Geochemical and Hydrological Controls on the Availability and Delivery of Phosphorus Associated with Aquifer and Sediment Legacy Stores to Surface Waters
Abstract
Fresh surface waters globally are threatened by eutrophication caused by elevated nutrients, particularly phosphorus (P), loads. P that has accumulated in landscapes over decades, referred to as legacy P, makes it challenging to reduce contemporary P loads and to make long-term water quality predictions. This thesis focuses on understanding geochemical and hydrological controls on P availability from different legacy stores and its potential delivery to surface waters. The specific objectives are to i) investigate factors influencing spatial and temporal variability in soluble reactive P (SRP) concentrations in the shallow aquifer of an alluvial riparian zone, ii) explore the potential for stream pH changes to influence retention and release of SRP from streambed sediments, and iii) evaluate long-term P loads to nearshore lake waters from decommissioned septic systems. These objectives were addressed through a combination of field investigations, numerical modeling, and data analysis.
For the first objective, field investigations conducted over a 19-month period illustrate the complexity of SRP retention and release processes in a shallow riparian aquifer. While SRP concentrations were generally low, high SRP concentrations were intermittently observed that generally could not be linked to factors known to control SRP release. The data highlight the need for caution in interpreting SRP inputs to streams based on discrete groundwater sampling performed at locations not proximal to a stream. This is because SRP released in a riparian aquifer may be rapidly re-sequestered. For the second objective, data analyses indicate that stream pH changes at multiple temporal scales (decadal, seasonal, diurnal, hydrological events) may mediate the release and retention of SRP from streambed sediments. The findings highlight the need to reconsider the role of stream pH changes in future models and analyses of stream P dynamics. For the third objective, field and numerical investigations show that groundwater P plumes from decommissioned septic systems are persistent and may contribute P loads to lakes for long periods (thousands of years) after decommissioning. Overall, the thesis provides important insights into factors controlling legacy P and its potential discharge to surface waters. Recognizing the importance of legacy P is critical for developing effective water quality management programs and establishing realistic expectations regarding the timescale for P load reductions.