Groundwater discharge in agricultural areas contributes significantly to nutrient levels in headwater streams. By studying two catchments in Ontario, Canada, researchers found that groundwater discharge during certain periods accounted for 30% of streamflow and 20-30% of nutrient loading. Previous estimates suggest groundwater may contribute even more. Overall, groundwater likely contributes 40% of the annual soluble reactive phosphorus load and 50-60% of the nitrate load in these streams. This highlights the important role of groundwater in nutrient pollution in headwater streams, influenced by legacy nutrient reservoirs in the soil.