Pore Water Chemistry Research Articles

Seasonal cryogenic processes control supra-permafrost pore water chemistry in two contrasting Cryosols

Over the last decades, Arctic landscapes have experienced intense warming leading to permafrost degradation and rapid ecosystem changes. Active layer thickening, widespread melting of ground ice and thermo-erosion have affected the mobilization of organic and mineral elements. While the carbon and nitrogen cycles are intensively studied, the soil weathering has been less documented. In the present study, we monitored the chemistry of soil capillary and gravitational pore waters, rainfall and stream waters daily during the growing season in two experimental sites under tussock tundra vegetation in the low-Arctic region, in Salluit (Nunavik, Canada). We aimed to investigate the seasonal thaw controls on the evolution of concentrations of major organic and inorganic elements in the active layer (i.e., seasonally thawed surface layers) of two permafrost soils (Cryosols) differing in parental materials: an ombrotrophic bog (i.e., Histic Cryosol) and post-glacial marine sediments continuously waterlogged (i.e., Turbic Cryosol). In the Histic Cryosol, the electrical conductivity was <100 µS cm−1 and Cl− and Na+ were the dominant soluble ions originating from atmospheric depositions. In the Turbic Cryosol, decarbonated in the first 40 cm, Ca2+ and Mg2+ were the dominant soluble ions in the capillary water reflecting the dissolution of soil minerals, while Cl− and SO42− dominated in gravitational water, illustrating inputs from uphill. In the two soils, Ca2+ and Mg2+ concentrations as well as Mg/Na and Ca/Na increased with depth. Along summer, the soil pore water chemistry evolved with thaw front and water table depths in the two sites. Particularly in the Histic Cryosol, electrical conductivity, solute concentrations, Mg/Na and Ca/Na ratios increased with the thaw front deepening. Our observations suggest that the active layer thickening and increasing supra-permafrost flow contribution, expected to increase with Arctic warming, could lead to a shift in chemistry of pore waters in organic and mineral permafrost soils, differently depending on permafrost landform settings.

Indicator of redox in soil (IRIS) films as a water management tool for rice farmers

Rice is a crucial part of the world's food supply but is also susceptible to uptake of contaminants including arsenic (As) and cadmium (Cd) depending on the soil redox potential. Careful control of soil redox state by implementing alternate wetting and drying (AWD) water management can decrease mobility of soil As and Cd, but can be difficult to manage. Indicators of reduction in soil (IRIS) tubes and films have been studied by pedologists for wetland delineation; here, we explore the use of the IRIS film technology as passive samplers of soil redox potential in rice paddies. The goal of this study was to test the response time of IRIS films under different water management (i.e., variable soil redox potentials). After paddy soils were exposed to severe or safe AWD, where rice paddies were allowed to dry to >30 cm below the soil surface and 15 cm below the soil surface, respectively, IRIS films, coated with Fe oxide or Mn oxide paint, were installed. Immediately following IRIS film installation, soils were reflooded, and percent removal of Fe or Mn oxides were monitored on films that were removed every 12 h for Fe films, and every 6 h for Mn films. Porewater was collected at installation and every 12 h during the studies to observe correlations between IRIS film paint removal and porewater chemistry. We observed quicker paint removal for Mn films than Fe films, and paint removal varied due to growing season and water management. Moreover, correlations between porewater chemistry and Mn paint removal were observed. While further work is still needed to understand kinetics of IRIS paint removal as it relates to porewater parameters, this work illustrates that IRIS films are a low-cost tool that rice farmers can use to better manage water and we highlight considerations for possible implementation strategies for the future.

Effects of seasonal weathering on dewatering and strength of an oil sands tailings deposit

Considerable research has been conducted over the past decade by oil sands mining companies to improve the dewatering and strength properties of fluid fine tailings deposits in an effort to meet the regulatory and closure requirements. Commercially employed dewatering treatment technologies (inline flocculation, thickening, and centrifugation) may not be sufficient to develop the strength for the creation of trafficable landscape without the use of soft soil capping technologies. These treated tailings are continuously deposited creating soft and saturated deep deposits. Seasonal weathering may be an additional promising technology to further dewater the treated tailings and promote the development of shear strength at the surface. This paper presents the effects of seasonal weathering on dewatering and strength that were investigated by performing multiple cycles of freeze–thaw and alternate drying–wetting cycles on two types of treated tailings deposit. The results indicate that multiple cycles of seasonal weathering significantly increased the dewatering and strength properties. However, different parameters such as freezing gradient, number of seasonal cycles, and pore water chemistry play an influential role in changing the magnitude of the strength. The results also suggest that a minimum threshold strength value is required where the effects of rainfall rewetting had a minimal impact on strength reduction (the strength corresponding to the moisture content approaching the plastic limit).

