Abstract
This study evaluates the cause of salinization in an irrigation scheme of 100 ha supplied from a reservoir. The scheme is located in Gumselasa catchment (28 km2), Tigray region, northern Ethiopia. The catchment is underlain by limestone–shale–marl intercalations with dolerite intrusion and some recent sediments. Water balance computation, hydrochemical analyses and irrigation water quality analyses methods were used in this investigation. Surface waters (river and reservoir) and groundwater samples were collected and analyzed. The water table in the irrigated land is ranging 0.2–2 m below the ground level. The majority of groundwater in the effective watershed area and the river and dam waters are fresh and alkaline whereas in the command area the groundwater is dominantly brackish and alkaline. The main hydrochemical facies in the groundwater in the effective watershed area are Ca-Na-SO4-HCO3, Ca-Na- HCO3-SO4, and Ca-Na-Mg-SO4-HCO3. The river and dam waters are Mg-Na-HCO3-SO4 and HCO3-SO4-Cl types, respectively. In the command area the main hydrochemical facies in the groundwater are Ca-Na-HCO3-SO4 and Ca-Na-Mg-SO4-HCO3. Irrigation water quality analyses revealed that salinity and toxicity hazards increase from the effective watershed to the irrigated land following the direction of the water flow. The results also showed that the analyzed waters for irrigation purpose had no sodicity hazard. The major composition controlling mechanisms in the groundwater chemistry was identified as the dissolution of carbonate minerals, silicate weathering, and cation exchange. One of the impacts of the construction of the dam in the hydrologic environment of the catchment is on its groundwater potential. The dam is indirectly recharging the aquifers and enhances the groundwater potential of the area. This increment of availability of groundwater enhanced dissolution of carbonate minerals (calcite, dolomite, and gypsum), silicate weathering and cation exchange processes, which are the main causes of salinity in the irrigated land. The rising of the brackish groundwater combined with insufficient leaching contributed to secondary salinization development in the irrigated land. Installation of surface and subsurface drainage systems and planting salt tolerant (salt loving) plants are recommended to minimize the risk of salinization and salt accumulation in the soils of the irrigated land.
Highlights
Salinization refers to a rise of excess soluble salts concentration in the soil
In irrigated areas consist of waterlogged soils usage of brackish/saline water from a reservoir and/or brackish/saline groundwater from wells for irrigation is the cause of salinization
The rises of brackish/saline groundwater to the depth of the root zone from an unconfined aquifer, perched aquifer, or leaky aquifer may cause salinization in irrigated areas covered by poorly drained soils
Summary
Salinization refers to a rise of excess soluble salts concentration in the soil. These salts occur in the soil as ions: Cl−, SO42−, HCO3−, CO32−, NO3−, Na+, Ca2+, Mg2+, and K+. Salinization occurs in most climatic regions in non-irrigated areas, irrigated areas, and coastal areas It can be caused by geogenic processes and anthropogenic impacts [1]. In the non-irrigated area, which is common in dryland regions and is known as dryland salinity, the cause is the rising of brackish/saline groundwater to the surface or depth of root zone from unconfined, perched, or leaky aquifers. The rises of brackish/saline groundwater to the depth of the root zone from an unconfined aquifer, perched aquifer, or leaky aquifer may cause salinization in irrigated areas covered by poorly drained soils. In coastal areas it can be caused due to flooding of the land by seawater and salt water intrusions In all these areas a practice of excessive fertilization with very soluble forms of fertilizer and excessive usage of pesticides can cause salinization. The author showed that soluble fertilizers differ in the degree to which they contribute to the process of salinization
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