Isotopic ratios could solve Mississippi mystery

Every summer a hypoxic layer now develops in the sea along the coast of Texas and Louisiana, where the Mississippi discharges into the Gulf. This is believed to be the result of massive nutrient inputs from the intensively cultivated catchment of the Mississippi, which drains around 40% of the US.

Mineralized soil nitrogen (N) and soil fertilisers are the main source of N in the Mississippi basin, but it is not known how closely contributions made by individual sources mirror final nitrate (NO3) inputs into the Gulf. Such is the complexity of the N cycle, that by the time water reaches the sea, much of the N could have been recycled, or converted into other compounds.

William Battaglin, whose research group is working on the problem at the USGS, said: “It might be possible to determine the relative contributions of major N sources to the nitrate in river water by measuring the stable isotopic ratios Isotope 15N and Is.18O of the NO3 ion. A handful of researchers have used the Is.15N and/or Is.18O isotope ratios to determine sources of NO3 in groundwater and small rivers, but very little is known about the isotopic composition of NO3 in large rivers like the Mississippi.”

So far, Battaglin’s researchers have established that only 18% of N entering the Mississippi River each year is from municipal and industrial sources, including WwTPs. The majority is from new and old fertilisers leached from the soil. The rest is thought to be from plant biomass, animal waste, leguminous fixation of atmospheric N, and N present in rainfall.

In the latest study, hundreds of water samples have been taken from 18 of the Mississippi’s major tributaries and analysed for Is.15N and Is.18O. If the isotopic signatures of catchments with contrasting land use patterns are found to differ significantly, an entirely new model of contributions made by individual sources to the Mississippi’s NO3 outputs – whether agricultural, municipal or industrial – will be generated from the results.

This could have important repercussions for farmers and wastewater utilities across the US, depending on the reaction of the US EPA, which is part-funding the project. The EPA would welcome a rapid solution to the so-called ‘dead zone’ problem in the Gulf of Mexico.