Some sodium bicarbonate waters at depth in the Atlantic and Gulf Coastal Plains have the same bicarbonate content as the shallower calcium bicarbonate waters in the same formation and appear to be the result of replacement of calcium by sodium through the action of base-exchange minerals. Others, however, contain several hundred parts per million more of bicarbonate than any of the calcium bicarbonate waters and much more bicarbonate than can be attributed to solution of calcium carbonate through the action of carbon dioxide derived from the air and soil.

As the waters in the Potomac group (Cretaceous) are all low in sulphate and as the environmental conditions under which the sediments of the Potomac group were deposited do not indicate that large amounts of sulphate are available for solution, it does not seem probable that carbon dioxide generated by chemical or biochemical breakdown of sulphate is responsible for the high sodium bicarbonate waters in this area.

Sulphate as a source of oxygen is not necessary for the generation of carbon dioxide by carbonaceous material. Oxygen is an important constituent of carbonaceous material and carbon dioxide is a characteristic decomposition product of such material—as, for example, peat and lignite.

Experimental work showed that distilled water, calcium bicarbonate water, and sodium bicarbonate water, after contact with lignite, calcium carbonate, and permutite (a base-exchange material), had all increased greatly in sodium bicarbonate content and had become similar in chemical character and in mineral content to high sodium bicarbonate waters found in the Coastal Plain. The tests indicated that carbonaceous material can act as a source of carbon dioxide, which, when dissolved in water, enables it to take into solution more calcium carbonate. If base-exchange materials are also present to replace calcium with sodium, a still greater amount of bicarbonate can be held in solution. The presence of carbonaceous material, together with calcium carbonate and base-exchange minerals in a formation is, therefore, sufficient to account for the occurrence in it of high sodium bicarbonate waters.