The existence of nitrates in groundwater is common in many parts of the world. It’s when the concentration exceeds the level considered safe by the Environmental Protection Agency that there’s a problem.
Nitrates can occur naturally or they can come from over-applying nitrogen fertilizers, wastewater, manure and other sources of nitrogen. Crops greatly benefit from the boost nitrogen provides, but they can take up only so much. After that, the nitrogen can travel downward through the soil until reaches groundwater. There it lingers as nitrates. Nitrates found today in groundwater can be from fertilizer that was overapplied decades ago — called a “legacy pollutant.”