Cistern water: soft - and corrosive.

Cistern water: Soft--and corrosive

Last year, the Environmental Protection Agency (EPA) proposed corrosivity limits on large drinking-water systems to control the leaching of lead from household plumbing (SN: 8/20/88, p. 118) -- a decision prompted in a part by concerns that acid rain increased the corrosivity of drinking-water sources. But water softness may play nearly as big a role as acidity in leaching toxis metalsM a new study finds.

Researchers sampled water from 50 similarly designed cistern systems -- half in Kentucky and Tennessee, the rest on St. Maarten, in the Netherlands Antilles. Because sitting in plumbing overnight gives water more time to leach metals, the study's sampling included this "standing" tap water.

Results, reported in the March ENVIRONMENTAL SCIENCE AND TECHNOLOGY, show that although the Kentucky-Tennessee rainwater had a pH about 4.5 -- making it roughly 100 times more acidic than St. Maarten's -- water in all the cisterns had comparable pH levels: The mean pH ranged from a neutral 7 to a slightly alkaline 7.6. It appears that concrete or plaster lining the cistern tanks neutralized the U.S. acid rain, says Harvey Olem, a Washington, D.C., consultant who led the project while working as a Tennessee Valley Authority staff scientist in Chattanooga.

Cisterns also removed metals in rain or leached from rooftop collectors, precipitating them into their tank sediment. That's why the researchers were puzzled that higher levels of 11 constituents -- including lead, cadmium, zinc and copper -- came out of taps where rainwater had been acidic. This, the researchers say, suggests "a causative link" between leached plumbing metals and some aspect of acid rainwater other than pH -- such as water softness. High calcium levels, which typify hard water, tend to coat pipe interiors with a residue that protects plumbing metals from leaching, Olem notes. Dependent on soft (low-calcium) rainwater, both regions he studied suffered measurable leaching of plumbing metals. For example, in 72 percent of the Kentucky/Tennessee homes and 40 percent of the St. Maarten homes, lead levels in standing tap water exceeded the proposed limit -- 5 micrograms per liter of lead -- being considered by EPA for U.S. drinking waters.