explanation to what's observed, that the porous aquifer to the west connects to the lake more on the side, but the karst aquifer to the east connects to the lake more on the lake's bottom? Your samples were taken at 0.5 m below lake surface, and the lake is quite deep. Fast flow-through in the lake also encourages mixing when groundwater enters the lake. Therefore, the groundwater discharge to the lake is much easier detected closer to the lake shore. If this is the case, then it explains why there is larger inflow from groundwater in the east but lower concentration of 222 Rn captured in the lake. Please discuss.

3, The
LGD is controlled by both lake and aquifer materials. Maybe add a table or figure explaining the spatial distribution of e.g. conductivity of both materials.
Minor comments.
L. 518. The conclusion is rather short, maybe consider including more quantitative results.
Figures and tables.