Biological elimination of H2S and NH3 from wastegases by biofilter packed with immobilized heterotrophic bacteria

Biotreatment of various ratios of H₂S and NH₃ gas mixtures was studied using the biofilters, packed with co-immobilized cells (Arthrobacter oxydans CH8 for NH₃ and Pseudomonas putida CH11 for H₂S). Extensive tests to determine removal characteristics, removal efficiency, removal kinetics, and pressure drops of the biofilters were performed. To estimate the largest allowable inlet concentration, a prediction model was also employed. Greater than 95% and 90% removal efficiencies were observed for NH₃ and H₂S, respectively, irrespective of the ratios of H₂S and NH₃ gas mixtures. The results showed that H₂S removal of the biofilter was significantly affected by high inlet concentrations of H₂S and NH₃. As high H₂S concentration was an inhibitory substrate for the growth of heterotrophic sulfur-oxidizing bacteria, the activity of H₂S oxidation was thus inhibited. In the case of high NH₃ concentration, the poor H₂S removal efficiency might be attributed to the acidification of the biofilter. The phenomenon was caused by acidic metabolite accumulation of NH₃. Through kinetic analysis, the presence of NH₃ did not hinder the NH₃ removal, but a high H₂S concentration would result in low removal efficiency. Conversely, H₂S of adequate concentrations would favor the removal of incoming NH₃. The results also indicated that maximum inlet concentrations (model-estimated) agreed well with the experimental values for space velocities of 50–150 h⁻¹. Hence, the results would be used as the guideline for the design and operation of biofilters.     

Toxicity of HC Orange No. 1 to Daphnia magna, zebrafish (Brachydanio rerio) embryos, and goldfish (Carassius auratus)

HC Orange No. 1 (HCO1; 2-nitro-4′-hydroxydiphenylamine) (CAS No. 54381-08-7) is used as a color additive in hair dyes and can be released into aquatic environments in wastewater. In this paper, the effects of HCO1 on aquatic organisms were studied using a battery of toxicological tests. These included measuring immobilization of Daphnia magna, inhibition of zebrafish embryo development, and acute lethality in zebrafish and goldfish, which are different species belonging to different trophic levels. HCO1 was toxic to all of the organisms studied. In our experiments, HCO1 remarkably restrained the mobility of D. magna, which may cause subsequent death. The EC₅₀ value for restrained the mobility of D. magna at 48 h was 1.54 mg HCO1 l⁻¹. In addition, HCO1 showed toxicity in zebrafish and goldfish, where LC₅₀values at 96 h were 4.04 and 5.37 mg l⁻¹, respectively. The results also indicated that HCO1 remarkably retarded the development of zebrafish embryos, which may cause embryo abnormality and even lethality. The most sensitive toxicological endpoint in the development of the embryos was failure to hatch, which had an EC₅₀ of 0.19 mg HCO1 l⁻¹. These results indicated that HCO1 is a potential teratogen to zebrafish embryos. In addition, as HCO1 concentrations increased, the outcomes of each of these toxicity tests changed in a concentration-dependent manner. Together, the results revealed that HCO1 appears to be toxic to multiple different species of aquatic organisms. The EC₅₀ (LC₅₀) values contain sufficient discriminatory power for risk assessment of HCO1 in aquatic environments. Based on the present results, more efficient risk assessment procedures for HCO1 will be designed in the future, integrating more flexible testing methods into the testing schemes that employ only the necessary tools for each case.     

Can aquatic distribution of human pharmaceuticals be related to pharmacological data?

