FR :

La distribution de la grosseur des particules, la composition géochimique et la concentration en métaux (Cu, Cr, Pb, Co, V, Ni, Hg et Zn) furent déterminés dans des carottes datées (à l'aide de matériel radioactif) provenant du lac Saint-Louis et du fleuve Saint-Laurent. Environ 20 cm de sédiments fins se sont accumulés durant les 30 dernières années à deux endroits situés à proximité du centre du lac. Par contre, une couche de sédiment de 20 cm d'épaisseur s'est déposée pendant les cinq dernières années à deux différents endroits de la partie sud du lac. Les profils de concentration des éléments majeurs et de la plupart des métaux présents dans les carottes de sédiment indiquent un apport continu de matériel de nature géochimique semblable aux aires d'échantillonnage. Une distribution uniforme de la grosseur des particules à l'intérieur du profil de sédiment montre que ces aires constituent des zones de dépôt pour des particules de propriétés hydrodynamiques semblables que transportent les eaux du fleuve. Les profils de concentration du mercure (Hg) et du zinc (Zn) dans les sédiments sont significatifs d'une charge provenant d'une source locale située sur le lac Saint-Louis. Les concentrations en métaux mesurées dans les sédiments du lac Saint-Louis dépendent du tri des particules, du transport et du dépôt des particules de la fraction limoneuse (taille comprise entre 2 µg et 63 µg) qui peuvent contenir de plus faibles concentrations en métaux comparativement aux particules de grosseur comparable à l'argile (taille < 2 µg) qui caractérisent les bassins de dépôt du lac Ontario.

EN :

Particle size distribution, geochemical composition and concentrations of metals (Cu, Cr, Pb, Co, V, Ni, Hg and Zn) were determined in radiodated cores collected from Lake St. Louis, St. Lawrence River, Quebec, Canada, to assess the significance of the lake as a sink of contaminants transported through the St. Lawrence River. The cores were collected at four stations in the depositional areas identified by earlier surveys of Lake St. Louis. Stations Nos. 3 and 9 were located in the northern part of the lake, and stations 17 and 21 along the southern shore. A modified Kajak-Brinkhurst curer was used to collect the sediment cores. The concentrations of major elements and metals were determined by X-ray fluorescence spectrometry. The concentrations of Hg and inorganic C were determined by cold vapour atomic absorption spectrometry and Leco carbon analyzer, respectively. The total Pb-210 and Ra-226 levels in one centimeter sediment core sections were measured by low-energy gamma-ray spectrometry for calculation of sediment age. The effects of mixing on sediment age has been estimated.

About 20 cm of fine grained sediment accumulated during the past 30 years at two sites near the centre of the lake. However, a 20 cm thick sediment layer was deposited over the past five years at two different areas in the southern part of the lake. Calculated sedimentation rate for surface sediments were 207, 296, 1837, and 2912 mg cm^-2 y^-1 for stations 3, 9, 17, and 21, respectively. The concentration profiles of ten major elements (Si, Al, Fe, Ca, Mg, Na, K, Ti, P, and Mn), organic and inorganic C and most of the investigated metals were similar and consistent in the sediment profiles at all stations, and indicated a continuous input of geochemically similar material at the sampling sites.

A uniform particle size distribution within the sediment profile showed that these areas are settling zones for particles of similar hydrodynamic properties transported by the St. Lawrence River. The concentration profiles of Hg in the cures indicated e continuous input of Hg along the southern shore of the lake. The concentration profile of Hg in the core from the northern part of the lake had a similar pattern to that from Lake St. Clair and Lake Ontario, showing a decrease of Hg in the surficial sediments. This decrease corresponds to implemented contrat of Hg losses by industry in Canada and the USA over the past 15 years. The concentration profiles of Hg and Zn in the sediments indicated an input from local sources along Lake St. Louis.

The concentrations of Cr, V and Co in sediments in the depositional zones were similar to both background and surficial sediment concentrations in depositional basins of Lake Ontario. The concentrations of Cu, Ni and Pb were lower than those in surficial sediments in Lake Ontario depositional basins. Observed concentrations of investigated metals in Lake St. Louis sediments were ascribed to the particle sorting, transport and deposition of silt-size (2 to 63 µm grain size) particles which may contain smaller concentrations of metals than clay-size (< 2 µm grain size) particles in the depositional basins of Lake Ontario. It was concluded that Lake St. Louis provides only a temporal storage for metals associated with clay-size sediment particles which may eventually become resuspended and moved downstream.

