Modelling saline soil remediation through flushing using Hydrus-1D and an analytical solution (CDE)
Soil contamination is not only a great threat to the environment but to sustainable agriculture as well; it can be devastating for the economical production of crops crucial to humankind’s survival all around the world. Out of the different soil remedial techniques, soil flushing may be the most environmentally friendly and cost effective, as well it poses very little risk for the on-site workers. It uses water to accelerate geochemical dissolution reactions such as adsorption/desorption and solution/precipitation along with accelerating a number of subsurface contaminant transport mechanisms, normally found in unsaturated flow systems, including advection, dispersion, molecular diffusion and volatilization or solubilization. In this study irrigation water use efficiency (WUE) for salt flushing was estimated by calculating liters of water/kg of salt flushed by modeling solute transport in HYDRUS 1-D. Soil flushing was simulated for a period of five years ranging from 1997 to 2001 under two different climatic regions Ottawa and Saskatoon, to flush out 80% of three different initial masses of salt, based on three different soil salinity levels of EC equaling 4, 8 and 12, under natural conditions using the actual daily average precipitation, maximum and minimum temperature data for sand, silt loam and clay loam soils and four different irrigated conditions. For irrigation condition 1, during the month of June 10 mm/day of water was applied continuously along with the precipitation data, which amounted to 300 mm/year for sand, silt loam and clay loam. For irrigation condition 2, during the month of June water was applied continuously along with the precipitation data, at a rate equal to 1/4th value of Ks, which amounted to 810 mm/year for silt loam and 468 mm/year for clay loam. For irrigation conditions 3.a, during the months of May and June water was applied continuously along with precipitation data, at a rate of 10 mm/day, which amounted to 610 mm/year for sand only. And for irrigated condition 3.b, during the month of June water was applied continuously along with precipitation data, at a rate of 20 mm/day, which amounted to 600 mm/year for sand only. To validate HDRUS-1D, first an initial solute concentration was developed in MATHCAD through an analytical solution of CDE for resident concentration under evaporation conditions. Then the same initial concentration was used in HYDRUS-1D (by developing steady-state conditions) and in MATHCAD through a convolution-based analytical solution of CDE using identical flow and transport conditions. Comparisons of results showed that HYDRUS-1D duplicated the analytical solution nearly identically. Under Ottawa weather the objective of flushing 80% of the initial solute mass was achieved for sand, silt loam and clay loam under the natural condition. In terms of WUE, sand did not need any irrigation added to the precipitation data to achieve the target of flushing 80% of the solute mass for all three salt concentrations, even under irrigated conditions; 1, 3.a and 3.b. In terms of total time to flush out 80% of initial solute mass silt loam and clay loam showed a improvement of 34% under irrigated condition 1 versus natural condition. WUE improved by 26% under irrigated condition 1 versus condition 2 for Silt loam, whereas clay loam showed 16.5% increase in WUE under irrigated condition 2 versus irrigated condition 1. Under Saskatoon conditions for sand again the objective of flushing 80% of the solute mass was achieved under the natural conditions. In terms of total time to flush out 80% of the initial solute mass, the objective was achieved a lot quicker (65%) under the irrigated condition 1 versus natural conditions; however, poor WUE (7441, 3720 and 2480 lit/kg of salt for mass 1, 2 and 3 respectively) was estimated by HYDRUS-1D. If time is not a big constraint then even under Saskatoon conditions, flushing of salts from sand can be done quite efficiently without applying any irrigation water. The target to flush out 80% of the initial solute mass was not achieved for silt loam and clay loam under natural condition. Silt loam and clay loam used 5.5% and 1% more water/kg of salt flushed under irrigated condition 2, versus irrigated condition 1. The major finding of the study was that under both climatic regions for all three different soil types, WUE in terms of liters of water/kg of salt flushed, was found to be directly proportional to the increase in the initial salt mass in the soil profile.