To a large degree, nitrate concentrations in ground water reflect land use practices at the time the water infiltrated into the soil, and these practices may not have been the same as when the ground water was sampled. For example, low tritium concentrations in water from 21 percent of the wells sampled in 1993 indicate that this water entered the ground before 1953 when nitrogen application rates also were low. Because nitrogen fertilizer application rates in the Willamette Basin have increased in recent decades, nitrate concentrations in ground water pumped during 1993 probably reflect application rates that were less than 1993 rates. Water pumped in the future may contain higher nitrate concentrations than in 1993, as the increasing fertilizer application rates of the past continue to influence ground water quality downgradient from application areas. Because of the time lag between infiltration of water into the ground and its arrival at a downgradient well, effects of past and present ground water management practices may take years or decades to become apparent in ground water pumped from shallow domestic wells.
Irrigation can accelerate transport of nitrate to shallow ground water in heavily fertilized areas.
Nitrogen fertilizer application rates in the Willamette Basin have increased since 1945. Site locations are shown on the land use map on page 24. The wells selected for sampling were completed in alluvium because more than 80 percent of ground water used in the Willamette Basin is pumped from alluvial aquifers. Site locations are shown on the hydrogeology map on page 24.