FR :

L'étude des tableaux de données hydrochimiques acquises au cours de campagnes synchrones (« instantanés ») ou de suivis diachroniques à pas régulier (hebdomadaire, quotidien, horaire) s'opère généralement en résumant l'information par des méthodes statistiques. Ces méthodes descriptives, qui négligent nécessairement une partie de l'information initiale, permettent l'interprétation de la structure du tableau de données en termes de fonctionnement hydrocinématique (BAKALOWICZ, 1979, 1982, MUDRY et BLAVOUX, 1988, ROLET et SEGUIN, 1986 8 et b).

Ces méthodes, fondées sur l'analyse d'une seule population statistique (bi ou multidimensionnelle) impliquent un mode de raisonnement déductif.

Leur application, ainsi que l'examen du tableau des données brutes (ou de ses représentations graphiques), peut mettre en évidence des sous-groupes fondés sur des critères hydrogéologiques. La réalité de ces sous-groupes peut être testée à l'aide de méthodes statistiques basées sur l'analyse de la variance. Certaines méthodes utilisent le rapport des variances qu'elles comparent à la distribution de Snedecor (analyse de la variance à une ou deux voies), d'autres comparent des variances multidimensionnelles intraclasses à la variance interclasses, c'est le cas de l'analyse discriminante. Les sous-groupes constituent une variable qualitative dont la pertinence peut être démontrée par la calcul. L'analyse discriminante apparaît donc comme un outil décisionnel. Le présent article présente brièvement la méthode du point de vue statistique et montre deux exemples d'application à des sources karstiques.

Le premier exemple traite de l'appartenance chimique d'une phase de basses eaux à la petite crue qui la précède et non à un tarissement au sens hydrocinématique. L'analyse discriminante permet d'affirmer qu'une recharge peu perceptible sur l'hydrogramme de la source amène une évolution chimique irréversible de l'eau de la réserve, responsable des phénomènes d'hystérésis observés sur les courbes concentration-débit. Ce cas est celui de l'aquifère de la Fontaine de Vaucluse (Sud-Est de la France) pendant un suivi quotidien d'étiage.

Le second exemple permet de rattacher, par son comportement physico-chimique hebdomadaire, une émergence karstique à une autre et non à une troisième. Ce cas est celui du karst de la Rochefoucauld (Charente), avec les sources du Bouillant, de la Font de Lussac et de la Lèche. Les sources du Bouillant et de la Font de Lussac ont un comportement physico-chimique semblable, alors que la Lèche réagit de manière totalement indépendante. Elle constitue un système globalement distinct du point de vue hydrocinématique, ce qui permet de minimiser les relations mises en évidence par traçage artificiel entre les deux systèmes.

EN :

The study of the hydrochemical data tables obtained during synchronous sampling (weekly, daily or hourly) is generally carried out by resuming the information by statistical methods. These methods, that disregard part of the initial information, allow to explain the structure of the data table in terms of hydrokinematics (BAKALOWICZ, 1979, 1982; MUDRY et BLAVOUX, 1986; ROLET et SEGUIN, 1986 a et b).

These descriptive methods, based on the analysis of a single (bi or multivariate) statistical population, imply deductive reasoning. Their application, as well as the study of the untreated data table (or of its scattergrams), can show the presence of sub-groups based on hydrogeological criteria. The existence of such sub-groups can be tested by statistical methods based on variance analysis. Several methods use the variance ratio and compare it to Snedecor's distribution (single or double path variance analysis), others compare multidimensional intragroup variances to intergroup variance. This is the case with discriminant analysis.

This paper describes the method from a point of view of statistics and presents two examples of application to karst springs.

The first study deals with the chemical relationship of a low-water period with the preceding period of small floods and not to a hydrokinematical water drying up. The discriminant analysis allows to say Chat a recharge less visible on the hydrograph of the spring induces a non reversible chemical evolution of the reserve water. This is the case of the Fontaine de Vaucluse karst spring (Southeastern France) during a daily low water sampling.