On the utilization of mica waste: The pore-fluid chemistry of mica soils and its implication for erosion susceptibility

Soil is a vital resource which is limited in availability and must be adequately managed for sustainable development. Despite the importance of soil and its limited availability, a huge amount of soil is classified as waste. As one of the world’s largest soil waste generators, the mining industry must transition to a circular economy model to meet sustainability requirements. As a readily available, large-volume waste material (about 10 million tonnes each year), waste mica has been considered for re-use as an alternative soil potassium source for plants, as landscaping material etc. One of the problems with mica waste is its susceptibility to erosion. To use mica as an agricultural or landscaping material, it is therefore important to understand the erodibility of mica and how its erodibility can be reduced. This study presents an experimental campaign to characterize mica erosion susceptibility. Mica particle-to-particle interaction forces and their effect on the macroscopic material behaviour were systematically investigated by changing pore fluid pH and ionic concentration. Sedimentation and liquid limit tests were first carried out to inform a conceptual model of the mica fabric. Triplets of 12 mica samples, compacted at different water contents and with different pore chemistry, were tested for erosion susceptibility using a Jet Erosion Test (JET) apparatus. The particle configuration of mica samples consistently varied with the pore water chemistry, regardless of whether the samples being tested were suspension sediments or compacted samples. For mica samples formed with neutral water at low ionic concentration, the particles are in a dispersed configuration. This implies a poor mechanical behaviour and high erosion susceptibility, as particles are eroded one by one. For porewater samples formed with an increased ionic strength or within the acidic pH range, particles tend to cluster together and organize in a non-dispersed configuration. This results in an improved mechanical behaviour and less erosion susceptibility, as group of particles must be eroded as opposed to individual ones. Similarly, the Jet erosion test results reinforced these observations showing that mica erodibility varies with porewater chemistry. Considering that mica erodibility varies with pore fluid chemistry and mica waste derived from mining operations are often slightly acidic, this paper paves the way for tailored assessments of individual mica waste materials to determine the appropriateness of their use.

Geochemical Characteristics of Sediment in Tropical Lake Sentani, Indonesia, Are Influenced by Spatial Differences in Catchment Geology and Water Column Stratification

Physical and (bio)chemical processes in the catchment as well as internal lake processes influence the composition of lacustrine sediments. Lake internal processes are a consequence of reactions and fluxes between sediment, porewater and the water column. Due to its separation into four interconnected sub-basins, Lake Sentani, Papua Province, Indonesia, is a unique tropical lake that reveals a wide range of geochemical conditions. The highly diverse geological catchment causes mineralogical and chemical differentiation of the sediment input into each sub-basin. Also, strong morphological differences between the sub-basins result in a unique water column structure for each sub-basin, ranging from fully mixed to meromictic. Given the strong differences in sediment composition and bottom water chemistry among the four sub-basins, Lake Sentani offers a unique chance to study multiple lacustrine systems under identical climate conditions and with a common surface water chemistry. We used sediment cores and water samples and measured physicochemical water column profiles to reveal the geochemical characteristics of the water column, the sediment and pore water for all four sub-basins of Lake Sentani. The chemical composition of the sediment reveals differentiation among the sub-basins according to their sediment input and water column structure. Catchment lithology mainly affects overall sediment composition, whereas pore water chemistry is also affected by water column structure, which is related to basin morphology and water depth. In the meromictic sub-basins the bottom water and sediment pore water appear to form a single continuous system, whereas in those sub-basins with oxygenated bottom water the sediment-water interface forms a pronounced chemical barrier.

