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Fertilizer is critical to agricultural supply chains, but its use results in downstream externalities in the form of aquatic hypoxic zones and algal blooms. Quantifying farm pollution is challenging due to its non-point nature and the lack of a temporally consistent, administrative-level dataset on water quality. This study offers a novel satellite-derived measure of algal bloom intensity that spans 30-plus years and encompasses lakes, riparian, and coastal aquatic resources across US counties. We show that fertilizer use is closely linked to our measure of water quality. Such farm pollution drives water quality impairment both locally and downstream from the fertilizer use, and impacts occur at an annual and longer-term timescale.