Nutrient Loads in Different Geological Conditions

https://doi.org/10.5194/hess-2021-364 Preprint. Discussion started: 7 September 2021 c Author(s) 2021. CC BY 4.0 License. 1 Contrasting lacustrine groundwater discharge and associated 2 nutrient loads in different geological conditions 3 Xiaoliang Sun1, Yao Du1,2*, Yamin Deng1,2, Hongchen Fan1,2, Teng Ma1,2 4 a Hubei Key Laboratory of Yangtze Catchment Environmental Aquatic Science, School of 5 Environmental Studies, China University of Geosciences, Wuhan 430078, China 6 b State Key Laboratory of Biogeology and Environmental Geology, China University of 7 Geosciences, Wuhan 430078, China 8 Abstract. The spatial patterns of lacustrine groundwater discharge (LGD) and associated nutrients 9 input is crucial for effective management and protection of lakes. Multiple factors have been 10 found to influence the spatial differences in LGD rates and associated nutrients loads, but the 11 influence of geological conditions on the differences have not been well understood. In this study, 12 we quantified LGD rates and associated nutrients loads in two sides with contrasting geological 13 conditions of East Dongting Lake (EDL) within central Yangtze catchment and discuss the 14 influence of geology on the spatial differences, through 222Rn mass-balance model, water 15 chemistry coupled with existing geological data. The results showed that LGD rates were 38.66 ± 16 21.07 mm d-1 in the east EDL which is characterized by hilly geomorphy, deep/fast/narrow 17 flowing, coarse-grained lakebed and large hydraulic gradients (0.004 - 0.006). Surprisingly, LGD 18 rates were higher (92.82 ± 51.98 mm d-1) in the west EDL which is characterized by 19 alluvial-lacustrine plain geomorphology, shallow/sluggish flowing, clayey or silty lakebed and low 20 hydraulic gradients (0.0002 - 0.0015). The remaining factor determining the higher LGD rates in 21 the west EDL is the permeability of the porous aquifer connected with the lake, which could be 22 enlarged by some preferential pathways including large-scale buried paleo-channel and 23 small-scale plant roots. The groundwater around the east EDL existed in a less confined 24 environment, and frequent flushing led to low concentrations of nutrients. On the contrast, rapid 25 burial of sediments and deposition of paleo-lake sediments since Last Deglaciation formed an * Correspondence to: Yao Du, School of Environmental Studies, China University of Geosciences, Jincheng Road 68, Wuhan 430078, China. E-mail address: [email protected] (Yao Du) 1 https://doi.org/10.5194/hess-2021-364 Preprint. Discussion started: 7 September 2021 c Author(s) 2021. CC BY 4.0 License. 26 organic-rich and reducing environment, which facilitated the enrichment of geogenic nutrients. As 27 a result, the loads of LGD-derived nutrients in the west generally exceeded that in the east by one 28 order of magnitude. In practice, future water resource management and ecological protection of 29 Dongting Lake should focus on groundwater discharge in west EDL. This study highlights an 30 important role of geological conditions in determining contrasting LGD rates and associated 31 nutrients loads in large freshwater lakes. 32 33 1 Introduction 34 Recent studies have shown that groundwater is an important component of lake water (Kong et al., 35 2019; Kidmose et al., 2013; Stets et al., 2010; Schmidt et al., 2009; Roy and Hayashi., 2008) and lake 36 chemistry (Kazmierczak et al., 2020; Kong et al., 2019; Luo et al., 2018; Burnett et al., 2017; 37 Meinikmann et al., 2015) globally. Lacustrine groundwater discharge (LGD) has received increased 38 interest among researchers due to its impacts to lake water. LGD and LGD-derived nutrients can result 39 in a deterioration of lake water quality and phytoplankton blooms (Holman et al., 2008; Krest et al., 40 2000), consequently negatively effecting the aquatic ecosystem of a lake. Therefore, there is an urgent 41 need for the quantification of the contributions of LGD to lake water and nutrients balances, thereby 42 providing a new perspective on lake water resources management and aquatic ecosystem protection. 43 LGD and associated nutrients input are often characterized by large spatial variability due to the 44 heterogeneity of geomorphy, surface hydrology, lakebed sediment lithology, hydrogeology, nutrient 45 concentration levels of groundwater around lakes, etc (Wallace et al., 2020; Tecklenburg and Blume, 46 2017; Hare et al., 2017; Rosenberry et al., 2015; Meinikmann et al., 2015, 2013; Blume et al., 2013; 47 Schneider et al., 2005). For example, Schmidt et al. (2010) found that LGD rates were related to the 48 drainage area/lake area ratio which promotes greater surface connectivity. Meinikmann et al. (2015) 49 observed that both the intensity of the contamination and its proximity to the lake inhibit nutrient 50 retention within vadose zone and aquifer and allow significant phosphorus loads to be discharged into 51 the lake. Tecklenburg and Blume (2017) found that large-scale LGD patterns were correlated with 52 topography and groundwater flow field, whereas small-scale patterns correlated with grain size 53 distributions of the lake sediment. However, the influence of geological conditions on spatial 54 differences in LGD and associated nutrients input have not been well understood. Moreover, the 55 geological factor could be internally inter-played with hydrogeology, groundwater quality and even 2 https://doi.org/10.5194/hess-2021-364 Preprint. Discussion started: 7 September 2021 c Author(s) 2021. CC BY 4.0 License. 