The second study allows to relate a karst spring to another one, thanks to its weekly physico-chemical behaviour, and not to a third one : this is the case of the La Rochefoucauld karst system (Charente, Western France) with the Bouillant, Font de Lussac and Lèche springs. The Bouillant and Font de Lussac springs behave in the same way hydrochemically, whereas the Lèche works independently. It is, hydrokinematically, a separate karst system, even if there is a certain relationship between all of them.

FR :

La nappe aquifère de Hesbaye située au Nord-Ouest de Liège (Belgique) est logée dans les craies du Crétacé, surmontées d'une épaisse couche de limon. L'alimentation en eau de la région est assurée par les 60 000 m³ extraits Joumel!ement des galeries captantes et des puits.

Un modèle mathématique par éléments finis a été construit pour simuler numériquement le comportement de l'aquifère en régime transitoire. La complexité de la géologie locale a nécessité l'emploi d'une méthode par éléments finis permettant une discrétisatlon 30 souple et détaillée de la zone étudiée, modélisation réalisée avec un maillage fixe en condition de nappe libre.

Des développements numériques comprenant la simulation de la surface libre en mode transitoire, ont été introduits dans le modèle LAGATHER du programme LAGAMINE (Université de Liège). Le programme a été testé sur des exemples bidimensionnels avant d'étre appliqué en 30 à l'aquifère de Hesbaye.

La discrétisation et les principaux résultats sont présentés.

EN :

The Hesbaye aquifer located in Cretaceous chalks near Liège (Belgium), is recharged by infiltration through the overlying loess and conglomerate.

Wells and collecting adits produce a daily flow of 60 000 m³ out of this aquifer, to provide the alimentation of the area of Liège and its suburbs.

More Man 500 boreholes, plezometers and wells give many informations for the idealization and the calibration procedures. Variation ranges of permeability and effective porosity are determined from 150 pumping tests. The main drainance axis of the aquifer (called « dry valleys ») are located and characterized by higher permeabilities.

Finite element modelling has been developed to foresee the evolution of the water table and to get some additional informations especially about these main drainance axis.

The modelling method is based on the LAGAMINE code (from Liège University). The LAGATHER code is one of the module of the LAGAMINE which is initially adapted to treat non-linear thermic problems. Here, it has been developed to compute flows in porous media in transient conditions. The water table is calculated with a fixed mesh structure using a new method developed by (CHARLIER et al., 1988). Constitutive laws of the flow are changed to take into account the state of saturation or no-saturation of the media. Then the flow equation (the Darcy's law) becomes non-linear. The transition from unsaturated to saturated field is treated at a constant zero pressure, according to the analogy with thermodynamical phase changes at constant temperature. The method has been checked on uniaxial and two dimensionnal sections.

3D discretisation and modelling of the whole aquifer has been realized. The problem requires about 2 670 8-nodes isoparametric brick finite elements. The complexity of the geological conditions justifies fully the use of finite element method by the diversity and the wide range of the hydrogeological parameters to introduce in the model.

The discretisation is described and some of the must significant results are presented.

FR :

L'acumulation de sédiments dans les retenues de barrages hydro-électriques nécessite de procéder à des opérations régulières de désenvasement indispensables à leur bon fonctionnement.

Ces opérations de vidange ont le plus souvent un effet destructeur sur la faune et la flore aquatique.

Cet effet destructeur est principalement lié à une diminution de la concentration en oxygène dissous et une augmentation de la teneur en matières en suspension.

La méthodologie présentée a pour but d'estimer préalablement à toute vidange les risques écotoxicologlques correspondant à une telle opération. Elle implique d'une part la description et l'analyse physico-chimique des sédiments en place, d'autre part la mesure en laboratoire de la consommation en oxygène dissous des matériaux remis en suspension au cours du temps.

La fiabilité d'une telle procédure a été montrée en particulier à l'occasion de vidanges de retenues situées sur le cours de l'Isère.

Le protocole est décrit précisément de façon à pouvoir guider les exploitants de retenues à l'occasion des opérations de vidange qu'ils dirigent.

EN :

Sediment silting up in hydroelectric dams Implies regular draining operations in order to prevent disfunction of the dams.

These operations often lead to drastic lethal affects on aquatic fauna and flora.

A toxicological study has clearly shown two main factors responsible for acute toxicological effects : dissolved oxygen deficit and suspended solids increase.

These effects have been quantified by experimental tests on Brown trout fry (Salmo trutta fario) the results of which are summarized.

