The federal Bureau of Mines and the U.S. Geological Survey measured the changes in formation water chemistry during the initial stages of in situ uranium leaching at a site in south Texas, using (NH4)2CO3 as the leach solution. Cores and ground water were analyzed before leaching. Water samples were taken from test wells between injection and recovery wells as leach solution replaced the ground water. Measurements were made at least daily of temperature, ph, U, Na, K, Ca, Mg, SiO2, NH4, HCO3, CO3, SO4, and Cl. The data were needed for developing and verifying a computer simulation model of the geochemical reactions involved in in situ uranium leaching.