ABSTRACT The San Antonio segment of the karstic Edwards Aquifer provides an example of the impact of aggregate quarries on nitrate loads in karst aquifers. Previous published reports have not evaluated aggregate quarries as a possible cause of nitrate concentrations in the Edwards Aquifer above normal background levels and instead attribute nitrate to recent urban development directly over the aquifer’s recharge zone. Quarries are known to be sources of nitrate pollution of groundwater resulting from use of ammonium nitrate fuel–oil (ANFO) mixtures as explosives in mining operations. Ammonium nitrate is highly soluble and leaches readily into the formation being mined. Large quantities are used, up to 30 percent of which is not consumed by blasting, thus constituting a potential groundwater pollutant. Groundwater data from the Texas Water Development Board and stream data from the Texas Commission on Environmental Quality show that urbanization cannot explain the geographic distribution and historical changes of nitrate in the aquifer. The geographic distribution of numerous limestone quarries fits with the data, and nitrate concentrations in the Edwards Aquifer have risen since ANFO use as an explosive began on a large scale, which indicates quarries are a major contributor of nitrate to the aquifer. Water-quality measurements of nitrate in the Edwards Aquifer mostly remain at a level safe for humans. However, some freshwater aquatic organisms are harmed by prolonged exposure to lower nitrate levels. Recent nitrate measurements show that most measurements have reached a level that might be harmful to some groundwater species.