Demand for aggregate has been on the rise since 1992, leaving producers increasingly forced to quarry aggregate in karst environments. With the increase in karst quarrying has come growing concern for the potential environmental damage that can be done to these regions, which cover about 10% of the world's land surface.

Bill Langer has been a research geologist with the U.S. Geological Survey since 1971. At the January 2003 meeting of the Chicago Section of the Society for Mining, Metallurgy and Exploration, Langer examined the potential damages from quarrying and a strategy for mitigating damage to these sensitive environments.

Karst environments are formed over the course of hundreds or thousands of years, through a process of dissolution. Carbonate rocks (limestone, dolomite and marble) contain openings and fractures that allow water to penetrate the rock surface. Slightly acidic water starts a chemical reaction with the rock, causing openings to dissolve and widen — allowing more water to penetrate deeper into the rock. The process self-perpetuates, eventually creating topography that contains caves, sinkholes and underground drainage systems.

Aboveground, karst systems vary widely in appearance from region to region. “Karst areas are generally areas of incredible scenic beauty,” says Langer, “but they are incredibly environmentally sensitive.” It doesn't take much to disrupt the balance of a karst environment. Small changes can bring significant impact, even the death of species dwelling within the dark voids of the terrain.

It's no secret that as our nation grows, so does our need for aggregate. During the last century, the number of metropolitan areas with populations more than 1 million grew from six to 49; the overall population has more than tripled. During the same time, aggregate production exploded, from 50 million mtpy to more than 2.75 billion mtpy. The result: compared with the early 1900s, today we have three times as many people demanding 50 times as much rock — and three times as many folks who don't want a quarry in their backyard.

All that rock has to come from somewhere, and that means more quarrying in sensitive areas. Which, Langer says, brings us to the question: “How can we meet our needs for carbonate rocks and still maintain our expected level of environmental quality, especially when quarrying in areas of karst?

“To protect the environment from potential impacts,” Langer says, “you need to know what the potential impacts are, and understand how they occur.” He emphasizes that while quarrying can cause harm to sensitive environments, there are many other human activities that also cause environmental impacts, including urbanization, deforestation, tourism and recreation, military actions, dams, groundwater pumping, agriculture and industry.

Environmental impacts on karst are categorized into two types: direct and cascading.

Direct impacts (or engineering impacts) are changes caused by engineering activity, and can often be remedied by another engineering activity. For example, aggregate quarrying can produce unwanted dust and noise levels. Engineering activities can alleviate the problems: enclosing operations in sound-deadening structures, building sound-control berms, and adjusting operation schedules can reduce noise. Dust can be controlled with filtering techniques and watering strategies. When recognized, the effect of direct impacts can be minimized or eliminated with engineering solutions.

Cascading impacts are not as easy to address. Again, engineering activities can be the culprits behind cascading impacts, which begin with the alteration of a natural system. The system responds, setting off another response and creating a chain of events that result in enormous consequences. For example, blasting can cause fugitive dust that covers leaf surfaces in surrounding foliage. The leaf stomata are blocked, limiting gas exchange, thus limiting photosynthesis and killing the plant, altering the terrain and its inhabiting wildlife.

Another effect of blasting is vibration and shock waves, which can cause cave roofs to collapse or crack; cracks in the quarry face or floor increase permeability and can induce flooding. Finally, because karst environments often contain pinnacled bedrock, poorly designed or executed blasts can cause hazardous flyrock. Cascading impacts can manifest immediately or many years after the mining operation has ceased, and can spread far beyond the boundaries of the quarry.

Environmental impacts in karst can be triggered by a variety of events: drilling, blasting, runoff, load changes, vegetation removal, etc. Each trigger can produce a variety of impacts.

One example involves terrestrial ecosystems that develop in dark karst caves and are often inhabited by species that lack many of the essential functions of aboveground creatures, such as sight. The darkest areas of karst experience a complete lack of light, 100% humidity, low air exchange and high carbon dioxide concentrations.

Cave inhabitants are very sensitive to change, Langer says. “These little creatures are not adaptable, so you can destroy species by opening a fracture and changing light or water conditions.”

