The influence of road salt on seasonal mixing, redox stratification and methane concentrations in urban kettle lakes.

Influxes of saline water from roads treated with deicers can alter the density structure of urban lakes. This can diminish or halt turnover events, such that lakes may transition from dimixis to monomixis or meromixis. In nutrient-rich lakes, this lack of turnover can produce persistent hypolimnetic anoxia. We hypothesized that diminished turnover in urban lakes impacted by road salt inputs would lead to increased accumulation of methane in the hypolimnia, with the potential for greater release of methane to the atmosphere via ebullition and from larger storage fluxes of methane when turnover events do occur. The lake water columns of two urban lakes (Woods Lake and Asylum Lake), previously suggested to have transitioned to meromixis and monomixis because of road salt deicer inputs, were sampled monthly from March 2016 to June 2017. A nearby rural lake (North Lake) less likely to be impacted by road salt and maintaining seasonal mixing, was also sampled for comparison. Lake column water was analyzed for conductivity, temperature, dissolved oxygen, ferrous iron, manganese, sulfide, calcium, magnesium, sodium, chloride and methane concentrations as a function of depth. All three lakes are eutrophic with at least seasonally anoxic hypolimnia. Our data are consistent with prior studies suggesting that Woods Lake has transitioned to meromixis and Asylum Lake to monomixis due to an influx of dense saline water from roads treated with deicers. In contrast, rural North Lake, which had much lower chloride, sodium and conductivity levels, was dimictic. The diminished or absent turnover in the two urban lakes during fall and spring resulted in persistently anoxic, redox-stratified hypolimnia, with much larger accumulations of methane compared to the rural lake. This study demonstrates that road salt deicers impact lake mixing and biogeochemistry, especially methane concentrations, with the potential for significant increases in greenhouse gas emissions from urban lakes.

Competitive Adsorption of Cd(II), Cr(VI), and Pb(II) onto Nanomaghemite: A Spectroscopic and Modeling Approach.

A combined modeling and spectroscopic approach is used to describe Cd(II), Cr(VI), and Pb(II) adsorption onto nanomaghemite and nanomaghemite coated quartz. A pseudo-second order kinetic model fitted the adsorption data well. The sorption capacity of nanomaghemite was evaluated using a Langmuir isotherm model, and a diffuse double layer surface complexation model (DLM) was developed to describe metal adsorption. Adsorption mechanisms were assessed using X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Pb(II) adsorption occurs mainly via formation of inner-sphere complexes, whereas Cr(VI) likely adsorbs mainly as outer-sphere complexes and Cd(II) as a mixture of inner- and outer-sphere complexes. The simple DLM describes well the pH-dependence of single adsorption edges. However, it fails to adequately capture metal adsorption behavior over broad ranges of ionic strength or metal-loading on the sorbents. For systems with equimolar concentrations of Pb(II), Cd(II), and Cr(VI). Pb(II) adsorption was reasonably well predicted by the DLM, but predictions were poorer for Cr(VI) and Cd(II). This study demonstrates that a simple DLM can describe well the adsorption of the studied metals in mixed sorbate-sorbent systems, but only under narrow ranges of ionic strength or metal loading. The results also highlight the sorption potential of nanomaghemite for metals in complex systems.

Surface complexation modeling of Cd(II) adsorption on mixtures of hydrous ferric oxide, quartz and kaolinite.

Cadmium adsorption was measured as a function of ionic strength (0.001-0.1M NaNO(3)), and spanning a range of sorbate/sorbent ratios, on pure hydrous ferric oxide (HFO), kaolinite, and quartz and also on binary and ternary mixtures of the three solids. Diffuse- layer surface complexation models (DLMs) were parameterized to fit Cd sorption data for the pure kaolinite and quartz systems. Cd adsorption on kaolinite was modeled using a two-site DLM, with formation of a monodentate Cd complex on a variable charge site and Cd binding to a permanent exchange site; Cd adsorption on quartz was described using a one-site DLM with formation of a mondentate Cd complex on a variable charge site. These DLMs, together with the Dzombak and Morel DLM for HFO, were used to predict Cd adsorption on the binary and ternary mineral mixtures using a simple component additivity approach. In general, the predicted adsorption edges were in good agreement with measured data, with statistically similar goodness of fit compared to that obtained for the pure mineral systems. However, in some cases the model overpredicted Cd sorption, possibly indicating that interaction of the solids may prevent Cd from accessing all of the sorption sites.

Surface complexation modeling of Cu(II) adsorption on mixtures of hydrous ferric oxide and kaolinite.

