Transport Characteristics of Nitrite in a Shallow Sedimentary Aquifer in Northwest China as Determined by a 12-Day Soil Column Experiment

Abstract

The transport behaviour of nitrite in a shallow sedimentary aquifer in northwest China was investigated in the laboratory by conducting a soil column experiment under flow velocity and temperature conditions likely to be encountered in field situations. The transport characteristics of nitrite are not well known scientifically and the purpose of the study was to provide data that are considered essential for the development of programs for aquifer remediation and groundwater quality protection. The soil column experiment was performed at a constant flow rate for a period of 12 days using a 40 cm column. It involved a 1.5 mmol/L solution of sodium nitrite to which sodium bromide had been added as a chemically conservative tracer. The data were analysed using the CXTFIT 2.0 model code for the one-dimensional convective–dispersive transport of solutes. The breakthrough curve (BTC) for bromide revealed an average linear flow velocity of 0.3445 cm/h for the experiment and a longitudinal dispersivity of 4.16 cm for the sediment. The effective porosity was determined to be 29 %. The BTC for nitrite showed a significant delay compared to the bromide BTC, presumably due to some form of chemical reaction. Significantly, outflow concentrations of nitrite approached 1.5 mmol/L (the input value) towards the end of the experiment (after 10 days) indicating that the nitrite emerging at that time had not undergone significant chemical transformation during its passage through the column. Curve matching showed that nitrite behaviour could be represented by invoking chemical retardation using a retardation factor that begins at 2.5 and gradually decreases over time to a value of 1. However, the BTC for nitrite is better explained by the pH-dependent, chemical transformation of nitrite, presumably to nitrate or nitrogen gas, at the leading edge of the advancing chemical front that gives the appearance that nitrite is being retarded. Nitrite in more alkaline conditions behind the advancing front is significantly less affected by chemical transformation such that the apparent retardation effect diminishes. Further work is required to provide details on the nature of the reactions taking place and the constraints under which they operate.