It is a well-established problem that phosphorus (P) enrichment of freshwater ecosystems initiates eutrophication and thus deterioration of the water quality. Transport of P is often associated with surface hydrological pathways, due to the common assumption that P is immobilized in the vadose zone during percolation. However, opposing this common perception, emerging studies suggest that groundwater may contain elevated P concentrations, subsequently posing an important and unrecognized threat to freshwater ecosystems. This Ph.D. study explores this under-investigated issue, both by detailed field investigations and method development of the application of stable ¹⁸O in orthophosphate (δ¹⁸O_P) as a tracer for P origins in freshwater.
The study presents research results from Nørresø, a Danish hypereutrophic lake. A yearlong record of the lake’s water levels, stream and spring discharges, and geochemical parameters was sampled and used to determine the water balance of the lake. The relative importance of different sources of external waterborne P to the lake was inferred from this data, combined with P measurements. Overall, the results showed that even though groundwater only accounts for about half of the total water contribution, it is still by far the biggest external contributor of P entering the lake.
Enriched P concentrations in groundwater can mainly be attributed to two mechanisms:
i) A downward migration of contaminating P, originating from anthropogenic surficial deposited P, or ii) Mobilization of geogenic P within the aquifer. Nevertheless, geogenic P is only rarely considered and documented, especially when it comes to groundwater-P
transported to freshwater ecosystems.
The aquifer in this study, which is locally discharging groundwater into the lake, is confined by a thick layer of clay. This suggests that the elevated P concentration in the groundwater has a geogenic origin, as it was deemed unlikely that surface deposited P has percolated down to the aquifer. This assumption is supported by paleolimnological data of the lakebed sediment, indicating that Nørresø has always been meso-eutrophic, even before
the introduction of agriculture at about 6000 years BP. Thus, we presumed the lake to be naturally eutrophic.
By comparing the P internally released from the lake sediment with the total external P contribution, we found internal-phosphorus cycling to be the immediate controller of the trophic state of the lake. Yet, we demonstrated that geogenic groundwater-borne P is the decisive cause of P accumulation in the sediment.
The findings from Nørresø underline the significance of the impact of groundwaterborne geogenic P on the trophic state of freshwater ecosystems, and highlights the necessity of taking this into consideration when designing effective mitigation and restoration measures.
The ability to identify where the P originates from is equally essential for this mitigation and restoration. The concentration of δ¹⁸O_P has been suggested as a possible tracer for the origin of P. However, currently there are still some methodological challenges involved in accurately determining δ¹⁸O_P. This is especially true when it comes to its applicability for freshwater ecosystems. As a part of the Ph.D. research, numerous samples were collected and tests conducted to investigate the potential and the pitfalls of the use δ18OP as a tracer in freshwater ecosystems. From this, we constructed a protocol compiling experience from previous studies in combination with our own experience.