Nutrient and sediment runoff from agricultural landscapes with varying suites of conservation practices in the Mississippi Alluvial Valley

Abstract

Increasing concern regarding environmental degradation in coastal areas that experience annual hypoxic zones has led to the need for mitigation of nutrient laden runoff from inland landscapes. An annual occurrence of a hypoxic zone in the Gulf of Mexico has led to the development and implementation of nutrient reduction strategies throughout the Mississippi River Basin (MRB). With federal, state, and private financial and technical assistance, landowners have implemented best management practices (BMPs) to combat nutrient and sediment nonpoint source pollution; however, the effectiveness of these BMPs alone or utilized together has not been quantified. This study uses a field-scale, paired watershed approach in two watersheds in the Mississippi Alluvial Valley to test for differences in sediment and nutrient runoff concentrations between four management systems. A total of 774 samples (415 baseflow samples and 359 stormflow samples) were collected from 2011 to 2015. Median baseflow concentrations across all sites within this study were 52 mg L⁻¹ for total suspended solids (TSS), 0.38 mg L⁻¹ for total phosphorus (TP), 0.09 mg L⁻¹ for nitrate-nitrite (NO₃⁻-NO₂⁻), and 0.81 mg L⁻¹ for ammonium (NH₄⁺). Median sediment and nutrient concentrations from stormflow samples across all sites within the study were greater than baseflow concentrations. Median stormflow concentrations across all sites were 985 mg L⁻¹ for TSS, 1.21 mg L⁻¹ for TP, 0.32 mg L⁻¹ for NO₃⁻-NO₂⁻, and 1.04 mg L⁻¹ for NH₄⁺. Results showed no strong improvements in water quality from agricultural landscapes where suites of BMPs had been implemented, rather the data showed variability in runoff concentrations indicative of strong influences from environmental and management variables. Study outcomes highlight opportunities to better capture nutrient dynamics at the field scale through adaptive management of BMPs for increased effectiveness of nonpoint source pollution reduction and monitoring.