Abstract
The general perception has long been that lake eutrophication is driven by anthropogenic sources of phosphorus (P) and that P is immobile in the subsurface and in aquifers. Combined investigation of the current water and P budgets of a 70 ha lake (Nørresø, Fyn, Denmark) in a clayey till-dominated landscape and of the lake’s Holocene trophic history demonstrates a potential significance of geogenic (natural) groundwater-borne P. Nørresø receives water from nine streams, a groundwater-fed spring located on a small island, and precipitation. The lake loses water by evaporation and via a single outlet. Monthly measurements of stream, spring, and outlet discharge, and of tracers in the form of temperature, δ18O and δ2H of water, and water chemistry were conducted. The tracers indicated that the lake receives groundwater from an underlying regional confined glaciofluvial sand aquifer via the spring and one of the streams. In addition, the lake receives a direct groundwater input (estimated as the water balance residual) via the lake bed, as supported by the artesian conditions of underlying strata observed in piezometers installed along the lake shore and in wells tapping the regional confined aquifer. The groundwater in the regional confined aquifer was anoxic, ferrous, and contained 4–5 µmol/L dissolved inorganic orthophosphate (DIP). Altogether, the data indicated that groundwater contributes from 64% of the water-borne external DIP loading to the lake, and up to 90% if the DIP concentration of the spring, as representative for the average DIP of the regional confined aquifer, is assigned to the estimated groundwater input. In support, paleolimnological data retrieved from sediment cores indicated that Nørresø was never P-poor, even before the introduction of agriculture at 6000 years before present. Accordingly, groundwater-borne geogenic phosphorus can have an important influence on the trophic state of recipient surface water ecosystems, and groundwater-borne P can be a potentially important component of the terrestrial P cycle.
Highlights
Eutrophication poses a major threat to freshwater lake ecosystems worldwide [1,2,3] and is assumed to be controlled by input of phosphorus (P) [4,5]
Even in cases where groundwater only makes up a minor part of the lake’s annual water budget, it can still control the trophic state of the lake, if the discharging groundwater contains a high concentration of P [6,20,21]
The electrical resistivity tomography (ERT) profiles indicated that the deep-lying sand layer appears to be connected to the terrain through a thin sloping sand layer
Summary
Eutrophication poses a major threat to freshwater lake ecosystems worldwide [1,2,3] and is assumed to be controlled by input of phosphorus (P) [4,5]. The general perception is that surface and near-surface hydrological pathways are the main transport routes for external P. This is due to the common assumption that P is retained readily in the aquifer sediments by adsorption and metal complex precipitation, predominantly in the oxic zone [10,11,12]. There has been increasing awareness that P-rich groundwater can play an important role regarding the trophic state of lakes [14,17,18,19] especially for groundwater-fed aquatic systems [8,9,17,18]. Even in cases where groundwater only makes up a minor part of the lake’s annual water budget, it can still control the trophic state of the lake, if the discharging groundwater contains a high concentration of P [6,20,21]
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