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Soil aquifer treatment (SAT) is an effective and sustainable technology for wastewater or stormwater treatment, storage, and reuse. During SAT, the vadose zone acts as a pseudo-reactor in which physical and biochemical processes are utilized to improve the infiltrated-water quality. Dissolved oxygen (DO) is necessary for aerobic microbial oxidation of carbon and nitrogen species in the effluent. Therefore, to enhance aeration, SAT is generally operated in flooding and drying cycles. While long drying periods (DPs) lead to better oxidizing conditions and improve water quality, they reduce recharge volumes. As the population grows, the quantity of effluent directed to SAT sites increases, and increasing recharge volumes become a concern and often a limiting factor for SAT usage. In this study, direct subsurface air injection SAT (Air-SAT) was tested as an alternative to long-DP operation. Six long-column experiments were conducted (2 m column) that aimed to examine the effect of air injection on the soil's water content, oxidation–reduction potential (ORP), DO concentrations, infiltrated amounts, and ultimate outflow quality. In addition to basic parameters, such as dissolved organic C (DOC) and N species, the effluent quality analysis also included an examination of three emerging water contaminants: ibuprofen, carbamazepine, and 1H-benzotriazole. Pulsed-air-injection experiments were conducted during continuous flooding using different operation modes (i.e., air pulse durations, frequencies, and airflow rates). Our results show that Air-SAT operation doubled the time during which infiltration was possible (i.e., the infiltration was continuous with no downtime) and allowed up to a 46 % higher mean infiltration rate in some cases. As a result, the infiltration volumes in the Air-SAT modes were 47 %–203 % higher than conventional flooding–drying operation (FDO). A longer air pulse duration (60 min vs. 8 min) and higher airflow rate (∼2 L min -1 vs. ∼1 L min -1) led to a higher mean infiltration rate, whereas a high pulse frequency (4.5 h -1) led to a lower mean infiltration rate compared with low-frequency operation (24 h -1). Air injection also allowed good recovery of the ORP and DO levels in the soil, especially in the high-frequency Air-SAT experiments, where steady aerobic conditions were maintained during most of the flooding. Consequently, the mean DOC, total Kjeldahl N (TKN), and ibuprofen removal values in these experiments were up to 9 %, 40 %, and 65 % higher than those with FDO, respectively. However, high-frequency Air-SAT during continuous flooding also led to significant deterioration of the mean infiltration rate, probably due to enhanced biological clogging. Hence, it may be more feasible and beneficial to combine it with conventional FDO, allowing a steady infiltration rate and increased recharge volumes while sustaining high effluent quality. While these results still need to be verified at full scale, they highlight the possibility of using air injection to minimize the DP length and alleviate the pressure on existing SAT sites. [ABSTRACT FROM AUTHOR]

The capillary fringe (CF) is characterized by transient and steep redox gradients and is thought to be a hot spot for biogeochemical processes. Understanding chemical fate and transport in the CF is significant, however, biogeochemical dynamics at the CF are poorly understood because of the difficulty to measure representatively with high spatio-temporal resolution at depths under dynamic hydrologic regimes. Hydrogeophysics is a developing field that uses minimally intrusive and quick response methods to monitor hydrological properties. Two geoelectrical methods [spectral induced polarization (SIP) and self-potential (SP)], which are sensitive to the solid–liquid interfaces (SIP) and biogeochemical processes (SP) can address the above difficulty. The challenge lies on linking the geoelectrical responses with biogeochemical processes, where many different processes contribute to the signals. We conducted a soil column experiment under five hydrologic regimes focusing on nitrogen transformations with SIP and SP measurements: (1) a static regime with a stable water level; (2) an infiltration regime with periodic pulse infiltration events with a constant water level and (3) fluctuating regimes with water level fluctuations under three drying-wetting frequencies (6/12/18-day-cycle). This is the first large lab-scale work in a well-controlled and highly instrumented soil column. The dynamic hydrologic conditions stimulated complex biogeochemical processes at the CF, and therefore the SIP and SP signals result from many physical and biogeochemical processes. Therefore, we relied on statistical analysis in this study for a novel interpretation. Spearman correlation analysis supported water content played the most important role in real conductivity (σ ′) dynamics in the vadose zone, whereas fluid conductivity dominated σ ′ in the saturated zone. Both correlation analysis and spatial moment analysis implicated that water content was the driving factor for both σ ′ and imaginary conductivity (σ ″). A multiple linear regression model indicated the gradient of redox potential, the gradient of soil matric potential and water content were the three main influencing factors for the SP signals. We proposed that the water level fluctuation can efficiently facilitate microbial electron transfer through ions transport between the different redox zones, and aggregate redox processes to create SP signal gradients. Depth zonation analysis, using six environmental indexes (E h and nitrogen species; water content; real conductivity; imaginary conductivity; SP signal; microbial community composition), suggested that water content induced by soil hydrology was the most dominant factor, captured by all the indexes. In turn, it led to indirect inference on biogeochemical processes and resultant geoelectrical signals. Applying geoelectrical methods to such biogeochemical processes will not only lead to a better understanding of the mechanistic meanings of the geoelectrical signals, but also build relationships between geoelectrical signals and biogeochemical parameters to facilitate a novel way to monitor biogeochemical processes. [ABSTRACT FROM AUTHOR]

