A New Technique Of Ammonium-nitrogen Isotope Measurement For Groundwater And Its Application

Ammonium (NH4+) is a common contaminant in groundwater suffering from some types of contamination sources. It can provide scientific basis for proventing groundwater from ammonium contamination to understand sources of NH4+ contamination and mechanism of nitrogen transport and transformation in groundwater. Ammonium-nitrogen isotopes provide a direct means of source identification and nitrogen transport/transformation mechanism studies. In this paper, the azide method for ammonium-nitrogen isotope measurement was introdued for groundwater ammonium contamination studies based on reviewing literatures. A new technique of ammonium-nitrogen isotope measurement for groundwater using the azide method was established. A technique of sample preparation for soil ammonium-nitrogen isotope measurement using the azide method was explored. It was studied for sorption-desorption kinetics and characteristics of isotope fractionation of NH4+ in wastewater derived from the fertilizer plant. A lot of first-hand experimental data were obtained. The main findings and conclusions were as follows:1. There were many sample preparation methods for ammonium-nitrogen isotope measurement. Traditional NH4+-15N isotope measurement procedures included collection of NH4+ in samples, conversion of NH4+ to gas N2 and analysis of 15N isotope ratio mass spectrometry. Currently, under the premise of guaranteed some precision, analysis efficiency, easy operation and the sample size required for 15N isotope analysis were key factors for selection of measurement methods, while the azide method had advantages of high analysis efficiency, easy operation and small sample size.2. A new technique of ammonium-nitrogen isotope measurement for groundwater using the azide method was established in China, its precision being better than 0.5‰. The technique had many advantages of low NH4+ concentration, small sample size, easy operation and high precision.3. It was investigated whether soil extracts were suitable for analysis of NH4+-15N isotope using the azide method. Results showed that the yield of conversion of NH4+ to NO2- was low. Soil exacts must be pre-treated before using the method. Procedure of sandwiched diffusion sorption packets was selected for pre-treatmeat of soil exacts. Results showed that the pre-treatment procedure was expected to make soil NH4+-15N analysis using the azide method.4. Adsorption and desorption experiments were conducted for sediments of 8 different lithology samples to NH4+ in wastewater derived from the fertilizer plant using batch experiments compared to the solution of the same NH4 concentration prepared in the lab. Results showed sorption balance was aohived within a very short period (10 min) for any lithology and any solution. Different lithology had different, maximum adsorption capacity for NH4+ in wastewater derived from the fertilizer plant, being 1500-2800mg/kg dry for clay soil, and 174mg/kg dry soil for sand. Wastewater derived from the fertilizer plant had lower maximum adsorption capacity and reduced around 1000mg/kg compared to the prepared solution.5. The features of nitrogen isotope fractionation during NH4-sorption were studied using the new measurement technique of the azide method. The fractionation factorα(solid/liquid) for prepared solution concentrates between 1.00 to 1.02. With the sorption time increases,αvalue tended to 1, which indicated that the sorption fractionation was not obvious. The fractionation factors for wastewater ranged from 0.96 to 0.99, indicating that other composions in wastewater derived from the fertilizer plant can affect the isotope fractionation of soil to ammonium sorption. Moreover, with the increase of adsorption time, a values did not tend to 1, indicating significant isotope fractionation. Clay had higher degree of fractionation compared to silty clay.