Water quality at the end of the Mississippi River for 120 years: the agricultural imperative

Archived water quality data collected between 1901 and 2019 were used to reconstruct annual averages of various forms of C, N, P, and silicate concentrations and alkalinity in the lower Mississippi River. During this interval the average annual nitrate concentrations doubled pre-dominantly from fertilizer applications and tiling, silicate concentrations decreased by half as diatom sedimentation increased as dams were built, and alkalinity increased 16%. Variances in silicate concentrations were proportional to river discharge before 1980 and concentrations have been stable since then. Average annual temperatures, discharge and alkalinity increased simultaneously around 1980; this suggests that there was greater weathering thereafter and is supported by the positive relationships between variations in alkalinity and variations in nitrate, phosphate, and silicate concentrations. The conversion of forests and grasslands into farmlands and improved drainage resulted in less evapotranspiration, a higher percent of precipitation going into streams and altered soil water bio-geo-chemistries. Field trials demonstrating soil health improvements resulting from more live roots and soil cover and greater biodiversity demonstrate water quality improvements and no effect on farm profitability. Lowering nitrate loading to the coastal waters will reduce summertime hypoxic waters formation offshore, but alkalinity in the river will increase further with climate warming.

Automated summary

This study reconstructed the annual averages of various elements and alkalinity in the lower Mississippi River using water quality data collected from 1901 to 2019. The results showed that nitrate concentrations doubled mainly due to fertilizer applications and tiling, while silicate concentrations decreased by half as a result of diatom sedimentation from dam construction. Alkalinity increased by 16%. Before 1980, the variations in silicate concentrations were proportional to river discharge, but stabilized afterwards. The study also found positive relationships between variations in alkalinity and variations in nitrate, phosphate, and silicate concentrations.