Electrokinetic retention, migration and remediation of nitrates in silty loam soil under hydraulic gradients

Contamination of groundwater by nitrates leaching from intensive agricultural and livestock operations have become a major concern for surrounding communities that use groundwater as their water supply. High levels of nitrate in drinking water poses a significant risk to human health, i.e., methaemoglobinaemia (“blue baby” syndrome).

The traditional pump-and-treat method is ineffective in medium to fine-textured agricultural soils due to the low hydraulic conductivity. This paper presents the results of a laboratory experiment investigating the feasibility of using electrokinetic treatment in retaining, accumulating, moving and remediating nitrates in a silty loam soil under hydraulic gradients.

A hydraulic gradient of 1.25 was applied to the horizontal soil columns to simulate a groundwater movement system. The study was performed in two stages. During the first stage of the experiment, the anode located at the inflow end of the columns was able to retard the movement of nitrates even under a hydraulic gradient of 1.25. After 15 days of flow, the effluent nitrate concentration in the control column rose to 90 mg l⁻¹ while no nitrates were detected in the effluent from columns subjected to the electrokinetic treatment.

After 15 days, the polarity of the electrodes was switched and this second stage lasted another 20 days. The cathode near the inflow end promoted the conversion of nitrates entering the column to other forms. The anode near the outflow end promoted the migration and accumulation of negatively charged nitrate ions towards the outflow end. By the 12th day, the nitrate concentrations in the electrokinetically treated columns were brought down to <5 mg NO₃-N l⁻¹. Electrokinetic treatment retarded nitrate movement against a hydraulic gradient of 1.25 and effectively restored a medium-textured soil contaminated with NO₃-N.

The NO₂-N level remained below 1 mg l⁻¹ throughout the experiment. The hydraulic conductivity varied between 1.0E–7 and 3.6E–7 m s⁻¹. The current requirement varied between 3 and 6 mA.     

Micro-scale modeling of saturated sandy soil behavior subjected to cyclic loading

Liquefaction induced damage to the built environment is one of the major causes of damage in an earthquake. Since Niigata earthquake in 1964, it has been popularly recognized that the liquefaction induced ground failures caused severe damage in various forms such as sand boiling, ground settlement, lateral spreading, landslide, etc. Since then, understanding the mechanism of liquefaction phenomena became very important to take measures against the liquefaction induced ground failures. To understand the mechanism of liquefaction, it is important to consider the soil as an assemblage of particles. A continuum approach may fail to explain some of the phenomena associated with liquefaction. Discrete approach, such as distinct/discrete element method (DEM), is an effective method that can simulate the mechanism of liquefaction and associated phenomena well at the microscopic level.