Data-driven approaches demonstrate legacy N accumulation in Upper Mississippi River Basin groundwater

Increases in nitrogen (N) fertilizer application, livestock densities, and human population over the last century have led to widespread nitrate contamination. While increases in riverine N loads are well documented, the total magnitude of N accumulation in groundwater remains poorly constrained. Here we provide a first data-driven estimate of groundwater N mass accumulation in the Upper Mississippi River Basin (UMRB), a region of intensive row-crop agriculture and the primary contributor to Gulf of Mexico hypoxia.

Using approximately 49,000 groundwater nitrate well concentration measurements spanning a range of depths, along with a suite of hydrogeologic and land-use predictors, we developed a Random Forest model to generate gridded predictions of depth-varying nitrate concentrations. Our results indicate that approximately 15 Tg of N (328 ± 167 kg-N ha⁻¹) is currently stored in UMRB groundwater recharged over the past 50 years.

For context, we compare these estimates to those from a lumped statistical model, which predicts accumulation of 387 ± 133 kg-N ha⁻¹, and to a simple basin-scale N mass balance, which places an upper bound of approximately 1000 kg-N ha⁻¹ for the period 1967–2017. These findings underscore the importance of legacy N when forecasting future water quality, as nitrogen stored in the subsurface will continue to degrade drinking water quality and elevate surface water N loads for decades.