Study of organic sulphur compounds (DMS,DMSP and CS2) in lagoon ecosystems: the case of the Venice lagoon

This study of the origin and fate of dimethyl sulphide (DMS) in a particular and complex lagoon ecosystem such as that of the Venice lagoon focuses on the temporal evolutions of DMS concentrations in surface water together with those of dimethylsulphoniopropionate (DMSP), carbon disulphide (CS2), nutrients (nitrate, nitrite, ammonium, phosphate, silicate), sulphate, chlorophyll a, chlorinity, water temperature and phytoplankton (composition and density). Measurements were made from 3 March 1997 to 23 July 1998 at three stations in the central part of the Venice lagoon. The temporal trends of DMS concentration showed an absolute maximum concentration in winter (65 nmol S/l, 19/2/1998, Stn. 1; 119 nmol S/l, 19/2/1998, Stn. 2; 29 nmol S/l, 17/2/1998, Stn. 3) and two relative maxima in the spring-summer period. The spring-summer secondary maxima of DMS concentration were related to the maxima of DMSP and chlorophyll a concentrations and consequently to phytoplanktonic abundance while the winter DMS maximum showed no relation to DMSP or to chlorophyll a suggesting that the production and the fate of DMS could be different for the two periods. According to previous studies the CS2 concentration increased in the spring, achieved its maximum in summer, decreased in autumn and fell to its minimum in winter.     

Temporal evolution of DMS and DMSP in Antarctic Coastal Sea water

The temporal evolution of concentrations of dimethylsulphide (DMS), its precursor dimethylsulphoniopropionate (DMSP) and chlorophyll a is surveyed weekly in the water column and in a landfast ice core at a coastal station of Gerlache Inlet (Terra Nova Bay, Antarctica) from 27 November 2000 to 14 February 2001. The DMS and DMSP profile concentrations in the water column are similar and show a clear temporal trend, with minimum values (<0.7?nM) at all depths occurring on 27 November 2000 and maximum values (4.8 × 10²?nM for DMS and 1.8 × 10²?nM for DMSP) in surface water on 27 December 2000 for DMS and on 19 December 2000 for DMSP. When the sea-ice cover is present, the temporal evolution of DMSP closely follows that of chlorophyll a in the water column, supporting the idea that DMSP, and therefore DMS, has a phytoplanktonic origin. However, when the ice cover breaks up during the late austral summer, a second phytoplankton bloom occurs, while the DMSP concentration in the sea-water column remains very low. In the ice core, the results show higher concentrations of DMSP than those of the underlying sea water, highlighting the important role of sea ice in the sulphur cycle of the Antarctic ecosystem.     

Sequential analysis of dimethyl sulfur compounds in seawater

A sequential method for the determination of dimethyl sulfur compounds, including dimethylsulfide （DMS）, dimethylsulfonio- propionate （DMSP） and dimethylsulfoxide （DMSO）, in seawater samples has been developed. Detection limit of 2.5 pmol of DMS in 25 mL sample, corresponding to 0.10 nmol/L, was achieved. Recoveries for dimethyl sulfur compounds were in the range of 68.6- 78.3%. The relative standard deviations （R.S.D.s） for DMS, DMSP and DMSO determination were 3.0, 5.4 and 7.4%, respectively.     

Chemometric studies in the Lagoon of Venice, Italy. Annual evolution of sulphur species and relationship to biogeochemical cycles in lagoon water

During the period March 1997-March 1998 dimethyl sulphide (DMS), dimethylsulphoniopropionate (DMSP) and carbon disulphide (CS2) were determined weekly in the water of the Lagoon of Venice, Italy (at three stations located in the Giudecca Canal, the San Secondo Canal and the Rio di San Nicolò). At the same time, the following hydrological and biological variables were also measured: tide height, temperature, transmittance, fluorescence, pH, salinity, chlorinity, sulphate, ammonia, nitrite, nitrate, phosphate, silicate, chlorophyll a, phaeopigments, phytoplankton (abundance and biomass). Principal component analysis (PCA), applied as a dimension reduction tool, made it possible to summarize multivariate information in a small number of components, which highlighted the relationships between the temporal evolutions of the sulphur compounds with hydrological and biological variables in the seasonal biogeochemical cycle of the lagoon. In particular the overall temporal cycle, which begins with the development of biological activity in late winter and spring, followed by the predominance of degradation processes during the late summer and the remineralization of nutrients in autumn, is clearly described in the plane of the first two principal components, together with the interrelationships between all the relevant variables.     

