Are nutrient concentrations in groundwater changing over time?
Nitrate concentrations increased in 7 of 24 groundwater studies re-sampled after about a decade. Median concentrations of nitrate measured in 495 wells increased from 3.2 to 3.4 mg/L (6 percent) between the first sampling period of 1988 to 1995 and the second sampling period of 2001 and 2004. In shallow groundwater beneath agricultural areas, the median nitrate concentration increased from 4.8 to 5.7 mg/L, whereas in deeper groundwater in major aquifers, the median nitrate concentration increased from 1.2 to 1.5 mg/L. The proportion of wells with nitrate concentrations greater than the U.S. Environmental Protection Agency’s drinking-water standard (MCL of 10 mg/L as nitrogen) increased from 16 to 21 percent a decade later. Nitrate concentrations in deep oxic (dissolved oxygen concentrations greater than 0.5 mg/L) aquifers used as a source of water supply are likely to increase in the future as shallow groundwater with high concentrations moves deeper into aquifers. These increases will
