地下水硝酸盐污染是国内外学者共同关注的重要议题和热点话题。近年来实施的压采、人工补水等地下水位恢复工程进一步强化了水位上升区地下水硝酸盐变化机理及污染防控的重要性。目前大部分野外监测和模拟研究工作强调了水位上升后通过包气带淋滤过程带来的物理性增加，忽略了水位上升引起的地球化学条件的改变。在此背景下，我们在北京地区选择地下水位恢复的永定河流域，综合地下水位监测、水化学、同位素和微生物等技术手段，探讨了水位上升条件下地下水硝酸盐衰减的主要机理及影响因素，研究成果可为地下水硝酸盐防控和治理提供一定的科学指导。主要研究成果如下：

（1）研究区水化学类型以Na-SO₄型为主。氘氧同位素数据表明研究区潜水含水层在补给过程中表现出蒸发信号，承压水含水层与地表水力联系较弱，说明承压水含水层具有较好封闭性，能为反硝化作用提供厌氧场所。从水化学数据来看，NO₃⁻和DO沿着地下水流向，表现为先增加后减少的趋势，进一步证实了承压水含水层可能发生反硝化作用。地下水样品测得的氮氧同位素值与粪肥与生活污水和土壤有机氮的预测范围重叠，根据简单同位素混合模型估算粪肥与生活污水占92.0%，土壤有机氮占8.0%。扣减反硝化所贡献的比例后得到结果表明粪肥与生活污水占44.5%，土壤有机氮占55.5%。

（2）氮氧同位素数据与微生物群落结构及功能数据揭示了空间尺度上承压水含水层显著的反硝化作用。下游承压水含水层样品中δ¹⁵N-NO₃⁻值与NO₃⁻浓度呈负相关，斜率为-0.05(R²=0.48, p<0.05)。此外，在承压水含水层中δ¹⁵N-NO₃⁻与δ¹⁸O-NO₃⁻值（εN/εO比值）之间有明显的关系，斜率为2.2:1。根据Rayleigh分馏方程计算空间尺度的富集系数为-5‰。微生物16S rDNA测序结果表明地下水中反硝化作用占是氮总转化功能的28.3%以上。确定了10种反硝化功能基因，其相对丰度顺序为：narH、narG、narL、norB、nosZ、nirK、norC、napA、napB、nirS。冗余分析表明DO和NO₃^-浓度（尤其是NO₃^-浓度）是控制反硝化功能基因丰度的重要因素（p<0.05）。且NO₃^-浓度与反硝化相关功能基因的相对丰度呈现正相关关系（R²=0.90），与同位素结果互相验证。

（3）地下水1976-2022年长时间序列硝酸盐变化与定点月尺度硝酸盐变化展示了反硝化作用在时间尺度上对硝酸盐衰减的重要作用。J9站点1976-2022年长时间序列的NO₃⁻浓度结果显示，2017-2022年地下水位回升8m左右，而NO₃⁻浓度明显下降。氚同位素结果显示研究区地下水年龄在大致在35~45年。J9与J7站点月尺度数据δ¹⁵N-NO₃⁻与δ¹⁸O-NO₃⁻值具有显著的统计学相关性，εN/εO比值分别为1.4:1和0.9:1，计算其时间尺度上的富集系数为-27.6‰。分析硝酸盐年际变化规律发现NO₃⁻浓度呈现出季节性变化明显的两个高峰：2020年9-10月和2021年3-4月，推测硝酸盐随着降水间歇式补给而间歇式输入，但这一观点仍需要进一步验证。

本研究提供了地下水位上升区含水层中硝酸盐衰减机制的同位素和微生物证据。以往在硝酸盐输入有限的实地调查研究中，同位素与微生物两种方法的共同佐证或相互支撑尚不多见。希望后续研究能结合同位素和微生物方法在多个条件不同的类似地点从不同时间尺度角度，阐明含水层中硝酸盐衰减过程，以最大限度地发挥这些技术的优势。