The recognition of pharmaceuticals as significant environmental contaminants has only been a recent phenomenon. Therefore there is a paucity of data relating to the fate and effects of pharmaceuticals once they enter an aquatic receiving system. The amount of work that needs to be done in terms of risk assessment for pharmaceuticals required by regulatory agencies is substantial. This paper has determined the environmental partitioning coefficient (K_d) of 13 diverse human pharmaceuticals in three model systems of differing combinations of solid phases and solutions. The K_d values were then compared with distribution values of the pharmaceuticals in the human body determined from pharmacological studies. This was done to assess the functional relationship between K_d and distribution values in the human body (V_D). K_d values ranged from 3 to 2450 L kg⁻¹. Regression coefficients ranged from r² = 0.62–0.72, indicating that V_D values are a useful indicator for the K_d values of the tested pharmaceuticals within the batch sorption systems. The relationship between K_d and V_D should therefore be further explored to determine whether this relationship can be applied to a broader range of pharmaceuticals in more diverse environmental systems. Exploiting available human pharmacological data in such a way would be of great benefit in prioritising human pharmaceuticals as environmental contaminants in the risk assessment process.     

Some pharmacokinetic aspects of PCDDs and PCDFs in mammals after administration of a fly ash extract from a municipal incinerator

Fly ash extracts were fed to male hamster (single dose), male rat (single dose and multiple dose), pregnant and lactating female rat (multiple dose). The retention of four isomers, 2,3,7,8-TCDD, 2,3,7,8-TCDF, 1,2,3,7,8-PnCDD and 2,3,4,7,8-PnCDF, was studied in the liver of the adults, foetuses and liver of the sucklings.

Liver retention was structure dependent and different for both species. Transportation of the isomers via the mother milk was 50–100 times more effective than via the placenta.

After a single intravenous dose of fly ash extract to male rats the elimination of these four isomers was studied in the liver, during a period of 10 days. Elimination rates for 2,3,7,8-TCDD, 1,2,3,7,8-PnCDD and 2,3,4,7,8-PnCDF were in the same range. Pharmacokinetic calculations were done on both tetra congeners, to obtain information about the validity of the published K_e values in the multiple dose experiments with male rats.

For 2,3,7,8-TCDF the K_e value was applicable, but for 2,3,7,8-TCDD the validity of the K_e could not be determined.     

Spray irrigation of landfill leachate: estimating potential exposures to workers and bystanders using a modified air box model and generalised source term

Generalised source term data from UK leachates and a probabilistic exposure model (BPRISC(4)) were used to evaluate key routes of exposure from chemicals of concern during the spraying irrigation of landfill leachate. Risk estimates secured using a modified air box model are reported for a hypothetical worker exposed to selected chemicals within a generalised conceptual exposure model of spray irrigation. Consistent with pesticide spray exposure studies, the key risk driver is dermal exposure to the more toxic components of leachate. Changes in spray droplet diameter (0.02-0.2 cm) and in spray flow rate (50-1000 l/min) have little influence on dermal exposure, although the lesser routes of aerosol ingestion and inhalation are markedly affected. The risk estimates modelled using this conservative worst case exposure scenario are not of sufficient magnitude to warrant major concerns about chemical risks to workers or bystanders from this practice in the general sense. However, the modelling made use of generic concentration data for only a limited number of potential landfill leachate contaminants, such that individual practices may require assessment on the basis of their own merits.     

Generic scenario for the impact of cooling lubricants into the hydrosphere

According to directive 93/67/EEC of the European Commission, this paper deals with the generic exposition evaluation of the environmental concentration of cooling Lubricant chemicals from the metal working industry into the hydrosphere. After considering the relevant life-cycle steps and the selection of a representative point source for the 'reasonable worst case', the emission per day Elocal(water) is determined. It leads to the predicted environmental concentration (PEClocal(water)) for the local stage in the compartment water. In order to gain the PEClocal(water) for an example--a corrosion inhibitor as additive in a cooling lubricant--, the relevant emission paths and the corresponding representative point source are described for the reasonable worst case. For non-water-miscible cooling lubricants, none of the operations in the life cycle leads to a release into the compartment water. To evaluate the hazard potential for cooling lubricant chemicals, the complete risk assessment has to be done. Also, the assessment has to be done for all high production volume chemicals, new substances and existing hazard chemicals. This means that even industrial categories like chemicals used in the textile industry or biocides and others have to be evaluated.     

