Competitive transport processes of chloride, sodium, potassium, and ammonium in fen peat

Nitrogen loading resulting from major floods and sediment resuspension to a large coastal embayment

Major floods pose a severe threat to coastal receiving environments, negatively impacting environmental health and ecosystem services through direct smothering with sediment and nutrient loading. This study examined the short and long-term impacts of the February 2022 major flood event on mud extent and sediment nitrogen flux in Moreton Bay (the Bay), a large, sub-tropical embayment in Southeast Queensland, Australia. Short-term impacts were assessed three days after the flood peak by sampling surface water at 47 sites in the direction of the predominant circulation pattern. Longer-term impacts were assessed by undertaking an intensive sediment survey of 223 sites and a nutrient flux experiment using sediment core incubations to simulate calm and resuspension conditions for the four key sediment classes. Short-term impacts revealed elevated turbidity levels extended across the Bay but were highest at the Brisbane River mouth, ammonium concentrations varied inversely with surface turbidity, whereas nitrate concentrates closely tracked surface turbidity. The sediment survey confirmed fine sediment deposition across 98 % of the Bay. Porewater within the upper 10 cm contained a standing pool of 280 t of ammonium, with concentrations more than three orders of magnitude higher than overlying surface waters. The nutrient flux experiment revealed an order of magnitude higher sediment ammonium flux rate in the sandy mud sediment class compared to the other sediment classes; and for simulated resuspension conditions compared to calm conditions for sand, muddy sand, and mud sediment classes. Scaling across the whole Bay, we estimated a mean annual sediment flux of 17,700 t/year ammonium, with a range of 13,500 to 21,900 t/year. Delivery of fine sediments by major floods over the last 50 years now impact >98 % of the benthic zone and provide a major loading pathway of available nitrogen to surface waters of Moreton Bay; representing a significant threat to ecosystem health.

Solute depletion and reduced hydrological connectivity in subarctic patterned peatlands disturbed by mine dewatering

Patterned bog and fen peatlands of the Hudson Bay Lowlands, which form one of the largest continuous peatland complexes in the world, are globally significant stores of carbon and important water conveyance and storage features on the landscape. However, expansion of resource exploration and extraction combined with warmer temperatures associated with climate change may result in reduced water availability to these peatland complexes, potentially disrupting peatland hydrological connectivity and hydrogeochemical cycling. A case study on the effects of reduced water availability on peatland hydrological and geochemical function was conducted near the De Beers Victor Diamond Mine, located 90 km west of Attawapiskat. Active dewatering occurred here over a 12-year period (2007-2019) during which a 1.5 km transect was monitored within the mine impacted radius. Hydrological (streamflow and groundwater levels) and chemical (porewater and surface water samples) parameters were collected at the impacted transect and two nearby unimpacted reference sites. Results demonstrated that impacted peatlands had depleted water storage and spent an average of 50 % less time hydrologically connected than unimpacted peatlands. By the end of the study period, increasingly depleted water storage within the dewatering radius resulted in disproportionately lower flowrates in two tributaries downgradient of the mine-impacted peatlands when compared with the reference sites. Moreover, diminished water storage allowed solute-depleted precipitation to reach greater depths within the peat profile, while stronger downwards gradients suppressed upwards flow into fens, limiting the amount of solute-enriched water reaching the surface. The recovery of fen solute concentrations will be a prolonged process (i.e., decades to centuries) due to the slow rate of upwards diffusion, which may result in the transition of these systems towards ombrotrophic bogs. Further studies should focus on the susceptibility of these impacted systems to further reductions in water availability due to climate change.

Multiphase flow behavior of diesel in bog, fen, and swamp peats

Hydrocarbon fate and transport in various categories of peatlands is complicated by the botanical origin, and thus variations in the hydraulic structures and surface chemistry of its peat soils. There has been no systematic evaluation of the role of different peat types on hydrocarbon migration. Thus, two-phase, and three-phase flow experiments were performed for living and partially decomposed peat cores from bog, fen, and swamp peatlands. Numerical simulations of water drainage were performed using HYDRUS-1D, diesel-water and diesel-water-air flow using MATLAB Reservoir Simulation Toolbox (MRST). Five water table (WT) fluctuations were imposed to explore its potential to reduce residual diesel saturation in peat columns. Our results demonstrate a good match of the relative water permeability (k_rw) - saturation (S) relations estimated using the unsaturated hydraulic conductivity-S relation derived from HYDRUS-1D modeling of two-phase flow, and k_rw - S from MRST for three-phase flow, for all tested peat columns. Thus, we recommended using two-phase system based k_rw - S predictions if multiphase data are unavailable for peatland sites' spill management plans. We found the discharge of water and diesel both increase with increasing hydraulic conductivity, while residual water and diesel were within the range of 0.42-0.52 and of 0.04-0.11, respectively. High diesel discharge rates suggest that quick spill-response is required to manage its spread in peatlands. Up to 29% of residual diesel saturation was yielded by the five WT fluctuations, and thus we strongly recommend WT manipulation as a first step towards diesel decontamination progression in peatlands.

Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater

Changes in sulfate (SO₄^2-) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO₄^2- deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO₄^2- export from a peatland-dominated catchment as a function of changing SO₄^2- deposition in a long-term (1998-2011), whole-ecosystem, control-impact experiment. Annual SO₄^2- deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO₄^2- additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO₄^2- addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO₄^2- inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO₄^2- deposition declines.

Measurement of contaminant adsorption on soils using cycling modified column tests

An innovative cycled column test with supporting batch equilibrium and kinetic analysis for adsorption or desorption were developed for evaluation of adsorption behavior of soils. Non-equilibrium adsorption was observed in the cycled column tests as the traditional testing methods. The isotherm of local equilibrium of the soil was conducted based on the testing results within a relatively short duration with simple analysis. The concentration curves of influent and effluent of the cycled column tests were simulated by dual-porosity (DP) model with a modified inlet boundary. Based on the modeling results, the isotherm of local equilibrium is close to that of the mobile phase adsorption capacity, whereas the immobile phase of the soil is nearly inactive in the retardation of the contaminants. The testing results from cycled column tests are hardly interfered by desorption or the sorption rate according to the modeling for corresponding scenarios. The cycled column test can be used as an alternative or supplementary method to the traditional column test for the determination of local equilibrium isotherm, with advantages of shorter testing duration and easier data analysis.

Assessing benzene and toluene adsorption with peat depth: Implications on their fate and transport

After a hydrocarbon spill in a peatland, dissolution of water-soluble compounds including benzene and toluene introduces a dissolved-phase plume to the peatland groundwater system, while the adsorption of these solutes onto the peat matrix restrains their distribution velocity. The adsorption of benzene and toluene and its dependency on peat depth, thus degree of decomposition, are investigated. The batch adsorption experiments revealed that benzene and toluene adsorption isotherms in peat are linear, with adsorption coefficients ranging from 16.2 to 48.7 L/kg and 31.6-48.7 L/kg, respectively. In a vertical peat profile benzene adsorption decreased with depth, while toluene adsorption increased. Considering toluene adsorption onto cellulose is significantly less than toluene adsorption onto humic substance, the increase in toluene adsorption was attributed to decreasing cellulose and increasing humic substances with depth. Negligible competition for adsorption was observed between benzene and toluene at the measured concentrations. The retardation factors of benzene and toluene ranged respectively from 3.5 to 10.7 and from 5.4 to 17.7, both increasing with depth. Higher retardation in deeper peat coupled with lower hydraulic conductivity will lead to a weaker solute velocity in deeper peat, thus preferential migration of these dissolved-phase contaminants in shallow layers. The results can help predict the behavior of dissolved hydrocarbons in peatlands after a hydrocarbon spill.

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The dual-porosity structure of peat and the extremely high organic matter content give rise to a complex medium that typically generates prolonged tailing and early 50% concentration breakthrough in the breakthrough curves (BTCs) of chloride (Cl^-) and other anions. Untangling whether these observations are due to rate-limited (physical) diffusion into inactive pores, (chemical) adsorption or anion exclusion remains a critical question in peat hydrogeochemistry. This study aimed to elucidate whether Cl^- is truly conservative in peat, as usually assumed, and whether the prolonged tailing and early 50% concentration breakthrough of Cl^- observed is due to diffusion, adsorption, anion exclusion or a combination of all three. The mobile-immobile (MiM) dual-porosity model was fit to BTCs of Cl^- and deuterated water measured on undisturbed cores of the same peat soils, and equilibrium Cl^- adsorption batch experiments were conducted. Adsorption of Cl^- to undecomposed and decomposed peat samples in batch experiments followed Freundlich isotherms but did not exhibit any trends with the degree of peat decomposition and sorption became negligible below aqueous Cl^- concentrations of ~310 mg L^-1. The dispersivity determined by fitting the Cl^- BTCs whether assuming adsorption or no adsorption were significantly different than determined by the deuterated water (p < .0001). However, no statistical differences in dispersivity (p = .27) or immobile water content (p = .97) was observed between deuterated water and Cl^- when accounting for anion exclusion. A higher degree of decomposition significantly increased anion exclusion (p < .0001) but did not influence the diffusion of either tracer into the immobile porosity. Contrary to previous assumptions, Cl^- is not truly conservative in peat due to anion exclusion, and adsorption at higher aqueous concentrations, but the overall effect of anion exclusion on transport is likely minimal.

