Biomonitoring groundwater contamination : application to a karst area in Southern France

In order to access the descriptive value of subterranean organisms with regard to groundwater contamination, faunal sampling was carried out at two unimpacted sites and one sewage polluted site located in the south eastern part of the Lez karst system from 1991-1993. Invertebrates were collected by means of an air lift pump in 8 deep monitoring wells which intersected Cretaceous and/or Jurrassic limestones. Wells of the unimpacted sites had faunal assemblages dominated by crustaceans and they harboured a high number of stygobite species which usually represented a major component of the total number of invertebrates. At the sewage polluted site, the wells had significant relative abundances of oligochaetes, lowstygobite richnesses and their groundwater fauna consisted mainly of stygoxene taxa whose relative abundance could be as high as 88%. The spatio temporal distribution of organisms was related to the groundwater ans sewage flow patterns. During low water periods sewage polluted water moved preferentially through the conductive fractures of the saturated zone whose faunal assemblages were dominated by stygoxenes such as the polysaprobiont oligochaete Tubifex tubifex. When floods occured, these stygoxenes were flushed out of the site and were associated with stygophiles. As the source of sewage pollution decreased in intensity with time, preliminary signs of groundwater fauna recovery were observed. Oligochaetes became less abundant but recolonization of groundwater by stygobites was not yet observed. On the basis of our results and those of other authors we emphasize the advantages which could result from the development of biomonitoring programmes for the assessment of karstic groundwater contamination. / Dans la partie sud est du système karstique du Terrieu (Hérault), des prélèvements de faune ont été effectués sur trois sites de 1991 à 1993 : deux sont exempts de pollution et un site s'avère pollué par des eaux usées domestiques. Les sites non pollués sont dominés par les crustacés et par une faune d'espèces souterraines de crustacés et d'oligochètes. Dans la zone polluée, les oligochètes se révèlent dominants et la richesse spécifique des espèces souterraines de crustacés et d'oligochètes diminue fortement. A la suite d'une réduction de la source de pollution, la restauration du milieu s'eat traduite principalement par la diminution de la densité des oligochètes. En revanche, la recolonisation du milieu par des espèces souterraines n'a pas encore été observée.