Читать книгу River of Lost Souls - Jonathan P. Thompson - Страница 17

Perfect Poison

ACID MINE DRAINAGE MAY BE THE PERFECT POISON. It kills fish. It kills bugs. It kills the birds that eat the bugs that live in streams tainted by the drainage. It lasts forever. And to create it, one needs no factory, lab, or added chemicals. One merely needs to dig a hole in the earth.

The hole, or mine, exposes once-buried, sulfide-bearing rocks such as iron pyrite (FeS2) to oxygen. As the hole gets deeper, it will penetrate aquifers or intersect groundwater-carrying faults and fractures, and even draw groundwater to it since it provides the path of least resistance. Put those three innocent ingredients—oxygen, water, and iron pyrite—together, and they engage in an atom- and ion-swapping molecular-level orgy. Hydrogen, sulfide, and oxygen come together to form sulfuric acid (H2SO4).²⁰ Thus, the water becomes acidic, or its pH drops. Iron and oxygen hook up to form iron oxide (Fe(OH)3). The iron oxidizes, or rusts, giving the Gold King slug, and acid mine drainage in general, that striking ochre hue.²¹

As the now-acidic groundwater moves through the mine, it dissolves and picks up naturally occurring metals in the rocks over which it flows, a process known as metal loading. Mine drainage is typically loaded with iron, zinc, cadmium, lead, copper, aluminum, arsenic, and silver. Mercury can get mixed in, too, sometimes even uranium.

It gets worse. As the pH level of the water drops below 4.8, acidophilic bacteria begin feeding off the metals, releasing more acid into the solution and causing metal loading to occur up to one million times faster than in water with higher pH. The result is acid mine drainage, mining’s most insidious, pervasive, and persistent environmental hazard. The water trickling, gurgling, and sometimes gushing out of a mine portal can have a pH equal to that of battery acid or worse. This concoction can carry hundreds of pounds per day of toxic heavy metals with it, picking up even more as it courses over or through the mine-waste rock dump piled outside nearly every mine portal.

“Probably all the waters met with in the mines of the Silverton quadrangle . . . are meteoric waters variously modified by the materials through which they have passed,” noted Frederick Ransome, in a 1901 U.S. Geological Survey report about the region. “The descent of the meteoric water through masses of pyrite and other ore minerals is often sufficient to give it a strong acid reaction and render it highly ferruginous.” The water at the Yankee Girl Mine, just over the ridge from the Gold King, was so acidic that “Candlesticks, picks, or other iron or steel tools left in this water became quickly coated with coppers (sulfate salts). Iron pipes and rails were rapidly destroyed, and the constant replacement of the piping and pumps necessary to handle the abundant water was a large item in the working expenses.”

Acid mine drainage is ubiquitous in mining country, from Idaho’s contaminated Coeur d’Alene River basin to the eerily colored mine pool of the Berkeley Pit in Butte, Montana, where thousands of migrating snow geese perished in one fell swoop after landing on the contaminated “lake.” The waters flowing from the Richmond Mine in California are more corrosive than battery acid and will devour a shovel in twenty-four hours. Left alone, the mine will continue to spew into the Sacramento River watershed for thousands of years to come. An ancient mine in southern Spain, abandoned four millennia ago, continues to ooze acid mine drainage into the aptly named Rio Tinto to this day.

A “normal” and healthy stream will have a pH of 6.5 to 7.5. Drop below that (or go above it), and the creatures that live in or rely on the stream begin to struggle. Mayflies, stoneflies, and caddisflies can’t survive below pH 6; rainbow trout perish at pH 5.5. Hardier fish species can hang on in water with a pH as low as 4.5, but their health will be impacted, their eggs won’t hatch, and their food sources will be diminished. When a stream’s pH drops below 4, all deals are off for aquatic life. Water draining from the Gold King Mine prior to the blowout had a pH ranging from 2 to 3. Since the Silverton Caldera’s geology offers up no natural buffers such as limestone, Cement Creek retains its high acidity (pH 3.5, similar to Mountain Dew) all the way to its juncture with the Animas River. Cement Creek (and, presumably, Mountain Dew) is uninhabitable, extremophiles notwithstanding.

After the waters of Cement Creek, Mineral Creek, and the Animas River come together, and as the now combined streams leave the Silverton Caldera, the pH rises quickly thanks to dilution and natural geologic buffering. Even the Gold King slug’s acidity diminished as it moved downstream, measuring in at a healthy pH 6.8 as it slithered through Durango. Acid, however, is not the only, or even most harmful, element of acid mine drainage. The metals contained in that water can be far more deleterious in the long run. And those metals remain in the river, either in solution or as tiny, suspended particles, for miles and miles downstream, even as the acidity dissipates.

Zinc, copper, and cadmium, in both dissolved and solid states, are particularly toxic to trout and other fish and the bugs on which they depend. In very high concentrations, these dissolved metals can bring death in a fell swoop; a single high runoff event at California’s Richmond Mine killed forty-seven thousand fish. At lower concentrations, it’s a slow ecocide. The metals accumulate in the food chain, hindering fish growth, stifling reproduction, and wiping out more sensitive species altogether. Some metals are more persistent than others, and aren’t broken down by natural processes. So they bioaccumulate, building up in a fish’s organs over time without killing it, making the fish toxic to whatever eats it, including humans. Mercury is especially pernicious in this regard, because it tends to build up in the fish’s muscles, the part most commonly eaten, and mercury is also biomagnified as it moves up the food chain. That’s why tuna, near the top of the chain, is likely to have higher mercury concentrations than sardines. Changes in water chemistry and temperature can make metals more or less bioavailable and toxic. Zinc and cadmium have a synergistic relationship, meaning in combination they are more pernicious than alone. Relatively benign iron becomes toxic at lower concentrations when the pH drops below 5, and warmer water temperatures generally increase metals’ toxicities, yet another reason to fear climate change.

Metals in solid form are not as easily ingested or absorbed, so pose less threat to bugs or fish as toxins. Yet they can snuff out a stream’s oxygen and light, they can build up on and kill plants, and accumulate on or abrade a fish’s gills. Iron is not especially toxic, but it is abundant in acid mine drainage and as it precipitates out of solution it settles onto the stream bed and hardens into ferricrete, thus damaging bug habitat and cementing over gravel in which fish spawn. Cement Creek probably got its name from the ubiquitous coating of ferricrete throughout its length.

Acid mine drainage spews into western watersheds daily from thousands of abandoned hardrock, coal, and uranium mines. It amounts to a round-the-clock defilement of aquatic ecosystems. Yet except on rare occasions—such as when three million gallons of it got backed up in the Gold King Mine, then came blasting out over just a few hours—acid mine drainage is invisible, and goes mostly unnoticed. When mines are active in a region, however, a far more apparent form of pollution, mill tailings, or slimes, can cause even more damage—call it acid mine drainage, supersized.