The nitrogen in chemical fertilizer does two things incredibly well. It supercharges crop growth, and it produces nitrates, chemicals that are ultra-soluble in water and easily pass through soil to accumulate in groundwater. Once there, nitrates can persist for decades and increase in concentration as more fertilizer is added.
Ingestion of nitrates by infants has been shown to lower levels of oxygen in the blood, leading to the potentially fatal blue-baby syndrome. And several studies have shown that consumption of nitrate-contaminated water can cause cancers in animals.