Capillary electrophoretic analysis of dimethylsulfoniopropionate in sugarcane and marine algal extracts

A simple and high-resolution analytical method for the determination of dimethylsulfoniopropionate (DMSP) in sugarcane and marine algae is described. Effective extraction of DMSP from plant samples was also investigated using organic solvents, 5% perchloric acid or deionised water. To increase the sensitivity, DMSP in the extracts was first converted to a phenacyl ester, and the reaction mixture was applied directly to capillary electrophoresis without any pretreatment. Water extraction followed by esterification in a pH 4 reaction buffer was found most suitable for the measurement of alkaline-labile DMSP. This method was applied to the determination of DMSP levels in marine algae samples collected from the seashore of Nagasaki, Japan. An increase in DMSP content in Ulva pertusa in the winter period was observed.     

Concentration levels and temporal variations of heavy elements in the urban particulate matter of Navi Mumbai, India

Coarse and fine fractions of particulate matter (PM) were collected and analysed for trace elements using Instrumental Neutron Activation Analysis and Energy Dispersive X-ray Fluorescence techniques. The result showed high concentrations of Fe, S, Zn and Pb in both the size fractions. The elemental data obtained is used to analyze the temporal and seasonal variations. The trend showed maximum concentrations of PM and metals during winter and minimum during the monsoon season. Enrichment Factor (EF) and source analysis was performed for the same data set to identify the strength of contribution of anthropogenic sources and the possible contributing sources in the study area. EF studies showed high enrichments of Zn, Pb and As in the fine fraction particles. Crustal, vehicular and industrial emissions are identified as the major contributing sources of PM in the study area.     

云南阳宗海砷污染水平、变化趋势及风险评估

云南阳宗海砷污染事件引起社会广泛关注.为了解事件发生后阳宗海砷污染水平及变化趋势,分别于2008年12月、2009年2月、5月及9月四次采样,研究了阳宗海湖水、底泥、周边井水、土壤、农作物及水生生物中的砷含量及其变化趋势.研究结果显示:湖水平均砷浓度分别为176.9、147.3、159.3和161.1μg/L(算术平均),底泥平均浓度分别为32.87、62.41、62.99和46.96μg/g(算术平均).阳宗海湖水砷浓度经历了先升后降再到平稳的变化过程,底泥砷含量迅速升高后缓慢下降,湖水和底泥间砷交换还在进行.阳宗海附近土壤中砷最高浓度为23.33μg/g,未超过国家土壤环境质量三级标准.大米、玉米、花椰菜、小油菜等农作物可食用部分中砷的最高值为0.35μg/g,均未超过国家无公害食品标准.水生植物中砷水平大多在100～200μg/g之间,最高为苦草,砷含量超过300μg/g,说明该植物对砷有一定的富集能力.虾、鱼类等可食用水生动物砷浓度范围为1.52～11.4μg/g.     

The Behavior of Trivalent and Pentavalent Methylarsenicals in Lake Biwa

Seasonal changes in the distribution of arsenic species were observed in a dredged area in the southern basin of Lake Biwa, Japan. The concentrations of dimethylarsenic acid [DMAA(V)] became comparable with those of inorganic forms during a stratification period. DMAA(V) increased not only in the photic zone but also in the hypolimnion. In the photic zone, an increase in DMAA(V) was observed with the yearly maximum of water temperature. In the hypolimnion, the seasonal changes in methylarsenicals differed from those in surface waters. DMAA(V) maxima appeared seasonally under sub-anoxic conditions and developed regionally at the redox boundary and above the sediment surface. The DMAA(V) concentrations increased in the initial period of oxygen depletion and just after the disappearance of anoxia, while they diminished in the anoxic hypolimnion by midsummer. The seasonal behavior of trivalent methylarsenicals, which are readily oxidized in oxic environments, was similar to that of DMAA(V). The total arsenic concentration in the surface layer rose to a maximum in late summer. Methylarsenicals did not increase in such a way that the total arsenic concentration increased during summer. © 1997 by John Wiley & Sons, Ltd.     

