In November 1986, a fire occurred at the Sandoz agro-chemical warehouse at Schweizerhalle (near Basel) in Switzerland. At the time of the fire, the warehouse contained c. 9831 of agrochemical products such as organophosphates, chlorinated organic compounds, and organic mercury compounds as well as 354 t of formulation auxiliaries and other chemicals. A majority of the stored chemicals were destroyed in the fire, but large quantities were released into the atmosphere, into the Rhine River through runoffofthe fire fighting water (10 000-15 000 m3), and into the soil and groundwater at the site.
The amount of chemicals released into the Rhine was not known but was estimated to be between 1% and 3% of the stored chemicals. The estimated discharge of the dominant compounds was 3000-8900 kg for disulfoton, 1200-3900 kg for thiometon, 160-1900 kg for propetham-phos, 50-290 kg for parathion, 2.5-300 kg for fenitrothion, and 250-1900 kg for oxadixyl. The concentrations of chemicals in the Rhine River were measured on the day of the accident at Village Neuf, 173 km downstream of the Sandoz chemical warehouse. The concentrations of dis-ulfoton, thiometon, propethamphos, parathion, and oxadixyl were present at 600, 500, 100, 200, and 80 mgl _1, respectively. A comparison ofwater concentrations ofthe EC50s and LC50s suggests that certain chemicals could have contributed to the damage to biota, and cumulative or possibly even synergistic toxic effects resulted from the mixture of pesticides.
After the accident, the aquatic life in the Rhine was greatly damaged up to several hundred kilometers downstream of the spill site. The toxic plume destroyed the entire eel population (approximately 2001) up to 400 km downstream of the incident along with inflicting severe damage to other fish species such as grayling, trout, and their food organisms. Populations of aquatic invertebrates were initially devastated, and the populations of a range of invertebrates were greatly reduced. Various chirono-mid populations were among the first to recolonize, followed by the recovery of caddis fly. It seems that for chironomid and caddis fly populations, no more than one generation was required for recovery and a second generation for complete recovery.
That the damage caused by the spill to the Rhine was not more extensive may be explained by the chronic contamination that had already eliminated many sensitive species. With regard to the ecotoxicological aspects of the spill, up to 400 km from the spill site, grayling and trout still died at concentrations 1000 times less than the LC50s of individual organophosphate compounds. It was probably the mixture ofchemicals that was responsible for the fish kills, rather than the effect of any given compound. Some researchers concluded, in their analysis of the chemical spill into the Rhine River, that acute toxic effects occur at concentrations much smaller than LC50 values suggest and synergistic effects ofthe substances involved must be considered, as well as the fact that they added to the existing chronic pollution of the river water. They also postulated that environmental catastrophes, as long as they are not too big and do not occur too often, are less harmful on the whole than is the chronic intoxication of ecosystems.
As for lessons to be learned from the accident, new regulations have been established for large chemical plants requiring holding ponds to be built to retain fire water runoff.
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