Spatiotemporal assessment of humic substance-rich stream and shallow karst aquifer interactions in a boreal catchment of northern Estonia

Abstract

Artificial and natural tracers have been frequently used to study the water interactions in the river-karst aquifer continuum. Dissolved organic matter (expressed in the form of total or dissolved organic carbon) has proven to be an effective soil-related parameter when studying the recharge behavior of karst aquifers in many cases. In this study we discuss the use of more catchment-specific dissolved organic matter groups like humic substances to characterize the hydrodynamic and chemical interactions of the sinking Tuhala River and the shallow Nabala-Rakvere karst aquifer hosting the Tuhala karst system in a boreal catchment of northern Estonia. A network of 15 surface water and groundwater monitoring points was sampled for humic substances, major ions and stable isotopes for seven field campaigns (138 samples in total) under different hydrological conditions over the period of one year. In the context of relatively homogenous major ion chemistry and stable isotope signatures, humic substances proved to be an effective conservative tracer for delineating and characterizing the mixing of the waters of the Tuhala River and shallow aquifer in the karst system. A two-component mass balance mixing model utilizing humic substances was used to evaluate the mixing fractions in the main perennial spring group of the karst system and in other relevant monitoring points if possible. It was deduced that on average 88 ± 16% of the recharge for the main perennial spring group of the karst system was provided by the Tuhala River. In the case of the wet September of 2017, only 54% of the spring discharge originated from the river, i.e. 46% could have been provided by the hosting aquifer. The river-aquifer mixing fractions were found to be in good agreement (R2 = 0.7; n = 7; p = 0.019) with the ratio of intersection of the potentiometric surfaces (hydraulic gradient ratio) of the karst system and the hosting karst aquifer, suggesting their significant hydraulic interdependence.