Contribution of local factors to the status of a groundwater dependent terrestrial ecosystem in the transboundary Gauja-Koiva River basin, North-Eastern Europe

A pilot study was carried out in the Matsi spring fen, southern Estonia to test a proposed methodology (Terasmaa et al., 2020) for identifying potential effects of groundwater bodies on groundwater dependent terrestrial ecosystems (GDTE). A comprehensive hydrochemical and isotopic dataset was collected and analyzed using multivariate statistical methods to assess the origin of the groundwater discharging in the spring fen and to determine both the quantitative and qualitative aspects of GDTE-groundwater interdependence. The hydrochemical findings were complemented with the hydrogeological, hydrodynamical and botanical observations, and summarized into a conceptual model of the study site. The results show that the status of the spring fen is influenced both by groundwater inflow from the regional Middle Devonian sandstone aquifer (D2gj-ar) in the transboundary Gauja/Koiva River basin, but also by discharge of the shallow aquifer found in Quaternary (Q) deposits of local extent. By using multivariate statistical methods, the mixing of the two main groundwater sources in the spring fen discharge was quantified. On average, the regional D2gj-ar and local Q aquifer contributed 67% and 33%, respectively, to the spring fen discharge. The groundwater was generally more mineralized in the D2gj-ar aquifer while more nutrient-rich in the shallow Q aquifer. Agricultural activities in the arable lands adjacent to the spring fen have resulted in the elevated loads of macronutrients and halogens in the shallow Q aquifer (Ntot=1.95 ± 2.6 mg N/l, Ptot=0.59 ± 0.8 mg P/l). Although generally in pristine state, the elevated macronutrient loading from the Q aquifer and perhaps the beaver activity, has led to deterioration of the status of the spring fen communities in the upstream parts of the main fen polygon. The N:P ratios were occasionally above the 16:1, indicating nitrogen limitation and phosphorus excess, which is likely due to increased upland loading of phosphorus from fertilizers. The results of the study show that poor chemical status of a GDTE can result from local pollution sources and land-use even in cases when the related regional GW body itself is in good chemical status. Also, the usefulness of analyzing minor groundwater components (e.g Ba, Sr, Al, F, SiO2) in a GDTE assessment in establishing the end-member mixing relationships has been shown. All in all, it is one of the first comprehensive studies of the interactions between a groundwater body and a groundwater-dependent terrestrial ecosystem in the Estonian context.