Hydrochemistry of sediment pore water in the Bratsk reservoir (Baikal region, Russia)

This study aimed to identify the factors responsible for the major ion composition of pore water from the bottom sediments of the Bratsk water reservoir, which is part of the largest freshwater Baikal-Angara water system. In the Bratsk reservoir, the overlying water was characterized as HCO3–Ca–Mg type with the mineralization ranging between 101.2 and 127.7 mg L−1 and pore water was characterized as HCO3–SO4–Ca, SO4–Cl–Ca–Mg and mixed water types, which had mineralization varying from 165.9 to 4608.1 mg L−1. The ionic composition of pore waters varied both along the sediment depth profile and across the water area. In pore water, the difference between the highest and lowest values was remarkably large: 5.1 times for K+, 13 times for Mg2+, 16 times for HCO3−, 20 times for Ca2+, 23 times for Na+, 80 times for SO42−, 105 times for Cl−. Such variability at different sites of the reservoir was due to the interrelation between major ion concentrations in the pore water and environmental parameters. The major factor responsible for pore water chemistry was the dissolution of sediment-forming material coming from various geochemical provinces. In the south part of the reservoir, Cl−, Na+ and SO42− concentrations may significantly increase in pore water due to the effect of subaqueous flow of highly mineralized groundwater.

Sorption and electrokinetic properties of ASR product and C-S-H: A comparative modelling study

Alkali-silica reaction (ASR) is one of the most important concrete durability issues worldwide. ASR products formed at high temperature have a structure similar to the natural mineral shlykovite. Reactive Grand Canonical Simulations were used to investigate sorption of K+, Cl− and Ca2+ on C-S-H and ASR products at different pH (10.0 to 13.0) and pore water chemistry. Divalent ions can overcompensate for the negative surface charge, in particular for C-S-H, which leads to a co-adsorption of negative species such as Cl−. At high K/Ca in solution, monovalent K+ can desorb calcium from C-S-H and shlykovite surface. The calculated Ca/K partition coefficient shows a higher affinity of ASR product for K+, while C-S-H favours Ca2+. The uptake of K+ by ASR products lowers the alkali concentration in the solution and could thus slow down the rate of ASR.

Influence of pore water chemistry on GCL self-healing with hydration from silica sand

The self-healing of a GCL with a circular hole is examined in experiments where the GCL, overlain by geomembrane, is hydrated from a silica sand subgrade (SSS) having three different pore water chemistries. Factors considered included: hole size, subgrade initial moisture content wfdn, GCL mass per unit area, and overburden stress (20–100 kPa). GCL self-healing is better for wfdn = 16% than for wfdn = 10%, which is better than for 5%, when the SSS pore water has negligible cations (ionic strength, I < 0.1 mM). However, only the 14.3 mm-diameter hole fully self-healed and only when wfdn = 16%. In contrast, when the GCL is hydrated from SSS with pore water having an ionic strength, I, of 20 and 30 mM, the self-healing for wfdn = 5% is better than for wfdn = 10%, which is better than for wfdn = 16%, although none of the holes self-healed. When a ~0.5 m hydraulic head was applied above the GCL under σv = 20–100 kPa, a 38.1 mm-diameter hole self-healed with water having I < 0.1 mM, a 25.4 mm-diameter hole self-healed with pore water with I = 20 mM and 30 mM, but none self-healed with simulated synthetic landfill leachate (SSL). Post-hydration hydraulic conductivity (k) tests with SSL suggest that a hole up to 14.3 mm-diameter would not pose a significant adverse impact on the k compared to an intact GCL; however, this is not the case for the larger holes tested.

Assessing the Impacts of Land Spreading Water-Treatment Residuals on the Anecic Earthworm Lumbricus terrestris, Soil Microbial Activity, and Porewater Chemistry.

