An intensive study was designed to determine the activity of different radionuclides in water, including strontium-90 (90Sr), cesium-134 (134Cs), cesium-137 (137Cs), plutonium-239-240 (239+240Pu), and tritium (3H). Activity levels detected were between 0.6 and 21.3 mBq/L for 90Sr; less than 0.5, 1.4, and 327 Bq/L for 134Cs, 137Cs, and 3H, respectively; and between 0.14 and 21 microBq/L for 239+240Pu. Other nonradioactive parameters were measured including pH; dry residue: conductivity; calcium concentration, [Ca++]; magnesium concentration, [Mg++]; potassium concentration, [K+]; iron(II) and (III) concentration, [Fe2+,3+]; and ammonium concentration [NH4+]. Only the aquifer corresponding to the refrigeration reservoir of the nuclear power plant of Almaraz (Extremadura, Spain) showed greater-than-average concentrations of 3H and 239+240Pu compared with those found in the remaining aquifers. The greater presence of dissolved Fe in the waters analyzed clearly indicated greater activity associated with 90Sr, 137Cs, and 239+240Pu in dissolution. In addition, the water samples that were highly mineralized aided the dissolution of 90Sr and (to a lesser extent) 137Cs and made dissolution of 239+240Pu more difficult.