The New York Times-20080129-Agricultural Practices Found to Contribute To Bicarbonate in River

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Agricultural Practices Found to Contribute To Bicarbonate in River

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The Mississippi may be known informally as the Big Muddy, but it's the Big Bicarbonate, too. It contains a huge amount of dissolved inorganic carbon in the form of bicarbonate, or HCO3-, ions, produced by chemical weathering of rock.

In chemical weathering, atmospheric carbon dioxide dissolves in rainwater, creating an acid that reacts with minerals in the rock, forming calcium bicarbonate and other compounds that through the flow of groundwater eventually enter rivers and end up in the ocean. It's an important means by which CO2 is removed from the atmosphere.

The amount of bicarbonate in the Mississippi has increased substantially in the last 50 years, and the question is why. Peter A. Raymond and Neung-Hwan Oh of Yale and colleagues report in Nature that it's due to agricultural practices more than any increase in precipitation.

The researchers used a 100-year record of alkalinity (a measure of the bicarbonate concentration) at water-treatment plants in New Orleans, which gets its drinking water from the Mississippi. They analyzed this data along with precipitation data for the region and information on water discharge in parts of the Mississippi watershed.

They found that in areas with a high percentage of agricultural land, the water discharge, and the bicarbonate flux, increased even in years of average precipitation. They say that agricultural practices like irrigation, increased use of fertilizer and lime and changes in crop type and rotation may contribute to the increases -- by causing more water to flow through mineral-rich soil, where it picks up more bicarbonate on its way to the river.

[Illustration]PHOTO: The amount of bicarbonate in the Mississippi River has been increasing. (PHOTOGRAPH BY MARK HIRSCH FOR THE NEW YORK TIMES)