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Abstract

All water bodies, either above or below the surface, are in constant mutual interaction.
Therefore, if we are to tackle successfully environmental issues such as water scarcity and
quality, both of which are global concerns, it is necessary to understand the interactions
that occur between groundwater and surface water bodies and to treat them as a single
system. This is also crucial in implementing properly the European Union water policy
established in the EU Water Framework Directive, the purpose of which is to improve
chemical and quantity status of surface and groundwater bodies.
The aim of this doctoral dissertation was to investigate the patterns of nutrients and
other hydrological issues affecting groundwater and river water in order to identify stressors
and the impact that they have on groundwater in an alluvial aquifer and on river water.
The work goes further, since by understanding groundwater and surface water as a single
system it is possible to explain the impact that these stressors have on groundwater-surface
water interactions. The thesis looks at two case studies, the Sava River Basin (SRB) and a
small aquifer within the SRB—the Ljubljansko polje, which is located in central Slovenia
and is an important source of drinking water supply for the local population.
This research took an interdisciplinary approach to understanding groundwater-surface
water interactions by combining geochemistry, hydrology, statistics, and by modeling percolation
and river water and groundwater flow. It is important to stress that a combination
of the various different tools used in this thesis creates a clear picture of the water cycle
in the study area in space and time––not only in the past, but also in the future. First,
the long-term data for the Sava River reveals a decreasing nutrient content. The ranges of
isotope values (δ15NNO3 and +δ18ONO3) suggest that nitrates in the Sava River water derive
mainly from soil nitrification, sewage and/or manure—a finding supported by microbial
source tracking. Second, multiple-tools were used to develop a conceptual model of the
Ljubljansko polje aquifer system. The δ18O and δ2H values reveal the diverse impacts
that the Sava River water and local precipitation have on the aquifer. According to a statistical
analysis of the hydrological and meteorological chronological data, such impacts
are noticeable within 24 hours at Jarški prod or up to a month later at Klece, the time
difference being due to pressure changes in the aquifer. Additionally, a short (up to 8-year)
groundwater mean residence time was estimated using 3H and 3H/3He methods. The thesis
also involved the comprehensive numerical modeling of the steady and transient state of
groundwater flow using FEFLOW, which was coupled with MIKE 11 and WaSiM software
for modeling the Sava River water flow and percolation, respectively. This comprehensive
modeling framework was also used to predict the groundwater and surface water behavior
for the period from 2036 to 2065. This period was defined based on climate projections
using the three regional climate models SMHI-RCA4, KNMI-RACMO22E and CLMcom-
CCLM4-8-17 provided through the EURO-CORDEX initiative. Modeling shows that the
Ljubljansko polje will retain sufficient water capacity and will not suffer due to water
scarcity in the period 2036–2065.
This thesis describes a set of important tools that provide us with a deeper under standing of water cycling in the Ljubljansko polje and partly in the SRB. The same tools
can be further applied in water protection and management strategies in the future.