Iron (Fe²⁺), manganese (Mn²⁺), and ammonium (NH₄⁺) are the three most common contaminants in anaerobic groundwater and are typically removed in rapid sand filters in a series of simultaneous, uncontrolled, and interconnected redox reactions. In this study, we demonstrated separation of these oxidation processes, including reversing the order of NH₄⁺and Mn²⁺oxidation, allowing Mn²⁺to oxidize before NH₄⁺. To achieve this uncommon sequence, the filter was operated with low O₂ concentrations (∼0.02 mmol/L, ∼0.5 mg/L) and a high pH (∼8). Under these conditions, Mn²⁺ oxidation is consuming all available O₂, suppressing the occurrence of NH₄⁺oxidation. In the filter with low O₂ (0.08 mmol/L, ∼3 mg/L) and low pH (∼6.8), the opposite was observed, as Mn²⁺ oxidation was delayed under these conditions, resulting in complete O₂ consumption by NH₄⁺-oxidizing bacteria. Reactive transport modelling and parameter estimation revealed that Mn²⁺ oxidation is one order of magnitude faster in absence of NH₄⁺ oxidation (1.4 × 10⁻² vs 2.5 × 10⁻³ mmol/L), whereas NH₄⁺ oxidation seemed to be accelerated by simultaneous Mn²⁺ oxidation (6.8 × 10⁻³ vs 2.9 × 10⁻² s⁻¹). This interconnection between Mn²⁺ and NH₄⁺ oxidation was further emphasized by the observation of Mn²⁺ release in the presence of NO₂⁻. In conclusion, this study has shown that a shift from conventional aerated groundwater treatment to sequential oxidation in separate filters offers (i) a more controllable system, (ii) the potential to optimize the rates of each oxidation process separately, which would ultimately result in higher flows and less backwashing.

Removal of carcinogenic arsenic (As) from groundwater is essential for providing safe drinking water. Arsenate (As(V)) is more effectively removed in groundwater filters than arsenite (As(III)), making the oxidation of As(III) to As(V) a key step in the treatment process. This study distinguishes between surface-catalytic and biological As(III) oxidation on natural manganese oxide (MnO_x) coated filter sand, since it is unknown which pathway dominates in filters. The MnO_xcoated sand was collected from a full-scale groundwater filter and consisted of a mixture of different abiotically and biologically formed Mn oxides, such as Birnessite and Todorokite. A lab-scale filter setup was operated with As(III)-containing water. Within 3 weeks, a shift from surface-catalytic to biological As(III) oxidation was observed. Initially, surface-catalytic As(III) oxidation (k_CHEM= 0.318 min^–1) was coupled to Mn(II) release at a ratio of 0.96, approximating the stoichiometric ratio of 1. This coupling disappeared over time, indicating the biological nature of the reaction, as confirmed by microbial inhibition. An increase in relative abundance of the known As-oxidizing families Comamonadaceae, with Polaromonas as the dominant genus, and Microscillaceae were found post experiments. Except for these changes, the microbial community on the sand grains stayed relatively similar prior to and post experiments. No significant changes in the physical-chemical properties of the MnO_xcoating were found post experiments. A first-order biological As(III) oxidation rate constant k_BIOof 4.64 min^–1was found, yielding a half-life of 9 s. This represents a 14-fold acceleration compared with surface-catalytic oxidation, revealing that kinetic limitations rather than surface passivation can be attributed to the loss of surface-catalytic oxidation. Our study demonstrates that biological oxidation of As(III) can outpace the acknowledged oxidizing power of MnO_x, offering a potential new pathway for the development of effective As removal systems.

Iron (Fe²⁺), manganese (Mn²⁺), and ammonium (NH₄⁺) oxidation processes were studied in three single media and three dual media full-scale rapid sand filters (RSFs) using reactive transport modelling (RTM) in PHREEQC and parameter estimation using PEST. Here, we present the insights gained into the spatial distribution of Fe and Mn mineral coatings in RSFs and its influence on the oxidation sequence and rates. Fe²⁺ and Mn²⁺ oxidation predominantly occurred simultaneously in the RSFs, contrary to the expected sequential oxidation based on Gibbs free energy calculations. During backwashing, RSF grains become fully mixed, which initiates heterogeneous Mn²⁺ oxidation on Mn-coated grains that end up in the top layer. The resulting grains have a mixed Fe/Mn mineral coating, which is limiting heterogeneous Mn²⁺ oxidation due to the limited Mn mineral surface available. Mixed coatings did not seem to affect Fe²⁺ oxidation rates, instead oxidation rates were increasing at lower pH. We found that RSFs can be designed to spatially separate Fe²⁺ and Mn²⁺ oxidation, which results in optimal conditions for Mn²⁺ oxidation. The RSF needs to consist of two layers with varying density to inhibit mixing and complete Fe²⁺ oxidation should occur in the top layer. The developed RTM can be used to estimate the depth at which Fe²⁺ oxidation is complete, and thus the ideal intersection depth of the two layers. A novel perspective is provided on how mineral coating distribution in single and dual media filters influence removal rates and the sequence of oxidation, which contributes to the design of more efficient groundwater filters.

