Short-term nitrate variations in streams are common, but the extent of groundwater contributions to such variability has been unclear. Analysis of well, spring and stream nitrate concentrations plus well water levels and stream flow for a chalk aquifer in Dorset (UK) shows that nitrate variation in streams lags discharge peaks by days to weeks. Spring monitoring also shows a short lag time, showing that high nitrate concentrations are rapidly transmitted from aquifers to streams. The lag time reflects the difference between the celerity (pressure wave speed) and the advective velocity of groundwater flow through the fracture network. Results give celerities of ∼500 m/d, advective velocities of 100–200 m/d, and kinematic porosities of ∼0.001. In addition, long-term nitrate data gives time lags of several decades, reflecting the matrix porosity of ∼0.3. Consequently, nitrate can be used an environmental tracer for both preferential flow through fracture networks and for retarded matrix flow and storage in this dual-porosity bedrock aquifer. Dual porosity is common in bedrock aquifers, so it is possible that this methodology may be widely applicable to understand the hydraulic characteristics of bedrock aquifers and to explain short-term nitrate variation in streams.

中文翻译：

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季节性硝酸盐变化作为基岩含水层优先流的示踪剂


溪流中硝酸盐的短期变化很常见，但地下水对这种变化的贡献程度尚不清楚。对英国多塞特郡白垩含水层的井、泉水和溪流硝酸盐浓度以及井水位和溪流流量的分析表明，溪流中硝酸盐的变化滞后于排放峰值数天至数周。春季监测也显示出短暂的滞后时间，表明高浓度的硝酸盐迅速从含水层转移到溪流中。滞后时间反映了地下水流通过裂缝网络的速度（压力波速度）与平流速度之间的差异。结果显示速度为 ∼500 m/d，平流速度为 100-200 m/d，运动孔隙度为 ∼0.001。此外，长期硝酸盐数据给出了数十年的时间滞后，反映了~0.3的基质孔隙率。因此，硝酸盐可用作环境示踪剂，用于通过裂缝网络的优先流动以及在该双孔隙基岩含水层中延迟基质流动和储存。双孔隙度在基岩含水层中很常见，因此该方法可能广泛适用于了解基岩含水层的水力特征并解释溪流中的短期硝酸盐变化。