Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/772143
Bill Langer is a consulting research geologist who spent 41 years with the U.S. Geological Survey before starting his own business. He can be reached at Bill_Langer@hotmail.com CARVED IN STONE 44 AGGREGATES MANAGER / January 2017 B reakfast table talk with my wife, Pam, recent- ly included the discussion of topics for future Carved in Stone articles. Pam had just finished reading an article about permeable pavement when she suggested, "How about writing some articles on uses of aggregate for environmental purposes?" I am NOT making this up! Why was she reading that? Would that topic work? I cast about on the internet using the search string "aggregate environment," and Google came up with about 89,100,000 hits in 0.31 seconds. I was pleased to see that my publication Aggregate and the Environment was near the top of the list. It can be downloaded for free from the American Geosciences Institute at www.americangeosciences.org/ sites/default/files/aggregate.pdf. Unfortunately, it didn't take long for me to become somewhat alarmed. Clicking on most of the hits took me to sites describing the terrible things that aggregate mining does to the environment. I tried other search strings like "environmental applications for aggregate," and "environmental uses gravel;" all with no better results. That did it! I know there are many situations where the use of aggregates actually benefits the environment. I need to vent, so, this year, Carved in Stone will contain a number of articles on the environmentally beneficial uses of ag- gregate. I will begin with the one Pam read about — permeable pavement. Does the idea of permeable pavement hold water? Let's see. In the simplest of terms, permeable pavement is a hard surface used in applications such as parking lots or roads, with a base and subbase that allow the movement of storm water through the surface. The permeable pavement Pam was reading about was made from aggregate, crumb rubber, and asphalt. Permeable pavement can also be made with no crumb rubber; just aggregate bound with asphalt or concrete. Some low-traffic permeable pavement can be made from unbound aggregate or grass with, or without, geotextiles and interlocking pavers. The drawing shows the general design of a permeable pavement. Don't be fooled by the simplicity of the drawing; each individual system has its own specific requirements. Here is the problem. During a storm, the first 1 1/2 inches of rainfall mobilizes about 90 percent of surface pollutants. In- stead of going into treatment facilities, runoff typically is channeled directly into storm sewers and then into local water bod- ies. The pollutants can cause an increase in algae content, harm aquatic life, and require expensive treatments to make the water potable. If that initial runoff can somehow be retained, some of the ill effects of storm water runoff can be reduced. Conventional pavement causes increased runoff by preventing water from infiltrating into the soil. By contrast, pervious pavement captures rainwater in a network of voids, thus reducing runoff. In addition, the captured water can slowly percolate into the underlying soil and filter out pollutants carried by the runoff. In some geologic settings, that water recharges local aquifers. If pervious concrete or asphalt roadways and parking lots cover enough area, they can double as water retention structures. That may reduce or eliminate the need for traditional storm water retention ponds, thus lowering overall project cost. In addition to storm water control, pervious pavements aid in reducing the urban heat-island effect. The open-cell structure of the pervious pavement allows free air circulation, allowing more rapid cooling of the pavement. So, it looks like permeable pavement really does hold water. Thanks for the idea, Pam. In 2017, this column will highlight some of the environmentally beneficial uses of aggregates. Aggregates and the Environment