The second step consisted in the prediction of water quality evolution downstream during dam draining operation, regarding both factors suspected, in order to assess ecotoxicological hazard.

The proposed methodology based on a sedimentological study made in Grangent dam, located on river Loire and immediately downstream of St. Etienne urban area, and in St. Hilaire dam situated on the river Isère downstream part

It consists in sampling cored sediments and in measuring dissolved oxygen kinetic by mixing sediments with water In a reactor.

The sample conservation as carried out in jar glasses kept at 4 °C.

Results are reproducible as long as residual dissolved oxygen concentration is higher than 3 mg/l. If this condition is respected, oxygen consumption is strictly dependent on suspended sediment concentration.

An increase in test water temperature makes the oxygen consumption rise.

In accordance with these results, recommendations for dissolved oxygen measures are about 5 g/l suspended sediment concentration, with an experimental temperature reaching the temperature observed during dam draining.

The variability of the results for different samples depends on the dam studied. Surface tore samples collected in St. Hilaire dam consume dissolved oxygen faster than the same bottom tore samples.

However, in Grangent dam, only one surface core sample collected near the dam construction has clearly shown reducing properties. The difference between two dam sediments is very important : Grangent dam sediment have, on average, dissolved oxygen consumption three more times higher than St. Hilaire in the same duration.

Taking into account dissolved oxygen kinetics by mixed sediments, and a simple aeration model using water and sediment deposits from downstream dam, may expert both suspended sediment and oxygen consumption evolution.

Hazardous mortality of trout fario fry in downstream dam are quickly graphically visualized.

Dam draining operator may so assess, for different suspended sediment land the harmful exposition to fish at any distance from it.

This knowledge allows him to plan more strictly floodgate working : pointly dissolved oxygen continuous measurement on downstream dam is sufficient to give a mark related to model hazard assessment.

The model reliability has been demonstrated by the examination of data collected during St. Hilaire dam draining operation.

FR :

Un réacteur de 10 litres, automatisé pour la culture continue en anaérobiose de bactéries photosynthétiques, a été réalisé et mis au point. Ce réacteur parfaitement agité a été utilisé dans différentes conditions de fonctionnement en système fermé (batch), en système semi-ouvert (atouts de substrats concentrés en discontinu), en système ouvert (chémostat) avec et sans recyclage de la biomasse, afin d'étudier la consommation, par les bactéries, d'un substrat carboné, le lactate. La production d'hydrogène par la bactérie photosynthétique Rhodobacter capsulatus, souche B10, résultant de la dégradation du lactate, est optimale pour des cultures en continu diluées, limitées en source azotée. Ainsi, à un taux de dilution de 0,04 h^-1, avec 5 mM glutamate dans le milieu nutritif, la densité bactérienne étant de 2,1 è 660 nm, on a observé une production continue moyenne de 65 ml • h^-1• l^-1 pendant une période de 200 heures. Pour des concentrations bactériennes élevées, la limitation d'énergie lumineuse entrain une perte d'activité nitrogénase et, de ce fait, une chute de la production d'hydrogène.

EN :

A photobioreactor was set up to cultivate a photosynthetic bacterium in continuous cultures. The bioreactor was designed so as to 1) allow the capture of light energy by bacteria through a spiral transparent flexible tube placed under the light, in a water bath maintaining the growth temperature at 30 °C; 2) male the suspension of bacteria circulate continuously in the reactor with a volumetric pump to maintain the medium homogeneous; 3) allow degassing of the suspension in a degassing chamber; 4) feed the culture with nutritive media, add neutralizing solution (pH 7) and withdrax aliquots white maintaining constant the volume of the culture; 5) recycle the bacteria by filtration when the bioreactor was used as e closed system (batch).

The photosynthetic bacterium was Rhodobacter capsulatus strain B10 is known to lie a good H₂ producer [Hillmer and Gest (1977) J. Bacteriol. 129, 724-731]. The bioreactor was run using 10 l of a synthetic medium containing lactate as carbon source and glutamate as nitrogen source. It was studied for its capacity to degrade lactate. Glutamate was the growth-liliting substrate allowing a maximum derepression of nitrogenase, the enzyme catalysing the reduction of protons to H₂. The bacterial suspension was continuously circulated in the photoreactor, conceived as a plane light captor of 1 m², to avoid bacterial self-shading and allow regeneration of ATP by photophosphorylation at high rates. The circuit was tightly closed to avoid air entry, which would prevent H₂ production due to respiration of the bacteria.

