A Further Study of the Zinc-Polluted River Ystwyth

Abstract

One of the outstanding features of pollution by heavy metals is its persistence. A river polluted by organic matter can recover lower down its course when the offending substances have been oxidized but once soluble salts of lead, zinc or copper find their way into a river no natural agency can remove them, though dilution by unpolluted tributaries can lessen their concentration. Metallic pollution can be very persistent in time and the river Ystwyth in north Cardiganshire is an interesting example for it remains seriously polluted by zinc thirty-five years after the cessation of mining activities in its valley. The first scientific examination of the main stream was made by Carpenter (1924) who found that the fauna seemed limited to nine species, all belonging to the animal groups Insecta, Hydracarina and Crustacea-Entomostraca. The flora was also very poor, the only common algae being Batrachospermum and Lemanea (Saccheria). This was attributed to pollution by lead salts, and analyses made in 1921-22 showed that the river carried 0 4-0-5 mg/l. of lead in solution (Carpenter 1924, p. 12). The presence of zinc was not suspected at this time, though it was well known that the Ystwyth mines yielded lead, zinc and silver ores and no tests for zinc were made. In 1939-40 the writer (Jones 1940) made a study of the river which proved to be polluted by lead and zinc, the former in concentrations up to 0-05 mg/l., the latter in concentrations of 0 2-1 2 mg/l. It was shown that the main sources of pollution were the Cwm Ystwyth, Frongoch and Wemyss mines and a brief description was given of the river's fauna and flora. The present paper carries the investigation some stages further; it includes a resurvey of the sources of pollution, further observations on the flora and the chemical condition of the main stream, the results of quantitative sampling of the fauna and an account of the feeding habits of the chief insect species.