A RARE CASE OF DRINKING WATER CONTAMINATION BY THALLIUM: PIPE MONITORING ALONG DISTRIBUTION NETWORKS IN PIETRASANTA (LU)

Thallium is a metal widely distributed in nature even though its abundance is very low. Although its presence in water distribution systems (WDSs) as a result of raw water contamination is an extremely rare condition, its toxicity has aroused concern in consumers. In September 2014, a severe contamination was detected in two WDSs of Pietrasanta, an Italian town (about 24,900 inhabitants including hamlets) of the province of Lucca in Tuscany region. The two WDSs, fed by one drainage gallery, three springs and five wells, were connected each other by a pipe and a reservoir. Concentrations up to 14 μg/L were reached in distributed drinking water, well above the US-EPA maximum contaminant level of 2.0 μg/L. The contamination was delivered inside the waterworks by groundwater collected from one of the three springs, located close to an abandoned mining site in a mountainous area. The contaminated spring was immediately disconnected from the first WDS but a rapid increase in the concentration of thallium (up to 60 μg/l) was detected in the distributed water due to its migration from the internal surface of pipes where thallium had accumulated as sediments, sludge and adsorption products. Although system flushing for several weeks and complete replacement of steel pipes along the first WDS were performed, contamination of drinking water distributed by the second WDS remained. 150 pipe core samples (steel, cast iron, lined cement-mortar and high-density polyethylene) were collected from the second WDS to determine the extent of the remaining contamination. Samples were processed with a multiple sequential extraction, previously optimized [1]. In details, every pipe core sample was subjected to three consecutive ultrasonic-assisted extractions with reagents of increasing reactivity and eventually mineralized with concentrated nitric acid. Following the application of this extraction procedure, the concentration of Tl released in weakly acid condition (that is, the first extractant) was determined by inductively coupled plasma – mass spectrometry. The maximum detected value was 180 μg/cm2, which represented about 5-9% of the total content present on the pipe surface. The contamination of the second WDS decreased rapidly as the distance from the two interconnection points with the first WDS increased. A negligible background concentration was found at a distance greater than 1 Km.