To run it under automated conditions, the bioreactor was equipped with two temperature sensors, two pH electrodes, a water level detector, a manometer and a computer-controlled electric valve. The bioreactor, of the well-mixed type, was used under various working conditions, namely as a closed (batch) system, as a fed-batch system (discontinuous additions of concentrated substrates), and as an open system (chemostat) with or without biomass recycling.

Control of key parameters (pH, temperature, dilution rates) allowed us to define the culture conditions producing maximal amounts of molecular hydrogen. The production of H₂ accompanying lactate degradation was maximal in diluted nitrogen-limited continuous cultures. It was observed at a dilution rate of 0.04 h^-1 with 5 mM glutamate in the influent medium, the optical density of the culture being 2.1 at 660 nm. Under these conditions an average production H₂ of 85 ml • h^-1 • l^-1 was observed over a 200 h period. At higher bacterial concentrations, the limitation of light energy resulted in a decrease in nitrogenase activity and therefore in a drop in the production of hydrogen.

The interdependence of various parameters (pH, dilution rates, N and C sources light intensity) renders the system complex and not easily controlled by computer. Indeed, we observed that during recycling of the bacteria by the use of an ultrafiltration cell, the bacteria became a fermentative-type of metabolism accompanied by a decrease in nitrogenase activity and therefore in a drop in the production of hydrogen.

FR :

La dégradation du lactate par la bactérie photosynthétique Rhodobacter capsulatus cultivée dans un photobioréacteur en anaérobiose, à la lumière, en limitation d'azote, a été mesurée ainsi que la production d'hydrogène par la nitrogénase. En réacteur ouvert à faible taux de dilution (D = 0,04 h-1), le lactate initialement à 30 mM est dégradé à près de 90 % avec une faible accumulation (< 1-2 mM) des produits de dégradation (formiate et acétate). C'est dans ces conditions qu'on observe la meilleure production d'hydrogène (44 ml • h-1 • h-1). Lorsque la culture bactérienne devient trop dense en réacteur fermé ou an recyclage des bactéries, Il y a apparemment passage à un métabolisme de type fermentatif, chute de la production d'hydrogène et accumulation d'acides organiques (formique, acétique, propionique) provenant du lactate. Dans ces conditions, il n'y a pas diminution de la charge de carbone organique. Outre la présence de ces acides à courtes chaînes, les bilans carbonés; font apparaître la présence, dans le surnageant de la culture, de substances carbonées exocellulaires non identifiées pouvant représenter jusqu'à 50 % du flux carboné sortant.

EN :

A 10 l photobioreactor, consisting of a PVC tubing, spiral-coiled so as to form a plane light captor (DELACHAPELLE et al., 1990), was used to study the degradation of lactate by the photosynthetic bacterium Rhodobacter capsulatus strain B10. The bacterial culture was continuously circulated in the reactor so as to maintain a homogeneous suspension, to optimize illumination of the cells, and make a well-mixed reactor for optimal nutrient transfers and degas ing of the medium. The bacterium was cultivated anaerobically under photoheterotrophic and N-limited conditions. The bioreactor was operated in three modes : as an open system (chemostat), as a closed system (batch), and as an open system with phases of bacteria recycling through the use of an ultrafiltration cell.

When the bioreactor functioned as a chemostat, at low dilution rate (D < 0.04 h-1) and at relatively low bacterial density (A660 nm < 2.5), up to 90 % of the added lactate (initial concentration 30 mM) was degraded; the concentration of degradation products (formate, acetate) remained low (<2 mM). Under these conditions nitrogenase-mediated H2 production was maximal (44 ml • h-1 • l-1). In batch cultures or when the cells were « recycled » in the chemostat by ultrafiltration, the increase in absorbancy led to a decrease in cell illumination and the bacteria apparently switched from a photosynthetic of a fermentative type of metabolism; lactate was converted into formate + acetate + propionate, no H2 was produced. From the determination of the carbon balance between the influent and the effluent medium, the presence of non identified extracellular carbon compounds (up to 50 % of the C content of the effluent) was proved.