Rapid sand filters (RSF) are an established and widely applied technology for the removal of dissolved iron (Fe²⁺) and ammonium (NH₄⁺) among other contaminants in groundwater treatment. Most often, biological NH₄⁺oxidation is spatially delayed and starts only upon complete Fe²⁺ depletion. However, the mechanism(s) responsible for the inhibition of NH₄⁺oxidation by Fe²⁺ or its oxidation (by)products remains elusive, hindering further process control and optimization. We used batch assays, lab-scale columns, and full-scale filter characterizations to resolve the individual impact of the main Fe²⁺ oxidizing mechanisms and the resulting products on biological NH₄⁺ oxidation. modeling of the obtained datasets allowed to quantitatively assess the hydraulic implications of Fe²⁺ oxidation. Dissolved Fe²⁺ and the reactive oxygen species formed as byproducts during Fe²⁺ oxidation had no direct effect on ammonia oxidation. The Fe³⁺ oxides on the sand grain coating, commonly assumed to be the main cause for inhibited ammonia oxidation, seemed instead to enhance it. modeling allowed to exclude mass transfer limitations induced by accumulation of iron flocs and consequent filter clogging as the cause for delayed ammonia oxidation. We unequivocally identify the inhibition of NH₄⁺oxidizing organisms by the Fe³⁺ flocs generated during Fe²⁺ oxidation as the main cause for the commonly observed spatial delay in ammonia oxidation. The addition of Fe³⁺ flocs inhibited NH₄⁺oxidation both in batch and column tests, and the removal of Fe³⁺ flocs by backwashing completely re-established the NH₄⁺removal capacity, suggesting that the inhibition is reversible. In conclusion, our findings not only identify the iron form that causes the inhibition, albeit the biological mechanism remains to be identified, but also highlight the ecological importance of iron cycling in nitrifying environments.

Iron-based adsorbents are commonly used to remove arsenic (As) from water for drinking water purposes. Here, we study the role of biological As(III) oxidation on iron-based adsorbents in filters and its effect on overall As uptake. A lab-scale filter with iron oxide coated sand (IOCS), a commonly used adsorbent, was operated with water containing As(III) and As(V), while water samples were taken periodically over its height. As(III) oxidation initiated after approximately 10 days and increased to a first order rate constant of 0.09 s⁻¹ after 57 days resulting in full oxidation of As(III) in <50 s. Consequently, the filter shifted from an As(III) to an As(V) adsorbing filter. Oxidation was not observed after inhibiting the microbial activity using sodium azide confirming its biogenic nature. This implies that As(III) oxidizing biomass can grow on iron-based adsorbents in water filters without requiring inoculation. As the experimental conditions were similar to full-scale As treatment plants, we believe that biological As(III) oxidation is widely overlooked in these systems. Occurrence of biological oxidation is, however, beneficial for removal, as at pH <8 the adsorption capacity for As(V) can be up to 10-fold higher than for As(III). With these new insights, arsenic treatment using iron-based adsorbents can be further optimized. We suggest a more robust new design with a biological active As(III) oxidizing top layer and an As(V) adsorbing bottom layer.

Well clogging was studied at an aquifer storage transfer and recovery (ASTR) site used to secure freshwater supply for a flower bulb farm. Tile drainage water (TDW) was collected from a 10-ha parcel, stored in a sandy brackish coastal aquifer via well injection in wet periods, and reused during dry periods. This ASTR application has been susceptible to clogging, as the TDW composition largely exceeded most clogging mitigation guidelines. TDW pretreatment by sand filtration did not cause substantial clogging at a smaller ASR site (2 ha) at the same farm. In the current (10 ha) system, sand filtration was substituted by 40-μm disc filters to lower costs (by 10,000–30,000 Euro) and reduce space (by 50–100 m²). This measure treated TDW insufficiently and injection wells rapidly clogged. Chemical, biological, and physical clogging occurred, as observed from elemental, organic carbon, 16S rRNA, and grain-size distribution analyses of the clogging material. Physical clogging by particles was the main cause, based on the strong relation between injected turbidity load and normalized well injectivity. Periodical backflushing of injection wells improved operation, although the disc filters clogged when the turbidity increased (up to 165 NTU) during a severe rainfall event (44 mm in 3 days). Automated periodical backflushing, together with regulating the maximum turbidity (<20 NTU) of the TDW, protected ASTR operation, but reduced the injected TDW volume by ~20–25%. The studied clogging-prevention measures collectively are only viable as an alternative for sand filtration when the injected volume remains sufficient to secure the farmer’s needs for irrigation.

