Nitrate (NO3-) is a common contaminant of groundwater due to the intensive agricultural activities and special overfertilization. Nitrate itself is relatively nontoxic, but it can be reduced to nitrite (NO2-) which may cause cancer and methemoglobinemia. Nitrate itself is relatively nontoxic, but it can be reduced to nitrite (NO2-) which may cause cancer and methemoglobinemia.