Prevalent chemical character

These waters are derived from both pristine and polluted carbonate systems. The analyses are ordered according to their increasing TDS contents. The freshest waters have a prevalent chemical character (PCC) of the calcium-bicarbonate type. As TDS values increase, the waters become relatively more enriched in Na, Cl, and/or SO4. The high Na and Cl content of analysis number 9 in Table 6.7 (TDS = 1269 mg/L) is derived from its pollution by sewage. The even higher Na and Cl concentrations of analysis number 10 reflect the fact that waters in the X-Can well, which is in coastal Yucatan, have been mixed with seawater. 

Weathering of continental rocks increases their TDS and concentrations of calcium and bicarbonate relative to sodium and chloride. The composition of streams so affected plot to the left of both diagrams and include the Columbia, Mississippi, Yukon, and Thames rivers. Evaportranspiration from arid climate drainage basins and streams such as the Colorado, Pecos, and Jordan rivers, which receive soil runoff and irrigation return waters, further increase the Na and TDS content of streams. Concomitant precipitation of CaC03 further shifts the prevalent chemical character of such streams back toward NaCl and the chemistry of seawater. 

The chemical differences between lanthanide and actinide derivatives are mainly due to some differences in their electronic structures. The 5f orbitals of early actinides are incompletely shielded from external influences by the 6s and 6p electrons, thus they have sufficient radial expansion for some overlap with ligand orbitals. As a consequence, organoactinides show a consistent covalent contribution to bonding, even though the prevalent ionic character remains. In the later actinides the increased nuclear charge causes contraction of 5f orbitals so that the metal-ligand overlap decreases and the trivalent oxidation state dominates in the more ionic derivatives. 

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