Soil aquifer treatment (SAT) is an effective and sustainable technology for wastewater or stormwater treatment, storage and reuse. During SAT, the vadose zone acts as a pseudo reactor in which physical and biochemical processes are utilized to improve the infiltrated water quality. Dissolved oxygen (DO) is necessary for aerobic microbial oxidation of carbon and nitrogen species in the effluent. Therefore, to enhance aeration, SAT is generally operated in flooding and drying cycles. While long drying periods (DPs) lead to better oxidizing conditions and improve water quality, they reduce recharge volumes. As the population grows, the quantity of effluent directed to SAT sites increases and increasing recharge volumes become a concern and often a limiting factor for SAT usage. In this study, direct subsurface air injection SAT (Air-SAT) was tested as an alternative to long DPs operation. Six long column experiments were conducted, aiming to examine the effect of air injection on the soil's water content, oxidation-reduction potential (ORP), DO concentrations, infiltrated amounts and ultimate outflow quality. In addition to basic parameters such as dissolved organic-C (DOC) and N species, the effluent quality analysis also included an examination of three emerging water contaminants – Ibuprofen, Carbamazepine and 1H-benzotriazole. Pulsed air injection experiments were conducted during continuous flooding at different operation modes (i.e., air pulse durations, frequencies and airflow rates). Our results show that the Air-SAT operation doubled the infiltration time (i.e., the infiltration was continuous with no off-time) and allowed up to 46 % higher infiltration rate in some cases. As a result, the infiltrated volumes in the Air-SAT modes were 47–203 % higher than the conventional flooding-drying operation (FDO). Longer air pulse duration (60 vs. 8 min) and higher airflow rate (~2 vs. ~1 SLPM) led to a higher infiltration rate, while a high pulse frequency (4.5−1 h−1) led to a lower infiltration rate compared to low-frequency operation (24−1 h−1). The air injection also allowed good recovery of the ORP and DO levels in the soil, especially in the high-frequency Air-SAT experiments, where steady aerobic conditions were maintained during most of the flooding. Consequently, the mean DOC, total Kjeldahl N (TKN), and Ibuprofen removals in these experiments were higher than in FDO by up to 9, 40, and 65 %, respectively. However, high-frequency Air-SAT during continuous flooding also led to significant deterioration in the infiltration rate, probably due to enhanced biological clogging. Hence, it may be more feasible and beneficial to combine it with the conventional FDO, allowing a steady infiltration rate and increased recharge volumes, while sustaining high effluent quality. While those results still need to be verified at full scale, they open the possibility of using air injection to minimize the DPs length and alleviate the pressure over existing SAT sites. [ABSTRACT FROM AUTHOR]