Any quarrying activity that intersects or changes water levels or flow patterns in any way can cause significant impact. Not only can blasting collapse caves and other passageways, but resultant noise and air concussion from as far away as 1,500 meters can disrupt colonies of bats and swiflets or interfere with their ability to communicate.

Water quality also can be impacted in a variety of ways through quarrying activities. The most common concern is pollution, which can be carried quickly throughout a karst system. Increases in runoff and sedimentation can deteriorate groundwater.

In cases where quarried stone acts as a protective layer over an aquifer, removing that rock allows surface water to infuse groundwater, bringing contaminants along with it. Blasting can open and close passages, changing the groundwater flow, which also can expose the water to contaminants, dust or silt. Dewatering quarries can lead to lower water tables, which can flood quarries or cause sinkholes.

Sinkhole collapse is one of the most obvious impacts of quarrying in karst environments, and can occur gradually or suddenly. Reducing the water table below the ground surface commonly contributes to sinkhole collapse. And a number of triggers can set off the process including water level fluctuations, soil de-saturation, volume shrinkage from material compaction, an influx of surface water into groundwater through piping, and increased groundwater velocity.

Construction activities can aggravate many of the causes of sinkhole collapse, increasing the risk of damage. Diverting natural drainage can further increase groundwater velocity and other effects that cause collapse. Removing overburden can weaken the site. Drilling, auguring or coring provides more opportunities for surface water to penetrate the subsurface. Vibrations from blasting or other activities can weaken karst structures and cause collapse, as can loading from heavy equipment.

With all the possibilities for environmental impacts in karst, it benefits operators to approach each site using “systems analysis,” a method designed to examine all possible scenarios and effects from a quarrying operation. Systems analysis is comprised of three phases: site characterization, risk analysis and quarry operation feedback. “Each karst environment varies greatly,” Langer says, “and deserves its own analysis — not only the surface, but in three dimensions.”

During site characterization, the site is evaluated and problems are identified. Information on land surface, geomorphology, geology and groundwater systems is developed in a database and each system is conceptualized and characterized, considering possible environmental impacts.

In risk analysis, mining and processing methods are assessed for risks. The system helps determine the likelihood that initiating events will result in consequences, and methods and risks are re-evaluated until a final mine design and operation plan are developed. Quarry operation feedback starts with the onset of operation, and starts another process of re-evaluation. By taking information garnered through operation and using it for continuous study, quarry operation is constantly improving.

Reclamation can breathe new life into old quarries, alleviating negative aesthetic effects in the form of sculpture parks, public gardens and golf courses, to name just a few. Langer says landform replication is an increasingly popular type of reclamation: carefully planned and strictly controlled blasting creates screed piles at the quarry face. The piles are re-planted with natural vegetation, which grows to resemble the surrounding area and eventually blends the mined site with adjoining terrain.

One of the most creative reclamation efforts Langer has experienced is at Rättvik, Sweden. The Dalhalla festival stage was built in Draggängarna, a former limestone quarry.

Operas, jazz and big band concerts, and symphonic and chamber music are performed with accompanying stage and light settings adapted to the site's mighty rock walls. As one colleague describes it, says Langer, “Dalhalla demonstrates that it is possible to fill a hole in the ground — even with music.”

When it comes down to it, Langer says, damage to karst environments is as often a result of other activities as of quarrying. “Generally, quarrying by itself does not make huge impacts. The problem is that when it goes wrong, it goes really wrong.” The relatively low incidence of damages can be attributed to both luck, he says, and preparation. But with more quarrying than ever before, the chances for damage to karst are higher, and operators can benefit from planning every operation with the environment in mind.

“The up-front costs of being prepared will offset the back-end costs if something goes wrong,” Langer says. After all, he adds, “geologists are cheaper than lawyers.”

Jennifer G. Prokopy is principal of Orange Grove Media and former associate editor of Rock Products.

Some material for this article was derived from USGS Open-File Report 01-0484: Potential Environmental Impacts of Quarrying Stone in Karst — A Literature Review by William H. Langer. To view the report, go to http://pubs.usgs.gov/of/2001/ofr-01-0484.