Biomotoring groundwater contamination: Application to a karst area in Southern France

Abstract

In order to assess 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 (Herault, France) from 1991–1993. Invertebrates were collected by means of an air-lift pump in 8 deep monitoring wells (from 43 to 90 m deep) which intersected Cretaceous and/or Jurassic limestones. Wells of the unimpacted sites had faunal assemblages dominated by crustaceans (from 92.4 to 98.3% of the total number of organisms), and they harboured a high number of stygobite species (from 15 to 24) which usually represented a major component of the total number of invertebrates (from 48.1 to 81.5%). At the sewage-polluted site, the wells had significant relative abundances of oligochaetes (from 8.4 to 40.4% of the total number of invertebrates), low stygobite richnesses (from 3 to 7 species) and their groundwater fauna consisted mainly of stygoxene taxa (from 20 to 34) whose relative abundance could be as high as 88%. The spatio-temporal distribution of organisms was related to the groundwater and sewage flow patterns. During low-water periods sewagepolluted water moved preferentially through the conductive fractures of the saturated zone whose faunal assemblages were dominated by stygoxenes such as the polysaprobiont oligochaeteTubifex tubifex. When floods occurred, these stygoxenes were flushed out of the site and were also disseminated throughout the thinly fissured part of the saturated zone where they 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.