Identification of nutrient sources and sinks in irrigation water in southern Alberta using stable isotope techniques

Excessive growth of aquatic vegetation in irrigation canals can negatively affect the timely conveyance of irrigation water to crops, impacting crop yield and quality. This vegetative growth is often caused by elevated concentrations of dissolved nutrients in irrigation water. Stable isotope measurements of carbon and nitrogen containing compounds, including oxygen isotope ratios of nitrate, were used to identify predominant nutrient sources (fertilizer, manure, agricultural soils, agricultural drainage water, and municipal effluent) and sinks (periphyton, macrophytes and canal sediment) in southern Alberta (Canada) irrigation water during a 2-year study of four irrigation districts. The δ 13 C DIC values of irrigation water ranged from −25.5 to −0.8‰, while δ 15 N NO3 values ranged from −1.8 to + 18.8‰, and δ 18 O NO3 values ranged from −9.5 to + 21.1‰. These isotopic signatures suggest that the main sources of carbon in irrigation water were the dissolution of carbonate rock and inputs from drainage water, while irrigation canal sediment was found to be a carbon sink. Sources of nitrate were mixed and varied among irrigation districts. In districts characterized by a high density of livestock operations, nitrate was primarily derived from manure and manure-amended soil especially when nitrate-N concentrations in irrigation water exceeded 0.2 mg/L. In districts characterized by cropland agriculture, the main contributions of nitrate in irrigation water were soil-derived nitrate and inorganic fertilizers. Knowledge of the source of nutrients in irrigation water enables proactive decision making by water managers about nutrient management practices on land surrounding irrigation canals. This can reduce the risk of excess aquatic vegetation growth within irrigation canal systems.