Transport of geogenic phosphorus to a groundwater-dominated eutrophic lake

Abstract

The estimation of groundwater-driven lacustrine dissloved inorganic phosphorus (DIP) loading is a challenging task associated with uncertainties related to simplifications used in the empirical and numerical methods. A multi-disciplinary approach can result in a better understanding of a system and therefore the improved estimate of external DIP loading into lakes. Groundwater discharge and DIP loading to a naturally eutrophic lake were estimated using three methods based on field measurements: (1) the lake segment approach, (2) three-dimensional (3D) flow and transport modelling, and (3) integrating mapped water chemistry with the 3D flow model. Field measurements and 3D flow modelling show the groundwater contribution to the lake water budget 75–81%. The in situ measured spatial distribution of the groundwater discharge rate, based on seepage meters and a conservative tracer (Cl−) budget approach, is well reproduced by the 3D flow model. DIP fluxes between the aquifer and the surface water vary between 220 and 351 kg yr−1. The integrated approach is associated with the lowest degree of uncertainty and therefore predicts the spatial distribution of the mass loading into the lake with the highest degree of accuracy. 3D hydrological information is crucial to estimate the magnitude of DIP transport. Despite the high measured and simulated DIP concentration in groundwater beneath gyttja covering the lake bottom, the actual contribution of this area into annual groundwater-driven DIP loading is low due to low groundwater discharge rates. DIP loading is concentrated instead in the narrow littoral high discharge zones. Low release rate of DIP within the old lake bottom (0.14 µg L−1) predicted with the use of the zero-order release term in MT3DMS results in a high average DIP concentration in the groundwater (60 µg L−1) and is one of the major factors responsible for the long-term lake eutrophication.