Water-treatment residuals (WTRs), by-products of drinking water clarification, are increasingly recycled to land to promote circular economy and reduce disposal costs, yet there is a lack of published literature on their effects on soil ecology. In the present study, the effects of WTRs on earthworm growth, soil respiration, and soil porewater chemistry were investigated throughout a 7-wk outdoor mesocosm trial. We derived WTRs from both aluminum and iron coagulants and applied them to a loam soil at 0 to 20% (w/w). In addition, soil from a field that had received long-term WTR applications and that of an adjacent nontreated reference field were included in the study. Earthworm mass increase was significantly higher in all but one laboratory-treated soil when compared to the control. Furthermore, a linear regression model was used to predict increases in weekly soil respiration based on the application rates of both Al and Fe WTRs. In addition, a significant increase in soil respiration was observed from the treated farm soils during the first 4 wk of the trial. Measured sodium, magnesium, potassium, and iron porewater concentrations were higher in the treated farm soils than the reference site soil in a majority of samples, although these differences may be related to land management. Laboratory-treated soils had elevated porewater arsenic concentrations (e.g., ~17 µg L-1 in controls vs ~62 µg L-1 in the 20% w/w Al WTR treatment in week 1), whereas porewater nickel concentrations were, respectively, elevated and lowered in Al WTR- and Fe WTR-amended samples. Overall, observed disturbances to soil ecology were determined to be minimal. Environ Toxicol Chem 2021;40:1964-1972. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Simplified biogeochemical numerical model to predict pore fluid chemistry and calcite precipitation during biocementation of soil

Microbially induced calcite precipitation (MICP) technique has gained attention recently as a novel method to enhance the engineering properties of soils, especially sandy soils. However, the applicability of this method to field scale is challenging and requires understanding of the factors affecting MICP process under variable subsurface conditions. This study presents a laboratory investigation and numerical predictive model to assess pore-water chemistry and calcite precipitation during the biocementation process. Laboratory experiments were conducted to assess the microbial treatment of Narmada sand in plastic tubes using three bacterial strains and two cementation media concentrations. Calcite precipitation via ureolysis as a result of biogeochemical reactions was measured. The effects of pH and electrical conductivity (EC) on the rate of urea hydrolysis and calcite precipitation were assessed. The presence of calcite crystals was analyzed using scanning electron microscopy (SEM). The SEM images confirmed the formation of calcite at the surface and between the sand particles. A simplified numerical model was developed to estimate the rate of urea hydrolysis and their effects on the biocementation of sand. Three stages of MICP process were identified: bacterial ureolysis, dynamic equilibrium between liquid-gas interface and oversaturation of ions, and calcite precipitation. The variations of pH and EC at these three stages were modeled. The predicted pH, EC, and calcite precipitation based on the simplified model were found to be in close agreement with the experimental results. The numerical model can be used to assess and optimize the system variables for effective MICP field applications.

Relevance of Iron Oxyhydroxide and Pore Water Chemistry on the Mobility of Nanoplastic Particles in Water-Saturated Porous Media Environments

The increasing use of plastic products and its inevitable decomposition after improper disposal has led to large numbers of nano- and microplastic in aqueous environments. There is currently a critical need to investigate the transport and retention mechanisms of nanoplastic particles in water-saturated porous media (e.g., aquifers or sediments) to better understand residence times and ecosystem exposure of these particles in aqueous environments. In this study, we performed a set of column experiments in order to investigate and understand the primary controls on the mobility of nanoplastics in a controlled laboratory environment. As part of the experiments, we used polystyrene nanoplastic particles (PS-NPs, 50 nm) in combination with iron oxyhydroxide–coated sand, which is known for its high surface reactivity and often can be found in natural systems in environmentally relevant amounts. We also adjusted pore water chemistry (pH, ionic strength, cation species) to represent non-uniform geochemical conditions in nature and to understand how these conditions quantitatively affect the transport of nanoplastics. Mobility and retention of PS-NPs were assessed by analyzing breakthrough curves. For negatively charged iron oxyhydroxide coatings (at pH > pHpzc), only little retention of PS-NPs could be observed. In contrast, positively charged iron oxyhydroxide coatings (pH < pHpzc) provided favorable deposition sites for the negatively charged PS-NPs. DLVO theory was used to show that high pH and low ionic strength increased the energy barriers between PS-NPs and the porous media. In contrast, low pH and high ionic strength decreased the barriers and thus increased retention in the columns. Finally, bridging agents, such as Ca2+ and Ba2+, resulted in the significant deposition of nanoplastics by forming bonds between O-containing functional groups on both the plastic and sediment surfaces. These findings indicate that the deposition and fate of nanoplastic particles are strongly affected by the water chemistry and soil components in subsurface environments.

