Nitrate increased at low streamflows throughout the basin, except for the Ohio and Illinois Rivers.
"These results show that solving the problem of the dead zone will not be easy or quick. We will need to work together with our federal and state partners to develop strategies to address nitrate concentrations in both groundwater and surface water," said Lori Caramanian, Department of the Interior deputy assistant secretary for water and science.
USGS says groundwater is likely the dominant source of nitrate during low flows, and it may take several years before full water-quality effects of either increased groundwater contamination, or of improved management practices, are evident in the rivers.
The permanent nitrate monitoring stations include: Mississippi River at Clinton, Iowa; Iowa River at Wapello, Iowa; Illinois River at Valley City, Ill.; Mississippi River below Grafton, Ill.; Missouri River at Hermann, Mo.; Mississippi River at Thebes, Ill.; Ohio River near Grand Chain, Ill.; and Mississippi River above Old River Outflow Channel, La.
The USDA in recent years has pledged to assist farmers in mitigating nutrient problems by constructing wetlands and helping farmers employ cover crops and other conservation methods.
In August, USDA's Natural Resources Conservation Service estimated that controlling erosion and managing nutrients has reduced the edge-of-field losses of sediment by 35%, nitrogen by 21% and phosphorous by 52%
The report, part of USDA's Conservation Effects Assessment Project, or CEAP, also found that conservation practices have resulted in an estimated 17% reduction in nitrogen and 22% reduction in phosphorus entering the Gulf of Mexico annually.
USDA said a key takeaway from the study was the importance of the appropriate rate, form, timing and method of application for nitrogen and phosphorous.
To read the full USGS report, click here.