Exploration of optimal disinfection model based on groundwater risk assessment in disinfection process.

Under the influence of different types of disinfectants and disinfection environments, the removal level of pathogens and the formation potential of disinfection by-products (DBPs) will have a dual impact on the groundwater environment. The key points for sustainable groundwater safety management are how to balance the positive and negative relationship and formulate a scientific disinfection model in combination with risk assessment. In this study, the effects of sodium hypochlorite (NaClO) and peracetic acid (PAA) concentrations on pathogenic E. coli and DBPs were investigated using static-batch and dynamic-column experiments, as well as the optimal disinfection model for groundwater risk assessment was explored using quantitative microbial risk assessment and disability-adjusted life years (DALYs) models. Compared to static disinfection, deposition and adsorption were the dominant factors causing E. coli migration at lower NaClO levels of 0-0.25 mg/L under dynamic state, while disinfection was its migration factor at higher NaClO levels of 0.5-6.5 mg/L. In contrast, E. coli removed by PAA was the result of the combined action of deposition, adsorption, and disinfection. The disinfection effects of NaClO and PAA on E. coli differed under dynamic and static conditions. At the same NaClO level, the health risk associated with E. coli in groundwater was higher, whereas, under the same PAA conditions, the health risk was lower. Under dynamic conditions, the optimal disinfectant dosage required for NaClO and PAA to reach the same acceptable risk level was 2 and 0.85 times (irrigation) or 0.92 times (drinking) of static disinfection, respectively. The results may help prevent the misuse of disinfectants and provide theoretical support for managing twin health risks posed by pathogens and DBPs in water treatment.

Source identification and apportionment of the nitrogen in groundwater based on isotope methods in the Beilin region of Suihua basin, northeastern China.

Multi-isotope method was used to analyze the migration and transformation characteristics of nitrogen in groundwater in the center of a typical confined water basin, and a simplified isotope mixing model was established to quantify the contribution of potential nitrate sources in the center of the basin. Based on the water quality monitoring results, the contour map of nitrate concentration in groundwater in the center of the basin was drawn. The results showed that the nitrate concentration in groundwater in the center of the basin increased gradually from upstream to downstream. The high value area of nitrate concentration in phreatic water is mainly affected by agricultural activities and infiltration of sewage discharge from upstream urban areas. The high value area of nitrate concentration in confined water is mainly due to the water level depression funnel caused by large exploitation of confined water. The quantitative results of N-O isotope mixing model for potential nitrate sources show that the main recharge sources of groundwater in the center of the basin are atmospheric precipitation, agricultural irrigation water, and the lateral inflow of upstream groundwater. Agricultural irrigation water has the highest contribution rate of 67.01%. The main recharge sources of confined aquifer in the center of the basin are phreatic water leakage and lateral inflow of upstream confined water. The contribution rate of upstream confined water is between 45.55% and 56.35%, which is basically maintained at about 50%. Compared with the calculation results of D-O isotope mixing model, the accuracy of the established N-O isotope mixing model meets the basic requirements. The results of this study can provide technical reference and theoretical support for the identification and quantitative research of potential nitrate sources in groundwater under the same type of hydrogeological conditions. PRACTITIONER POINTS: Multiple isotope fingerprint comparison to identify nitrate source contribution ratio. Migration and transformation of nitrogen in the center of a typical confined water basin Simplified the traditional isotope mixing model to quickly quantify the source of contamination.

Co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions.

To investigate co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions, NH4+ was selected as the target contaminant, and silicon and humic acid (HA) were selected as typical organic and inorganic colloids in groundwater. Column experiments were then conducted to investigate the transport of NH4+ colloids under various hydrochemical conditions. The results showed that because of the different properties of colloidal silicon and HA after combining with NH4+, the co-transport mechanism became significantly different. During transport by the NH4+-colloid system, colloidal silicon occupied the adsorption sites on the medium surface to promote the transport of NH4+, while humic acid (HA) increased the number of adsorption sites of the medium to hinder the transport of NH4+. The co-transport of NH4+ and colloids is closely related to hydrochemical conditions. In the presence of HA, competitive adsorption and morphological changes of HA caused NH4+ to be more likely to be transported at a higher ionic strength (IS = 0.05 m, CaCl2) and alkalinity (pH = 9.3). In the presence of colloidal silicon, blocking action caused the facilitated transport to be dependent on higher ionic strength and acidity (pH = 4.5), causing the recovery of NH4+ to improve by 7.99%, 222.25% (stage 1), and 8.63%, respectively. Moreover, transport increases with the colloidal silicon concentrations of 20 mg/L then declines at 40 mg/L, demonstrating that increased concentrations will lead to blocking and particle aggregation, resulting in delayed release in the leaching stage. Graphical abstract.

Fe-colloid cotransport through saturated porous media under different hydrochemical and hydrodynamic conditions.

To investigate the effect of different colloids on Fe migration in saturated porous media under different hydrochemical and hydrodynamic conditions, experiments were performed using colloidal silicon (inorganic) and colloidal humic acid (HA, organic), which are representative of the colloids in groundwater. Transport of Fe with and without colloid was investigated by column experiments using various porous media, colloid concentrations, ionic strengths (ISs), cation valences, and flow rates. The results show that colloidal silicon promotes and colloidal HA inhibits Fe transport, which is mainly because of their different bonding ratio, bonding modes with Fe and opposite surface charges between Fe-colloidal silicon and Fe-colloidal HA. Almost 100% of HA binds to Fe through the deprotonated functional groups, whereas only 13.3% of colloidal silicon binds to Fe, which is by electrostatic forces. Cotransport is also dependent on the hydrochemical and hydrodynamic conditions. For the Fe-colloidal silicon system, increasing the colloid concentration and flow rate, and decreasing the IS enhances Fe transport. Compared with colloidal silicon concentration = 10 mg/L, flow rate = 0.25 mL/min, and IS = 0.05 with CaCl2, a higher colloidal silicon concentration (20 mg/L), a higher flow rate (0.50 mL/min), and a lower IS (<0.0005 M) increase Fe recovery by 1.69%, 94.49% and 38.92%, respectively. Fe migration is also different in different porous media. For the Fe-colloidal HA system, Fe recovery decreases by 81.46% as the colloidal HA concentration increases from 0 to 20 mg/L. The type of porous medium and flow rate conditions have the same effects on Fe-colloidal HA transport as for colloidal silicon, although the electrical conditions have the opposite effect. With increasing IS, Fe-colloidal HA transport is enhanced because of competitive adsorption of the cations and Fe to colloidal HA and the porous medium.