Sources of pollution in agro-ecosystems are usually difficult to identify using pollutant concentrations alone. Most of these sources are not attributed to a specific location (non-point) and are typically in solution (non-particulate). Therefore, compound concentrations can inform about the level of pollution (low or high), temporal and spatial dispersal but cannot directly confirm the actual source and transformation of the pollution. To unambiguously identify sources of pollution, a more advanced approach is needed, e.g., using tracers that carry unique fingerprints of each potential source. Frequently stable isotope compositions of elements forming the chemical compound in pollution are used as environmental tracers. In conjunction with the major ion ratios and stable hydrogen and oxygen isotope compositions of water molecules, these isotope tracers can be successfully used to identify sources of pollutants found in surface and groundwaters (Kendall 1998; Marshall et al. 2008). The application of mass balance calculations using isotope tracers often allows for not only qualitative but also quantitative estimations of the relative contributions from different sources of pollution and the discrimination of agricultural inputs from other contaminants (Kendall 1998). The stable isotope signatures and ion ratios also provide information that is independent of solute concentrations, thereby disentangling the water budget from the pollution budget. An accurate identification of sources and transport of pollutants from soil to water in agro-ecosystems will help develop appropriate soil and water management practices to minimize agropollutants to surface and groundwaters.

Two primary challenges need to be addressed to improve our ability to monitor and quantify sources and transport of pollutants in agro-ecosystems using stable isotope tracers. The first lies in the reliable and routine use of stable isotope methods to identify pollutants and distinguish mixing from stable isotope fractionation. The second challenge lies in the comprehensive implementation of a multi-tracer approach relying on two or more elements and their stable isotope compositions to characterize and quantify sources and reactive transport of solutes in agro-ecosystems. In the present book, we address these two challenges by focusing on the stable H, C, N, O and S isotope compositions in several chemical compounds to trace and monitor sources and transport of macronutrients and micro-contaminants in soil and waters under different land uses and hydro-climatic conditions. Stable isotope tracers have great potential to improve the understanding, management and protection of surface and groundwater and to help in the qualitative and quantitative prediction of water resources. The six chapters included in this book present a critical overview of the stable isotope tracer methods forming a hydrochemical toolbox to be applied in the broad range of agro-contaminant studies. We thus anticipate that this book will stimulate researchers and managers to further develop and use hydrochemical and stable isotope techniques worldwide, complementarily to conventional monitoring and mass balance approaches.

The most commonly used analyses of the stable isotope composition are those of the solutes in water [δ(15N)NO3, δ(18O)NO3, δ(34S)SO4, δ(18O)SO4, δ(13C)DIC, δ(18O)PO4, δ(13C)pesticide, δ(15N)pesticide], the particulate organic matter [δ(13C)POM, and δ(15N)POM], together with water molecules [δ(2H)H2O and δ(18O)H2O], as well as chemical ion concentrations, and the major ion ratios (e.g., Na+/Ca2+, Cl−/SO42−, Ca2+/SO42−). All these analyses are particularly useful for tracing the dynamics of pollution contributions from different sources, usually in reference to local baselines and the original signatures of the contributing pollution sources. The major advantages of stable isotope methods are their relatively low cost and high effectiveness, even when used in areas lacking long-term monitoring or a comprehensive sampling network.

The stable hydrogen, carbon, nitrogen, oxygen and sulfur isotope compositions (HCNOS) of various compounds can be used to study agricultural pollution in surface and groundwaters by applying mass balance calculations and other models to assess the progress of dispersal or decomposition (Chap. 3). Different stable isotope tracers usually carry different and complementary information about pollution sources, water flows and mixing, as well as about pollutant chemical transformations in the aquatic environment (Chaps. 4–7). Various tracers can be combined to obtain a more unambiguous characterization of pollution sources and to disentangle water budgets from solute budgets, or they can be used independently to address different environmental questions (e.g., Szynkiewicz et al. 2015; Elsner and Imfeld 2016; Dogramaci et al. 2017; Pfahler et al. 2022). This brief overview provides a list of the commonly used stable isotope tracers discussed in this book, along with their major applications, the general ranges of the δ-values observed in nature and the expected analytical uncertainty (Coplen et al. 2002a, b). The analytical uncertainty, along with the natural variability of the δ-values in each source, needs to be considered for the correct solution of the mass balance mixing model equations. One critical consideration is whether the δ-values in the sources potentially contributing to the analysed mixture are significantly different from each other. If the δ-values of different sources are similar, models cannot be solved with high confidence to distinguish sources. On the other hand, transformation processes may be traced over space and time if δ-values in the sources are similar.

