A Comprehensive Evaluation Model of Ammonia Pollution Trends in a Groundwater Source Area along a River in Residential Areas

Solution, exchangeable and fixed ammonium in natural diatomite as a simulated PRB material: effects of adsorption and bioregeneration processes

Ammonia nitrogen (NH₄⁺-N) is widely found in aquifers with strong reducibility or poor adsorptivity as a dissolved inorganic nitrogen pollutant. The application of adsorbents with effective long-term in situ bioregeneration as permeable reactive barrier (PRB) media for nitrogen removal has raised concern. In this study, the advantage of natural diatomite as a PRB material was investigated by exploring its NH₄⁺-N adsorption and desorption characteristics, and the ability of diatomite and zeolite to be loaded nitrifying bacteria was also compared. The results showed that the exchangeable ammonium from chemical-monolayer adsorption was the main form of NH₄⁺-N and was adsorbed by diatomite. Moreover, the adsorption process was limited with a maximum adsorption capacity of 0.677 mg g^-1. However, diatomite demonstrated an excellent loading of aerobic-heterotrophic microorganisms, even stronger than zeolite. Compared with zeolite reactors, a higher OD₆₀₀ value of nitrifiers, a faster NH₄⁺-N degradation rate and more abundant functional genes were observed during the bioregeneration process of diatomite. Both the solution and exchangeable ammonium forms were bioavailable, and the regeneration of diatomite was more than 80.0% after two days. Moreover, desorption-biodegradation was systematically analysed to determine the bioregeneration mechanism of diatomite. Diatomite with good regeneration ability can be used as a competitive alternative to address sudden nitrogen pollution.

Prediction of the impact of benzo[a]pyrene on shallow groundwater during natural infiltration of reclaimed water-receiving rivers: A case study of Liangshui, China

The quality of groundwater along rivers is greatly affected by long-term infiltration from surface water, especially reclaimed water-receiving rivers. To predict the degree of influence of contaminated river water on groundwater quality, the spatiotemporal distribution and migration evolution prediction of benzo[a]pyrene (B(a)P) was monitored and simulated by Hydrus-coupled Groundwater Modeling Systems (GMS) model in terms of reclaimed water-receiving Liangshui River. The prediction results indicated the goodness-of-fit of this coupled model, according to the model efficiency (E: 0.78-0.93), the mean absolute error (MAE: 0.01-0.32 m) and the root-mean-square error (RMSE: 0.06-0.35 m). The vertical infiltration rate of B(a)P in the vadose zone was 0.102 m^-1, which was only 0.73% that of water. B(a)P penetrated the 16 m depth vadose zone for 63 years owing to the attenuation function of adsorption and biodegradation, with contribution ratios of 78.4% and 19.3%, respectively. However, once B(a)P intersects with groundwater, the migration of B(a)P is dominated by horizontal migration due to downward movement along the groundwater flow direction. The migration rate of B(a)P in groundwater was 6.65 m/y in the horizontal direction, which was 2.42 and 16.22 times higher than the dispersion rate in the longitudinal and vertical directions, respectively. The spatiotemporal distribution indicated that the B(a)P concentration decreased with the crow-fly distance from river with attenuation rate constants of 1.19 × 10^-4, 3.05 × 10^-4, and 3.67 × 10^-3 m^-1 over horizontal, longitudinal, and vertical direction, respectively, which were negatively correlated with migration rate. However, the B(a)P content increased over the extension of infiltration time with an accumulation rate of 7.3 × 10^-2 d^-1. The migration and accumulation of B(a)P induced potential health risks to groundwater-based drinking water safety, which resulted in the groundwater safety utilization range decreasing from 450 m, 283 m, and 20.1 m-583 m, 338 m, and 28.2 m far from the river over the horizontal, longitudinal, and vertical directions, respectively, 20 years later. This study provides a numerical modeling solution for the viable spatiotemporal evolution of B(a)P in groundwater and an effective decision-making tool for the safe utilization of groundwater as drinking water.

Groundwater Nitrate Pollution Sources Assessment for Contaminated Wellfield

Nitrates are one of the most common groundwater contaminants and they come from different sources. The paper presents a study of groundwater quality at Varaždin wellfield in the north part of Croatia. The nitrate concentration at this location has been above the maximum allowed concentration for several decades, which has made the opening of new wellfields costly. Based on the previously developed groundwater flow model, a model that covers the narrow area of the wellfield is developed. The influential zone of the observed wellfield in working conditions is determined. Based on the developed model, the sources of nitrate pollution are located, which can be generally divided into non-point and point sources. Considering the time of groundwater retention in the horizontal flow, it is concluded that the water protection zones are marked following the applicable ordinance. Based on the developed groundwater flow model at the observed wellfield, a simulation of nitrate pollution propagation by advection and dispersion processes is performed. The simulation results point out the location of the poultry landfill as the largest source of nitrate pollution. However, poultry farms, which are located in the influence area of the wellfield, also contribute significantly to the nitrate concentration at the wellfield.