Hydrochemical signatures of springs for conceptual model development to support monitoring of transboundary aquifers

Neighboring states sharing transboundary aquifers should carry out joint assessment of the common groundwater resources to fulfill the EU Water Framework Directive's and Water Convention's aims. Therefore, the establishment of a representative cross-border groundwater monitoring network is essential. The transboundary catchments of Estonia (EE) and Latvia (LV) are sparsely populated and feature a relatively scarce monitoring network. Springs are natural groundwater outflows that may represent a significantly greater catchment area than monitoring wells, and their monitoring is more cost-effective. But a thorough evaluation is required to select the most representative springs for particular groundwater bodies/transboundary aquifer systems. In this study, 59 springs were investigated in the EE-LV transboundary area for 37 hydrochemical parameters. Additionally, we assessed 32 monitoring wells to define the aquifer system end-members. In total 409 groundwater samples were analyzed. The sampled springs were pre -classified to one of the three aquifer systems: Quaternary (Q), Upper-Devonian (D3) and Middle/Upper-Devonian (D2). Significant differences among the pre-classified groups in terms of spring elevation, Q thickness and discharge were detected. Multivariate and machine learning techniques implementing barium as a tracer, were applied to link the studied springs to their main contributing aquifer systems. This study shows that the application of diverse hydro -chemical and statistical methods help to evaluate the sources of spring water in an area with relatively homogenous groundwater chemistry. The developed conceptual models provide new generalized interpretation of transboundary aquifers, needed to improve groundwater monitoring networks in data-scarce areas using springs.