Regional Wetland Plant Responses to Sulfur and Other Porewater Chemistry in Calcareous Rich Fens

Many wetland environmental gradients structure plant community composition, yet controls of plant community composition within rich fens, botanically diverse groundwater-fed wetlands, are still incompletely understood. Porewater chemistry and plant community composition were recorded for eight calcareous rich fens encompassing both calcium carbonate and calcium sulfate geological inputs in the Central New York State region. As expected, porewater sulfate and sulfide concentrations were, on average, higher for wetlands overlying calcium sulfate than for wetlands overlying calcium carbonate. However, within-wetland heterogeneity in porewater chemistry was high. Moss species density, moss cover, and total plant cover decreased with increased sulfide. Moss, dicot, and total cover also had a negative relationship with calcium. There were a number of species-level responses to calcium, sulfide, phosphorus, and ferrous iron. Plant height had a positive relationship with nitrogen. The strength and relative importance of some plant responses to sulfide and calcium in the current regional study differed from a previous sub-hectare scale study. The observed decrease in some metrics of site-level plant diversity with increased sulfide variability across fens highlighted the need to characterize species-environment relationships across spatial scales.

Methods in Capturing the Spatiotemporal Dynamics of Flow and Biogeochemical Reactivity in Sandy Beach Aquifers: A Review

Sandy beach aquifers are complex hydrological and biogeochemical systems where fresh groundwater and seawater mix. The extent of the intertidal mixing zone and the rates of circulating flows within beaches are a primary control on porewater chemistry and microbiology of the intertidal subsurface. Interplay between the hydrological and biogeochemical processes at these land-sea transition zones moderate fluxes of chemicals, particulates, heavy metals, and biota across the aquifer-ocean interface, affecting coastal water quality and nutrient loads to marine ecosystems. Thus, it is important to characterize hydrological and biogeochemical processes in beach aquifers when estimating material fluxes to the ocean. This can be achieved through a suite of cross-disciplinary measurements of beach groundwater flow and chemistry. In this review, we present measurement approaches that have been developed and employed to characterize the physical (geology, topography, subsurface hydrology) and biogeochemical (solute and particulate distributions, reaction rates) properties of and processes occurring within sandy intertidal aquifers. As applied to beach systems, we discuss vibracoring, sample collection, laboratory experiments, variable-density considerations, instrument construction, and sensor technologies. We discuss advantages and limitations of typical hydrologic field sampling methods when used to investigate beach aquifers and provide a measurement framework for researchers seeking to sample and collect data from these systems.

Sulfur amendments to soil decrease inorganic arsenic accumulation in rice grain under flooded and nonflooded conditions: Insights from temporal dynamics of porewater chemistry and solid-phase arsenic solubility.

Rice cultivation under flooded conditions enhances arsenic (As) solubility and favors As accumulation in rice grain that poses an indisputable threat to human health worldwide. The reduction of sulfur may induce processes that decrease As solubility, but its impact on rice grain As species remains unresolved. Herein, we investigated the influence of sulfur (S)-containing materials, including chicken manure and elemental sulfur powder on As accumulation and speciation in rice grain as well as the dynamics of the porewater chemistry and solid-phase As solubility throughout the entire growth stage under continuous flooding and intermittent flooding conditions in pot experiments. The S amendments (200mgSkg-1) to the soil significantly decreased inorganic As in rice grain under continuous flooding (~65% decrease) as well as under intermittent flooding (~70% decrease). The chicken manure amendment promoted sulfur reduction and enhanced dissolvable Mn, Fe, and As at an earlier growth stage. The sequential extraction results corroborated a decrease in the soluble and exchangeable As (F1) and an increase in residual As (F5) fractions in the S-amended treatments. Solubility data suggested that As adsorption onto Fe oxides was the primary mechanism controlling As solubility rather than the formation of AsFe sulfides. Porewater As, considered to represent the most bioavailable As fraction, failed to explain the grain As accumulation. The time-averaged concentration of oxalate-extractable As explained grain arsenite best, suggesting that poorly crystalline Fe oxides may be the primary dissolvable reactive phases that control As bioavailability in the soil-rice system. Our results suggest that the application of S-containing soil amendments can effectively decrease inorganic As accumulation in rice grains grown under the flooded conditions, which are most widely applied in paddy rice production.

