Summary:
The present diploma thesis entitled "Tracing Nitrogen Sources and
Transformations by using the Nitrate dual Isotopic Composition of Groundwaters from
Atalanti, Fthiotida, Greece", was prepared within the framework of the Master's
Program "Earth Sciences & Environment" of the Department of Geology &
Geoenvironment of the National and Kapodistrian University of Athens, with
Specialization "Mineral Resources - Petrology & Environmental Management".
The purpose of the thesis is to identify the existing nitrate pollution in the wider
area of Atalanti, in the prefecture of Fthiotida and for the first time to determine the
sources of nitrate ions, through isotopic analyzes of the elements nitrogen (δ
15N-NO3-) and oxygen (δ18O-NO3-), in the molecule of nitrate ions of the groundwater of the area, as well as the processes of the isotopic synthesis and transformation of nitrogenous compounds.
The study area is defined as the wider area of the catchment area of Atalanti,
Fthiotida Prefecture. It is an intensely rural area with a relatively small urban fabric,
which nevertheless does not have an organized system for the collection and treatment
of domestic wastewater. Geotectonically, it belongs to the Sub-Pelagonic Unit, as the
basin is covered by alluvial sediments of the Quaternary, while the surrounding rocks
are Neogene argillaceous marls, greywackes - quartzites of the Paleozoic, with lenses
of peridotites and additional limestones - dolomites of Jurassic age.
Nitrate pollution is defined as the pollution of the waters of an aquifer system by
the presence of nitrate ion, above the permitted value of 50 mg L-1
for drinking water,
defined by the World Health Organization (WHO) and the corresponding European
Directive (2020/2184). The intense presence of high concentrations of nitrate ions in
the groundwater seems to be found mainly in rural areas. Both the use of nitrate and
ammonium fertilizers and manure, as well as the absence of an organized system for
the collection and treatment of domestic wastewater, are typical and common sources
of nitrate pollution.
A single sampling campaign was carried out in June 2021, i.e., during the dry
season. In total, 41 groundwater samples were collected from productive irrigation
boreholes, several of which were also used for drinking water supply purposes. The
depths of the boreholes ranged from 20 to 200 m, with an average depth of 60 m. During
sampling, in-situ parameters were determined such as pH, dissolved oxygen (DO), redox potential (Eh), electrical conductivity (EC) and total dissolved solids (TDS),
using a pre-calibrated portable multiparameter meter. All samples were analyzed for
major cations (Ca2+, K+, Mg2+, Na+) and trace elements, nutrient ions (NO3-, NO2-, NH4+, PO43-) and main anions (Cl-, HCO3-, SO42-). In addition, 32 of the 41 samples, were analyzed for isotopic composition of nitrate ions (δ15N and δ18O of NO3-). Τhe 32
samples were selected with the intention to cover the whole range of nitrate
concentration values from the lowest to the highest ones. The determination of nitrate
ions concentrations was done in the laboratory of Environmental Chemistry, of the
Department of Chemistry of NKUA, using an ion chromatography system, whereas the
samples were analyzed for nitrate isotopes at the Isotope Bioscience Laboratory –
ISOFYS, Ghent University, Gent, Belgium using a bacteria denitrification method
(Casciotti et al., 2002). The analytical uncertainties were ± 0.2 ‰ and ± 0.4 ‰ for δ15N and δ18O, respectively. Also, the rest of the chemical analyzes (ICP-OES, ICP-MS) of
the samples in major cations and trace elements, were carried out at the University of
Life Sciences Prague Faculty of Environmental Sciences, Department of
Environmental Geosciences, in the Czech Republic.
The pH ranged from 7.00 to 7.69 indicating neutral to slightly alkaline conditions.
The Dissolved O2 (DO) and Eh values ranged from 1.47 to 8.06 mg L-1 (median 6.13
mg L-1) and from -333 to 291 mV (median 89.3 mV), respectively. These indicated that
most of the samples are under oxic conditions. All chemical ion concentrations were
below the threshold values for drinking water (WHO, 2004), except for nitrate ions.
Overall, the nitrate values ranged from below detection limit (DL = 0.5 mg L-1) to 337 mg L-1
(median 68.8 mg L-1), that is, they exceeded the threshold value of 50 mg NO3-L-1
(WHO, 2004) in ~66 % of the samples. The δ15N-NO3- values in the groundwater
samples ranged between +2.0 ‰ and +14.5 ‰, with an average of 6.5±3.3 ‰, whereas
the δ18O-NO3- values were between ±0.3 ‰ and +11.0 ‰, with an average of 4.7±2.4
‰. Most of the samples (~72 %) fell within the overlapping ranges for organic N in
soil, NH4+ fertilizers, manure, and sewage effluents, indicating that multiple nitrate
sources are responsible for the nitrate contamination of the aquifer. The δ15N-NO3-
and δ18O-NO3- values exhibited spatial variability reflecting diverse N sources in the
aquifer. For example, the groundwater samples located closer to the urban area
“Livanates”, showed the highest δ15N-NO3- values, whereas those from the central part
of the basin, which is mostly agricultural, had much lower. In some samples the combination of high δ15N-NO3- values and low DO and Eh values reflecting the possible
occurrence of denitrification processes under reducing conditions in some parts of the
aquifer (Nikolenko et al., 2017). On the other hand, the samples, mainly from the central
part of the basin, showed lower δ18O-NO3-, and high DO values, which could be
indicative of nitrification processes (Nikolenko et al., 2017).
In this study, two major nitrate sources were identified in the Atalanti basin during
dry season using chemical and isotope tracers: fertilizers are mostly responsible for
nitrate pollution in the central part of the basin, whereas leaching of untreated wastes
from septic systems deteriorated the groundwater near the settlement of Livanates. The
relationship between the physicochemical parameters, such as redox (Eh) and dissolved
oxygen (DO) and nitrate isotopes lead to the conclusion that the denitrification most
likely occurs in the northern part of the basin, near the Livanates settlement, whereas
nitrification was more evidently detected in the central part. As for the high values of
electrical conductivity, for the samples near the coastline, it seems that the sea sprays
(marine aerosols) are responsible, while the high values in Cr(VI), in some samples,
seem to be of indigenous origin (ultramafic rocks), or and its occurrence is also possible
from the use of phosphate fertilizers (Vogel et al., 2020).