Development of a new fluorescent probe: 1,3,5,7-tetramethyl-8-(4'-aminophenyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacence for the determination of trace nitrite

A new fluorescent probe, 1,3,5,7-tetramethyl-8-(4'-aminophenyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacence (TMABODIPY) has been developed for the determination of trace nitrite in terms of the reaction of nitrite with TMABODIPY first in acidic solution and then in alkaline solution to form diazotate, a stable and highly fluorescent reagent. The method offered the advantage of specificity, sensitivity and simplicity. The linear calibration range for nitrite was 8-300 nmol l-1s with a 3 sigma detection limit of 0.65 nmol l-1. The proposed method has been applied to monitor the trace nitrite in drinking water and vegetable without extraction.     

Exposure to 4,4'-methylenediphenyl diisocyanate (MDI) during moulding of rigid polyurethane foam: determination of airborne MDI and urinary 4,4'-methylenedianiline (MDA)

Occupational exposure to 4,4'-methylenediphenyl diisocyanate (MDI) was measured during moulding of rigid polyurethane foam. The aim of the study was to find out whether an MDI-derived urinary amine metabolite could be detected in the urine of workers exposed to apparently low levels of MDI. Airborne MDI was sampled on 1-(2-methoxyphenyl)-piperazine (2MP)-impregnated glass fibre filters and determined by high-performance liquid chromatography (HPLC) using ultraviolet (UV) and electrochemical (EC) detection. The limit of detection of MDI was 3 ng ml-1 for a 20 microliters injection. The precision of sample preparation, expressed as relative standard deviation (RSD), was 1.3% with UV detection and 2.1% with EC detection at a concentration of 70 ng MDI ml-1 (n = 6). The 2MP-MDI derivative was stable at +4 degrees C up to eight weeks. The accuracy of the method was validated in an international quality control programme. Workers (n = 57) from three different factories participated in the study. Urinary 4,4'-methylenedianiline (MDA) metabolite was determined after acid hydrolysis as heptafluorobutyric anhydride derivatives by gas chromatography-mass spectrometry using chemical ionisation and monitoring negative ions. The limit of detection in urine was 0.2 nmol l-1. The precision of six analyses for a urine sample spiked to a concentration of 1 nmol l-1 was 29% (RSD). The MDI concentrations were below the limit of detection in most (64%) of the air samples collected in the worker's breathing zone. Still, detectable amounts of MDA were found in 97% of the urine samples. Monitoring of urinary MDA appears to be an appropriate method of assessing MDI exposure in work environments with low or undetectable MDI concentrations in the workplace air.     

The Fundamental Experiments of Red Tide Chemical Forecast

Many food algae and red tide algae were cultivated in the f/2 medium, and the nitric oxide (NO) concentration of the medium and the cell density were determined. The experiments on algae were conducted when different concentrations of NO were added into the medium using two methods. The results show that low concentrations of NO were self-produced by marine algae during the exponential growth period and were about nmol/L level. But at the end of the period, i.e., 2 or 3 days before the cell density reaches the maximum, an NO peak appeared, with the NO concentration reaching 10 nmol/L. The NO threshold concentration exists according to the influence of exogenous NO on the marine phytoplankton growth. One type is the threshold concentration that can promote algae growth, and its value is between 10 and 1 nmol level, or even lower. The other type can inhibit the phytoplankton growth, which is about μmol level or higher. The results indicate that red tide algae are far more sensitive to NO than are food algae. The fundamental experimental outcome above may provide a new clue for red tide chemical forecast by inspecting the NO change.     

