Nitrate Source Tracking with Stable Isotopes — A Guide for Catchment Managers
From concentrations to causes: separating septic, manure, and fertiliser inputs using δ¹⁵N, δ¹⁸O and biological indicators.
Having reliable, accurate data is fundamental to good catchment decisions — but nitrate concentration data alone rarely tells you where nitrate is coming from or why it persists. Stable isotope analysis (SIA), especially when paired with aquatic vegetation/periphyton and simple tracers, provides a practical, defensible way to diagnose nitrate sources and underlying processes in Irish catchments.
Why are isotopes a useful tool for nitrate management?
Nitrate pollution typically arises from multiple overlapping pressures, including agriculture, septic systems, and legacy soil nitrogen. These pressures vary across space and time, and their signals can be obscured by rainfall, flow conditions, and seasonal changes in land use. As a result, routine grab sampling often struggles to identify dominant sources with confidence.
Stable nitrogen isotopes provide an additional layer of information by reflecting the processes through which nitrate is produced and cycled. When measured in aquatic vegetation or periphyton, δ¹⁵N values integrate nitrate exposure over weeks to months, smoothing short-term variability and helping identify the dominant pressures that matter most for ecological status and long-term management.
What to sample?
Effective nitrate isotope studies usually combine water samples with biological indicators. Water samples provide essential information on nitrate concentrations and short-term dynamics, and can be supplemented with nitrate isotope analysis (δ¹⁵N–NO₃⁻ and δ¹⁸O–NO₃⁻) where process-level interpretation is required.
Aquatic vegetation and attached algae, by contrast, assimilate nitrate continuously and record its isotopic signature in their tissues. This makes them particularly valuable in systems where nitrate concentrations fluctuate strongly or where chronic low-level inputs are suspected. In practice, water samples describe current conditions, while plant or periphyton samples provide a time-integrated record of recent nitrate exposure.
Where to sample?
Sampling locations should be chosen to reflect likely nitrate sources and pathways rather than convenience alone. This typically includes inflow streams and drainage channels, shoreline or river reaches adjacent to housing clusters, and areas influenced by agricultural land or field drains. Where possible, reference sites upstream or outside suspected impact zones are essential for interpreting whether observed isotope values are elevated in a local context. A well-designed spatial network does not need to be large, but it does need to span the key pressure gradients within a catchment or lake system.
When to sample?
Timing is central to interpreting nitrate isotope data, particularly when separating septic system inputs from agricultural sources. Septic-derived nitrate is often groundwater-fed and most apparent during baseflow or dry-weather conditions. Manure and diffuse agricultural inputs, in contrast, are commonly mobilised during or following rainfall events.
For this reason, many studies focus on two complementary sampling windows, or adopt a quarterly approach that captures both low-flow and high-flow conditions. Aligning sampling with hydrological conditions is often more informative than increasing sampling frequency alone.
Sample preparation
Water samples for nitrate isotope analysis require specific handling and preservation, and laboratories should be consulted early to ensure appropriate protocols are followed. Aquatic vegetation and periphyton samples are typically rinsed, dried or frozen, homogenised, and analysed for δ¹⁵N using standard EA-IRMS techniques.
Simple supporting measurements, such as nitrate concentration, conductivity, chloride, or basic nutrient chemistry, can substantially strengthen interpretation at relatively low additional cost. Atomic Ecology provides guidance on sampling protocols and can coordinate analytical workflows with accredited laboratories.
Data interpretation
Nitrate isotope results are most powerful when interpreted as part of a weight-of-evidence framework rather than as standalone indicators. Interpretation typically integrates δ¹⁵N values in vegetation or periphyton with spatial patterns, hydrological timing, and supporting chemical tracers.
Where nitrate dual isotopes are used, coupled changes in δ¹⁵N and δ¹⁸O can help identify denitrification or other in-catchment processes that modify isotope values. These process indicators are critical for avoiding misattribution of sources in wetlands, lagoons, and low-oxygen environments.
Common pitfalls (and how to avoid them)
table isotope analysis is not a silver bullet, and its effectiveness depends on appropriate study design. Local reference data are important for identifying whether δ¹⁵N values are elevated in a meaningful way. In some systems, denitrification can enrich remaining nitrate in heavy isotopes, complicating direct source interpretation unless dual isotopes or supporting evidence are available.
Clear management questions and a sampling design aligned to those questions are essential. A short scoping discussion can often prevent unnecessary sampling and improve the usefulness of results.
Working with Atomic Ecology
Working with Atomic Ecology means working directly with me, Brian Hayden, throughout the project. I support clients in designing fit-for-purpose sampling programmes, selecting appropriate analytical approaches, and interpreting results in a way that is scientifically robust and management-relevant.
Projects can range from rapid screening assessments that provide an initial indication of dominant nitrate sources, to more detailed diagnostic studies that support regulatory decision-making, mitigation planning, and stakeholder engagement. Indicative costs Costs are indicative and provided for planning and scoping purposes, though projects can tailored to management questions and available budgets.
Screening assessment
Aquatic vegetation or periphyton δ¹⁵N analysis with spatial interpretation.
Indicative cost: €6,000–8,000
Full diagnostic assessment
Adds nitrate dual isotopes and supporting tracers.
Indicative cost: €9,000–12,000
Extended assessment
Expanded spatial coverage and repeat sampling.
Indicative cost: €12,000–15,000
If you are considering how stable isotope analysis could support nitrate management in your system, I’m happy to discuss your objectives and help scope an appropriate approach.