In search of the phosphorus legacy: merging hydrological and biogeochemical approaches to understand phosphorus dynamics in streams.

Abstract

Nutrient and sediment pollution of surface waters remains a critical challenge for improving water quality. Phosphorus and sediment export to lakes and rivers has resulted in diminished water quality placing drinking water supplies, human health, recreation opportunities, and aquatic ecosystems at risk. Understanding both biogeochemical processing and transport of phosphorus and fine sediments (a major sorbent of phosphorus) in stream channels is required to understand the legacy effects of agricultural land management decisions and the effects of mitigation strategies. I focused my thesis on sediment and phosphorus dynamics in Midwestern USA watersheds by examining three parts: (1) using the Nutrient Tracking Tool (NTT) with physical site characteristics to target areas for revegetation, and to compare this field-scale tool with watershed export data; (2) a combination of tracer experiments with transient-storage modeling and biogeochemical assessment to understand phosphorus and sediment dynamics at base flow within Plum Creek, WI; and (3) the inventory and assessment of fallout radionuclides as a potential sediment fingerprint in the upper Midwest. To assess potential load reductions using revegetation, the nutrient tracking tool (NTT) was used with a scoring system to identify areas where vegetation mitigation could be implemented within three selected Fox River, WI sub-watersheds. A corn-soybean rotation, an implementation of a 10-m vegetated buffer, a full forest conversion, and tiling were modeled and assessed. The corn-soybean results were aggregated and compared to watershed level gauge data in two sub-watersheds. Edge of field data was compared to modeled results using multiple parameterization schemes. The agricultural areas that scored higher and were untiled showed greater potential nutrient and sediment export reduction when vegetation mitigation was implemented in the model. Aggregated watershed results showed disparities between modeled and measured phosphorus exports but modeled sediment export fell within observed gauge data ranges. Field specific parameter adjustments resulted in more accurate modeled results compared to measured edge of field data, but needed further refinement. Targeted mitigation using vegetation based on the scoring system was shown to be a helpful tool for nutrient and sediment reductions when modeled. Using a field scale model aggregated to the watershed scale presents tradeoffs regarding processes found beyond the edge of field. To understand in-channel processes, a fluorescent fine particle surrogate, bromide, and phosphate were injected and sampled under base flow conditions within two stream reaches that are representative of the lower and upper Plum Creek, WI, watershed. Grab samples were analyzed and breakthrough curves were modeled utilizing the one-dimensional transport with inflow and storage (OTIS) model. Sediment and stream water samples were analyzed for equilibrium phosphate concentration determination to gain a better understanding of phosphorus dynamics and the potential time lags associated with phosphorus delivery from agricultural fields to downstream water bodies when used in conjunction with transport parameters determined by OTIS. Results indicate sediments within Plum Creek have a large potential to sorb dissolved phosphorus entering the stream channel and reduce dissolved loads at the individual reach scale. These results suggest a large potential for discrepancies between mitigation implementation and noticeable water quality improvements when considering base flow transport and storage metrics. Fallout radionuclides beryllium-7 and lead-210 were measured at the St. Paul Weather Station, MN and at the Marcell Experimental Forest by collecting bulk deposition. Event-based sampling was completed in St. Paul whereas weekly sampling was done at the Marcell Experimental Forest in order to compare results with data from the National Atmospheric Deposition Program. Results showed that neither beryllium-7 nor lead-210 were correlated with precipitation inputs, but showed seasonal patterns with peak deposition occurring during the summer months. Beryllium-7 was weakly correlated with precipitation at the Marcell Experimental Forest and showed some correlation with sulfate and chloride deposition but the correlation was not strong enough that either sulfate or chloride could be used as a predictor of beryllium-7 deposition. Cumulative deposition followed a linear trend and could be useful for predicting deposition or filling data gaps in areas with scant depositional records or in remote settings.