Material transport between the atmosphere and sediment of the Lake Balaton

A monitoring system was developed to gain information on the present level of pollutants in the Lake Balaton, Hungary. Determination of 13 polycyclic aromatic hydrocarbons (PAHs) and inorganic pollutants (mostly toxic metals) in aerosol, precipitation and sediment samples was carried out. The aim of collecting aerosol and precipitate samples in the same site at the same period was to determine the distribution of elements in two depositions. For the fractionation by particle size, aerosols were sampled by a cascade impactor. A simple three-stage sequential leaching procedure was applied to establish the distribution of metals among environmentally mobile, bound to carbonates and oxides, and environmentally immobile, (bound to silicates) fractions in aerosols. Sediment samples were collected from 17 different sites inside of the lake and 10 sites at harbors at 30–70 cm in depth. Core samples were cut to 10-cm pieces, dried at room temperature, and finally passed through a 63-μm sieve. Total concentrations of elements were determined by atomic absorption spectrometry (AAS) after an acidic digestion. The concentrations of PAHs were determined by HPLC method with fluorescence detection.In aerosol samples collected from September 6, 2002 to January 26, 2003, concentration of Cd was <0.1 μg/m³, and the majority of Cd has been found in the mobile fraction. Cadmium was associated to particles between 0.25 and 2 μm indicating the anthropogenic origin. Similar distribution of Pb was obtained in all seasons, and the highest concentration of Pb was found as 8.6 ng/m³ in particle size of 0.7 and 1.4 μm. Results of total concentration of elements of bottom sediments of the Lake Balaton and harbors were compared to Interim Sediment Quality Guideline (ISQG) values and the Probable Effect Level (PEL) values. Data showed that the average concentrations of elements were usually less than those of ISQGs and other background data for soils and geochemical values. The sediment is not polluted and its disposal is feasible. There is no direct correlation between the concentration of elements deposited onto the surface of the lake from dry and wet deposition and the upper part of the sediment. So, from the budget of the deposition, the concentration of elements in the upper layer of the sediment cannot be predicted.Seasonal changes of the concentration of PAHs in aerosol was observed, samples collected at winter contained the highest values. In December–January 2002/2003, the wet deposition was found as 64 μg/m² period. Among the individual compounds, the wet deposition rate of phenantrane, fluoranthene and pyrene was dominant, while for dry deposition, these compounds were fluoranthene and pyrene. The concentrations of ∑PAHs found for all sites and depth of sediment samples ranged from 11 to 1734 μg/kg dry weight with an average of 132 μg/kg. These values represent a quite low pollution level compared to other sediment with anthropogenic influence. Based on the results, it can be definitely confirmed that the chemical quality of the water and sediment of the Lake Balaton is satisfactory.     

Biomethylation of thallium by bacteria and first determination of biogenic dimethylthallium in the ocean

To investigate a possible biomethylation of thallium, incubation experiments were carried out under aerobic conditions with a sewage sludge and with a mixed bacterial culture isolated from a sewage sludge, as well as under anaerobic conditions with a fresh‐water lake sediment, by adding Tl(I) nitrate to these systems. Only in the case of the anaerobic sediment was a significant production of dimethylthallium observed (after three‐weeks). Analysing different surface seawater samples and those from a single depth profile down to 4000 m, dimethylthallium was determined above the detection limit of 0.4 ng L⁻¹ in about 20% of all samples, ranging from 0.5 to 3.2 ng l⁻¹. The proportion of dimethylthallium in these samples, compared with the total thallium concentration, varied from 3 to 48%. The determination of such low dimethylthallium content was possible by applying a new sensitive analytical method of positive thermal‐ionization isotope‐dilution mass spectrometry connected with a species‐unspecific thallium enrichment by anion‐exchange chromatography and a species‐specific extraction step. This is the first time that dimethylthallium could be detected in environmental samples. In surface seawater samples the occurrence of dimethylthallium correlated well with relatively high concentrations of chlorophyll‐a, used as an indicator for bioactivity. Mostly, other biomethylated compounds such as trimethyl‐lead, monomethylcadmium and dimethyl sulphide were also found with peak concentrations at these locations. The depth profile of dimethylthallium shows a maximum in concentration between 40 and 200 m, which corresponds with the highest level of bioactivity normally found in the water column of the ocean. However, even at depths of 1000 and 4000 m significant amounts of dimethylthallium could be analysed, which suggest bacteria as the biogenic source in the deep sea. Copyright © 2000 John Wiley & Sons, Ltd.     