FR :

Une étude écotoxicologique a été menée à l'égard du Tébuthiuron (TB), un phytocide homologué inhibant la croissance de végétation nuisible, afin de mieux cerner son impact sur le milieu aquatique susceptible d'être affecté par des épandages terrestres. La toxicité du TB a été évaluée en réalisant des bioessais à trois paliers écologiques, soit avec la truite arc-en-ciel (Salmo gairdneri), avec l'algue Selenaslrum capricornutum et avec la bactérie (Photobacterium phosphoreum) du système Microtox®. Parmi ces trois indicateurs, les algues se sont montrées les plus sensibles (C150 = 0,08 mg • L^-1), suivies des truites (CL50 = 115 mg • L^-1) et enfin, des bactéries du système Microtoxe (C150 - 328 mg • L^-1). Des résidus maximaux variant de 0,091 à 0,18 mg • L^-1 rapportés pour le TB en milieu aquatique, suite à des applications expérimentales, laissent donc croire que seules les algues pourraient étre victimes d'une agression marquée. D'autre part, les essais réalisés avec le SOS Chromotest ont démontré que le TB était faiblement génotoxique sans activation métabolique. En revanche, des algues exposées à 1 mg • L^-1 de TB durant 4 heures n'ont pu ni (dés)actlver ni bioaccumuier l'herbicide. En général, notre enquéte corrobore certaines données générées pour fins d'homologation de ce produit, lesquelles concluaient en faveur de son innocuité relative à l'égard de l'environnement aquatique. Les effets chroniques que pourrait avoir une longue exposition de faibles concentrations de TB sur certains paliers écologiques devraient cependant faire l'objet d'lnvestigations futures.

EN :

A wide array of chemical products are commonly used to inhibit the growth of a diversity of undesirable vegetation types for numerous purposes. In forestry applications, herbicides can improve ranges, contribute to sylviculture and facilitate rights-of-way management, for example. Among congeners of the substituted urea class herbicides, Tebuthiuron (TB) has proven efficient for such activities. Since its commercial appearance in 1974, this broad-spectrum weed killer was employed to control a variety of herbaceous and woody plants. When applied on soil before or during the onset of plant emergence, TB irreversibly affects photosynthesis after being absorbed by roots and translocated to its target sites. Prior to its registration as a herbicide, TB had undergone extensive (eco)toxicological testing which had generally indicated low potential for environmental concern, with regards to terrestrial and avian fauna. Although TB is generally purported to be unproblematic towards fish, the overall impact of substituted urea class herbicides is still not fully documented, as far as various members of the aquatic community are concerned. The experimental results presented herein - specifically on TB - contribute both confirmatory as well as some new information in this respect.

In our study, TB toxicity was investigated at three ecological levels by undertaking acute bioassays with rainbow trout (Salmo gairdeneri), algae (Selenastrum capricornutum) and bacteria (Photobacterium phosphoreum). Among these bio-indicators, algae proved to be the most sensitive (EC50 = 0.08 mg • L^-1), followed by rainbow trout (LC50 = 115 mg • L^-1) and bacteria (EC50 = 328 mg • L^-1). Since maximum TB residues lying between 0.091 and 0.18 mg • L^-1 have been reported for aquatic systems following experimental terrestrial applications, our toxicity results suggest that only algae could be adversely affected following acute exposure to the herbicide.

Additional tests performed with the SOS Chromotest, a bacterial colorimetric assay for detecting DNA-damaging agents, first showed that TB is weakly genotoxic without metabolic activation. Since recent genotoxicity studios have revealed that vegetal systems can either detoxify, activate or uptake specific chemicals, we then explored this possibility by exposing S. capricornutum (106 cells • mL^-1) to 1 mg • L^-1 of TB for 4 h. Results of this acute exposure indicated an absence of positive (detoxication) or negative (activation, accumulation) phytoplanktonic interactions. Indeed, the genotoxic characteristics of TB, before and after algal exposure, were unaltered, as demonstrated by SOS Chromotest assays. In this same experiment, a similar assay on TB-exposed algal cells (i.e. SOS Chromotest on an algal cell solvent extract) detected no genotoxic activity.

In conclusion, our study corroborates existing data generated for TB registration purposes and essentially supports the notion that this chemical is relatively harmless towards the aquatic environment under normal use conditions. Nevertheless, an important caveat remains concerning chronic affects on specific organisms, which could result from long term exposure to low concentrations of TB. Since such potential effects have not yet been adequately addressed, further studios are warranted in this area.