Assessing Phosphorus Loading to Surface Waters from Groundwater: Challenges and Lessons Learnt

Excess phosphorus inputs to surface waters are a major driver of freshwater eutrophication. Although groundwater can be an important source of phosphorus to surface waters in some settings, its contribution remains poorly quantified.  Quantifying phosphorus loads to surface waters is particularly challenging due to heterogeneous groundwater flow pathways and concentrations, combined with the high reactivity of phosphorus with subsurface materials and dynamic biological uptake and release processes. This presentation will synthesize lessons learned from multi-scale studies we have conducted that aimed to quantify groundwater phosphorus inputs to streams and lakes and identify the factors that drive hot spots and hot moments of phosphorus release from groundwater to surface waters. The synthesis draws on studies spanning i) regional-scale longitudinal stream surveys using radon-222 and phosphorus measurements,  ii) year-round groundwater sampling in a riparian zone, and iii) high resolution measurements of streambed phosphorus and groundwater-surface water exchange. Collectively these studies highlight the importance of accounting for dynamic processes that control phosphorus delivery to surface waters and underscore the need for caution when upscaling localized groundwater measurements that do not capture spatial variability in phosphorus concentrations.