Abstract:
The water quality for irrigation purpose is currently evaluated by the indicators such as: EC (electrical conductivity), SAR (sodium adsorption ratio) and RSC (residual sodium carbonate). It is observed during soil and groundwater monitoring endeavors that the conventional water quality indicators are not always helpful in evaluating the water quality of a soil - water system to which the rhizosphere is actually exposed to. Comparison of the soil and groundwater data revealed that long term application of groundwater with permissible EC, SAR and even negative RSC have marked some evidences on soil sodification. It challenges the countability of inuse conventional water quality indicators. It is reasoned out that this infringement is attributable to some geochemical processes taking place in a soil - water system. The RSC takes care of calcite (CaCO3) precipitation only. It does not decide the fate of Ca2+ spared in case of negative RSC after neutralizing carbonates as calcite. The left over Ca2+ may precipitate with sulphate (SO42-) as gypsum (CaSO4). Perhaps this turns our soils gypsiferous where the water chemistry and the climatic conditions are conducive to the phenomena. The disappearance of Ca2+ as precipitates from a Na+, K+, Ca2+, and Mg2+ dominated soil - water system would increase the relative activity of Na+. The elevated activity of Na+ therefore, may sodicate the soil. Thus, groundwater with low initial SAR may turn to that of high SAR in soil solution due to precipitation of Ca2+. Similarly, dissolution of dolomite minerals furnishing Ca2+ ` ions may decrease the high initial SAR of groundwater. It is suggested that in addition to SAR and RSC, the SO42- and other ions in groundwater involved in precipitation need worth consideration as functional elements leading to soil sodification. It is also recommended that water quality must be spatialized in context of geochemical processes taking places in the soil. The paper also describes some empirical models, which take care of ion-exchange phenomena. Such ion transport models can be developed and can be used in soil/water monitoring studies as guidelines.
Page(s):
207-224
DOI:
DOI not available
Published:
Journal: Proceedings of 5th Pakistan Geological Congress , Volume: 0, Issue: 0, Year: 2007