Physicochemical substance properties as indicators for unreliable exposure in microplate-based bioassays

In the last years many efforts were made to transform standardized algal test protocols into low-cost microplate assays. While advantages were pointed out frequently, limitations are not systematically addressed, thus hindering a widespread utilisation. In this study a group of organic substances with a wide distribution of volatility (log K_AW from −6.53 to −2.13) and lipophilicity (log K_OW from 1.26 to 4.92) was investigated with respect to the influence of these physicochemical properties on their algal toxicity in different assays. Therefore the EC₅₀ values were determined with a microplate assay based on ISO 8692 protocol and the results were compared with those of an established algal growth inhibition test conducted in air tight glass vessels. Using the ratio of the EC₅₀ values, a clear connection between biological response and volatility as well as lipophilicity of test substances could be detected. Chemicals with a log K_OW higher than 3 or a Henry coefficient log K_AW higher than −4 were identified as less effective in the microplate assay than in the comparative assay. The loss in nominal concentration due to physicochemical properties could be shown to contribute to this using HPLC analysis. Consequently, when using microplate assay’s one should be aware that lipophilic and volatile chemicals might be underestimated in their toxicity, which could be indicated from evaluating related physicochemical properties modelled from structural information prior to an experimental investigation.     

Studies on lethal concentrations and toxicity stress of some xenobiotics on aquatic organisms

Three widely used xenobiotics pentachlorophenol (PCP), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-chloro-2,6-diethyl-N-(butoxymethyl) acetanilide (Butachlor) are evaluated for acute toxicity and stress behavior on freshwater fish (Heteropneustes fossilis, Clarias batrachus, Channa punctatus) and mosquito larvae (Culex pipiens fatigans). The experiment was carried out by medium treatment using intermittent flow-through system. Median lethal concentrations (LC50) were calculated by probit analysis. The LC50 values and 95% confidence intervals showed variable range for tested chemicals. Mosquito larvae generally appeared resistant than fish, while H. fossilis was found to be most sensitive. Stress signs in the form of behavioral changes are also observed. Both types of organisms are recommended as good bioindicator for the risk assessment of aquatic environment due to chemicals tested.     

Alternatives for the determination of the soil adsorption coefficient, KOC, of non-ionicorganic compounds - a review

The determination of the soil adsorption behaviour of an environmental chemical is very important to the evaluation of potential dangers for man and nature. One of the major problems for European regulators working in the field of environmental protection and risk assessment is the lack of complete and comparable soil sorption data, e.g. K_oc values. The large number of existing chemicals makes it necessary to develop and apply fast methods as an alternative to classical batch or column studies. Based on this context numerous approaches to estimate K_oc have been published during the past two decades.

In this paper more than 200 existing relationships for K_oc estimations have been reviewed and divided into four categories. Regression equations for the most important classes of non-ionic organic environmental chemicals able to estimate soil adsorption coefficients are summarised, and the advantages and drawbacks of the different approaches are discussed briefly.

Due to the fact that a broad application of the proposed approaches is limited by the quality and comparability of the underlying experimental K_oc values reference soils are postulated to create a common basis for the comparison of soil adsorption coefficients.     

Research to develop a predicting system of mammalian subacute toxicity (3) construction of a predictive toxicokinetics model

