Sediment is not the primary source of Phosphorus in one of Minnesota’s most heavily eroding watersheds

By Anna Baker and Jacques Finlay

For decades phosphorus (P) management in the United States has focused on upgrades to waste water treatment plants and erosion control as a primary means of reducing negative impacts of excess P loading to lakes and rivers. However, the role of non-point source dissolved-P and the impact of interactions between sediment and dissolved-P on whole-basin phosphorus budgets is much less well known.  Because sediment has been viewed as a critical target for water quality improvement in the Minnesota River Basin (MRB), we sought to understand the extent to which sediment loading was responsible for P inputs to the Le Sueur River, one of the biggest sources of sediment and P to the watershed and the state.  Previous work in the MRB has indicated that streambank sediment may have an important role in P loading and partitioning between dissolved and particulate form.  An investigation in the Blue Earth River (Sekely, Mulla, and Bauer, 2002) revealed that streambank slumping may contribute 7-10% of the basin’s P load, and research in the Le Sueur (Grundtner, Gupta, and Bloom, 2014) demonstrated that streambank sediment not only had high total-P but also a great capacity to bind P, making it both a source and vector for P to downstream waters.

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Jacques Finlay observes an eroding bluff on the Le Sueur River during a geomorphology field tour and sampling trip undertaken by WRS faculty Finlay, Karen Gran, and WRS graduate student Anna Baker. Photo credit: Evelyn Boardman

To advance our understanding of the role of sediment in river P dynamics, we developed a mass balance for sediment-derived P that explores the contributions of all major sediment sources to P loading in the Le Sueur watershed.  Building upon a sediment budget developed for the Le Sueur, (Gran, et al., 2011), we sampled the basin’s major erosional hotspots including upland agricultural fields and ditches, and near channel features including bluffs, streambanks, and ravines. Total- and water-extractable P were measured on these sediments and multiplied by sediment loads from each source to find sediment derived-P loads, which were then compared to measured loads from the network of gages along the Le Sueur River to determine how much erosion from these features was contributing to total-P loading from the basin.  Furthermore, we took steps beyond a traditional budget analysis by incorporating data describing the sorptive properties of these sediments into the budget.

Surprisingly, this analysis revealed that only 24% of the total-P exiting the Le Sueur River at its mouth was attributable to erosion from all potential sources, including farm fields, ditches, bluffs, streambanks, and ravines.  This was unexpected because, on an average annual basis, only 37% of observed total-P load was in dissolved form, with the balance (63%) occurring primarily in particulate forms.  This result suggested that sorption converts a large amount of dissolved-P to particulate form within highly erosive river channels. To explore this discrepancy, we applied the results of two measures of P binding potential to the budget: sorptive capacity from experiments by Grundnter (2013) and average sorbed concentrations under conditions typical of storm events in the Le Sueur, which were developed as part of this study.  The application of these results indicated that between 2% (based on average sorbed concentration) and 24% (based on sorptive capacity) of the total-P budget may be particulate-P that is forming in the channel as a result of the binding of P by sediment.

These results have important implications for management.  First, even if erosion was completely eliminated, only a 24% reduction of the total-P would result.  Second, it is likely that the binding of P by sediment in the channel corridor is effectively dampening the signal of dissolved-P inputs to the channel; in other words, there is a lot more P entering the channel in dissolved form than what we measure as dissolved-P at the basin outlet.  These findings suggest that, while sediment remains an important target due to its multiple impacts on water quality, we may need to manage sediment and P differently, with much more attention focused on investigating dissolved-P source and fate in this basin and beyond.