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.
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