Application of multimedia models for understanding the environmental behavior of volatile methylsiloxanes: Fate, transport, and bioaccumulation.

Multimedia fate and transport models (MFTMs) describe how chemicals behave in the environment based on their inherent properties and the characteristics of receiving systems. We critically review the use of MFTMs for understanding the behavior of volatile methylsiloxanes (VMS). MFTMs have been used to predict the fate of VMS in wastewater treatment, rivers, lakes, marine systems, and the atmosphere, and to assess bioaccumulation and trophic transfers. More widely, they have been used to assess the overall persistence, long-range transport potential (LRTP), and the propensity for atmosphere-surface exchange. The application of MFTMs for VMS requires particularly careful selection of model inputs because the properties of VMS differ from those of most organic compounds. For example, although n-octanol/water partition coefficient (KOW ) values are high, air:water partition coefficient (KAW ) values are also high and n-octanol/air partition coefficient (KOA ) values are relatively low. In addition, organic carbon/water partition coefficient (KOC ) values are substantially lower than expectations based on KOW . This means that most empirical relationships between KOC and KOW are not appropriate. Good agreement between modeled and measured concentrations in air, sediment, and biota indicates that our understanding of environmental fate is reasonable. VMS compounds are "fliers" that principally partition to the atmosphere, implying high LRTP, although they have low redeposition potential. They are degraded in air (half-lives 3-10 days) and, thus, have low overall persistence. In water, exposure can be limited by hydrolysis, volatilization, and partitioning to sediments (where degradation half-lives are likely to be high). In food webs, they are influenced by metabolism in biota, which tends to drive trophic dilution (i.e., trophic magnification factors are often but not always <1). Key remaining uncertainties include the following: (i) the strength and direction of the temperature dependence for KOC ; (ii) the fate of atmospheric reaction products; and (iii) the magnitude of emissions to wastewater. Integr Environ Assess Manag 2022;18:599-621. © 2021 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Increasing plant availability of legacy phosphorus in calcareous soils using some phosphorus activators.

Legacy phosphorus (P) in soil, accumulated over several years of fertilizer application in excess of crop demand, represents a huge and largely untapped resource. P activators can increase the availability of this P to plants by accelerating its transformation into soluble P fractions. In this study, we evaluated the potentials of four "P activators" (oxalic acid, lignin, phytase and ascorbic acid) to increase plant available P in a laboratory incubation experiment with two P-deficient calcareous soils used for wheat production. Samples were analysed for Olsen P, phosphomonoesterase and with Hedley sequential P fractionation. All four treatments had significant effects on different soil P fractions. Oxalic acid mainly enhanced inorganic P (Pi) solubility from the HCl-extractable P pool. Lignin enhanced P lability from the NaOH-, HCl- and residual-P pools. Phytase and ascorbic acid principally affected the organic P fractions (Po). Oxalic acid and lignin showed most potential to improve P (H2O-P, NaHCO3-Pi and NaHCO3-Po) availability, which increased by 110-419% and 4.1-122%, respectively. These findings illustrated the potential mechanisms responsible for P release associated with different P activators and reinforced the case for their use in increasing legacy P availability for agriculture in calcareous soils.

Microbial community composition and activity controls phosphorus transformation in rhizosphere soils of the Yeyahu Wetland in Beijing, China.

Microorganisms in the rhizosphere of wetland plants can have a significant impact on phosphorus (P) interception. We investigated the seasonal pattern of microbial community structure and its relationship with different P forms in the rhizosphere of three plants Scirpus planiculmis, Zizania latifolia, and Phragmites australis from the Yeyahu Wetland, China. Chloroform fumigation-extraction was used to determine the soil microbial biomass P (SMBP) and phospholipid fatty acids (PLFA) were used to characterize microbial community composition. P fractions in rhizosphere soil samples were also observed using sequential chemical fractionation. Results showed that the average total PLFA (TPLFA) contents of rhizosphere soils ranged from 34.9 to 40.7nmol·g-1 and were highest in summer. Bacteria were predominant in the rhizospheres of all three plants, accounting for >63% of TPLFA. Aerobic bacteria, represented by 16:0 PLFA, were most abundant. Both organic P (OP) and inorganic P (IP) accumulated in the rhizosphere during the winter die-back phase. Furthermore, both TPLFA and bacterial PLFA decreased with increases in highly resistant OP (HR-OP), occluded P (Oc-P) and Calcium-bound P (Ca-P). This suggests that bacteria play an important role in P transformation and can make use of various P forms. We also found that SMBP was significantly negatively correlated with labile OP (L-OP), moderately labile OP (ML-OP) and HR-OP, reflecting a high degree of cross correlation between SMBP and the PLFA indices.

Monitoring fluvial water chemistry for trend detection: hydrological variability masks trends in datasets covering fewer than 12 years.

