The monitoring program is the first of its kind in the country and requires mines to work with a third party to test the emissions coming out of their equipment.

Tests found the presence of an oxidized, vaporous form of mercury, but further tests are needed to determine how much.

"The oxidized form is the most reactive form, the form that is deposited more locally. That's the one that could potentially make it into Nevada waterways," Pistone said.

Mercury, a neurotoxin, is of particular danger to children and developing fetuses.

The Assembly this week unanimously approved Assembly Bill 115, which would require higher fees of mining companies to pay for two new positions at the state agency to regulate mercury emissions. The proposal has support from Gov. Jim Gibbons, Pistone said.

Nevada is the nation's top gold producer, producing 78 percent of the total amount mined in the country. The state produces 12 percent of the world's gold. The calculated value of gold produced in Nevada was $3.8 billion last year, up about $750 million from 2005.

In February, the Division of Environmental Protection announced its first enforcement action against a mine accused of violating new pollution standards. Agency administrator Leo Drozdoff said that announcement was made in part to emphasize the state means business about controlling mercury pollution.

The new rules require mines that release significant amounts of mercury to install scrubbers and similar technology to control smokestack emissions.

Mercury is released into the atmosphere during roasting and other refining procedures used to extract gold from ore.