Inspection Services Blog

Research Update: Nitrogen Fertilizer Loading to Groundwater in the Central Valley

Research Project: Nitrogen Fertilizer Loading to Groundwater in the Central Valley. Thomas Harter et al.
Status: Completed

Issue: Nitrogen, in the form of nitrate, is the most common pollutant found in Central Valley groundwater. Nitrate has a maximum contaminant level (MCL) in drinking water of 45mg/L (NO3). Under natural conditions, groundwater nitrate may occur in concentrations up to 10mg/L. Nitrate can also leach into groundwater from other nitrogen sources, such as fertilizer and manure.

This project conducted an assessment of Central Valley groundwater vulnerability to nitrate contamination from various sources. Researchers evaluated sources of groundwater nitrate, historical trends, and methods to reduce nitrate loading. Previous work by these researchers showed that the sources of most nitrogen loading to groundwater in the Tulare Lake Basin and the Salinas Valley are nitrogen fertilizer applications and dairy manure.


Through this project, researchers found that, while growers have become more efficient with their water and fertilizer applications, there remain opportunities for improvement. They also identified the most significant nitrogen inputs and outputs in these agroecosystems to be the following:

The largest nitrogen fluxes into the agricultural landscape:

  • Synthetic Fertilizers
  • Land application of manure & wastewater effluent
  • Nitrogen fixation in alfalfa
The largest nitrogen fluxes out of the agricultural landscape:

  • Harvested nitrogen
  • Potential nitrogen losses to groundwater
  • Atmospheric nitrogen losses

Outlook: Based on the magnitude of nitrogen fluxes, there are a few options to reduce the potential for nitrogen loading to groundwater from cropland across the Central Valley:

  • Increase the amount of harvest without increasing the amount of synthetic or organic fertilizer
  • Reduce nitrogen input to the agricultural landscape. Synthetic fertilizer is the main flux that can be reduced significantly without changing Central Valley land use.

Due to the slow-moving nature of groundwater recharge, improvements in groundwater quality from better management practices adoption may take decades. However, improvements in agricultural management and resource efficiency are important in protecting groundwater quality.

As part of this work, FREP’s work, and others, a wide range of agricultural processes have been documented that significantly improve crop nitrogen use efficiency. Moving forward, a key goal will be to extend this knowledge to growers and effectively disseminate research findings.

Final Report:

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