Unveiling the hydrological response of NO3-rich springs to seasonal snowmelt in a karstic carbonate upland
•Modelling of groundwater recharge influenced by seasonal snowmelt in a karstic area.•Spring functioning unraveled using time-series analysis and multivariate statistics.•Springs have long response times and solute transport dominated by piston flow.•Changes in water quality and NO3− are seasonal an...
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| Published in: | Journal of hydrology (Amsterdam) Vol. 641; p. 131724 |
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| Main Authors: | , , , , , , , , , , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
Elsevier B.V
01-09-2024
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| Online Access: | Get full text |
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| Summary: | •Modelling of groundwater recharge influenced by seasonal snowmelt in a karstic area.•Spring functioning unraveled using time-series analysis and multivariate statistics.•Springs have long response times and solute transport dominated by piston flow.•Changes in water quality and NO3− are seasonal and related to main recharge periods.•NO3− is associated both with older pre-event water and recent event water.
Groundwater in karstic aquifers can be especially vulnerable to pollution, that includes nutrients from diffuse agricultural sources. The study focuses on the functioning of three nitrate-rich springs in Estonia, north-eastern Europe. The springs are located in a catchment on an upland comprised of karstic carbonate rocks in cold temperate climate with seasonal snow cover. The upland is an important agricultural area with arable land comprising > 50 % of the land cover. The main aim of the study was to unveil the processes that cause seasonal changes in spring hydrochemistry and NO3− concentrations. The springs were monitored for over 2 years for hydrodynamic and hydrochemical parameters. In addition, a precipitation-runoff model PRMS-IV was used to model daily groundwater recharge. Modelled recharge was subsequently used to study the response of spring discharge and water quality to recharge events using spectral analysis. The hydrodynamic and hydrochemical observations together with the relevant climatic data were used to delineate intra-annual phases of spring functioning using multivariate statistical methods. We found that the electrical conductivity of spring water and NO3− concentrations increase during high-flow periods that coincide with autumn precipitation and melting of the seasonal snow cover in early spring. The NO3− originates from two distinct sources. At the start of the high-flow period in late autumn, the springs discharge pre-event water from the bedrock aquifer and NO3− is released from storage. At the end of the high-flow period, during the melting of the snow cover, NO3− is also leached from the overlying reservoirs (soil, sedimentary cover and the epikarst). The springs have a long memory and response times to groundwater recharge (2–3 months) and only show seasonal (not event-based) changes in discharge and water quality. These patterns can be explained by 1) seasonality of groundwater recharge in the area; 2) the absence of large lateral flow channels in the bedrock and 3) the importance of different reservoirs where the water is stored before discharging in the springs (bedrock, epikarst zone, sedimentary cover). The findings emphasize that the spring discharge is a mixture of water from different sources (sedimentary cover, epikarst, bedrock) and their relative importance changes seasonally. Groundwater recharge in the study area does not cause a quick transmission of water from the recharge to the discharge area but mainly the release of pre-event water stored in the subsurface reservoirs. By better understanding the functioning of these NO3-rich springs, more effective measures can be developed to protect shallow groundwater and groundwater dependent surface water ecosystems in the region. |
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| ISSN: | 0022-1694 |
| DOI: | 10.1016/j.jhydrol.2024.131724 |