This paper sub-samples four 35 year water quality time series to consider the potential influence of short-term hydrological variability on process inference derived from short-term monitoring data. The data comprise two time series for nitrate (NO(3)-N) and two for DOC (using water colour as a surrogate). The four catchments were selected not only because of their long records, but also because the four catchments are very different: upland and lowland, agricultural and non-agricultural. Multiple linear regression is used to identify the trend and effects of rainfall and hydrological 'memory effects' over the full 35 years, and then a moving-window technique is used to subsample the series, using window widths of between 6 and 20 years. The results suggest that analyses of periods between six and eleven years are more influenced by local hydrological variability and therefore provide misleading results about long-term trends, whereas periods of longer than twelve years tend to be more representative of underlying system behaviour. This is significant: if such methods for analysing monitoring data were used to validate changes in catchment management, a monitoring period of less than 12 years might be insufficient to demonstrate change in the underlying system.

Effects of triclosan on soil microbial respiration.

The antimicrobial substance triclosan has widespread use in personal care products and can enter the terrestrial environment if sewage sludge is applied to soil. The inhibitory effects of triclosan on basal and substrate-induced respiration (SIR) of three different soils were investigated. Soils were dosed and later redosed with four nominal triclosan concentrations, and respiration rates were measured over time. In each soil, a significant depression in basal respiration was noted after initial dosing, followed by a recovery. The initial extent of respiration inhibition was positively related to the triclosan dose, i.e., respiration was most inhibited at highest triclosan concentration. Differences in respiration inhibition between soils at equivalent dose were inversely correlated with organic matter and clay content, suggesting that the bioavailability of triclosan might have been reduced by sorption to organic carbon or by physical protection in micropores. Substrate-induced respiration was also reduced by the addition of triclosan and subsequently recovered. After redosing with triclosan, basal respiration was enhanced in all soils, suggesting that it was acting as a substrate. However, redosing resulted in SIR inhibition in all treatments above 10 mg triclosan kg(-1) in all three soils, although all soils appeared to be more resistant to perturbation than following initial dosing. The present study suggests that triclosan inhibits soil respiration but that a subsequent acclimation of the microbial community occurs.

Multimedia fate of petroleum hydrocarbons in the soil: oil matrix of constructed biopiles.

A dynamic multimedia fugacity model was used to evaluate the partitioning and fate of petroleum hydrocarbon fractions and aromatic indicator compounds within the soil: oil matrix of three biopiles. Each biopile was characterised by four compartments: air, water, soil solids and non-aqueous phase liquid (NAPL). Equilibrium partitioning in biopile A and B suggested that most fractions resided in the NAPL, with the exception of the aromatic fraction with an equivalent carbon number from 5 to 7 (EC(5-7)). In Biopile C, which had the highest soil organic carbon content (13%), the soil solids were the most important compartment for both light aliphatic fractions (EC(5-6) and EC(6-8)) and aromatic fractions, excluding the EC(16-21) and EC(21-35). Our starting hypothesis was that hydrocarbons do not degrade within the NAPL. This was supported by the agreement between predicted and measured hydrocarbon concentrations in Biopile B when the degradation rate constant in NAPL was set to zero. In all scenarios, biodegradation in soil was predicted as the dominant removal process for all fractions, except for the aliphatic EC(5-6) which was predominantly lost via volatilization. The absence of an explicit NAPL phase in the model yielded a similar prediction of total petroleum hydrocarbon (TPH) behaviour; however the predicted concentrations in the air and water phases were significantly increased with consequent changes in potential mobility. Further comparisons between predictions and measured data, particularly concentrations in the soil mobile phases, are required to ascertain the true value of including an explicit NAPL in models of this kind.

Predicting accurate and ecologically relevant regional scale concentrations of triclosan in rivers for use in higher-tier aquatic risk assessments.

Triclosan (TCS) is used in a range of domestic products and may reach aquatic environments after disposal "down-the-drain". Reach-specific TCS concentrations were predicted using the stochastic water quality model LF2000-WQX for two heavily urbanised regions in England, encompassing some of the most densely populated areas (including London and Birmingham). The model was calibrated using effluent TCS concentration data derived from a national monitoring programme which included 21 sewage treatment plants (STPs). The assessment represents a significant improvement on previous higher-tier modelling studies of TCS because it is applied to a much wider area compared with that hitherto evaluated. Initial results suggest that total TCS concentrations exceed a conservative Predicted No Effect Concentration of 69 ng L(-1) in <15% of river reaches. However, TCS is an ionisable compound with a pKa of about 8. In many rivers, a significant fraction of TCS will exist as the phenolate anion, which has much lower toxicity than neutral TCS. Concentrations of the neutral form of TCS were, therefore, calculated using measured data on river water pH. Predicted neutral concentrations in most river reaches were significantly lower than those for total TCS, often by >50%, reducing the associated risks. Further work is needed to better characterise the aquatic toxicity of TCS to fresh water organisms under a range of environmentally relevant conditions.