A new predictive toxicokinetics model was developed to estimate subacute toxicity (target organs, severity, etc.) of non-congeneric industrial chemicals, where the chemical structures and physico-chemical properties are only available. Thus, a physiological pharmacokinetics model, which consists of blood, liver, kidney (these were experimentally found as major toxicological targets), muscle and fat compartments , was established to simulate the chemical concentrations in organs/tissues with pharmacokinetic parameters by means of Runge-Kutta-Gill algorithm. The pliarmacokinetic parameters, i.e. absorption rate, absorption ratio, hepatic extraction ratio of metabolism and renal clearance were calculated by using separately established Quantitative Structure-Pharmacokinetics Relationship equations. The developed predictive model was then applied to simulations of 43 non-congeneric industrial chemicals. The chemical concentrations in organs/tissues after single oral administration were simulated, and their maximum concentrations (Cmax's) and area tinder the concentration-time curves (AUC's) were calculated.Fast Inverse Laplace Transform was newly applied for the purpose of simulation of 28-day repeated dose toxicity.Simulated concentrations of 28 days repeated dose were, however, found to be the same as those of simple repetitions of a single administration per day because of the short half-lives of non-congeneric industrial chemicals.A comparison of subacute toxicity data with Cmax's and AUC's in a single dose scenario suggested that the organs/tissues with relatively high concentrations of tested chemical substances were the most sensitive targets within a chemical.Chemical concentrations in liver, for instance, were correlated with the severity of hepatotoxicity among the chemicals. It was also suggested that to improve and widen the present approach, data of metabolite and reactivity of non-congeneric industrial chemicals to organs/tissues, receptors, etc. should be incorporated into the model.     

Assessing joint toxicity of chemicals in Enchytraeus albidus (Enchytraeidae) and Porcellionides pruinosus (Isopoda) using avoidance behaviour as an endpoint

Contamination problems are often characterized by complex mixtures of chemicals. There are two conceptual models usually used to evaluate patterns of mixture toxicity: Concentration Addition (CA) and Independent Action (IA). Deviations from these models as synergism, antagonism and dose dependency also occur. In the present study, single and mixture toxicity of atrazine, dimethoate, lindane, zinc and cadmium were tested in Porcellionides pruinosus and Enchytraeus albidus, using avoidance as test parameter. For both species patterns of antagonism were found when exposed to dimethoate and atrazine, synergism for lindane and dimethoate exposures (with the exception of lower doses in the isopod case study) and concentration addition for cadmium and zinc occurred, while the exposure to cadmium and dimethoate showed dissimilar patterns.This study highlights the importance of dose dependencies when testing chemical mixtures and that avoidance tests can also be used to asses the effects of mixture toxicity.     

Short-term toxicity of pyraclofos used as a blackfly larvicide on non-target aquatic fauna in a tropical environment

Among the many larvicides tested for the control of s.l. larvae, the vector of human onchocerciasis in West Africa, pyraclofos proved to be 100% effective at 100 μg × L⁻¹ for 10 min in river, with a carry of 20 km at 100 m³ × sec⁻¹. Tests were then performed both in laboratory and field conditions to evaluate its toxicity on the non-target aquatic fauna. In experimental short-term gutter tests, the detachment of the total benthic insects was 35% at 100 μg × L⁻¹ for 10 min against 17% for temephos at the same dose and 59% for chlorphoxim at 50μg × L⁻¹ for 10 min. , and were the most affected organisms. The treatment of a river resulted in a considerable detachment of the same taxonomic groups, plus Orthocladiinae. On the other hand, investigations conducted in tanks showed that the 24-hr LC₅₀ for is 150 μg × L⁻¹ and that for 170 μg × L⁻¹, values which are not very different from the operational dose of the larvicide (100 μg × L⁻¹ for 10 min.). Nevertheless, in a river, no fish mortality was recorded. Based on fish LC₅₀ and drift of benthic insects, pyraclofos at 100 μg × L⁻¹ was judged to be less toxic to aquatic fauna in the short term than permethrin and carbosulfan.     

Combined toxic impacts of thiamethoxam and four pesticides on the rare minnow (Gobiocypris rarus)

