Comparing the discharge data with records of rainfall over the 100 year period showed that the increase in water volume could not be accounted for solely by increased rainfall and the authors suggest that farming practices such as tile drainage, fertiliser use and irrigation are significant contributors to the changing hydrology of the region.
Agricultural croplands cover around 42 per cent of the Mississippi watershed and they are intensively farmed so the extra water is likely to bring with it pollutants such as nitrogen and phosphorus arising from fertiliser use.
In addition to changes in water volume, the research found high concentrations of bicarbonate in the water. Rivers and oceans play an important part in the long term global carbon cycle by absorbing atmospheric CO2 which is stored in water in the form of bicarbonate.
Levels of bicarbonate began to rise in the mid 1950s, suggesting that agricultural practices, such as drainage, tillage practices and crop type, in the latter half of the twentieth century have had a stronger impact than previously thought.
The research also found a strong correlation between the size of each farmed watershed and the amount of water and bicarbonate discharged into the river, further indicating that farming is raising bicarbonate levels in the river.
The authors argue that bicarbonate can be seen as a marker for other waterborne pollutants, so the rise in bicarbonate levels is likely to signal a similar rise in pollutants such as nitrogen and phosphorus.
The authors warn that proposals for large scale changes to crop production to produce biofuels could worsen this process, by further increasing water flow.
Because first generation biofuel crops, such as maize, rely on intensive agricultural practices, this water would bring with it pollutants, such as nitrogen.