Corrosion and precipitationare physicochemical processes that reduce the amount of water flow in water transfer pipes and the effective life of water supply facilities, increase the energy consumption for water transfer, and cause some diseases among consumers. The present study was conducted to investigate the corrosion tendency and precipitationalong the Gorganroud River from highlands to the Gorgan Gulf.
Material and methods:
In this study, the results of the analysis of 11 physicochemical parameters (Calcium, Magnesium, Sodium, Potassium, Bicarbonate, Sulfate, Chloride, total dissolved solids, electrical conductivity, temperature, and pH) of the Gorganroud River during a period of 10 years were used (2004-2014).First, the annual mean changes in the qualitative parameters of water samples during the statistical period of the studied stations were investigated. Then, triangular diagrams were used to assess the Gorganroud River hydrochemical at the studied stations. Next, changes in water hardness as an important qualitative parameter in the industrial, agricultural, and drinking sectors were investigated along the Gorganroud River. Saturation indices were used to predict and the probability of precipitation or dissolution of some carbonate minerals (such as calcite, dolomite) and evaporites (such as anhydrite, gypsum, and halite) along the Gorganrood River. Then, the trend of changes in Langelier, Ryznar, Puckorius, Larson-Skold corrosion indices, corrosion ratio, and the calcium carbonate precipitation potential of Gorganroud River at four stations was investigated. Next, a one-way ANOVA test was used to investigate the significance level of indices in the studied stations. Finally, the relationship between mineral saturation indices and corrosion indices was investigated.
Results and discussion:
Based on the accumulation and distribution pattern of the samples in the triangular diagram, the dominant water type in the Gorganroud River is bicarbonate on the margin of heights, and as it enters the plain and the chloride ion concentration increases, it tends to reach full maturity, the sodium chloride type. Hardness increased along the water movement path. The results of saturation indices showed that the river’s water is supersaturated with carbonate minerals and undersaturated with evaporite minerals. Based on Ryznar, Puckorius and Larson-Skold indices, Gorganroud River water tends to cause corrosion. However, the precipitation rate increased from the margin of highlands to Gorgan Gulf. The results of the calcium carbonate precipitation potential and Langelier Index indicated that Gorganroud River tends to precipitation over the study area. Investigation of the relationship between saturation indices with corrosion indices and calcium carbonate precipitation potential showed linear relationships between qualitative indices and saturation indices. The results of the statistical test showed a significant difference between the calculated indices in the studied stations.
Although based on the hydrochemical results the main factor controlling water chemistry of the Gorganroud River was the water-rock reaction, factors such as saline water intrusion of Gorgan Gulf in the lower part of the river and inflow of untreated effluents into the river caused rapid hydrochemical evolution of the river and reached the sodium chloride type. Increasing the number of physicochemical parameters along the river path in addition to increasing the water hardness, has reduced corrosion and increased precipitation rate. Statistical results showed a clear linear relationship between saturation indices and corrosion and sequestration indices in water.