To examine pesticide mixture toxicity to aqueous organisms, we assessed the single and combined toxicities of thiamethoxam and other four pesticides (chlorpyrifos, beta-cypermethrin, tetraconazole, and azoxystrobin) to the rare minnow (Gobiocypris rarus). Data from 96-h semi-static toxicity assays of various developmental phases (embryonic, larval, juvenile, and adult phases) showed that beta-cypermethrin, chlorpyrifos, and azoxystrobin had the highest toxicities to G. rarus, and their LC₅₀ values ranged from 0.0031 to 0.86 mg a.i. L^?1, from 0.016 to 6.38 mg a.i. L^?1, and from 0.39 to 1.08 mg a.i. L^?1, respectively. Tetraconazole displayed a comparatively high toxicity, and its LC₅₀ values ranged from 3.48 to 16.73 mg a.i. L^?1. By contrast, thiamethoxam exhibited the lowest toxic effect with LC₅₀ values ranging from 37.85 to 351.9 mg a.i. L^?1. Rare minnow larvae were more sensitive than embryos to all the pesticides tested. Our data showed that a pesticide mixture of thiamethoxam–tetraconazole elicited synergetic toxicity to G. rarus. Moreover, pesticide mixtures containing beta-cypermethrin in combination with chlorpyrifos or tetraconazole also had synergetic toxicities to fish. The majority of pesticides are presumed to have additive toxicity, while our data emphasized that the concurrent existence of some chemicals in the aqueous circumstance could cause synergetic toxic effect, leading to severe loss to the aqueous environments in comparison with their single toxicities. Thence, the synergetic impacts of chemical mixtures should be considered when assessing the ecological risk of chemicals.

     

Assessing single and joint effects of chemicals on the survival and reproduction of Folsomia candida (Collembola) in soil

Chemicals are often found in the environment as complex mixtures. There has been a large effort in the last decade to assess the combined effect of chemicals, using the conceptual models of Concentration Addition and Independent Action, but also including synergistic, antagonistic, dose-level and dose-ratio dependent deviations from these models. In the present study, single and mixture toxicity of atrazine, dimethoate, lindane, zinc and cadmium were studied in Folsomia candida, assessing survival and reproduction. Different response patterns were observed for the different endpoints and synergistic patterns were observed when pesticides were present. Compared with the previously tested Enchytraeus albidus and Porcellionides pruinosus, the mixture toxicity pattern for F. candida was species specific. The present study highlights the importance of studying toxicity of chemicals mixtures due to the observed potentiation of effects and confirms that for an adequate ecologically relevant risk assessment different organisms and endpoints should be included.     

Assessment of combined antiandrogenic effects of binary parabens mixtures in a yeast-based reporter assay

To date, toxicological studies of endocrine disrupting chemicals (EDCs) have typically focused on single chemical exposures and associated effects. However, exposure to EDCs mixtures in the environment is common. Antiandrogens represent a group of EDCs, which draw increasing attention due to their resultant demasculinization and sexual disruption of aquatic organisms. Although there are a number of in vivo and in vitro studies investigating the combined effects of antiandrogen mixtures, these studies are mainly on selected model compounds such as flutamide, procymidone, and vinclozolin. The aim of the present study is to investigate the combined antiandrogenic effects of parabens, which are widely used antiandrogens in industrial and domestic commodities. A yeast-based human androgen receptor (hAR) assay (YAS) was applied to assess the antiandrogenic activities of n-propylparaben (nPrP), iso-propylparaben (iPrP), methylparaben (MeP), and 4-n-pentylphenol (PeP), as well as the binary mixtures of nPrP with each of the other three antiandrogens. All of the four compounds could exhibit antiandrogenic activity via the hAR. A linear interaction model was applied to quantitatively analyze the interaction between nPrP and each of the other three antiandrogens. The isoboles method was modified to show the variation of combined effects as the concentrations of mixed antiandrogens were changed. Graphs were constructed to show isoeffective curves of three binary mixtures based on the fitted linear interaction model and to evaluate the interaction of the mixed antiandrogens (synergism or antagonism). The combined effect of equimolar combinations of the three mixtures was also considered with the nonlinear isoboles method. The main effect parameters and interaction effect parameters in the linear interaction models of the three mixtures were different from zero. The results showed that any two antiandrogens in their binary mixtures tended to exert equal antiandrogenic activity in the linear concentration ranges. The antiandrogenicity of the binary mixture and the concentration of nPrP were fitted to a sigmoidal model if the concentrations of the other antiandrogens (iPrP, MeP, and PeP) in the mixture were lower than the AR saturation concentrations. Some concave isoboles above the additivity line appeared in all the three mixtures. There were some synergistic effects of the binary mixture of nPrP and MeP at low concentrations in the linear concentration ranges. Interesting, when the antiandrogens concentrations approached the saturation, the interaction between chemicals were antagonistic for all the three mixtures tested. When the toxicity of the three mixtures was assessed using nonlinear isoboles, only antagonism was observed for equimolar combinations of nPrP and iPrP as the concentrations were increased from the no-observed-effect-concentration (NOEC) to effective concentration of 80 %. In addition, the interactions were changed from synergistic to antagonistic as effective concentrations were increased in the equimolar combinations of nPrP and MeP, as well as nPrP and PeP. The combined effects of three binary antiandrogens mixtures in the linear ranges were successfully evaluated by curve fitting and isoboles. The combined effects of specific binary mixtures varied depending on the concentrations of the chemicals in the mixtures. At low concentrations in the linear concentration ranges, there was synergistic interaction existing in the binary mixture of nPrP and MeP. The interaction tended to be antagonistic as the antiandrogens approached saturation concentrations in mixtures of nPrP with each of the other three antiandrogens. The synergistic interaction was also found in the equimolar combinations of nPrP and MeP, as well as nPrP and PeP, at low concentrations with another method of nonlinear isoboles. The mixture activities of binary antiandrogens had a tendency towards antagonism at high concentrations and synergism at low concentrations.     

