IONS IN WATER The sulphate ion constitutes one of the most 



important elements to be considered in con- 

 Sulphate (SO4) nate water. It almost always has significance 



(Schoeller, 1955). The S0 4 ion is proof that 

 rocks contain gypsum and that anhydrite is or has been in contact with connate 

 water. A very small content of calcium sulphate in the rocks suffices to cause 

 a high concentration of sulphate in water. The solution of calcium sulphate is 

 rapid. Also, a stagnant or partly stagnant water, like that of connate water, 

 having passed through certain land areas which even may be very poor in 

 calcium sulphate, is itself capable of becoming highly saturated, even to the 

 point of complete saturation in calcium sulphate. 



It is the solubility of gypsum (CaS0 4 + 2H 2 0), one of the least soluble 

 sulphates, that governs the content of sulphate in water. The solubility of 

 gypsum is 2.095 grams per liter in pure water, but this is increased with the 

 concentration of NaCl up to 131 grams per liter of NaCl. The maximum 

 solubility is then 7.3 grams per liter of CaS0 4 , beyond which the solubility 

 decreases; but at saturation the content of sulphate is variable, because it 

 depends upon the calcium content in the water. Some calcium can be brought 

 into the water by the exchange of bases. Similarly, this process is capable of 

 reducing the content of sulphate. In some other instances, on the contrary, the 

 exchange of bases will replace calcium by sodium. The possibility then arises 

 of increasing the content of S0 4 . The amount of sulphate in connate water is 

 extremely variable. There is a tendency to saturation in CaS0 4 , as there is a 

 tendency to saturation in NaCl for the same reasons, and this saturation is 

 rapidly attained because of the ease of the solution of CaS0 4 , and of the degree 

 of stagnation of the water. 



The sulphate water in contact with petroleum accumulations can furnish the 

 reducing conditions which transform sulphate to sulphur. 



Although there is a tendency to saturation in CaS0 4 , this saturation does 

 not always exist. Much of the connate water is relatively low in S0 4 , as much 

 of it is low in NaCl. This condition exists in some of the pools of the Rocky 

 Mountains, the Gulf Coast, and many others. The reason is probably due to 

 the liberal circulation of meteoric waters, but the tendency to saturation is 

 extremely frequent. It has been observed in the waters of thrust zones and the 

 water from the slate beds of Boryslaw, the waters of the Carboniferous and 

 Permian anticline of Polasna-Krasnokamsk in Russia, the waters of Pechelbrown 

 (Schoeller, 1955), waters from the Permian Basin from Reagan County in West 

 Texas (Berger and Fash, 1934), from Oklahoma (Case, 1934) and from 

 Illinois. 



In saturated or nearly saturated water, the S0 4 content is very often lower 

 in number of milli-equivalents at the saturation point and frequently may be 



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