Irrigation with surface water carrying plant pathogens poses a risk for agriculture. Managed aquifer recharge enhances fresh water availability while simultaneously it may reduce the risk of plant diseases by removal of pathogens during aquifer passage. We compared the transport of three plant pathogenic bacteria with Escherichia coli WR1 as reference strain in saturated laboratory column experiments filled with quartz sand, or sandy aquifer sediments. E. coli showed the highest removal, followed by Pectobacterium carotovorum, Dickeya solani and Ralstonia solanacearum. Bacterial and non-reactive tracer breakthrough curves were fitted with Hydrus-1D and compared with colloid filtration theory (CFT). Bacterial attachment to fine and medium aquifer sand under anoxic conditions was highest with attachment rates of max. k _att1 = 765 day ^-1 and 355 day ^-1, respectively. Attachment was the least to quartz sand under oxic conditions (k _att1 = 61 day ^-1). In CFT, sticking efficiencies were higher in aquifer than in quartz sand but there was no differentiation between fine and medium aquifer sand. Overall removal ranged between < 6.8 log ₁₀ m ⁻¹ in quartz and up to 40 log ₁₀ m ⁻¹ in fine aquifer sand. Oxygenation of the anoxic aquifer sediments for two weeks with oxic influent water decreased the removal. The results highlight the potential of natural sand filtration to sufficiently remove plant pathogenic bacteria during aquifer storage.

An aquifer storage transfer and recovery (ASTR) system was studied in which tile drainage water (TDW) was injected with relatively high NO₃ (about 14 mg/L) concentrations originating from fertilizers. Here we present the evolution of denitrification kinetics at 6 different depths in the aquifer before, and during ASTR operation. First-order denitrification rate constants increased over time before and during the first days of ASTR operation, likely due to microbial adaptation of the native bacterial community and/or bioaugmentation of the aquifer by denitrifying bacteria present in injected TDW. Push-pull tests were performed in the native aquifer before ASTR operation. Obtained first-order denitrification rate constants were negligible (0.00–0.03 d⁻¹) at the start, but increased to 0.17–0.83 d⁻¹ after a lag-phase of about 6 days. During the first days of ASTR operation in autumn 2019, the arrival of injected TDW was studied at 2.5 m distance from the injection well. First-order denitrification rate constants increased again over time (maximum >1 d⁻¹). Three storage periods without injection were monitored in winter 2019, fall 2020, and spring 2021 during ASTR operation. First-order rate constants ranged between 0.12 and 0.61 d⁻¹. Denitrification coupled to pyrite oxidation occurred at all depths, but other oxidation processes were indicated as well. NO₃ concentration trends resembled Monod kinetics but were fitted also to a first-order decay rate model to facilitate comparison. Rate constants during the storage periods were substantially lower than during injection, probably due to a reduction in the exchange rate between aquifer solid phases and injected water during the stagnant conditions. Denitrification rate constants deviated maximally a factor 5 over time and depth for all in-situ measurement approaches after the lag-phase. The combination of these in-situ approaches enabled to obtain more detailed insights in the evolution of denitrification kinetics during AS(T)R.

A field injection experiment was performed in an anoxic sandy aquifer over 6 days to assess sorption characteristics of 7 commonly applied pesticides in agriculture and 2 frequently detected metabolites. Pesticide use changed considerably in the last decades, and there is insufficient knowledge of the fate of currently used pesticides in aquifers. Injected water arrival was monitored at 6 depth intervals of 1 m ranging from 11.4 to 32.2 m-below surface level with varying organic carbon contents (0.057–0.91%d.w.) to examine intra-aquifer variations in sorption. Observed pesticide concentrations were fit using a non-linear least squares routine to an advection-dispersion equation, from which retardation factors (R) were obtained. Pesticide degradation did not significantly influence the simulated R during the experiment. We observed that bentazon and cycloxydim were most mobile with R < 1.1 at all depths. Desphenyl chloridazon, methyl desphenyl chloridazon, and imidacloprid were, on average, less mobile, with maximum R of 1.5. Boscalid, chloridazon, fluopyram, and flutolanil showed a larger range of R, and R > 2.0 were observed in the shallowest part of the aquifer. Largest R were observed at the top of the aquifer and decreased with depth. K_oc values varied similarly, which indicates that sorption is not only influenced by sedimentary organic matter (SOM) content but also by its sorption reactivity. Obtained sorption parameters were substantially lower than reported in a widely used pesticide sorption database, which suggests that sorption parameters are influenced by methodological differences and variations in the sorption reactivity of SOM. The large intra-aquifer variations in pesticide sorption highlights that aquifer heterogeneity should be considered in groundwater risk assessments.