Do aquatic effects or human health end points govern the development of sediment-quality criteria for nonionic organic chemicals ?

The equilibrium partitioning theory may be used to describe the partitioning of nonionic organic chemicals between water, sediment, and aquatic biota. This paradigm was employed to compare the relative magnitudes of organic carbon-normalized sediment-quality criteria that are intended to protect either benthic organisms from the direct toxic effects of sediment-associated chemicals or humans from the indirect health effects posed by the ingestion of contaminated aquatic animals. Comparison of calculated sediment-quality criteria for a variety of hydrophobic chemicals suggests that human health-based end points often result in more restrictive criteria than aquatic effects-based values. Review of published field data indicates that the equilibrium partitioning paradigm may, depending on contaminant class, either over- or underestimate the extent to which sediment-associated contaminations are bioaccumulated. Despite the limitations of adopting this simple theory for criteria development, calculations reveal that regulatory decisions involving sediments contaminated with such chemicals may be dictated by human health concerns if current risk assessment methodologies are applied.     

Acute toxicity of cadmium to fish Labeo rohita and copepod Diaptomus forbesi pre-exposed to CaO and KMnO4

96-h LC₅₀ values of cadmium (Cd) to fish Labeo rohita and the copepod Diaptomus forbesi, determined by static bioassays, were, respectively, 89.5 and 10.2 mg/l. LC₅₀ values increased significantly when fish pre-exposed to 100–350 mg/l CaO or 0.5–1.5 mg/l KMnO₄ for 4 d and the copepod to 20–70 mg/l CaO or 0.25–1.0 mg/l KMnO₄ for same period. The LC₅₀ values also increased when the pre-exposure period of CaO was increased to 12 d at concentration 100 mg/l for fish and 20 mg/l for copepod. All fish died when pre-exposed to 1.5 mg/l KMnO₄ for 8 d. But LC₅₀ values of Cd to copepod increased when pre-exposure period of 0.5 mg/l KMnO₄ was increased from 4 to 8 d.     

