| Abstract | Održivo korištenje i upravljanje zagrebačkog vodonosnika kao strateškog resursa Republike Hrvatske, između ostalog, zahtijeva i identifikaciju i kvantifikaciju svih izvora njegovog prihranjivanja. U cilju kvantifikacije prihranjivanja vodonosnika iz oborine, kao jednog od izvora prihranjivanja, izrađeno je nekoliko je numeričkih dvodimenzionalnih modela tečenja vode i trasiranja čestica kroz nesaturiranu zonu zagrebačkog vodonosnika na području vodocrpilišta Velika Gorica. Pri definiranju geometrije, parametara, početnih i rubnih uvjeta, te za kalibraciju modela korišteni su podaci prikupljeni na istraživačkom poligonu koji se nalazi u prvoj zoni sanitarne zaštite zdenca B-1 vodocrpilišta. Na poligonu se kontinuirano mjere parametri nesaturirane zone pomoću TDR (engl. Time Domain Reflectometry) sondi te meteorološki uvjeti, a izvedena su i geofizička istraživanja te nekoliko eksperimenata trasiranja. Rezultati prognoznog modela za razdoblje od 8. 11. 2019. do 8. 5. 2023. utvrdili su količinu prihranjivanja vodonosnika iz oborine od 16 % i brzinu procjeđivanja oborine od tri godine i tri mjeseca, od trenutka infiltracije na površini terena do razine podzemne vode. Prognoznim modelom za buduće razdoblje (2019. - 2040.) predviđena je dvostruko veća količina procjeđivanja od 36 % što je u skladu s povećanjem učestalosti oborine u razdoblju do 2040. godine. Veća učestalost oborine uzrokovala je i kraće vrijeme procjeđivanja oborine, odnosno godinu dana i četiri mjeseca. Modelima tečenja, kontinuiranim mjerenjima parametara nesaturirane zone na istraživačkom poligonu i eksperimentima trasiranja utvrđeni su faktori koji utječu na količine i brzine procjeđivanja vode u nesaturiranoj zoni: učestalost, količina i intenzitet oborine, sadržaji vode u nesaturiranoj zoni neposredno prije oborinskog događaja, zatim heterogenost naslaga te pojava preferencijalnog tečenja. Pouzdano definiranje rubnog uvjeta infiltracije vode iz oborine do podzemne vode omogućava unaprjeđenje regionalnih numeričkih modela tečenja podzemne vode, a model tečenja vode kroz nesaturiranu zonu temelj je za buduće modele pronosa onečišćenja s površine do podzemne vode. |
| Abstract (english) | Sustainable management of the Zagreb aquifer, as a strategic resource of the Republic of Croatia, requires identifying and quantifying all sources of its recharge. In order to identify and quantify the recharge from precipitation, as one of the sources of recharge, a numerical two-dimensional model of water flow through the unsaturated zone was built in the area of the Velika Gorica wellfield. A research polygon, consisting of a meteorological station and a shaft, has been established at the Velika Gorica wellfield, where the parameters of the unsaturated zone are continuously measured at different depths using TDR (Time Domain Reflectometry) probes. Geophysical measurements were performed on the surface next to the shaft itself, and the method was electrical resistivity tomography. The measurements performed at the research polygon were used to define the unsaturated flow model geometry, parameters, initial and boundary conditions, and also for model calibration. The results of the flow model for the period from 11/8/2019 to 5/8/2023 pointed out that the precipitation contributes to aquifer recharge by 16 % amount in average. However, that amount varies greatly from year to year. For example, in the one-year periods from 11/8/2019 to 11/8/2020 and from 11/8/2020 to 11/8/2021, the aquifer recharge from precipitation amounts 20 and 19 %, while in the period from 11/8/2021 to 11/8/2022, that percentage is only 2%. The small amount of aquifer recharge from precipitation in 2022 is the result of relatively longer previous dry periods. In 2020, 17% less precipitation occurred than the multi-year average, in 2021 26% less, and in 2022 even 30% less. On top of that, 2022 was an above-average dry year. The most significant amounts aquifer recharge from precipitation occurs at the end and beginning of the year, that is, from December to April, depending on the hydrological year. The periods of significant aquifer recharge from precipitation coincide with the periods of high groundwater levels, high water contents in the unsaturated zone and low values of evapotranspiration. The results of the particle tracking model for the period from 11/8/2019 to 5/8/2023 showed a relatively long travel time of precipitation from the infiltration moment on the ground surface to the moment it reaches the groundwater, i.e., 3 years and 3 months. The factor that most affects the rather slow percolation of precipitation is dry hydrological conditions, that is, below-average amounts of precipitation and their unfavorable distribution (a small number of hours with precipitation). Other factors that affect precipitation percolation and cannot be ignored considering the research location are the heterogeneity of the observed unsaturated zone profile the appearance of preferential flow. The water flow forecast model for the future period (2019 – 2040) predicts a 36 % as the amount of aquifers recharge from precipitation, which is twice as much as the amount simulated by the model for the period from 11/8/2019 to 5/8/2023, which is in accordance with the predicted increase of the precipitation frequency in the future climate. Accordingly, particle tracking model simulated a much shorter travel time of precipitation through the unsaturated zone, i.e. 1 year and 4 months. Unsaturated flow models, continuous measurements of parameters of the unsaturated zone, as well as tracer experiments, showed that the amount of aquifer recharge from precipitation and the travel time of precipitation in the unsaturated zone from the ground surface to the groundwater is affected by the frequency, amount and intensity of precipitation, the water contents in the unsaturated zone before the precipitation event, then the heterogeneity of the observed unsaturated deposit and the appearance of preferential flow. Knowledge of the amounts and rates of precipitation and also of the percolation precipitation process through the unsaturated zone enables the quantification of aquifer recharge from precipitation, which further enables the improvement of regional groundwater flow models, by reliable definition of the boundary condition represented by aquifer recharge from precipitation. Additionally, the unsaturated flow model represents the basis for future models of contaminant transport through the unsaturated zone. |
