Nitrate contamination of groundwater remains a major concern despite all the measures and efforts undertaken over the last decades to protect water resources. We focused on a small catchment in Brittany (France) facing nitrate pollution with concentrations over the European drinking water standard of 50 mg.L⁻¹. This is a common situation in catchments where - supposedly effective - measures were applied for reducing the transfer of N to groundwater. At the scale of this small (~100 ha) basement aquifer, nitrate concentrations are very heterogeneous in the groundwater, sampled up to 15–20 m below the soil surface in several observation wells (hereafter referred as piezometers) and up to 110 m deep in a borehole drilled through a faulted area near the Spring (outlet of the catchment). We used complementary and robust approaches for exploring and constraining the driving parameters of nitrate transfer and distribution in groundwater. Detailed geological work and a geophysical electrical resistivity tomography survey identified the lithologies, tectonic structures and weathering layers. This highlighted a complex geological structure with several compartments delimited by faults, as well as the highly variable thickness of the weathered layer. It also illustrated the heterogeneity of the hydrosystem, some compartments appearing to be disconnected from the general groundwater flow. This was confirmed by geochemical analyses and by the mean apparent groundwater residence time based on CFCs-SF6 and noble-gas analyses, locally revealing old and nitrate-free groundwater, and very old water with a recharge temperature below than the current average temperature in the area, reflecting water dating back to the last period of glaciation (−19 to −17 ky). Nitrate isotopes clearly showed denitrification processes in a few piezometers, which was generally supported by microbiology and molecular biology results. This highlighted the presence of functional genes involved in denitrification as well as a capacity of the groundwater microbial community to denitrify when in situ conditions are favourable. This type of combined approach - covering chemistry, isotopic methods, dissolved gases, microbiological activity, geophysics and hydrogeology - appears to be indispensable for implementing the most relevant programme of measures and for accurately assessing their effectiveness, notably by considering the timeframe between implementation of the measures and their impact on groundwater quality.

尽管过去几十年采取了所有措施和努力来保护水资源，但地下水的硝酸盐污染仍然是一个主要问题。我们专注于布列塔尼（法国）一个面临硝酸盐污染的小集水区，其浓度超过欧洲饮用水标准 50 mg.L ^-1. 这是在集水区常见的情况，在这些集水区采取了据称有效的措施来减少氮向地下水的转移。在这个小型（约 100 公顷）地下含水层的规模上，地下水中硝酸盐的浓度非常不均匀，在几个观察井（以下称为渗压计）中采样到土壤表面以下 15-20 m，深度可达 110 m在穿过 Spring（集水区出口）附近断层区域的钻孔中。我们使用互补和稳健的方法来探索和约束地下水中硝酸盐转移和分布的驱动参数。详细的地质工作和地球物理电阻率层析成像调查确定了岩性、构造构造和风化层。这突出了一个复杂的地质结构，其中有几个由断层界定的隔间，以及风化层厚度变化很大。它还说明了水系统的异质性，一些隔间似乎与一般地下水流断开。地球化学分析和基于 CFCs-SF6 和惰性气体分析的平均表观地下水停留时间证实了这一点，局部揭示了陈旧且不含硝酸盐的地下水，以及补给温度低于当前平均温度的非常陈旧的水区域，反映了可追溯到最后一次冰川期（-19 至 -17 ky）的水。硝酸盐同位素在一些渗压计中清楚地显示了反硝化过程，这通常得到了微生物学和分子生物学结果。这突出了参与反硝化作用的功能基因的存在以及地下水微生物群落在原位条件有利时反硝化作用的能力。这种类型的组合方法——涵盖化学、同位素方法、溶解气体、微生物活动、地球物理学和水文地质学——似乎对于实施最相关的措施计划和准确评估其有效性是必不可少的，特别是通过考虑实施之间的时间框架措施及其对地下水质量的影响。