Seasonal variations in groundwater discharge and associated nutrient fluxes in Changhu Lake
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摘要:
为揭示地下水对湖泊水量和营养盐平衡的贡献及季节性变化, 以长江中游的长湖为研究对象, 通过丰、枯水期野外采样, 结合电导率(
EC )、稳定同位素(2H和18O)、水化学元素(Ca2+、Mg2+)和氡(222Rn)同位素对湖底地下水排泄(lacustrine groundwater discharge, 简称LGD)进行了多手段示踪, 并基于222Rn质量平衡模型量化了不同季节的LGD及其携带的营养盐通量。结果显示: 丰、枯水期LGD速率分别为64.52 mm/d和14.95 mm/d, 丰水期显著大于枯水期; 丰、枯水期地下水携带的总氮(TN)输入通量分别为25.68×106 g/d和5.58×106 g/d, 总磷(TP)输入通量分别8.14×106 g/d和0.17×106 g/d。丰、枯水期LGD强度的差异导致了地下水携带TN、TP输入的差异, 丰水期TP的输入量还受该时期农业活动的影响; 丰水期较强的降水和蒸发驱动了更大的LGD强度及其携带的TN、TP通量。本研究可为长湖区域水资源管理和水生态保护提供重要理论依据。Abstract:Objective To assess contribution and seasonal variation in groundwater discharge (LGD) to lake water and nutrient budgets,
Methods this study investigated Changhu Lake in the middle reaches of the Yangtze River. Field sampling was conducted during both the wet and dry seasons using multiple tracing techniques, including electrical conductivity (EC), stable isotope (2H and 18O), hydrochemical element (Ca2+ and Mg2+), and 222Rn isotope data. The 222Rn mass balance model was employed to quantify the LGD and associated nutrient fluxes in different seasons.
Results Results show that LGD rates during the wet and dry seasons were 64.52 mm/d and 14.95 mm/d, respectively, with significant differences between these seasons. Furthermore, during the wet and dry seasons, groundwater carried TN inputs of approximately 25.68×106 g/d and 5.58×106 g/d, respectively. The TP inputs were approximately 8.14×106 g/d and 0.17×106 g/d in the wet and dry seasons, respectively. The differences in the LGD rates between the wet and dry seasons lead to differences in groundwater carrying TN and TP inputs, and inputs of TP during the wet season is also influenced by agricultural activities during that period. Sronger precipitation and evaporation during the wet season drive greater LGD intensity and their carrying TN and TP fluxes.
Conclusion The research can provide scientific reference for water resource management and aquatic ecosystem preservation efforts in the Changhu area.
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Key words:
- lake /
- groundwater discharge /
- radon mass balance model /
- nitrogen /
- phosphorus /
- nutrient fluxes /
- seasonal variation
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图 1 研究区概况图
a.长湖的地理位置及其主要水系高程图，以及采样点分布，其中箭头指示地表水流方向；b.基于图a中A-B线展开的长湖水文地质剖面图
Figure 1. Map of the study area
图 2 丰、枯水期湖水和地下水中EC(a)、ρ(Ca2+)(b)、ρ(Mg2+)(c)、222Rn活度(d)的对比图
Figure 2. Comparison of the contentration of EC(a), Ca2+ (b), Mg2+(c), and 222Rn(d) in surface water and groundwater during wet and dry seasons
图 3 丰、枯水期湖水中保守指标与222Rn活度的相关性分析
n为样本量；r为相关系数；p为皮尔森相关系数
Figure 3. Correlation analysis between 222Rn activity and conservative tracers in lake water during the wet and dry seasons
图 4 丰、枯水期各采样位置湖水和地下水(包括井水和孔隙水)的δ18O(a)和δD(b)特征
Figure 4. The δ18O(a) and δD(b) characteristics of lake water and groundwater
图 5 丰、枯水期不同水体的ρ(TN)(a)、ρ(TP)(b)对比
Figure 5. Comparison of TN(a) and TP(b) concentrations in different end-member water bodies during the wet and dry seasons
图 6 研究区2021年1月至2022年11月的降水、蒸发和湖泊水位的月均数据(气象数据源于欧洲中期天气预报中心，水位数据来源于湖北省水利厅; 湖泊水位基准高程为30 m)
Figure 6. Monthly average precipitation, evaporation, and lake water level data in the study area from January 2021 to November 2022
表 1 222Rn质量平衡模型计算的相关参数
Table 1. Parameters of the radon mass balance model
参数 值 备注 丰水期 枯水期 湖水中222Rn的活度Cw/(Bq·m-3) 1 040.65± 334.26 147.16± 66.17 野外测试 大气中222Rn的活度Catm/(Bq·m-3) 48.70 16.20 野外测试 地下水中222Rn的活度Cgw/(Bq·m-3) 22 679.92±4 588.15 10 313.653±2 574.29 野外测试 沉积物培养孔隙水中222Rn的活度Ceq/(Bq·m-3) 38 233.62 17 494.31 式(5) 上清液中222Rn的活度A0/(Bq·m-3) 3 030 1 497.02 沉积物培养 大气温度Tatm/℃ 29.64 8.73 野外测试 湖水温度T/℃ 32.05 11.70 野外测试 风速μ10/(m·s-1) 3.43±1.95 4.22±1.99 气象站数据 湖底沉积物孔隙度n 0.48 0.51 实验室测量 湖泊深度d/m 3.24 2.94 湖北省湖泊志 沉积物扩散的222Rn通量Fsed/(Bq·m-2·d-1) 26.66±8.45 8.55±3.26 式(8) 大气弥散的 222Rn通量Fatm/(Bq·m-2·d-1) 879.82±428.55 84.40±45.28 式(9) 222Rn衰变量Fdec/(Bq·m-2·d-1) 610.28±196.02 78.31±35.75 式(13) 地下水排泄的222Rn通量Fgw/(Bq·m-2·d-1) 1 463.43±520.77 154.16±57.60 式(3) 
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表 2 地下水载荷的营养盐通量
Table 2. Nutrients fluxes loaded by groundwater discharge
ρB/(mg·L-1) 通量FB/(g·d-1) ρ(TN) ρ(TP) LGD LGD-TN LGD-TP 丰水期 3.12 0.99 8.23×106 25.68×106 8.14×106 枯水期 2.92 0.09 1.91×106 5.58×106 0.17×106
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