L'une des difficultés rencontrées couramment dans la conception des réseaux de mesure - au moins en ce qui concerne les micropolluants - porte sur la sélection des paramètres à mesurer. C'est notamment le cas pour les pesticides, dont plusieurs centaines sont utilisées en agriculture, mais qu'il est impossible de surveiller dans les eaux en totalité pour des raisons à la fois techniques et économiques. C'est la raison pour laquelle les autorités françaises ont fait procéder à la mise au point d'une méthode de sélection des matières actives utilisées en agriculture basée sur l'évaluation du risque. Dans cette méthode, l'exposition est figurée par un rang combinant les données relatives aux usages des matières actives (superficie, dose par ha) et leurs caractéristiques physico-chimiques. Le danger est représenté par la toxicité, soit pour l'homme, soit pour les espèces aquatiques. Cette approche a été appliquée à l'échelle nationale et dans un certain nombre de régions françaises, dont l'Alsace et la Lorraine. Les résultats des mesures de pesticides réalisées ensuite pendant un an ont été confrontés aux indices d'exposition obtenus. Les substances détectées le plus fréquemment correspondent effectivement à celles dotées des rangs d'exposition les plus élevés (ajustement exponentiel, r2≈0.82) ; cependant, le diuron apparaît à une fréquence plus élevée que celle attendue, en raison de ses usages non agricoles. La corrélation est moins bonne pour les substances dont les rangs d'exposition sont proches de la valeur considérée comme significative pour les eaux superficielles, ce qui peut provenir soit de l'utilisation de données erronées lors de la sélection, soit d'un poids insuffisant attribué à certains facteurs dans la méthode de sélection, soit enfin d'aléas météorologiques.
Monitoring of micropollutants is a rather recent activity (10-15 years), at least in surface waters; because of the need for sophisticated analytical methods and of the potential number of analytes, this type of activity is confronted with important economic constraints, which require that one make a selection among the range of substances to monitor. Among organic micropollutants, pesticides constitute a well-identified category, since they are used mainly in agriculture; this use on broad surfaces may have important impacts on the quality of surface water. Various methods have been used to select those pesticides likely to have the greatest impacts on water quality; some of these methods might be considered to be "hazard assessment", whereas others correspond to simplified "risk assessment" methods (this appears particularly true for pesticides, of which several hundreds are used in agriculture). Recently, a French panel of experts mandated by different Ministries designed a selection method called SIRIS, which allows one to define three different lists of pesticides according to the media to be monitored (surface or ground-water) and to the monitoring objectives (ecosystem protection, drinking water production). This paper deals with the application of the SIRIS method at a regional level, in the context of a permanent survey of river quality.As a simplified risk assessment method, SIRIS combines data on hazard and exposure; hazard is estimated by a single parameter, either toxicity for aquatic species or acceptable daily intake (ADI). Exposure represents the probability that a transfer to water bodies may occur; for surface water, this probability is influenced by the crop acreage, the applied dose (kg/ha), the solubility, the pesticide half-life, the hydrolysis and the distribution coefficient between water and organic matter (Koc). These factors are considered in this hierarchical order, and for each substance a score is assigned to each of these factors among three possible values ("o"=slight, "m"=medium, "d"=high, according to the relative influence on transfer); finally exposure is estimated by a relative rank obtained by a combination of these values following a "penalisation" principle. Two tables are available for applying this approach at a regional level: the first contains the values (o,m,d) assigned to more than 300 substances by the expert panel for solubility, half-life, etc., and should be completed with crop acreage and dose. The second table provides the ranks corresponding to the different combinations of o,m,d values. A final rank of 35 was considered by consensus to be a pragmatic threshold for the transfer to surface water. This method was applied in 1996 in two regions in France (Alsace and Lorraine) separately; most of the selected chemicals (but unfortunately not all, due to technical constraints) were then analysed monthly in surface waters (24 sampling points, yielding 144 samples in Alsace and 169 in Lorraine). Occurrences fell between 0% and 60% in Alsace, and between 0% and 90% in Lorraine; in both regions, the most frequently detected chemicals were atrazine and diuron.The relevance of the selection method may be discussed under several aspects: the choice of the factors, their order, the position of thresholds corresponding to o,m,d values, the value of the overall threshold, and the availability of the data. Some pesticides are not ranked only because no data were available concerning their solubility, hydrolysis rate or Koc, but the relative importance of such gaps cannot be appreciated with the current set of data. Other items may be assessed through the comparison of the exposure rank versus the occurrence. This relationship takes an exponential shape, with some anomalies: for example, the occurrence of diuron in Alsace is higher than expected, based on its exposure rank. This situation can be explained by the fact that there are non-agricultural uses of this substance, such that the exposure rank appears to be underestimated. For other substances, like aldicarb and chlorpyrifos-ethyl, discrepancies are observed between the exposure rank and occurrences, when comparing with substances with higher exposure ranks. This anomaly may be due to poor data quality. For carbendazime, the occurrence in Lorraine appears underestimated, probably because of a dry period deficit after the application. Finally, chlortoluron received the same rank in the 2 regions, but is more frequently detected in Lorraine; crop acreage may have been overestimated in Alsace. However, the dataset is still limited to one year of sampling; some discrepancies may appear less important when more data are available. For chemicals with ranks > 50, there is a good exponential fit between ranks and occurrences (y=0.0235*e0.0739x ; r2=0.82). This observation means that pesticides with ranks >50 are systematically encountered in surface waters; however, the current threshold (35) should be maintained, because some substances with ranks
