CHICAGO , Illinois ( CNN ) -- Environmentally friendly engineering is really right up Chicago 's alley -- in a city project called Green Alleys . Chicago workers resurface alleys with permeable concrete that cleans rainwater before it runs into Lake Michigan . The program aims to stop polluted rainwater in the city 's alleyways from ending up in adjacent Lake Michigan . Chicago 's 13,000 alleys , which allow rear access to buildings , date back to the 1800s . The 1,900 miles of back alleys ease access for garbage collection , loading and parking . They also make up about 3,500 acres of impermeable surface , which has created a big problem . Many of the alleys were built without connections to storm sewers . Rainwater puddles on the pavement and can run into buildings , sometimes flooding basements . `` We wanted to come up with a different way of solving that problem other than reconstructing , putting in a sewer system and adding water to our already full sewer system , '' said Janet Attarian , sustainability coordinator with the Chicago Department of Transportation . So , the extreme alley makeover started with the pavement . The alleys are being resurfaced with concrete , asphalt or paving stones that are permeable -- that is , water runs through them like a sieve to the dirt beneath . Microbes that thrive on stuff like fertilizer and oil are seeded into the pavement or migrate there naturally . They cleanse the water of pollutants from cars and lawns . The cleaner water makes its way into the groundwater , called an aquifer , and then flows into Lake Michigan . That 's a big deal , Attarian explained , because the city takes about a billion gallons of water out of Lake Michigan every year . But only about 1 percent of that water finds its way back to the lake , Attarian said . `` So by infiltrating that aquifer , we have a chance to [ give ] some of that water back . '' Additionally , green alleys feature pavement that 's high in albedo , or the ability to reflect light . The surface is lighter in color , reflecting more sunlight and absorbing less . `` It 's like putting on a white T-shirt instead of a black one on a hot summer day , '' Attarian said . `` It cools the city off and reduces our carbon footprint . '' Other features of the Green Alley program include the use of recycled materials in pavement and energy-efficient light fixtures that direct light downward to reduce light pollution . How green are those alleys ? Chicago has greened 40 of them , with 48 more scheduled to get the treatment this year .
Chicago covers alleys with special concrete that cleans polluted rain runoff . Microbes in concrete eat pollutants such as fertilizer and oil . Solution avoids having to rebuild city 's massive alley system . Unique concrete also helps to keep city cooler on sunny days .