Effect of drip and surface irrigation systems on contaminant transport in a soil affected by cracking pathway

The contaminant mass balance in a soil profile is highly necessary in long term agriculture management under different irrigation systems. The objective of this research was assessment of the preferential flow on the travel time and mass balance of NO3 under two irrigation systems.

Nine PVC-columns were employed with a 60 cm length and 16 cm diameter. Each column included 50 cm height of clay loam soil and two cracks with 50 cm length and 1 cm diameter that filled by course sand. The treatments were based on two groups at three levels. The main group was crack: soil with blocked cracks (M); soil with remove lateral infiltration to cracks (MC-S); soil with lateraled and vertical correlation and infiltration with cracks (MC-C). The subgroup was irrigation systems: surface irrigation (SI) with 2 cm water constant height and surface drip irrigation system (DI4 & DI2) with 2 and 4 l/h flux, respectively. Each experiment column was irrigated for 6 hours (1 h with distilled water; 2h with solution consist of 176 (mg) of NO3 and 61.8 (mg) of Cl in 1 liter distilled water and finally 3 h with distilled water). Before start of experiment, each column was irrigated with distilled water for 5 h to reach saturation condition. The drainage water separately collected from soil and cracks every 15, 30 and 60 min.

The results showed the value of mass balance of NO3 and Cl in SI and DI4 irrigation systems were more similarity. Based on the results, the mass of NO3 of drainage water in M inDI2 was approximately half in compare to SI and DI4. Whiles, this mass of Cl that transported under DI2was more in compare to SI and DI4. Also, the travel time of Cl in MC-C treatment, in crack section, was shorter of NO3. On the other hand, the results show the mass balance of these anions that transported from crack section in the most of the treatments was 1.5 times more. It indicates vertical flows in course paths of crack have the main role to move and transport of contaminants such as and Cl toward groundwater.

Over all, under high water velocity as SI, NO3 is transporting more rapidly but under condition with low water velocity like as DI2 with longer time of unsaturation condition, Cl is leaching more. The preferential flows in treatment with full interaction between soil matrix and cracks were happen more. Also, the preferential flow has stronger role to transport of Cl than NO3.