Interaction between nitrate and clay during an Aquifer Storage and Recovery cycle

Aquifer storage and recovery (ASR) is used to sustain the quality and quantity of groundwater by means of the injection of high-quality water. However, these systems face challenges such as physical clogging and poor recovered water quality. This study investigates the outcomes of injecting a low ionic strength water source into a clastic nitrate contaminated aquifer. The investigation is conducted by using a column experiment. We implemented six runs, representing a combination of two soil mixtures (loamy sand with clay content of 10% and 12% respectively), and three nitrate concentration in the injected groundwater (resp. 71, 114 and 187 mg/l).

 

Each of the six runs consists of two phases: in the first phase the contaminated groundwater flows through the soil column for an average of 13.8 pore volume units, which followed by a phase in which the low ionic strength water flows through for an average of 12.5 pore volume units. The results of the experiment indicate that the hydraulic conductivity decreases from an average of 0.31 cm/min to 0.25 cm/min after the introduction of the low ionic strength water. The nitrate breakthrough curves display a delay in the equilibrium which can be explained by dispersion and the creation of soil immobile zones.

 

Subsequently, these breakthrough curves are analysed using HYDRUS 1D with physical nonequilibrium transport model to determine the dispersivity, the immobile water content and the mass transfer coefficient. Our finding highlight that these parameters generally increased after the introduction of the low ionic strength water. Also, these parameters tended to increase, on average when the soil’s clay content increased. Our results enable the prediction of change in the recovery efficiency of the ASR systems.