Characteristics of Seasonal Changes in Organic Matter of Groundwater in Binhai, Jiangsu Province and Its Impact on Nitrogen Transport and Transformation
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摘要: 溶解性有机质（dissolved organic matter，DOM）在滨海湿地碳循环过程中发挥着重要作用，滨海地区水文过程的季节变化会影响DOM的组成，从而控制氮的迁移和转化途径.本研究选取江苏省连云港市滨海湿地作为研究区，基于地下水、河水以及海水的水化学数据，结合三维荧光光谱和紫外可见光谱，研究了DOM的季节特征及其对氮转化的影响.结果表明：滨海地区地下水DOM包括3种组分：陆源类富里酸（C1）、陆源类腐殖酸（C2）和微生物源类蛋白组分（C3）.研究区内NH4-N和DOC浓度随着距海岸越近逐渐升高，N浓度与DOM组分特征相关.丰水期地下水受到降雨和河水补给，外源大分子DOM入渗伴随着NH4-N进入地下水.含水层处于偏还原状态，硝化过程受到抑制.枯水期咸淡水之间的相互作用较弱，此时地下水中C3组分较高.同时含水层处于氧化性较强的环境中，促进了硝化作用.在潮间带地下水中，DOM的特点是腐殖化程度较高，NH4-N和DOC富集程度表明土壤含氮有机质的矿化.此外，较长的水体滞留时间和较强的微生物活动很可能会促进硝酸盐异化还原成铵（dissimilatory nitrate reduction to ammonium，DNRA），并导致NH4-N进一步富集.Abstract: Dissolved organic matter (DOM) plays a vital role in the carbon cycling process of coastal wetlands. Seasonal changes in hydrological process in the coastal area will affect DOM composition, thus controlling nitrogen transport and transformation pathways. In this study, the coastal area of Lianyungang, Jiangsu Province, was selected as the study area to investigate the seasonal characteristics of DOM and its impacts on nitrogen transformations based on the hydrochemical data of groundwater, river water, and seawater, combined with three-dimensional fluorescence spectroscopy and UV-visible spectroscopy. The results show that DOM in coastal area includes three components, terrestrial source-like fulvic acid (C1), terrestrial source-like humic acid (C2) and microbial source-like protein fraction (C3). In the study area, the NH4-N and DOC concentrations gradually increased with closer proximity to the coast, the N concentration is associated to the DOM component characteristics. During the wet season, the groundwater was recharged by rainfall and river water, with exogenous macromolecules DOM infiltrated accompanied by NH4-N into groundwater. The aquifer is in biased reducing condition, and the nitrification process is inhibited. During the dry season, the interaction between salty and fresh water was weak, at when the higher C3 component in the groundwater. Meanwhile, the aquifer is in more oxidizing environment, promoting nitrification. In the intertidal groundwaters, the DOM is characterized by higher degree of humification, the abundance of NH4-N and DOC suggests the mineralization of N contained soil organic matter. Also, a longer retention time of water as well as a strong microbial activity is likely to promote the dissimilatory nitrate reduction to ammonium (DNRA) and lead to further accumulation of NH4-N.
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Key words:
- coastal /
- wetland /
- groundwater /
- dissolved organic matter /
- seasonal variation /
- nitrogen /
- environmental geology
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图 1 研究区概况及采样点分布图(a)和水文地质剖面图(A-A'和B-B')(b)
Fig. 1. Overview of the study area and distribution of sampling sites (a) and hydrogeologic profiles (A-A ' and B-B') (b)
图 2 研究区丰、枯水期不同水样δ2H和δ18O分布
Fig. 2. Distribution of δ2H and δ18O of different water samples in the study area during wet and dry periods
图 3 研究区丰、枯水期地下水电导率EC与δ18O关系分布
Fig. 3. Distribution of groundwater conductivity EC in relation to δ18O in the study area during the wet and dry periods
图 4 研究区丰、枯水期地下水NO3-N和NH4-N浓度与海岸距离关系
Fig. 4. Groundwater NO3-N and NH4-N concentrations versus coastal distance during wet and dry periods in the study area
图 5 PARAFAC获得的DOM组分及荧光特征(荷载值无单位)
