Mapping agricultural water contamination risk in California’s Central Valley

California's San Joaquin Valley is host to some of the most productive farmland in the United States. The area is a patchwork of different crops, soils, and farming practices. However, the area also receives little rainfall and requires irrigation to meet the needs of crops on across an ever-increasing acreage of land. Large, unsustainable withdrawals of groundwater are one major issue resulting from high irrigation demand in these areas, and now it appears there is another: nitrate contamination.

A recent study revealed that irrigated cropland is the largest source of groundwater nitrate contamination in the southern San Joaquin Valley, contributing approximately 95 percent of the nitrate found in the region's groundwater. Nitrate contamination carries several health risks; for infants, excessive consumption of nitrate-contaminated groundwater can be fatal. In the San Joaquin Valley, drinking water contamination affects approximately 10 percent of the population, with the burden falling disproportionately on the poor. These communities are generally without infrastructure for water treatment, instead relying on domestic wells or small-scale community water systems.

Researchers from the University of California, Davis used a GIS-based tool to map the hazard index of leaching in California cropland — a single number that conveys the risk of nitrogen leaching from the crop to groundwater. The tool requires just three inputs: soil properties, crop properties, and the type of irrigation currently applied. The high-resolution model provides results at the scale of the individual field, identifying each field with a risk rating on a scale between 1 and 80.

The researchers used the tool to assess a four-county area in California's agriculturally dominated Central Valley, finding that nearly a third (31 percent) of the cropland was at high risk for nitrate leaching.

Leaching was associated with two primary factors: coarsely textured soils, which drain readily, and surface irrigation, which involves the flooding of fields. Crop type can also play a role in the risk of nitrate contamination. For example, wine grapes require little fertilization and their roots can grow to depths of 20 feet or greater, minimizing the risk of nutrients escaping. In contrast, spinach must be grown on coarse soils with heavy fertilization; its small and shallow root mass further increases the risk of nitrate leaching.

Initially, the researchers recommend incentivizing farmers in high-risk areas to switch over to less water intensive forms of irrigation, which could reduce high-risk acreage by a third. While leaching of nitrate does not guarantee transport to groundwater, it is certainly part of the process, they say. While the tool could not account for the specific management practices on individual farms, the tool does allow users make inferences about regional land management practices at a finer spatial scale than previous data allowed. Due to the tool's simplicity, the researchers hope that growers, regulatory agencies, or educators might use it in their own future work.