Quantifying Mechanisms of Aquifer Salinization at the Alto Piura Valley, Northern Part of Peru

The Alto Piura aquifer (Peruvian northern coastal area) supports the important agricultural activity in the region and provides water supply for 150,000 residents. Recent studies show a salinization process in the northwest area of the aquifer. Further groundwater degradation could create economic and social problems for the region. Aninfiltration-tracer test in a typical agricultural field of the Alto Piura region was carried out to explore water and solute transport mechanisms. The main objective was to mimic the mechanism of flood irrigation (25 cm head once a month) typically used by Peruvian farmers in banana plots. Soil and groundwater contaminations were monitored in a3x3 m area and a calcium chloride solution (3.07 m3) was injected as at racer during the irrigation event. The initial and final conditions of the soil profile were studied by drilling three boreholes inside and one outside the plot prior to, and 2, 4, 7 and 10 days following the irrigation event. In total, 289 soil samples were collected to follow the gravimetric water content (GWC) and electrical conductivity (EC)profiles during the experiment. Soil samples were extracted in 20 cm resolution from the upper 2 m and 50 cm resolution for the rest of soil profile to 4.5 m, the initial water table depth. Likewise, thirty groundwater samples were taken and analyzed for major ions, EC, and total dissolved solids (TDS). Bulk density and soil texture were also measured along the soil profiles. The GWC profiles indicated faster water movement through the unsaturated zone with respect to the salt movement. An infiltration test using double ring infiltrometer estimated Ks value of 0.24 m day-1.The EC soil profiles indicated a clear salt accumulation in the first 1 m layer that did not manifest changes until the end of the 10 day measurement period. Groundwater salinity varied within the 3x3 m zone. It increased, decreased or changed with time in respect to the initial salinity. This variability within the limited 3x3m zone, suggests that preferential flow played a major role in the salt transport. The preferential transport of salts in some areas suggests that fertilization and flood irrigation events will produce both soil and groundwater salinization. Hydrus 2D/3D model was used to simulate water flow and solute transport of the infiltration test. GWC data, TDS values and water table readings were used to calibrate soil hydraulic properties and dispersivity values. The simulation suggest a combined mechanism of salinization: the flood irrigation flushes down to the aquifer the accumulated solute from the first 1 m layer, producing new salt accumulation in the soil profile originating from external sources such as fertilizers. Simulations based on conventional Richards’ equation showed the necessity to incorporate preferential flow processes on a field-scale. Modeling of flow and transport in the aquifer, considering the irrigation return flow effect, presents the challenging avenue of the ongoing research.