Fig. 5. DOM fractions and fluorescence characteristics obtained by PARAFAC (loading values unitless)
图 6 研究区丰、枯水期不同水样荧光指数与组分占比关系
Fig. 6. Relationship between fluorescence index and fraction occupancy of different water samples in the study area during wet and dry periods
图 7 研究区丰、枯水期不同水样HIX及BIX与NH4-N浓度关系
Fig. 7. Relationship between HIX and BIX and NH4-N concentration in different water samples during wet and dry periods in the study area
图 8 研究区丰、枯水期地下水DOC与NH4-N浓度关系
Fig. 8. Relationship between DOC content and NH4-N concentration in groundwater during wet and dry water periods in the study area
图 9 研究区丰、枯水期不同水样NO3-N和NH4-N浓度与组分占比关系
Fig. 9. Relationship between NO3-N and NH4-N concentrations and fraction occupancy of different water samples in the study area during wet and dry periods
图 10 研究区丰、枯水期地下水NH4-N浓度与光谱指数关系(光谱指数无单位)
Fig. 10. Relationship between groundwater NH4-N concentration and spectral index during wet and dry water periods in the study area (spectral index unitless)
图 11 研究区氮迁移转化概念模型
Fig. 11. Conceptual model of nitrogen transport and transformation in the study area
表 1 研究区地下水主要水化学指标
Table 1. Main hydrochemical indicators of groundwater in the study area
指标 单位 地下水类型 丰水期
(2022年7月/2023年7月)枯水期
(2023年4/11月)最小值 最大值 平均值 最小值 最大值 平均值 水温 ℃ 区域地下水 16.5 27 20.87 15.9 24 18.84 潮间带地下水 25.30 34.70 29.71 15.20 24.80 19.28 pH 区域地下水 6.46 8.12 7.30 6.38 8.35 7.49 潮间带地下水 6.65 7.41 6.94 6.84 7.90 7.39 DO mg/L 区域地下水 1.26 11.84 4.02 1.33 6.31 3.92 潮间带地下水 0.77 5.25 2.57 0.27 7.15 3.65 EC μS/cm 区域地下水 586 25 177 3 846.45 505 16 675 6 047.08 潮间带地下水 2 291 53 647 2 6493.5 5 582 46 694 32 395.50 ORP mV 区域地下水 -148.3 108.7 8.57 -1 118.9 203 59.76 潮间带地下水 -258.40 -26 -140.67 -169 60.40 -34.83 NO2- mg/L 区域地下水 0.006 0.318 0.042 0.006 1.175 0.115 潮间带地下水 0.02 0.26 0.12 0 0.08 0.02 总Fe mg/L 区域地下水 0.02 2.95 0.33 0.05 0.65 0.19 潮间带地下水 0.69 10.10 3.26 0.05 5.90 2.13 Fe2+ mg/L 区域地下水 0.01 2.55 0.15 0 0.55 0.09 潮间带地下水 0.67 4.05 1.67 0.01 1.55 0.60 S2- μg/L 区域地下水 0 113 10.85 0 22 7.54 潮间带地下水 90 329 225.86 0 511 161.57 NH4-N mg/L 区域地下水 0 65 3.07 0.02 3.2 0.59 潮间带地下水 1.81 10.65 5.62 0.05 12.00 2.79 CO32- mg/L 区域地下水 0 105.02 3.18 0 18.75 1.56 潮间带地下水 0 0 0 0 0 0 HCO3- mg/L 区域地下水 122.02 903.71 399.60 165.60 743.56 504.38 潮间带地下水 198.28 1 254.52 467.70 183.21 2 394.64 970.56 Ca2+ mg/L 区域地下水 13.64 1 102.43 181.69 25.69 999.61 158.88 潮间带地下水 109.90 575.17 363.99 136.03 674.37 373.29 K+ mg/L 区域地下水 0.34 119.04 29.72 1.20 75.38 27.78 潮间带地下水 14.94 370.81 196.62 26.14 315.58 223.92 Mg2+ mg/L 区域地下水 13.341 738.21 95.65 18.34 794.75 123.48 潮间带地下水 49.07 1 475.91 673.14 161.83 1 389.21 950.42 Na+ mg/L 区域地下水 43.94 4 411.53 630.72 28.89 4 263.75 811.45 潮间带地下水 282.11 1 1004.8 5 078.59 1 054.6 8 642.97 6 499.26 Cl- mg/L 区域地下水 50.91 13 891.3 1 632.01 52.90 11 087.9 1 717.53 潮间带地下水 422.86 31 834.0 1 3811.7 1 906.5 22 121.9 15 322.48 Br- mg/L 区域地下水 0 30.88 5.01 0.20 43.66 8.10 潮间带地下水 10.46 133.54 60.90 6.37 161.58 59.16 NO3-N mg/L 区域地下水 0.52 73.76 25.85 0.89 95.96 20.76 潮间带地下水 0.52 50.72 22.32 3.99 96.45 36.09 SO42- mg/L 区域地下水 23.48 1 105.66 204.00 31.36 768.45 220.10 潮间带地下水 61.85 2 357.56 1 199.54 128.38 2 181.41 1 357.61 TDS g/L 区域地下水 0.25 21.04 2.89 0.37 19.05 3.26 潮间带地下水 1.18 47.81 21.98 4.21 34.90 26.05 DOC mg/L 区域地下水 2.76 11.61 4.86 1.08 8.22 3.47 潮间带地下水 5.06 15.37 10.98 3.80 13.30 7.55 18O ‰ 区域地下水 -9.68 -2.28 -6.74 -9.02 -3.42 -6.93 潮间带地下水 -6.78 -2.29 -4.29 -9.19 -1.29 -3.66 2H ‰ 区域地下水 -68.01 -21.53 -46.79 -63.20 -23.20 -46.27 潮间带地下水 -47.80 -18.23 -32.53 -55.16 -6.67 -23.15 
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