BACKGROUND: The application of surface complexation models (SCMs) to natural sediments and soils is hindered by a lack of consistent models and data for large suites of metals and minerals of interest. Furthermore, the surface complexation approach has mostly been developed and tested for single solid systems. Few studies have extended the SCM approach to systems containing multiple solids. RESULTS: Cu adsorption was measured on pure hydrous ferric oxide (HFO), pure kaolinite (from two sources) and in systems containing mixtures of HFO and kaolinite over a wide range of pH, ionic strength, sorbate/sorbent ratios and, for the mixed solid systems, using a range of kaolinite/HFO ratios. Cu adsorption data measured for the HFO and kaolinite systems was used to derive diffuse layer surface complexation models (DLMs) describing Cu adsorption. Cu adsorption on HFO is reasonably well described using a 1-site or 2-site DLM. Adsorption of Cu on kaolinite could be described using a simple 1-site DLM with formation of a monodentate Cu complex on a variable charge surface site. However, for consistency with models derived for weaker sorbing cations, a 2-site DLM with a variable charge and a permanent charge site was also developed. CONCLUSION: Component additivity predictions of speciation in mixed mineral systems based on DLM parameters derived for the pure mineral systems were in good agreement with measured data. Discrepancies between the model predictions and measured data were similar to those observed for the calibrated pure mineral systems. The results suggest that quantifying specific interactions between HFO and kaolinite in speciation models may not be necessary. However, before the component additivity approach can be applied to natural sediments and soils, the effects of aging must be further studied and methods must be developed to estimate reactive surface areas of solid constituents in natural samples.

Seasonal variations in pore water and sediment geochemistry of littoral lake sediments (Asylum Lake, MI, USA).

BACKGROUND: Seasonal changes in pore water and sediment redox geochemistry have been observed in many near-surface sediments. Such changes have the potential to strongly influence trace metal distribution and thus create seasonal fluctuations in metal mobility and bioavailability. RESULTS: Seasonal trends in pore water and sediment geochemistry are assessed in the upper 50 cm of littoral kettle lake sediments. Pore waters are always redox stratified, with the least compressed redox stratification observed during fall and the most compressed redox stratification observed during summer. A 2-step sequential sediment extraction yields much more Fe in the first step, targeted at amorphous Fe(III) (hydr)oxides (AEF), then in the second step, which targets Fe(II) monosulfides. Fe extracted in the second step is relatively invariant with depth or season. In contrast, AEF decreases with sediment depth, and is seasonally variable, in agreement with changes in redox stratification inferred from pore water profiles. A 5-step Tessier extraction scheme was used to assess metal association with operationally-defined exchangeable, carbonate, iron and manganese oxide (FMO), organic/sulfide and microwave-digestible residual fractions in cores collected during winter and spring. Distribution of metals in these two seasons is similar. Co, As, Cd, and U concentrations approach detection limits. Fe, Cu and Pb are mostly associated with the organics/sulfides fraction. Cr and Zn are mostly associated with FMO. Mn is primarily associated with carbonates, and Co is nearly equally distributed between the FMO and organics/sulfide fractions. CONCLUSION: This study clearly demonstrates that near-surface lake sediment pore water redox stratification and associated solid phase geochemistry vary significantly with season. This has important ramifications for seasonal changes in the bioavailability and mobility of trace elements. Without rate measurements, it is not possible to quantify the contribution of various processes to natural organic matter degradation. However, the pore water and solid phase data suggest that iron reduction and sulfate reduction are the dominant pathways in the upper 50 cm of these sediments.

Quantifying bioirrigation using ecological parameters: a stochastic approach†.

Irrigation by benthic macrofauna has a major influence on the biogeochemistry and microbial community structure of sediments. Existing quantitative models of bioirrigation rely primarily on chemical, rather than ecological, information and the depth-dependence of bioirrigation intensity is either imposed or constrained through a data fitting procedure. In this study, stochastic simulations of 3D burrow networks are used to calculate mean densities, volumes and wall surface areas of burrows, as well as their variabilities, as a function of sediment depth. Burrow networks of the following model organisms are considered: the polychaete worms Nereis diversicolor and Schizocardium sp., the shrimp Callianassa subterranea, the echiuran worm Maxmuelleria lankesteri, the fiddler crabs Uca minax, U. pugnax and U. pugilator, and the mud crabs Sesarma reticulatum and Eurytium limosum. Consortia of these model organisms are then used to predict burrow networks in a shallow water carbonate sediment at Dry Tortugas, FL, and in two intertidal saltmarsh sites at Sapelo Island, GA. Solute-specific nonlocal bioirrigation coefficients are calculated from the depth-dependent burrow surface areas and the radial diffusive length scale around the burrows. Bioirrigation coefficients for sulfate obtained from network simulations, with the diffusive length scales constrained by sulfate reduction rate profiles, agree with independent estimates of bioirrigation coefficients based on pore water chemistry. Bioirrigation coefficients for O2 derived from the stochastic model, with the diffusion length scales constrained by O2 microprofiles measured at the sediment/water interface, are larger than irrigation coefficients based on vertical pore water chemical profiles. This reflects, in part, the rapid attenuation with depth of the O2 concentration within the burrows, which reduces the driving force for chemical transfer across the burrow walls. Correction for the depletion of O2 in the burrows results in closer agreement between stochastically-derived and chemically-derived irrigation coefficient profiles.