LC-ESI-MS-MS method for the analysis of tetrabromobisphenol A in sediment and sewage sludge

Tetrabromobisphenol A (4,4'-isopropylidenebis(2,6-dibromophenol), TBBPA) is the most widely used brominated flame retardant in the world. Due to its low water solubility TBBPA released in aquatic ecosystems ultimately accumulates in sediments, but the lack of data on its environmental level and temporal trend in sediment cores precludes establishing if the input of TBBPA is an on-going environmental problem. We developed an analytical method involving HPLC-ESI-MS-MS (ion trap) with detection of the negative pseudo-molecular ion of TBBPA and its fragmentation pattern. Recovery of TBBPA from spiked marine sediment (both lyophilized and wet) and dehydrated sewage sludge was better than 95%. The current detection limit of TBBPA is 60 pg injected and the linearity of the response is at least three orders of magnitude, ranging from 7 ng ml(-1) to 7000 ng ml(-1). The method was also applied to the analysis of urban sewage sludge where TBBPA was detected at a concentration of 300 ng g(-1)(dry weight). With an analysis time of less than 20 min, this method is adequate for a rapid re-assessment of archived sediment samples avoiding cumbersome derivatization procedures.     

A method to test the detectability of GC/PFPD for an extended concentration range with respect to reduced sulfur compounds

In this study, a possible means to extensively expand the quantifiable range of reduced sulfur compounds (RSC) in air has been investigated by a combined application of GC with pulsed flame photometric detector (PFPD) and a multifunction thermal desorber (TD) system. To comply with the purpose of this study, gaseous RSC standards containing the equimolar concentrations of H(2)S, CH(3)SH, DMS, DMDS, and CS(2 )were prepared at 11 concentration levels (i. e., 10 ppb-10 ppm (over 10(3 )range)). These standards were then used to derive three-point calibrations based on the modified injection through a TD (MITD). If the mean calibration slopes of each concentration level are normalized to that of CH(3)SH, the relative ordering is found as 0.65 (H(2)S): 1 (CH(3)SH): 1.34 (DMS): 2.24 (DMDS). The reproducibility of MITD method, when assessed in terms of relative standard error (RSE) for all calibration slopes, had the most stable pattern for DMDS (5.77%) and the least stable one for H(2)S (12.8%). The sensitivity of the MITD-based calibration generally improved with an increase in concentration levels of standard gas. Based on our study, the MITD technique is useful to extend quantification of GC/PFPD by allowing RSC detection over a 10(3) range.     

Preliminary evidence for in vitro methylation of tributyltin in a marine sediment

Recent reports from our laboratory on the occurrence of methylbutyltins in marine sediments and seawater suggest that these compounds are formed in the environment by the methylation of both tributyltin (TBT) and that­of its degradation products, i.e. dibutyltin and monobutyltin, to give Me_nBu_(4?n)Sn for which n = 1, 2 and 3 respectively. We investigated the possibility of inducing methylation of TBT in seawater–sediment mixtures in experiments carried out in vitro using environmental materials collected from a yacht marina in Msida, Malta. Three water–sediment mixtures, which were shown to contain TBT, dibutyltin and monobutyltin but no other organotins, were spiked with tributyltin chloride (90 mg in 100 ml sea‐water/100 ml sediment); to one mixture was added sodium acetate and to another methanol, to act as possible additional carbon sources, and all mixtures were allowed to stand at 25 °C in stoppered clear‐glass bottles in diffused light for a maximum of 315 days. Speciation and quantification of organotins was performed using aqueous phase boroethylation with simultaneous solvent extraction followed by gas chromatography with flame photometric detection. The atmosphere inside the bottles quickly became reducing with abundant presence of H₂S, and after an induction period of about 112 days, and only in the reaction mixture containing methanol, methyltributyltin (MeBu₃Sn) was observed in both sediment (maximum concentration 0.87 µg_Sn g^?1) and overlying water (maximum concentration 6.0 µg_Sn l^?1). The minimum conversion yield of TBT into MeBu₃Sn was estimated to be 0.3%. MeBu₃Sn has a significantly lower affinity for sediment than TBT and, therefore, is more mobile in the marine environment, possibly also migrating into the atmosphere to generate a hitherto unsuspected flux of organotin into that phase. Copyright © 2001 John Wiley & Sons, Ltd.     

Quantitative determination of 2-methoxy-3-isobutylpyrazine in red wines and grapes of Bordeaux using a stable isotope dilution assay.