56 super-surface factors mentioned above, which may collaboratively lead to spatial variability in LGD 57 and associated nutrients input. The advancing of related understanding may require a comprehensive 58 analysis from the perspective of geology. 59 Dongting Lake had previously been recognized as the largest fresh lake in China by surface area. 60 However, the lake has gradually contracted due to natural sedimentation and land reclamation in recent 61 decades, and is currently recognized as the second largest fresh lake in China (Yu et al., 2020; Yi, 2017). 62 Dongting Lake also hosts one of the most ecologically important wetlands in the Yangtze Catchment. 63 This wetland has been referred to as the "kidney of Yangtze River" in recognition of its important 64 function in water purification, and also has vital roles in water conservation, climate regulation, and 65 biodiversity conservation (Hu et al., 2020; Zheng et al., 2016; Pan et al., 2013). Groundwater is 66 abundant around Dongting Lake (Huang et al., 2019), and the quality of this groundwater is poor due to 67 the influence of anthropogenic activities and natural processes (Huang et al., 2021; Long et al., 2021). 68 However, the role of groundwater in the balance of water and chemicals in Dongting Lake remains 69 poorly understood. Although some studies have identified the strong exchange between groundwater 70 and lake water (Zhan et al., 2014) and considerable contribution of groundwater to overall water and 71 nutrient balances of Dongting Lake (Sun et al., 2020), the spatial contrast of LGD and associated 72 nutrient loads has not yet studied. 73 East Dongting Lake (EDL) is a relatively independent part of Dongting Lake and is also 74 recognized as a wetland of international importance by the Ramsar Convention (Zou et al., 2019; Liu et 75 al., 2019). The EDL plays an important function in regulating the flood runoff of the Yangtze River and 76 also hosts rare birds and the Yangtze River dolphin (Zou et al., 2019; Xie, 2017; Xiong et al., 2016). 77 The current water resource and ecological statuses of the EDL are poor relative to historical conditions 78 due to a deterioration in water quality, ecological degradation, and a decline in species diversity as a 79 result of lake shrinkage and wetland degradation (Huang et al., 2018; Wang et al., 2015; Wang et al., 80 2015; Hu et al., 2015). The ecological sensitivity and important ecological role of the EDL emphasize 81 the need for an evaluation of LGD and associated nutrients input. The west side and east side of EDL 82 are characterized by contrasting geological conditions, with which significant differences can be 83 observed in geomorphy, surface hydrology, lakebed sediment lithology, hydrogeology, etc (details in 84 site description). The differences in these factors between the west and east EDL are expected to result 85 in spatial differences in LGD and associated nutrients input. 3 https://doi.org/10.5194/hess-2021-364 Preprint. Discussion started: 7 September 2021 c Author(s) 2021. CC BY 4.0 License. 86 Although various methods of quantifying LGD exist (Tecklenburg and Blume; 2017; Dimova et 87 al., 2015; Lewandowski et al., 2013; Stets et al., 2010), radioactive 222Rn tracing has been widely and 88 effectively used in recent years (Wallace et al., 2021, 2020; Dabrowski et al., 2020; Hare et al., 2017; 89 Petermann et al., 2018; Liao et al., 2018; Luo et al., 2018, 2016, 2014; Burnett et al., 2017; Dimova et 90 al., 2015, 2013; Gilfedder et al., 2015; Peterson et al., 2010; Schmidt et al., 2010, 2009). 222Rn is a 91 subset of 226Ra and is widely present in inland rocks and water bodies. Uranium-bearing minerals in 92 sediments continuously release 222Rn during decay, resulting in the 222Rn concentration in groundwater 93 typically being much higher than that in surface water. 222Rn is an inert gas that is chemically 94 conservative. Since 222Rn in lake water mainly originates from groundwater, the spatial distribution of 95 222Rn in lake water can to a certain extent act as an indicator of the spatial distribution of LGD if it is 96 assumed that the 222Rn concentration in groundwater is relatively uniform (Cheng et al., 2020; Wallace 97 et al., 2020; Burnett et al., 2017). A mass-balance model can be established based on the 222Rn source 98 and sink terms, thereby allowing LGD to be calculated as the single unknown term controlling 222Rn in 99 lake water.

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