The list of tracers presented below has been arranged considering their relevance to studies of agricultural pollution in terrestrial environments. The meaning of the categories is as follows:

  • analysed in—medium in which the stable isotope compositions can be analysed.

  • range of values—an approximate typical range of the δ-values frequently observed in natural terrestrial environments.

  • uncertainty—the typical uncertainty of the stable isotope analysis given as one standard deviation; it varies, respectively to the analytical method used.

  • applications—primary applications of the tracer.

  • zero value—the reference point for the scale characterized by the δ-value equal 0 ‰.

For the full range of variation in stable isotope compositions in all environments and compounds reported around the world, see the summary by Coplen et al. (2002a, b).

FormalPara δ(2H)H2O and δ(18O)H2O—stable hydrogen and oxygen isotope compositions of water

analysed in: surface, ground, precipitation and soil water

range of values: δ(2H) − 150 ‰ to + 50 ‰ and δ(18O) − 20 ‰ to + 10 ‰

uncertainty: δ(2H) ~ 0.5–1.0 ‰, δ(18O) ~ 0.05–0.10 ‰

applications: used for tracing sources, mixing models, assessing evaporative losses of water

zero value: the stable isotope compositions of the Vienna Standard Mean Ocean Water (VSMOW) are accepted as a zero-reference point (0 ‰) for δ(2H) and δ(18O) on the VSMOW2-SLAP2 scale. Therefore, δ(2H) and δ(18O) values close to zero are typical for seawater and moderately evaporated freshwater.

FormalPara δ(15N)NO3 and δ(18O)NO3—stable nitrogen and oxygen isotope compositions of nitrates

analysed in: NO3− ions dissolved in water or NO3− in solid substances, e.g., fertilizers, salt precipitates

range of values: δ(15N) -10 ‰ to + 25 ‰ and δ(18O) -15 ‰ to + 80 ‰

uncertainty: δ(15N) ~ 0.3–0.5 ‰ and δ(18O) ~ 0.3–0.5 ‰

applications: usually used to distinguish N-input sources of surface and groundwater pollution and to assess N-contributions from precipitation, natural plant decomposition, fertilizers, manure and human wastewater; significant fractionation can be caused by the nitrification–denitrification cycle influencing stable isotope composition and substrate concentrations

zero value: the stable isotope composition of atmospheric nitrogen is accepted as a zero-reference point for the nitrogen air scale [δ(15N) = 0 ‰]. The δ15N value of atmospheric N2 is constant around the world. The δ(18O) is usually reported on the VSMOW2-SLAP2 scale, which is the same as water.

FormalPara δ(34S)SO4, δ(18O)SO4—stable sulfur and oxygen isotope compositions of sulfates

analysed in: SO42− ion dissolved in water or SO42− in solid substances, e.g., fertilizers, salt precipitates; frequently, for reference, δ(34S)SO4 is analysed as well in sulfides and various fractions in soil

range of values: δ(34S) − 25 ‰ to + 40 ‰ and δ(18O) − 20 ‰ to 30 ‰

uncertainty: δ(34S) ~ 0.2–0.4 ‰, δ(18O) ~ 0.2–0.4 ‰

applications: used to assess inputs from fertilizers and wastewater and to distinguish them from acid rock and mine drainage and atmospheric pollution

zero value: the stable isotope composition of Vienna Canyon Diablo Troilite (VCDT), a meteorite mineral, is accepted as a zero reference for the sulfur scale [δ(34S) = 0 ‰]. The δ18O is usually reported on the VSMOW2-SLAP2 scale.

