Abstract:

To elucidate the hydrochemical characteristics, recharge sources, and genesis of high-ammonium (NH₄⁺) groundwater in the complex system of Lisha Island, a typical estuarine sandbar in the Pearl River Estuary, unconfined and confined groundwater samples were systematically collected during wet and dry seasons. A comprehensive study was conducted using hydrochemistry and multiple environmental isotope tracers (δ_2H, δ_18O, ³H, δ_13C, ¹⁴C, and δ_15N-NH4⁺). The results indicate that the groundwater system exhibits a clear ternary spatial structure of “terrestrial source-mixing-marine source,” with hydrochemical facies evolving from HCO₃-Ca type in the terrestrial recharge area to Cl-Na type in the marine-influenced area. Hydrogen and oxygen isotopes confirm that all groundwater originates from atmospheric precipitation but has experienced varying degrees of mixing with seawater. The combined use of ³H and ¹⁴C (corrected age 0 to 2200 a BP) tracers confirms that groundwater salinization results from the combined effects of residual Holocene paleo-seawater and modern seawater intrusion along a preferential pathway on the southwest side. High NH₄⁺ concentration is a significant regional characteristic; its δ_15N-NH4⁺ values show a wide range (from −2.7‰ to +26.7‰), with most samples far exceeding the typical range (+2‰ to +8‰) for organic matter mineralization. The extremely low NO₃⁻ content rules out nitrification as the cause for the ¹⁵N enrichment. The material source of the high NH₄⁺ is the degradation of natural organic matter in sediments, while the anomalous enrichment in δ_15N-NH4⁺ is primarily attributed to the isotopic fractionation effect of the microbial anaerobic ammonium oxidation (anammox) process. Furthermore, analysis of ionic deviation from conservative mixing reveals that NH₄⁺ concentration is significantly positively correlated with the positive deviations of Na⁺, K⁺, and HCO₃⁻ (p<0.05), and negatively correlated with the negative deviation of SO₄²⁻ (p<0.05). This study provides a scientific basis for the sustainable development of groundwater resources and the ecological protection of Lisha Island and similar estuarine islands.

Key words: high-ammonium groundwater, isotope tracing, anaerobic ammonium oxidation, hydrochemistry, seawater intrusion, Pearl River Estuary

摘要：

为阐明珠江口典型河口沙洲立沙岛复杂地下水系统的水化学特征、补给来源及高氨氮成因，于丰、枯水期系统采集了潜水与承压水样品，综合运用水化学、多同位素（δ_2H、δ_18O、³H、δ_13C、¹⁴C、δ_15N-NH4⁺）等环境示踪技术进行研究。结果表明，研究区地下水系统呈现清晰的“陆源－混合－海源”三元空间结构，水化学类型由陆源补给区的HCO³-Ca型向海源影响区的Cl-Na型演化。氢氧同位素证实地下水均来源于大气降水，但经历了与海水的不同程度混合。³H与¹⁴C（校正年龄为2200 a BP）联合示踪确认，地下水咸化是全新世古海水残留与现代海水沿西南侧优势通道入侵共同作用的结果。高浓度氨氮是该区域的显著特征，其δ_{15N-NH+ 4}值变化范围大（−2.7‰－+26.7‰），多数样品值远超有机质矿化（+2‰－+8‰）的典型范围，而极低的NO₃⁻含量排除了硝化作用导致¹⁵N富集的可能。高氨氮的物质来源是沉积物中天然有机质的降解，而δ_15N-NH4⁺值异常富集主要归因于微生物厌氧氨氧化（anammox）过程的同位素分馏效应。离子偏离分析进一步表明，NH₄⁺浓度与Na⁺、K⁺、HCO₃⁻的正向偏离呈显著正相关（p<0.05），与SO₄²⁻的负向偏离呈显著负相关（p<0.05）。该研究可为立沙岛乃至类似河口岛屿地区的地下水资源开发及生态环境保护提供理论支持。

关键词: 高氨氮地下水, 同位素示踪, 厌氧氨氧化, 水化学, 海水入侵, 珠江口