Degradation of 7 common pesticides (bentazon, boscalid, chloridazon, fluopyram, flutolanil, imidacloprid, and methoxyfenozide) and 2 metabolites of chloridazon (desphenyl-chloridazon, and methyl-desphenyl-chloridazon) was studied in an anoxic and brackish sandy aquifer before and during Aquifer Storage Transfer and Recovery (ASTR) operation. Fresh tile drainage water was injected and stored for later re-use as irrigation water. We hypothesized that electron acceptors (O₂, NO₃), dissolved organic carbon (∼24.7 mg/L), nutrients (NO₃: ∼14.1 mg/L, NH₄: ∼0.13 mg/L, PO₄: ∼5.2 mg/L), and biodegrading bacteria in tile drainage water could stimulate degradation of the pesticides and metabolites (ranging between 0.013 and 10.8 μg/L) introduced in the aquifer. Pesticide degradation was studied at 6 depths in the aquifer using push-pull tests lasting ±18 days before the onset of ASTR operation. Degradation was too limited to quantify and/or could not be assessed because of the potential occurrence of pesticide retardation. Utilizing push-pull tests to obtain degradation constants should only be considered in future studies for non-retarding pesticides with relative low half-lives (here <20 days). During ASTR operation, pesticide degradation was studied at the same depths during 3 storage periods equally spread over 1.5 years of ASTR operation. Overall, trends of degradation were observed, although with relatively high half-lives of at least 53 days. Microbial adaptation of the aquifer and/or bioaugmentation by the injected biodegrading bacteria did not result in enhanced degradation during consecutive storage periods. Operational monitoring data over longer periods and distances yielded half-lives of at least 141 days. The slow degradation mostly agrees with previous studies. The injected tile drainage water composition did therefore not notably stimulate pesticide degradation. The relatively persistent behavior of the studied pesticides/metabolites implies that ASTR abstracted water will have generally high pesticide concentrations, and non-abstracted water may form a contamination risk for the surrounding native brackish groundwater.

Push-pull tests (PPTs) were evaluated with 1-D radially axisymmetric multi-component geochemical reactive transport modelling (RTM) to assess aquifer reactivity controlling groundwater quality. Nutrient fate and redox processes were investigated in an Aquifer Storage and Recovery (ASR) system, in which oxic tile drainage water (TDW; nitrate ~14 mg/L; phosphate ~17 mg/L) was stored in an anoxic aquifer for later re-use as irrigation water. During the PPTs, the ASR system did not operate. PPTs were performed in two monitoring wells (MW2, MW3), with 1 m well screens in contrasting geochemical formations at different depths. In these wells, 300 L TDW was injected, and consecutively 720 L was abstracted within 12 days, during which water quality changes were studied. The RTM simulated cation exchange, precipitation of Hydrous Ferric Oxides, Iron (III)-phosphate and Calcium-phosphate minerals, and surface complexation as equilibrium processes. Oxidation of Pyrite, soil organic matter, and dissolved ferrous iron were simulated with kinetic rate expressions. Oxygen (within 2 days) and nitrate (within 4–7 days) were fully reduced during the PPTs. The main reductants were ferrous iron (Monitoring Well (MW) 2: 2%, MW3: 13%), soil organic matter (MW2: 93%, MW3: 6%), and Pyrite (MW2: 5%, MW3: 81%). The intra aquifer differences in dominant reduction pathways are remarkable as higher reduction rates coincided with lower contents of soil organic matter and Pyrite, respectively. Phosphate was mostly re-abstracted (MW2: 73%, MW3: 64%) and partially immobilized due to precipitation of Iron-hydroxyphosphates (MW2: 4.6%, MW3: 35%), Hydroxyapatite (MW2: 23%, MW3: 0%), and to a lesser extent by surface complexation on various minerals (MW2-3: <1%). The PPT-RTM approach enables a better understanding of reaction networks controlling water quality changes, and the reaction kinetics. PPT-RTM is a promising tool in exploratory studies or regular monitoring of water quality aspects of subsurface water technologies.