FormalPara δ(13C)POM and δ(15N)POM—stable carbon and nitrogen isotope compositions of particulate organic matter suspended in water (POM)

analysed in: particulate organic matter as a suspension in water or sediments, frequently for reference also analysed in plant material, and soil, usually POM is separated on fibreglass filters by water filtration

range of values: δ(13C) − 30 ‰ to − 10 ‰ and δ(15N) − 5 ‰ to + 20 ‰

uncertainty: δ(13C) ~ 0.10 ‰ and δ(15N) ~ 0.10 ‰

applications: primarily used to estimate C-inputs from different vegetation types (especially agricultural crops) and soil erosion to surface water and to distinguish terrestrial and aquatic C-sources; also frequently used to assess C3 to C4 plant input to soil or suspended particles

zero value: the stable isotope composition of Vienna Pee Dee Belemnite (VPDB), a carbonate marine fossil, is accepted as a zero-reference point for δ(13C) stable isotope scale. Therefore, marine carbonates have δ(13C) close to zero.

FormalPara δ(18O)PO4—stable oxygen isotope compositions of phosphate

analysed in: PO43− dissolved in water, in plant material, soil, fertilizers, sediments, rocks, detergents, rivers and vegetation

range of values: + 6 to + 31 ‰

uncertainty: ~ 0.3–0.5 ‰

applications: tracing phosphate sources and phosphate turnaround time in soil and surface water to assess fertilizer leaching and turnover

zero value: as above for water.

FormalPara δ(13C)DIC—stable oxygen isotope compositions of dissolved inorganic carbon (DIC)

analysed in: water, usually as total inorganic dissolved carbon (the ratio between CO2/HCO3−/CO32− in solution depends on pH)

range of values: − 20 ‰ to + 5 ‰

uncertainty: ~ 0.10–0.20 ‰

applications: primarily used to confirm decomposition of hydrocarbon pollution and methane oxidation and their contribution to DIC; also, to assess the contribution of aquatic organisms to diel cycles of DIC; to detect contributions from deep groundwater

zero value: as above for δ(13C)POM.

FormalPara δ(13C) and δ(15N)—stable carbon and nitrogen isotope compositions of pesticide molecules

analysed in: water, soil, sediment and plants

range of values: − 35 ‰ to 0 ‰ for δ(13C) and − 4 ‰ to + 4 ‰ for δ(15N)

uncertainty: ~ 0.5 ‰ for both δ(13C) and δ(15N)

applications: primarily used to evaluate in situ degradation, without considering concentration data of parent compound or transformation products, quantify pollutant degradation and, in some cases, identify reaction pathways

zero value: the stable isotope composition of Vienna Pee Dee Belemnite (VPDB), a carbonate marine fossil, is accepted as a zero-reference point for δ(13C) stable isotope scale. The stable isotope composition of atmospheric nitrogen is accepted as a zero-reference point for the nitrogen air scale [δ(15N) = 0 ‰]. The δ(15N) value of atmospheric N2 is constant around the world.

All stable isotope results in this book are presented in permille (‰) as 1000 isotope delta, on the stable isotope scale respective to the element (e.g., VPDB, VSMOW2-SLAP2, VCDT or AIR). The isotope delta (symbol δ) is defined by the isotope ratios of heavier (i) to lighter (j) isotope of an element (E) in substance (P) and isotope ratio of this element in an international standard (Std), defining the zero point for the international isotope scale, 0 ‰ (Eq. 1.1):

$$\delta \left( {{}_{{}}^{i/j} {\text{E}}} \right)_{{P,{\text{Std}} }} = \frac{{R\left( {{}_{{}}^{i/j} {\text{E}}} \right)_{P} }}{{R\left( {{}_{{}}^{i/j} {\text{E}}} \right)_{{{\text{Std}}}} }} - 1$$
(1.1)