Subsurface contamination is a significant problem due to excessive fertigation and industrial and domestic wastewater discharge. With numerical modeling and geophysical tool development, subsurface contaminant research has become easier to implement and study. However, there is still a gap in coupling the biochemical processes and geophysical signals. Such a coupling model is needed to facilitate understanding subsurface processes and provide further theoretical basis to practice and field monitoring. Thus, this research aims to simulate the self-potential (SP) signature in response to physical and biochemical dynamics in the subsurface. For the physico-bio-chemical model, the processes of water flow, solute transport, biochemical reactions, microbial dynamics, adsorption, and gas flow are considered. Specifically, the biochemical cycles related to C, N, Mn, Fe, and S are incorporated in the model. The physico-bio-chemical model is then coupled with the SP model. The SP model is addressed by Poisson’s continuity equation, based on streaming and redox potential contribution. The streaming potential is calculated by the effective excess charge density and the water flow velocity, while the Butler-Volmer equation solves the redox potential. The results show that redox processes dominate the SP signals. Oxygen and nitrate concentrations present positive relationships with redox potential and dominate the redox potential in the oxic and anoxic environment, respectively. Nitrification and dissolved organic carbon (DOC) aerobic oxidation rates show positive relationships with redox potential. In contrast, the denitrification rate presents a negative relationship. The higher reaction rates for different redox processes also correspond to their optimal redox potential ranges. The streaming potential affected by water content and flux contributes little to SP, and the negative values along with soil depth become less remarkable. Generally, the SP and redox potential model can better reflect redox species concentrations and reaction rates, while the streaming potential model can reflect the water content and flux dynamics. Thus, the research can guide the detection of redox-sensitive contamination and water leakage in the subsurface. [ABSTRACT FROM AUTHOR]

Soil aquifer treatment (SAT) is a tertiary process for wastewater treatment where the wastewater infiltrates through a thick vadose zone for purification and storage in the underneath aquifer. SAT infiltration basins are typically flooded intermittently, while maintaining a fixed ratio between the wetting and the drying stages. However, infiltration basins exhibit different physical and chemical properties, limiting the generalization of SAT operation to attain optimal efficiency. Since frequent sampling of the soil pore water to verify the SAT's biodegradation efficiency can be arduous, continuous monitoring of the SAT vadose zone's physico-chemical conditions is required. In this study, redox potential (Eh) was continuously monitored, together with other variables such as water content (θ), soil temperature, and gaseous oxygen (O2), at multiple depths of a SAT vadose zone throughout the year and while the system was constrained to different operational modes. Hydrological models were calibrated and validated to water content observations, and they illustrated the seasonal changes in water infiltration. Furthermore, it was shown that under long wetting stages during winter, there was a reduction in the SAT's drainage capabilities. The Eh observations, under long wetting stages, demonstrated larger variability and very negative values as ambient temperature increased. Assembling the daily Eh observations illustrated that a wetting stage should cease after about 30 hours, once suboxic conditions are established. A drying stage's optimal duration should be 36 hours, according to the Eh and O2 observations during summer and winter. Ultimately, the study shows that the length of wetting and drying stages should be defined separately, rather than by adhering to the wetting/drying ratio. [ABSTRACT FROM AUTHOR]

Breakthrough curves (BTCs) are a valuable tool for qualitative and quantitative examination of transport patterns in porous media. Although breakthrough (BT) experiments are simple, they often require extensive sampling and multicomponent chemical analysis. In this work, we examine spectral induced polarization (SIP) signals measured along a soil column during a BT experiment in a homogeneous and heterogeneous soil profiles. Soil profiles were equilibrated with an NaCl background solution and then a constant flow of CaCl2 solution was applied. SIP signature was recorded, and complementary ion analysis was performed on the collected outflow samples. Our results confirm that changes to the pore-water composition, ion exchange processes and profile heterogeneity are detectable by SIP: the real part of the conductivity-based BTCs clearly indicated the BT of the non-reactive ions as well as the retarded BT of Ca2+. The imaginary part of the conductivity-based curves reacted to the changes in ion mobility around the electrical double layer (EDL) and indicated the initiation and the termination of the Na+-Ca2+ exchange reaction. Finally, both the real and imaginary components of the complex conductivity reacted to the presence of a coarser textured layer in the heterogeneous profile. [ABSTRACT FROM AUTHOR]