Abstract
Exploring the interaction between precipitation, surface water, and groundwater has been a key subject of many studies dealing with water quality management. The Varaždin aquifer is an example of an area where high nitrate content in groundwater raised public concern, so it is important to understand the aquifer recharge for proper management and preservation of groundwater quality. The NW part of the Varaždin aquifer has been selected for study area, as precipitation, Drava River, accumulation lake, and groundwater interact in this area. In this study, groundwater and surface water levels, water temperature, water isotopes (2H and 18O), and chloride (Cl−) were monitored in precipitation, surface water, and groundwater during the four-year period to estimate groundwater recharge. Head contour maps were constructed based on the groundwater and surface water levels. The results show that aquifer is recharged from both Drava River and accumulation lake for all hydrological conditions–low, mean, and high groundwater levels. The monitoring results of water temperature, chloride content, and stable water isotopes were used as tracers, i.e. as an input to the mixing model for estimation of the contribution ratio from each recharge source. The calculation of mixing proportions showed that surface water is a key mechanism of groundwater recharge in the study area, with a contribution ratio ranging from 55% to 100% depending on the proximity of the observation well to surface water.
Highlights
Received: 19 November 2021Groundwater is a vital part of the hydrological cycle, as billions of people use groundwater for drinking worldwide
All observed parameters show similar values for both surface waters, as it is essentially the same water flowing from the Drava River into the accumulation lake Varaždin
Lower mean Cl− values are associated with wells situated closer to surface waters, while higher mean values are attributed to wells further away from surface waters and/or near the road
Summary
Groundwater is a vital part of the hydrological cycle, as billions of people use groundwater for drinking worldwide. Accurate estimation of groundwater recharge is extremely important for proper management of groundwater systems [1]. Groundwater recharge can be diffuse (from atmospheric precipitation that occurs quite uniformly over large areas) or focused (from surface water bodies such as streams, lakes, lagoons) [2,3]. Various methods are used to estimate the groundwater recharge, such as direct measurements of water level fluctuations, water budget methods, empirical relations, tracer techniques, and numerical modeling [4,5]. The application of multiple methods reduces the uncertainty of individual methods and improves the reliability of the overall recharge assessment. The Varaždin aquifer is a paramount source of drinking water for approximately
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