Nitrogen removal processes in lakes of different trophic states from on-site measurements and historic data

Freshwater lakes are essential hotspots for the removal of excessive anthropogenic nitrogen (N) loads transported from the land to coastal oceans. The biogeochemical processes responsible for N removal, the corresponding transformation rates and overall removal efficiencies differ between lakes, however, it is unclear what the main controlling factors are. Here, we investigated the factors that moderate the rates of N removal under contrasting trophic states in two lakes located in central Switzerland. In the eutrophic Lake Baldegg and the oligotrophic Lake Sarnen, we specifically examined seasonal sediment porewater chemistry, organic matter sedimentation rates, as well as 33-year of historic water column data. We find that the eutrophic Lake Baldegg, which contributed to the removal of 20 ± 6.6 gN m−2 year−1, effectively removed two-thirds of the total areal N load. In stark contrast, the more oligotrophic Lake Sarnen contributed to 3.2 ± 4.2 gN m−2 year−1, and had removed only one-third of the areal N load. The historic dataset of the eutrophic lake revealed a close linkage between annual loads of dissolved N (DN) and removal rates (NRR = 0.63 × DN load) and a significant correlation of the concentration of bottom water nitrate and removal rates. We further show that the seasonal increase in N removal rates of the eutrophic lake correlated significantly with seasonal oxygen fluxes measured across the water–sediment interface (R2 = 0.75). We suggest that increasing oxygen enhances sediment mineralization and stimulates nitrification, indirectly enhancing denitrification activity.

Carryover effects of silicon‐rich amendments in rice paddies

AbstractRice (Oryza sativa L.) readily accumulates the nutrient silicon (Si) and the toxin arsenic (As), elements that can interact antagonistically both in the plant and in the soil. Thus, many studies have been performed where Si‐rich materials are incorporated into soils to increase plant‐available Si and thereby decrease plant As. However, studies have not examined the dissolution kinetics of Si from Si‐rich materials over multiple seasons and the resulting effects on rice systems. Here, we analyzed the porewater chemistry, CH4 emissions, and rice elemental concentrations from rice paddies in Year 2 that had been amended with Si‐rich materials (silicate fertilizer, rice husk, and charred rice husk) in Year 1. After two crop cycles, plant‐available Si and plant Si concentrations remained elevated in Si‐amended paddies. Silicon‐rich amendments also decreased plant As concentrations, although this effect was only observed in the vegetative tissues. Husk treatment resulted in the lowest porewater redox, highest CH4 emissions, and highest porewater inorganic As. Porewater concentrations and dynamics of inorganic As, dimethylarsinic acid (DMA), and trimethylarsine oxide (TMAO) differed for each amendment, but Si‐rich amendments exhibited elevated inorganic and organic As early in the growing season. These Si‐rich amendments can continue to provide Si and impact rice systems two growing seasons after application.

Trace elements in sulfides and release to porewater from sulfide oxidation in a historical waste-rock pile, Ontario, Canada

Porewater chemistry and sulfide mineral characteristics were evaluated in a waste-rock pile at the Detour Lake gold mine, Ontario, Canada, that had weathered in situ for more than 30 years. Geochemical characterization suggests that the waste rock would be potentially acid generating. Porewater extracted from the waste rock ranged in pH from 7.05 to 8.35, suggesting on-going acid neutralization by carbonate-mineral dissolution. Of the trace elements, nickel occurs at the highest concentration in pyrrhotite (median of 2738 ppm in historical samples), the associated alteration rims, and in the porewater samples (median of 140 μg L−1). Nickel:cobalt and nickel:iron ratios in the sulfide grains and alteration rims, together with the porewater data, suggest no preferential retention of nickel over cobalt, and that sorption sites were saturated. X-ray absorption near edge structure (XANES) spectroscopy measurements at the iron-, sulfur- and nickel-edges also indicate trace element associations within the sulfide grains and alteration rims, and the presence of oxidized species of iron and sulfur.

Microbial controls on net production of nitrous oxide in a denitrifying woodchip bioreactor.

