Groundwater is a main source of drinking water for most Missourians and provides much of the water used to irrigate crops. Groundwater could be contaminated due to wells that have been abandoned and unused for Groundwater can become contaminated with high nitrate concentrations from agriculture and, to a lesser extent, failing septic systems. Nitrate levels are higher in groundwater under agricultural land than in water below urban areas. Contamination and shrinking water levels in the ground can affect bodies of water on the surface. The department monitors groundwater quantity and quality in a variety of ways.