Use of rain and dry deposition compositions for interpreting ground-water chemistry

ABSTRACT Waters from 91 springs in a 130 X 140 km area of west‐central Arkansas were analyzed for all the major ions. The yearly flux of these ions in the rain and pure‐water extract of the dry deposition 180 km north of this area was measured and assumed to represent the composition of the recharge to the ground water feeding the springs. Cation/Cl‐flux ratios were used as a norm for the concentration of ions in the spring waters by evapotranspiration. Ratios above the norm were interpreted as ion exchange or rock dissolution into the ground waters. Ratios below the norm were interpreted as ion adsorption or, for Ca2+, CaCO3 precipitation processes. Na+ and Mg2+ are invariably above the norm. K+, NH+4 and H+ are invariably below the norm; whereas Ca2+ values are about equally divided between above and below the norm. Equivalents of cations above the norm and below the norm were compared for a number of springs to determine the main ground‐water processes for each spring. The Na+/Cl‐ and K+/Cl‐ norms for the rain water form near‐perfect minimum and maximum envelopes, respectively, of the Na+ vs Cl‐ plots of the spring‐water compositions. This is due to the adsorption affinities of the soil for Na+ being low, and for K+ being high.