Nitrate is Wisconsin’s most common groundwater pollutant. Agriculture is the largest nitrate source, accounting for about 90% of that which leaches to groundwater (Shaw, 1994).
Only recently have the university and state agencies put together sufficient pieces of the nitrate puzzle to realize the extent of nitrate pollution (e.g., Central Wisconsin Groundwater Center, 1994; LeMasters and Baldock, 1997) and that it is increasing (e.g., Albertson and Shaw, 1998; Mason et al. 1990). A steep increase in groundwater nitrate began in the 1960s, coinciding with a large increase in chemical fertilizer-N use (Hallberg, 1989). Fertilizer-N use more-or-less leveled only about 15-20 years ago, but groundwater nitrate continues to increase. The continuing increase is due to the length of groundwater residence times (averaging decades to centuries, depending on the basin) relative to the short duration (~40 years) of increased fertilizer-N use. Groundwater nitrate concentrations and export from basins will continue to increase until aquifers equilibrate with modern nitrate loading rates.