Iron oxide is present in cracks in rocks
Detailed Description
Arsenic can attach to iron oxide (reddish-brown color) is present in cracks in rocks. The pH and redox conditions in an aquifer determine if the arsenic remains attached to the iron oxide or dissolves into the groundwater. Recharged oxygenated irrigation water and groundwater pumping commonly cause the water table to fluctuate, which can release arsenic from basin-fill sediment into the groundwater. Photo is from USGS Circular 1358, Water Quality in Basin-Fill Aquifers of the Southwestern United States.