Originally posted by MarkP
I think that the heavy metals are leached from the roofing and pipes by the acid rain rather than coming directly from the volcano. Is that not correct?
That is the primary concern especially for lead, but volcanic gases contain a whole set of hazardous materials that can end up in catchment water.
http://volcano.oregonstate.edu/book/export/html/151
Most Common Gases
Water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2) are the most common volcanic gases.
Other Gases
In lesser amounts, volcanoes release carbon monoxide (CO), hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2), hydrogen chloride (HCl), hydrogen (H2), methane (CH4), hydrogen flouride (HF), boron, hydrogen bromine (HBr), mercury (Hg) vapor, organic compounds, even gold. From Cadle (1980).
Mercury is released by most volcanoes and has been measured at Kilauea, Mauna Loa, Hekla, Erebus, at Mount St. Helens (Siegel and Siegel, 1987). Kilauea produces about 270 tons of mercury each year and has been identified as the source for mercury on Oahu, 320 km away.
https://books.google.com/books?id=hMA70VU36qUC&lpg=PA96&ots=AN3bnhx7ZW&dq=arsenic%20volcanic%20gas&pg=PA96#v=onepage&q=arsenic%20kilauea%20volcanic%20gas
3.6.8 Arsenic in volcanic gas emissions
Kilauea volcano, Hawaii, USA, erupted in January, 1983. During a two-week period including the eruption, atmospheric particulate matter (PM) around the colcano was collected on 0.4 um air filters. Measurements of the atmospheric particles yielded 0.0045 - 1.600 ng/m3 (nanograms of arsenic per cubic meter of air). Before the eruption, the average arsenic concentrations were 0.013 - 0.039 ng/m3.