This full notation, for example, for stable sulfur isotope composition in sulfate, respectively to VCDT scale can be expressed as follows (Eq. 1.2):

$$\delta \left( {{}_{{}}^{34/32} {\text{S}}} \right)_{{SO_{4} ,{\text{VCDT}} }} = \frac{{R\left( {{}_{{}}^{34/32} {\text{S}}} \right)_{{{\text{SO}}_{4} }} }}{{R\left( {{}_{{}}^{34/32} {\text{S}}} \right)_{{{\text{VCDT}}}} }} - 1.$$
(1.2)

To avoid excessive repetition of common terms, we used a shorthand version of the full notation (Eq. 1.3):

$$\delta \left( {{}_{{}}^{34} {\text{S}}} \right)_{{SO_{4} }} .$$
(1.3)

The delta values are reported without corrections or adjustments, exactly as in the source publications. Readers should be aware that the isotope delta values assigned for international standards undergo occasional changes, and discrepancies may arise while pooling values published during different years. To warrant the reusability of published data, all new stable isotope results should be published following minimum requirements for publishing HCNOS stable isotope delta results guidelines (Skrzypek et al. 2022) and correct terminology and notations endorsed by the International Union of Pure and Applied Chemistry (Cohen et al. 2007; Brand et al., 2010; BIMP 2019). Each stable isotope data set should be accompanied by a description of (1) analytical procedure, (2) traceability, (3) data processing and (4) uncertainty evaluation.

Other Tools for Tracing Anthropogenic Contaminants

Other less frequently used tracers for tracing anthropogenic contaminants in water and soil are boron [δ(11B) reflecting 11B/10B isotope ratio] and strontium [δ(87Sr) reflecting 87Sr/86Sr isotope ratio]. The main anthropogenic source of boron (B) is sodium perborate (NaBO3), which is used in laundry detergents and household cleaners; thus, boron is commonly found in household sewage. Sewage treatments generally do not remove boron or cause stable isotope fractionation (Barth 2000). Hence, δ(11B) can be considered a conservative tracer for partitioning agriculture pollution (e.g., hog manure, cattle feedlot runoff and synthetic fertilizers) from household wastewater. The use of δ(11B) coupled with δ(15N)NO3 and δ(18O)NO3 has proved to be an effective means of tracing agricultural nitrate sources to distinguish between two types of sewerage that were indistinguishable using δ(15N) alone, including washing powders and animal/human manure (Guinoiseau et al. 2018; Kruk et al. 2020). Boron in household pollutants has values like boron minerals [δ(11B) = 5 to 13 ‰] significantly different from values observed in pristine groundwater (~ 30 ‰) or seawater (~ 39 ‰) (Vengosh et al. 1994; Vengosh 1998) and can be reflected in plant tissues (Chang et al. 2016).

In natural waters, the geochemistry of dissolved Sr is analogous to that of Ca. The Sr isotope fractionations in geochemical processes are considered negligible due to small differences in isotope masses (~ 1 %), and the 87Sr/86Sr ratios in natural systems mainly reflect geological age and the initial Rb/Sr ratio in the rocks (Gosselin et al. 2004). Thus, in groundwater, the 87Sr/86Sr ratio often reflects water–rock interaction time or the aquifer matrix if the groundwater contact time is long enough to attain equilibrium. Strontium isotope composition could be used for assessing salt budget and sources of salinity, particularly to partition fraction coming from rock erosion and precipitation and fertilizers (Hosono et al. 2007; Dogramaci and Skrzypek 2015).

Overall, pollutants may originate from various sources related to different types of agriculture activities, including cultivation, aquaculture, livestock and dairy farms and related food processing industries. Evaluating their respective contributions to soil, surface- and groundwater bodies remains, however, challenging. In this context, stable isotope tracers have untapped potential to quantify agro-contaminant sources, transport and transformation and distinguish them from other sources of contaminants, as presented below.