Short Term Effects of Salinization on Compound Release from Drained and Restored Coastal Wetlands

Over the past two decades, great efforts have been made to restore coastal wetlands through the removal of dikes, but challenges remain because the effects of flooding with saline water on water quality are unknown. We collected soil samples from two adjacent coastal fen peatlands, one drained and diked, the other open to the sea and rewetted, aiming at assessing the mobility and export of various compounds. Microcosm experiments with constant flow-through conditions were conducted to determine the effluent concentrations of dissolved organic carbon (DOC), ammonium ( NH 4 + ), and phosphate ( PO 4 3 − ) during saline–fresh water cycles. Sodium chloride (NaCl) was used to adjust salinity (saline water, NaCl concentration of 0.12 mol L−1; fresh water, NaCl concentration of 0.008 mol L−1) and served as a tracer. A model analysis of the obtained chloride ( Cl − ) and sodium ( Na + ) breakthrough curves indicated that peat soils have a dual porosity structure. Sodium was retarded in peat soils with a retardation factor of 1.4 ± 0.2 due to adsorption. The leaching tests revealed that water salinity has a large impact on DOC, NH 4 + , and PO 4 3 − release. The concentrations of DOC in the effluent decreased with increasing water salinity because the combination of high ionic strength (NaCl concentration of 0.12 mol L−1) and low pH (3.5 to 4.5) caused a solubility reduction. On the contrary, saline water enhanced NH 4 + release through cation exchange processes. The PO 4 3 − concentrations, however, decreased in the effluent with increasing water salinity. Overall, the decommissioning of dikes at coastal wetlands and the flooding of once drained and agriculturally used sites increase the risk that especially nitrogen may be leached at higher rates to the sea.

Effects of surface-modified biochars and activated carbon on the transformation of soil inorganic nitrogen and growth of maize under chromium stress

Elevated chromium (Cr) level is challenging agricultural production and affecting soil biochemical process. This study evaluated the effect of amendments including surface-modified biochars (HBC: acid washing, Fe(III)-HBC: ferric iron loading, nZVI-HBC: nanoscale zero-valent iron loading) and activated carbon on hexavalent chromium (Cr(VI)) removal in soil and on N cycling enzyme activities, transformation of soil inorganic nitrogen, and growth of maize under Cr stress. The results showed that amendments increased Cr(VI) removal by 72.9%-96.34% at three levels of spiked Cr(VI) (low: 125 mg kg^-1, moderate: 250 mg kg^-1, high: 500 mg kg^-1). Under low Cr stress, amendments generally significantly decreased urease and nitrite reductase activities but increased nitrate reductase activity (p < 0.05). The NH₄⁺-N content had a significant positive correlation with urease activity (p < 0.01), while both NO₂^--N and NO₃^--N were absent correlations with N cycling enzyme studied. Amendments decreased NH₄⁺-N/NO₃^--N ratio under low Cr stress but increased it under moderate Cr stress, although the difference was not significant. Under high Cr stress, only Fe(III)-HBC significantly increased NH₄⁺-N/NO₃^--N ratio (p < 0.05). The decrease and increase of NH₄⁺-N/NO₃^--N ratios indicate the enhancement of nitrification and denitrification, respectively. The increase in Cr(VI) removal by amendments contributed to the increase in the migration of NO₃^--N from roots to shoots. Amendments (except for nZVI-HBC in soil under low Cr stress) increased maize height by 20%-59%. Under low Cr stress, however, nZVI-HBC significantly decreased maize height by 65% (p < 0.05), indicating the toxic effect of nZVI on maize growth overwhelmed low Cr stress.

Salt-Tolerant Superabsorbent Polymer with High Capacity of Water-Nutrient Retention Derived from Sulfamic Acid-Modified Starch

The application of superabsorbent polymers (SAPs) is hindered because their absorption capability is greatly affected by the electrolytes in a solution. A novel modified water-absorbent polymer was fabricated by solution polymerization of sulfamic acid-modified starch and acrylic acid; the swelling ratios of this absorbent polymer were 1026 g/g in deionized water and 145 g/g in 0.9% sodium chloride solution and increased by 99.5 and 13.4%, respectively, when compared with ordinary starch-grafted acrylic SAPs. The water absorption capacity was measured in water at different pH values, salt concentrations, and temperatures. In addition, water and fertilizer retentions were studied by simulated leaching tests in a soil column. The results showed that water absorption capacities of the modified SAP in salt solutions were improved due to the adsorption and transfer of water molecules by the sulfonic acid groups. Compared to the losses when there was no superabsorbent treatment, the water, nitrate, ammonium nitrogen, and water-soluble potassium losses during the salt-tolerant superabsorbent treatment were significantly reduced by 18.5, 22.8, 88.0, and 63.8%, respectively. The method introduced in this study could guide the development and wide application of salt-tolerant SAPs in agriculture and horticulture.