Temperature effect on the transport of nitrate and ammonium ions in a loose-pore geothermal reservoir

The concentration of NO3−―N in some of the geothermal water samples collected from Kaifeng City, Henan Province, China has exceeded the World Health Organization's guideline value for drinking water (10 mg/L). The NH4+―N concentration cannot also meet the demand for quality standard for groundwater (0.2 mg/L) in China. There is a need to predict the mobility of these two ions in a geothermal reservoir. Column experiments were performed at temperatures from 15 °C to 60 °C and the computer code CXTFIT was used to fit the experimental data and to determine the transport parameters and ammonium adsorption parameters. Effects of both the flow rate and temperature on nitrate transport parameters were evaluated. The convection–dispersion equation (CDE) was used to describe the nitrate transport process at temperatures from 15 °C to 60 °C, where the tracer transport was dominated by advection under experimental conditions. The solute hydrodynamic dispersion and dispersivity in the simulated sandy column were enhanced with the increase of temperature. The asymmetrical shape and tailing of the ammonium breakthrough curve showed that ammonium was subject to nonequilibrium transport. The amounts of adsorbed ammonium increased significantly, while the first-order kinetic rate decreased along with the drop of temperature.

► The loose-pore low-temperature geothermal reservoir environment was simulated.
► The effect of temperature on the transport of ammonium was studied.
► The effect of temperature on nitrate transport parameters was evaluated.
► The transport mechanisms and models of nitrate and ammonium ions were discussed.