Leaching of atrazine, metolachlor and diuron in the field in relation to their injection depth into a silt loam soil

A field experiment was conducted on a Calcaric Cambisol soil to study the consequences of the penetration depth and properties of pesticides on the risk of subsequent leaching. Three pesticides with different mobility characteristics and bromide were injected at 30 cm (where soil organic matter (OM) was 2%) and 80 cm (soil OM 0.5%) on irrigated plots without a crop. The migration of injected solutes was assessed for two years by sampling the soil solution using six porous cups installed at 50 and 150 cm depth and by relating solute contents to drainage water flux estimated by the STICS model (Simulateur mulTIdisciplinaire pour les Cultures Standard). Pesticides injected at 30 cm were strongly retained so that no metolachlor or diuron was detected at 50 and 150 cm. The ratio of atrazine peak concentration in the soil solution to concentration in the injected solution (C/C₀) was 1 × 10⁻³ and 0.2 × 10⁻³, respectively, at 50 and 150 cm. When injected at 80 cm, (C/C₀) of atrazine, metolachlor and diuron were 10 × 10⁻³, 1 × 10⁻³ and 0.3 × 10⁻³ at 150 cm, respectively; 1/(C/C₀) was correlated with K_oc values reported from databases. The ratio of drainage volume to the amount of water at field capacity in the soil layer between the injection point at 30 cm and the water sampling level (V/V₀) at 50 and 150 cm was 0.6 and 0.9, respectively, for bromide and 1.6 and 1.0 for atrazine. V/V₀ of the injected solutes at 80 cm was for bromide, atrazine, metolachlor and diuron 0.6, 0.9, 1.2 and 1.7, respectively; pesticide V/V₀ was correlated with K_oc. The retardation factor was a good indicator of migration risk, but tended to overestimate retardation of molecules with high K_oc. Atrazine desorption represented an additional leaching risk as a source of prolonged low contamination. The large variability in soil solution of bromide and pesticide concentrations in the horizontal plane was attributed to flow paths and clods in the tilled soil layer. This heterogeneity was assumed to channel water fluxes into restricted areas and thereby increase the risk of groundwater contamination. The methodology used in the field proves to provide consistent results.     

Predicting diffuse-source transfers of surfactants to surface waters using SWAT

Chemicals present in domestic wastewater can adsorb to solid phase materials during sewage treatment. If biosolids (or sewage sludge) are applied to land, these chemicals can be transferred to soil. Under some circumstances they can also be transferred to surface waters during storm events either in solution or attached to sediment. In this paper we describe the utility of the SWAT 2000 model to estimate diffuse-source surface water exposure to "down-the-drain" chemicals. The model was applied hypothetically to predict the behaviour of linear alkylbenzene sulphonate (LAS), an anionic surfactant commonly used in household detergents, in a small catchment in Bedfordshire, UK, where it has previously been successfully validated for stream flow, sediments and pesticides. LAS transfers were estimated for two scenarios: (1) realistic and (2) reasonable worst case, based on assumptions on sludge application rates and the concentration of LAS in sludge. In addition, the sensitivity of the model output to the proportion of the catchment to which sludge is applied was established. Soil wetness and the total quantity of biosolids applied were the biggest determinants of chemical transport from the catchment. The potential of SWAT as a higher-tier tool in environmental risk assessments is discussed.     

Comment on “Assessment of Crop Loss from Ozone”

Abstract

A mathematical model is proposed for assessing health risk rates of fluctuating concentrations. Each time-averaged concentration may be regarded as a dose that, when applied to the dose-response curve, produces a risk of an adverse effect. A theoretical derivation shows that the dose-response pattern is a cumulative lognormal curve because of the diversity of the individuals in the exposed population. Similarly, the concentration pattern is a log-normal distribution because of the diversity of emission sources and dispersive processes. The health risk is produced by the overlapping of the right tail of the concentration distribution and the left tail of the dose-response curve. The evaluation of the joint probability in this region has been performed by numerical integration by computer in terms of two generalized parameters. One represents the geometric standard deviation of the concentration distribution relative to that of the dose-response curve, and the other represents the distance between the geometric mean concentration and the concentration producing an adverse response in 50% of the exposed population. These results are presented graphically and in tabular form. If the two parameters of the dose-response curve are known, the health risk of the concentration pattern may be calculated conveniently for any geometric mean and geometric standard deviation values.