Groundwater nitrate pollution is an environmental issue and topic of concern all over the world. The impact of groundwater dynamics on nitrate variations has recently received much attention, especially after the successful adoption of measures to minimize impacts from groundwater exploitation. Most field studies conducted in areas with rising groundwater levels have found an increase in nitrate concentration owing to nitrate leaching, but they have not investigated the possible biogeochemical processes. In this study, a case study from the Yongding River Basin in Beijing with an apparent groundwater-level recovery trend over the past decade was selected to assess and quantify nitrate attenuation. This study advances our understanding of biologically mediated nitrate fate in aquifers that have undergone groundwater-level recovery. The findings can be used to guide policymakers in their nitrate contamination remediation efforts. Groundwater level monitoring, water chemistry, dual nitrate isotopes, and microorganisms were used and the results are as follows:

(1) The hydrochemical type of the study area is mainly Na-SO₄ type. The δ¹⁸O-H₂O and δ²H-H₂O values indicate evaporation during recharge for samples in the unconfined aquifer and a weak hydraulic connection with surface water for samples in the confined aquifer. This supports a possible anaerobic field for denitrification of the confined aquifer. The nitrate concentrations and DO concentrations exhibited an increasing trend in the unconfined aquifer and a remarkably decreasing trend in the downstream confined aquifer, further indicating denitrification in the confined aquifer. The measured dual nitrate isotopic values in groundwater overlapped with the predicted ranges from human/animal waste N with 92.0% estimated from a simple isotope mixing model 44.5% after eliminating the proportion from denitrification.

(2) Significant denitrification in the confined aquifer was revealed using the dual nitrate isotopes combined with microbial functional and taxonomic evidence on the space scale. A negative correlation between δ¹⁵N-NO₃⁻ values and the NO₃⁻ concentration with a slope of -0.05(R²=0.48, p<0.05) in downstream confined aquifer samples. Furthermore, there was a significant relationship between δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ values (εN/εO ratio) in the confined aquifer with a slope of 2.2:1. The spatial ε value was calculated to be -5.0‰. Denitrification was responsible for over 28.3% of the total nitrogen transformation functions. 10 functional genes involved in denitrification were identified with an order of relative abundance as follows: narH, narG, narL, norB, nosZ, nirK, norC, napA, napB, nirS. NO₃⁻ and DO, especially NO₃⁻, were identified as important contributors to regulating denitrification gene abundance (p<0.05). Interestingly, the abundances of denitrification functions identified through high-throughput sequencing analysis exhibited a strong positive linear relationship with nitrate concentration (R²=0.90).

(3) A long-term series of NO₃⁻ concentrations from 1976 to 2022 and monthly data highlighted denitrification in aquifers with a rising groundwater level plays an essential role in nitrate attenuation on the temporal scale. A long-term series of NO₃⁻ concentrations in site J9 from 1976 to 2022 has shown an obvious decline since 2017, as the groundwater level recovered by 8.0 m in the past seven years. The tritium ages in the confined aquifer were estimated to be 35~45 years. There were statistically significant correlations between δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ in the monthly samples from sites J9 and J7, with εN/εO ratios of 1.4:1 and 0.9:1, respectively. The enrichment coefficient (ε) for denitrification is -27.6‰ on the temporal scale. A pronounced seasonality was exhibited, with two NO₃⁻ concentration peaks beginning in September/October and March/April, suggesting the discrete input with groundwater recharge.

The present study presents isotopic and bacterial evidence of nitrate attenuation in an aquifer undergoing groundwater storage recovery; this type of evidence has rarely been observed in previous field investigations where there has been limited nitrate input. Both isotopic and bacterial approaches on multiple time scales at several similar sites with varying conditions should be applied to maximize the strengths of these tools to elucidate the nitrate attenuation processes in aquifers.