Soda ash [NaCO] can be present in natural water from the weathering of basalt which is an igneous rock. Lime [Ca(OH)] can be present in natural water when rain water comes in contact with calcined  minerals such as ash produced from the burning of calcareous coal or lignite in boilers. Anthropogenic use of soda ash also finally adds to the RSC of the river water.
Where the river water and ground water are repeatedly used in the extensively irrigated river basins, the river water available in lower reaches is often rendered not useful in agriculture due to high RSC index or alkalinity. The salinity of water need not be high.