Case study · Publication · Reactive transport

Why nitrate degrades at a constant rate in aquifers

Nitrate removal in aquifers involves flow, mass transfer between water and sediment, and microbial growth — yet at field sites the degradation rate often looks strikingly simple: a constant, zero-order rate, independent of how much nitrate is present. Practitioners use that constant every day; the theory said it shouldn’t be that simple.

What we did

We derived the steady-state concentrations of biomass, electron acceptors and electron donors in closed form, for denitrification fueled by organic carbon released from the sediment matrix, and confirmed the result with full numerical simulations. The math shows the bottleneck: the microbially mediated hydrolysis step that releases the electron donor from the matrix limits the whole chain. Neither the biomass nor the reduction rate depends on the nitrate concentration — the electron-yielding capacity of the matrix sets the pace.

The numerical model in action (video S2 of the paper): electron acceptor, reaction rate, electron donor and biomass along the flow path — the transient profiles converge to the closed-form analytical prediction (dashed red).

Why it matters

It gives a mechanistic justification for the zero-order rates used in site assessments — and tells you what to measure to predict them: the processes controlling microbial access to solid-phase electron donors, not the nitrate itself. The result applies to any dissolved electron acceptor reacting with donors released from the matrix.

Cantarella, V., Mellage, A., & Cirpka, O. A. (2025). The Electron-Yielding Capacity of the Matrix Can Explain Apparent Zero-Order Reduction of Electron-Acceptors in Aquifers at Steady State. ACS ES&T Water, 5(4), 1889–1896. Read the paper (open access) →