Quantitative analysis of 2-methoxy-3-isobutylpyrazine (MIBP) in grapes and wines was developed, using a stable isotope dilution assay. This was applied to red grapes and wines from the Bordeaux region. The grapes and the wines of the 1995 and 1996 vintages came from the three most frequently used varieties of the region, Merlot, Cabernet Franc and Cabernet Sauvignon. The wines made from Cabernet Sauvignon grapes exhibited levels of MIBP (mean concentration, 12 ng l-1 for 1996 vintage and 13 ng l-1 for 1995 vintage) close to or higher than its odour threshold in wines (10 ng l-1) and slightly higher than the amounts found in the Merlot wines (mean concentration, 8 ng l-1 for 1996 vintage and 4 ng l-1 for 1995 vintage), especially those of the 1996 vintage. The variation in the levels of MIBP in grape samples and in their corresponding wines was monitored at four different stages towards the end of maturation. MIBP was present in all grapes and wines analysed, even in surmaturation. A linear trend was observed between grapes and wines of the three cultivars during maturation.     

Effect of salinity on petroleum biodegradation

Summary The biodegradation of Ashtart crude oil by a mixed bacterial community (EH1) isolated from a marine sediment was investigated in varying concentration of sodium chloride (0 to 2 mol/l). Each fraction of Ashtart crude oil was biodegraded after a 30 day incubation period for a NaCl concentration equivalent to the one of seawater (0.4 mol/l). Saturates were more readily degraded than aromatics. The amount of oil degraded increased initially with increasing salt concentrations to a maximum level for 0.4 mol/l NaCl concentration. Thereafter the amount of oil degraded decreased with increasing salt concentrations. The polar fraction increased for the 0.4 to 1.4 mol/l NaCl concentration range. Asphaltenes were only slightly degraded (10%) for a NaCl concentration equivalent to the one of seawater (0.4 mol/l). NaCl concentrations seemed to affect the relative biodegradation of pristane and phytane. GC, UV fluorescence and FTIR techniques were used to study the evolution of each fraction of Ashtart crude oil.     

Seasonal Changes in Methylarsenic Distribution in Tosa Bay and Uranouchi Inlet

The distribution of arsenic species, including trivalent methylarsenicals, was observed in coastal seawater of Tosa Bay and Uranouchi Inlet Japan. In Tosa Bay, most arsenic was dissolved in the inorganic form throughout the year and the concentration of total dissolved arsenic was higher than that in Uranouchi Inlet. The sum of methylarsenicals found in surface waters comprised 2–25% and 10–82% of the total dissolved arsenic in Tosa Bay and Uranouchi Inlet, respectively. In Uranouchi Inlet, seasonal variations in the concentrations of arsenicals were observed both in the water column and in surface sediments. The maximum concentrations of methylarsenicals appeared during summer, and became comparable to those of inorganic arsenicals in surface water. The concentration of trivalent methylarsenicals was usually low, and their seasonal changes seemed to be independent of those of the pentavalent species. The variations in methylarsenic(V) concentration did not coincide with those of chlorophyll a in either Tosa Bay or Uranouchi Inlet. These results suggested that methylarsenic(V) in natural waters was produced not directly by the activity of phytoplankton but through decomposition of organic matter by bacteria.     

Occurrence and distribution of selected pharmaceutical compounds on sewage-impacted section of River Acheloos,Western Greece

The occurrence of eight pharmaceutical compounds (PhCs), two metabolites and caffeine was investigated in River Acheloos, located in Western Greece, during a twelve-month monitoring period (March 2007–February 2008). Solid-phase extraction (SPE) was used for the extraction and pre-concentration of the target pollutants and gas chromatography mass spectrometry (GC–MS) for the detection and quantification. Recoveries were determined between 74.0–100.4% for distilled water and 72.6–95.1% for the river water, whereas the relative standard deviation was less than 9.4% for distilled water and 8% for the river water, respectively. The limits of detection ranged between 1–40 ng L^?1. Two pharmaceuticals (paracetamol and carbamazepine), caffeine and the metabolite (salicylic acid), have been detected in all the analysed samples. Maximum concentration levels determined in river samples reached 305 ng L^?1 recorded for paracetamol. The concentrations of target compounds were significantly higher in the samples collected at the sampling station situated after the wastewater treatment plant (WWTP) of Agrinio City compared to the samples collected in upper and lower parts of the river. Seasonal variations were attributed mainly to river flow variations and removal percentages by WWTP. Risk quotient method for median environmental concentrations revealed minimal to median risk with the exception of triclosan, ibuprofen and diclofenac, which presented high risk when maximum environmental concentrations were used.