AI-generated summary reviewed by our newsroom.<\/p>\n
Read our AI Policy.<\/a><\/p>\n By year\u2019s end, Raleigh-Durham International Airport<\/a> will finish building one of the state\u2019s largest paved parking lots, with nearly 11,000 spaces over 110 acres of asphalt and concrete.<\/p>\n Read more Hurricanes need to fix these 5 problems as they head into Game 2 against Vegas<\/a><\/p>\n The rain that falls on all that pavement, and the gunk it collects on the way, will flow into a new type of stormwater system that is still considered experimental by state regulators.<\/p>\n It\u2019s called a submerged gravel wetland. There will be seven of them scattered around RDU\u2019s Park Economy 3<\/a>, ranging from a quarter to a full acre, each fenced off and surrounded by gravel and grass.<\/p>\n Water from the lot will enter each wetland from a pipe at one end, then settle through a layer of sandy soil planted in sneezeweed, rushes, swamp sunflowers and other wetland plants, down to gravel. Water filtered by the plants and microbes in the soil and gravel will eventually drain out the other end toward Crabtree Creek.<\/p>\n RDU\u2019s contractors consulted with N.C. State University researchers on the design of the system. Over the next year, the university will monitor the wetlands and the quality of water leaving two of them.<\/p>\n \u201cThere\u2019s a lot of stuff that\u2019s being learned,\u201d said Bill Hunt, a stormwater specialist in NCSU\u2019s Biological and Agricultural Engineering Department<\/a>. \u201cWe\u2019re confident it\u2019s going to work. We just don\u2019t know exactly how well \u2014 that\u2019s why we\u2019re doing the monitoring.\u201d<\/p>\n State regulators allowed RDU to use the wetlands as a \u201cpilot study\u201d in conjunction with the NCSU research, according to Mike Lawyer, stormwater program supervisor at DEQ\u2019s Division of Energy Mineral and Land Resources<\/a>.<\/p>\n While the approach is not approved for use in North Carolina, the engineering and technical material provided by the airport showed the system would \u201cprovide equal or better stormwater control and equal or better protection of surface waters,\u201d Lawler wrote in an email.<\/p>\n Submerged gravel wetlands were developed and tested by engineers at the University of New Hampshire Stormwater Center<\/a> more than a decade ago. They\u2019re widely used throughout New Hampshire and are spreading to other states, including New Jersey and Maryland.<\/p>\n But treating stormwater with submerged wetlands is still novel in North Carolina and other Southern states, even though, Hunt said, they\u2019re likely to work better in a warmer climate.<\/p>\n \u201cThe biological transformation does better the warmer it is,\u201d he said. \u201cSo if it worked well, which it did, in New Hampshire, it\u2019s going to work even better here.\u201d<\/p>\n A handful of small submerged gravel wetlands have been built in North Carolina, including one at the Walnut Creek Wetland Park<\/a> in Raleigh. But RDU will be the technology\u2019s biggest test by far in the state; an inch of rain falling on 110 acres of pavement will produce nearly 3 million gallons of runoff, according the U.S. Geological Survey<\/a>.<\/p>\n On a recent rainy day, the most complete of the RDU wetlands looked like a pond. It had been raining for hours, and about 4\/10ths of an inch had hit the pavement and washed into the wetland, covering the sandy surface with water.<\/p>\n Small plants stood in neat rows. They will eventually grow several feet high, creating a mini forest that covers the wetland surface.<\/p>\n The water doesn\u2019t remain at the surface for long, filtering down through the sand and plant roots to the gravel layers below. That\u2019s an important feature, because ponded water can become a breeding ground for mosquitoes and attract geese, something an airport wants to avoid, Hunt said.<\/p>\n Read more Do Canes players buy tickets for family & friends to watch the Stanley Cup?<\/a><\/p>\n The soil, roots and microbes throughout the wetland help collect sediment and remove nitrogen and phosphorous from the water, Hunt said. Oil, gas and grease from the parking lot binds to organic matter at the wetland surface, where microbes go to work on it.<\/p>\n \u201cThese microbes, as oil and greases start coming into the wetland, their population will begin to explode,\u201d Hunt said. \u201cAnd they will eat the oil and greases and break them down to just generic water, carbon and gas. It\u2019s cool.\u201d<\/p>\n The system has limits. A major petroleum spill would overwhelm the microbes, for example.<\/p>\n And the wetlands are designed to handle one to two inches of rain over a 24-hour period. When a hurricane hits, the basin around the wetland will fill to the level of the outfall pipe and water will drain out untreated. Storms have dropped more than 2 inches of rain on RDU twice in the past year, according to the National Weather Service.<\/p>\n \u201cIt doesn\u2019t clean the big storms,\u201d Hunt said. \u201cIt just passes the big storms safely without causing erosion.\u201d<\/p>\n Downstream erosion is a concern for Jean Spooner, head of the Umstead Coalition<\/a>, a group that aims to protect neighboring William B. Umstead State Park<\/a>. Spooner says water flowing from the lot during construction created gullies and washed large amounts of sediment into Lake Crabtree, and she hopes the airport is taking steps to prevent that from happening after the lot is finished.<\/p>\n Spooner is a retired NCSU extension professor who specialized in soil science and water quality. She says her review of how submerged gravel wetlands have performed elsewhere make her skeptical they will adequately remove nitrogen from Park Economy 3\u2019s stormwater. At the same time, she says she worked with Hunt for years and trusts his judgment.<\/p>\n \u201cI\u2019m not convinced they\u2019re the best for this application,\u201d she said of the wetlands. \u201cBut if you\u2019re going to do these, the best person to oversee the construction and monitoring of them in the country is Dr. Bill Hunt.\u201d<\/p>\n The day-to-day task of monitoring the wetlands falls to Anna Dias, a student working toward a master\u2019s degree in biosystems engineering. Sensors allow Dias to monitor water levels at all seven wetlands to determine if they\u2019re getting clogged or not working properly.<\/p>\n And at two wetlands, Dias collects water samples for lab tests to measure pollutants such as phosphorous, nitrogen and \u201ctotal suspended solids\u201d or TSS, which is essentially how much sediment is coming in and leaving.<\/p>\n \u201cI was looking at my water quality results earlier this week, and it\u2019s reducing TSS a lot,\u201d she said recently.<\/p>\n Dias expects to continue collecting data, particularly after storms, through the summer of 2027. It will help the NCSU team determine if the wetlands have the right mix of plants, for example, or are the right size to handle the pavement. RDU is building 1 acre of wetlands for every 21.7 acres of pavement.<\/p>\n \u201cThis practice is very likely going to be approved across the state,\u201d Hunt said. \u201cSo what we\u2019re doing is fine-tuning the design standards that everyone else in North Carolina are going to use.\u201d<\/p>\n