Denitrifying woodchip bioreactors are potential low-cost technologies for the removal of nitrate (NO3 - ) in water through denitrification. However, if environmental conditions do not support microbial communities performing complete denitrification, other N transformation processes will occur, resulting in the export of nitrite (NO2 - ), nitrous oxide (N2 O), or ammonium (NH4 + ). To identify the factors controlling the relative accumulation of NO2 - , N2 O, and/or NH4 + in denitrifying woodchip bioreactors, porewater samples were collected over two operational years from a denitrifying woodchip bioreactor designed for removing NO3 - from mine water. Woodchip samples were collected at the end of the operational period. Changes in the abundances of functional genes involved in denitrification, N2 O reduction, and dissimilatory NO3 - reduction to NH4 + were correlated with porewater chemistry and temperature. Temporal changes in the abundance of the denitrification gene nirS were significantly correlated with increases in porewater N2 O concentrations and indicated the preferential selection of incomplete denitrifying pathways ending with N2 O. Temperature and the total organic carbon/NO3 - ratio were strongly correlated with NH4 + concentrations and inversely correlated with the ratio between denitrification genes and the genes indicative of ammonification (Σnir/nrfA), suggesting an environmental control on NO3 - transformations. Overall, our results for a denitrifying woodchip bioreactor operated at hydraulic residence times of 1.0-2.6d demonstrate the temporal development in the microbial community and indicate an increased potential for N2 O emissions with time from the denitrifying woodchip bioreactor.

Gene abundances of AOA, AOB, and anammox controlled by groundwater chemistry of the Pearl River Delta, China

Ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and anaerobic ammoniaoxidation (anammox) bacteria are very important contributors to nitrogen cycling in natural environments. Functional gene abundances of these microbes were believed to be well relevant to N-cycling in groundwater systems, especially in the Pearl River Delta (PRD) groundwater with unique high intrinsic ammonia concentrations. In this research, 20 sediment samples from two in the PRD were collected for porewater chemistry analysis and quantification of N-cycling related genes, including archaeal and bacterial amoA gene and anammox 16S ribosomal Ribonucleic Acid (rRNA) gene. Quantitative Polymerase Chain Reaction (qPCR) results showed that gene abundances of AOA, AOB, and anammox bacteria ranged from 3.13×105 to 3.21×107, 1.83×104 to 2.74×106, and 9.27×104 to 8.96×106 copies/g in the sediment of the groundwater system, respectively. Anammox bacteria and AOA dominated in aquitards and aquifers, respectively, meanwhile, the aquitard-aquifer interfaces were demonstrated as ammonium-oxidizing hotspots in the aspect of gene numbers. Gene abundances of nitrifiers were analyzed with geochemistry profiles. Correlations between gene numbers and environmental variables indicated that the gene abundances were impacted by hydrogeological conditions, and microbial-derived ammonium loss was dominated by AOA in the northwest PRD and by anammox bacteria in the southeast PRD.© 2021 China Geology Editorial Office.

A comparison of porewater chemistry between intact, afforested and restored raised and blanket bogs

Afforestation is a significant cause of global peatland degradation. In some regions, afforested bogs are now undergoing clear-felling and restoration, often known as forest-to-bog restoration. We studied differences in water-table depth (WTD) and porewater chemistry between intact, afforested, and restored bogs at a raised bog and blanket bog location. Solute concentrations and principal component analysis suggested that water-table drawdown and higher electrical conductivity (EC) and ammonium (NH4-N) concentrations were associated with afforestation. In contrast, higher dissolved organic carbon (DOC) and phosphate (PO4-P) concentrations were associated with deforestation. Drying-rewetting cycles influenced seasonal variability in solute concentrations, particularly in shallower porewater at the raised bog location. WTD was significantly deeper in the oldest raised bog restoration site (~9 years post-restoration) than the intact bog (mean difference = 6.2 cm). However, WTD in the oldest blanket bog restoration site (~17 years post-restoration), where furrows had been blocked, was comparable to the intact bog (mean difference = 1.2 cm). When averaged for all porewater depths, NH4-N concentrations were significantly higher in the afforested than the intact sites (mean difference = 0.77 mg L−1) whereas significant differences between the oldest restoration sites and the intact sites included higher PO4-P (mean difference = 70 μg L−1) in the raised bog and higher DOC (mean difference = 5.6 mg L−1), EC (mean difference = 19 μS cm−1) and lower SUVA254 (mean difference = 0.13 L mg−1 m−1) in the blanket bog. Results indicate felled waste (brash) may be a significant source of soluble C and PO4-P. Mean porewater PO4-P concentrations were between two and five times higher in furrows and drains in which brash had accumulated compared to other locations in the same sites where brash had not accumulated. Creating and maintaining brash-free buffer zones may therefore minimise freshwater impacts.