The influence of Na+ and Ca2+ on the migration of colloids or/and ammonia nitrogen in an unsaturated zone medium

Interplay of legacy irrigation and nitrogen fertilizer inputs to spatial variability of arsenic and uranium within the deep vadose zone

The vadose zone is a reservoir for geogenic and anthropogenic contaminants. Nitrogen and water infiltration can affect biogeochemical processes in this zone, ultimately affecting groundwater quality. In this large-scale field study, we evaluated the input and occurrence of water and nitrogen species in the vadose zone of a public water supply wellhead protection (WHP) area (defined by a 50-year travel time to groundwater for public supply wells) and potential transport of nitrate, ammonium, arsenic, and uranium. Thirty-two deep cores were collected and grouped by irrigation practices: pivot (n = 20), gravity (n = 4) irrigated using groundwater, and non-irrigated (n = 8) sites. Beneath pivot-irrigated sites, sediment nitrate concentrations were significantly (p < 0.05) lower, while ammonium concentrations were significantly (p < 0.05) higher than under gravity sites. The spatial distribution of sediment arsenic and uranium was evaluated against estimated nitrogen and water loading beneath cropland. Irrigation practices were randomly distributed throughout the WHP area and presented a contrasting pattern of sediment arsenic and uranium occurrence. Sediment arsenic correlated with iron (r = 0.32, p < 0.05), uranium negatively correlated to sediment nitrate (r = -0.23, p < 0.05), and ammonium (r = -0.19 p < 0.05). This study reveals that irrigation water and nitrogen influx influence vadose zone geochemistry and mobilization of geogenic contaminants affecting groundwater quality beneath intensive agricultural systems.

Effects of colloids on ammonia nitrogen release under different ion conditions in natural sediments of Lake Taihu, China

Colloids act as vectors for accelerating contaminant movement in natural porous media such as lake sediments. Releasing characteristics of colloids and colloid-adsorbed ammonia nitrogen from lake sediments with the presence of monovalent and divalent cations were studied by an indoor anaerobic flooding incubation experiment. Results show that release of colloids was influenced by valence state and strength of the cation. In the presence of Na⁺ and Ca²⁺, the concentrations of the colloids in the overlying water were 11-163 mg/L and 13-88 mg/L during the entire incubation process and their values increased by 12.7%-122.3% and 1.5%-29.1%, respectively, compared to the control group, which indicated that the promoting effect of monovalent cations on release of colloids was more obvious than that of divalent cations. However, the total mass of colloids release reduced with the increasing ionic strength. Colloid-adsorbed ammonia nitrogen in the overlying water reached 0.15-1.72 mg/L and 0.15-1.12 mg/L with the presence of Na⁺ and Ca²⁺ and was higher 61.7%-161.7% and 21.3%-80.9%, respectively, than in the control group, indicating a consequent effect of ion conditions on the release of ammonium nitrogen from sediments. A significant positive correlation between colloids and ammonia nitrogen concentration further shows that colloidal activity determinately resulted in the increase or decrease in the ammonia nitrogen concentration in the overlying water, which could adsorb ammonia nitrogen and act as vehicles to carry ammonia nitrogen together into the aqueous medium or sink into the sediment. The release of ammonia nitrogen is possibly enhanced by colloidal behavior and varies with spatiotemporal ionic conditions in natural sediments. These findings are essential for improving the understanding of the geological fate of environmental colloids and associated nutritive salts, which provide scientific basis and technical support for the control of endogenous pollution and the comprehensive treatment of water bodies in Lake Taihu.

Aluminum hydroxide colloid facilitated transport of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) in porous media

In this study, aluminum hydroxide colloids, which widely exist in soils, were selected to investigate their effect on the infiltration of an abundant congener of PBDEs (BDE-47) to groundwater. The batch and column experiments were conducted to study the co-migration of aluminum hydroxide colloid and BDE-47 in two sand media with particle sizes of 2-4.75 and 0.15 mm. The results indicated that the colloid significantly increased the transport of BDE-47 to 24.32% and 65.84% in the vadose zone of coarse and fine sand columns, respectively. The adsorption and blocking effect were found to be the two main functions during the co-migration of aluminum hydroxide colloids with BDE-47. Specifically, BDE-47 that adsorbed on colloids moved faster in the coarse porous media, and the breakthrough peak of BDE-47 appeared early in the media at an approximate pore volume of 0.15. In comparison, colloids that adsorbed onto the fine porous media formed a layer that blocked the adsorption of BDE-47 onto the fine porous media. This weakened the protection capacity of the vadose zone and led to a greater than 80% amount of BDE-47 breakthrough to the vadose zone.

Co-transport of chromium(VI) and bentonite colloidal particles in water-saturated porous media: Effect of colloid concentration, sand gradation, and flow velocity

The transport of pollutants inside the groundwater system is profoundly affected by absorption and transmission via colloid or soil particles. Therefore, it is essential to investigate the significant pollutants (Such as hexavalent chromium (Cr(VI))) transfer in the presence of colloid particles that can facilitate or retain this transfer. For this purpose, an experiment is carried out in a saturated porous media column to study the bentonite concentration, flow velocity and sand grain size effects on co-transport of Cr(VI) with bentonite. The results of this study demonstrated that the colloid particles facilitate the transfer of Cr(VI) by 30% in 200 mg/l bentonite colloids concentration. The amount of transmitted Cr(VI) is decreased by increasing the bentonite colloids concentration from 200 mg/l to 300 mg/l. As the flow velocity increased from 2 cm/min to 3.3 cm/min, the amount of transferred Cr(VI) increased by 7%. The results show that with reducing the sand grain size, the amount of transmitted bentonite and Cr(VI) is reduced that this effect is more sensible in bentonite transport. As a result, it can be noted that the bentonite colloidal particles according to its concentration and experimental conditions, may facilitate or retain the Cr(VI) transport and sand gradation has a significant impact on colloid and pollutant transmission.