Tracing of the sources of dissolved organic matter in coastal groundwater using fluorescence indices and end-member mixing analysis
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摘要:
滨海地下水中溶解性有机质（DOM）长期受到陆源、海洋源等多种有机质混合，定量评估其来源对认识滨海地区碳迁移转化具有重要意义。选取了江苏连云港市为研究区，利用同位素示踪和光谱指数识别了地下水DOM来源，并通过光谱指数与端元混合模型（EMMA）定量评估了不同端元对滨海地下水DOM的贡献占比。结果表明：滨海地下水DOM主要由河水DOM、海水DOM和沉积物可溶性有机质混合影响，3个端元贡献占比分别为44%±22%、33%±10%和22%±13%。研究区北侧地下水受到海水混入和淡水补给共同影响，地下水DOM具有较高海水DOM贡献占比和自生源特征；而研究区南侧地下水咸化严重，高盐度地下水在径流过程中促进了沉积物可溶性有机质释放，地下水DOM具有较高的沉积物可溶性有机质贡献占比和腐殖化特征。研究成果为滨海地下水DOM来源的计算提供了评估方法。
Abstract:Objective Dissolved organic matter (DOM) in coastal groundwater is long-term mixed with from multiple sources, including terrestrial input, marine intrusion, and organic leachates from sediments. Quantitative estimation of the contributions of different sources is crucial for understanding carbon transport and transformation processes in coastal aquifers.
Methods In this study, coastal groundwater in Lianyungang City, Jiangsu Province was investigated using stable isotopic tracers, fluorescence indices, combined with end-member mixing analysis (EMMA) to identify and quantify DOM sources.
Results The results showed that DOM in coastal groundwater mainly derives from DOM in river water, seawater, and sediments, contributing 44% ±22%, 33% ± 10%, and 22% ± 13%, respectively. Groundwater in the northern part of the study area is affected by both seawater intrusion and freshwater recharge, exhibiting higher proportion of seawater DOM and stronger autochthonous characteristics. In contrast, groundwater in the southern part shows pronounced salinization, where elevated salinity enhances the mobilization of sediment-derived soluble organic matter, resulting in groundwater DOM with a higher proportion of sediment-derived sduble organic matter contribution and humification characteristics.
Conclusion This study highlights that DOM in coastal groundwater is jointly controlled by hydrodynamic and hydrogeochemical conditions. The integration of fluorescence indices with EMMA provides a reliable quantitative and efficient approach for source apportionment of DOM, offering new insights into the land-ocean continuum of the carbon cycle.
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图 1 研究区概况和采样点位置图（a）和水文地质剖面图（b）
Ⅰ, Ⅱ. 含水岩组组号；Qh. 全新统；Qp3. 上更新统；Qp2. 中更新统；Pt2−3Dh. 中-新元古界；K2p. 上白垩统跑马岗组
Figure 1. Overview of the study area and location of the sampling points (a), and hydrogeological cross-section (b)
图 2 地下水、河水、海水和雨水的δ2H-δ18O关系(a)，地下水与海水和河水混合线关系(b)，以及地下水、河水和海水的δ18O-ρ（Cl−）关系（c）
GWML. 全球大气降水线；LMWL. 当地大气降水线；A. 低盐分区；B. 高盐分区
Figure 2. Relationships of δ2H and δ18O among groundwater, river water, sea water and precipitation (a), relationship between groundwater and the mixing line of sea water and river water (b), and relationships of δ18O and ρ(Cl−) among groundwater, river water and sea water (c)
图 4 三维荧光光谱−平行因子分析（EEM-PARAFAC）得到的DOM组分及其荧光特征
C1，C2，C3为3种荧光组分，其中C1 和 C2 为类腐殖质组分，C3 为类蛋白质组分，下同
Figure 4. DOM components and their fluorescence characteristics identified by EEM-PARAFAC
图 5 地下水以及各端元DOM的光谱指数变化范围
HIX，BIX，FI均为光谱指数，下同
Figure 5. Variation in fluorescence indices of DOM in groundwater and different endmembers
图 6 地下水中DOM和各端元的HIX-C3占比关系（a），地下水DOM各端元贡献占比（b）及端元贡献占比空间分布图（c）
Figure 6. Relationships between HIX and C3 ratio of DOM in groundwater and different endmembers (a), boxplot of the contribution proportions of endmembers to groundwater DOM (b), and spatial distribution of endmember contribution proportions (c)
图 7 水样品的荧光参数和主要离子的主成分分析的变量分布和得分图（a，b）及各排序轴荷载图（c）
Figure 7. Principal component analysis (PCA) of fluorescence indices and major ions in groundwater: Loading (variables) and score (samples) plots (a, b), and loading plot of principal components (c)
表 1 各双光谱指数端元混合模型对滨海地下水DOM来源占比分析的适用性
Table 1. Applicability of different dual fluorescence index combinations for endmember mixing analysis of DOM sources in coastal groundwater
双光谱指数端
元混合模型模型参数 G MRE/% RMSE C1占比-BIX C1占比 −0.238 10.223 0.119 BIX 0.227 11.439 0.069 C1占比-C3占比 C1占比 0.445 7.572 0.059 C3占比 0.678 9.491 0.058 C2占比-BIX C2占比 −0.239 8.701 0.119 BIX −1.079 13.688 0.068 C2占比-C3占比 C2占比 −0.559 21.193 0.059 C3占比 0.591 16.01 0.065 HIX−BIX HIX 0.0913 27.141 2.023 BIX 0.096 6.573 0.102 HIX−C3占比 HIX 0.715 9.589 1.132 C3占比 0.712 10.642 0.054 注：G. 预测优度；MRE. 相对误差；RMSE. 均方根误差；下同 
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