|The bright orange tint of the runof in this temporary drainage stream reveals significant iron hydroxide contamination.|
A visit to Laurel Highlands, a major rural tourist area consisting of three counties in the southwest quadrant of Pennsylvania, appears at first glance to be an escape from so much industry located only about an hour away in Pittsburgh. However, the ecological balance in the area — a region that was once heavily mined for coal after the turn of the century — is being threatened. Those mines that used to provide fuel for the Pittsburgh steel industry's blast furnaces are still releasing a small quantity of iron and turning storm water runoff acidic, degrading the habitability of nearby lakes and streams for aquatic life.
Rick Lowe, construction inspector for Mountain Watershed Association, Melcroft, PA, has lived in this region all of his life and built a log cabin here when he retired in early 2008. He then went back to work for the association on a failed acid mine drainage system remediation project in Fayette County's Springfield Township. The 34-year Army Corps of Engineers veteran notes that Indian Creek, which is located near the drainage system and the Youghiogheny River into which the creek empties, are populated with some trout despite the contamination of their waters. Still, the area's economy is now based entirely on tourism — and clean water for leisure activities such as whitewater rafting and fishing.
Like much of the state, the area has numerous abandoned coal mines with a great deal of pyrite residue from the mining. When pyrite is exposed to air and water, sulfuric acid and iron hydroxide are formed, lowering the pH of storm water flowing through the mines and coating stream bottoms with iron hydroxide. The Pennsylvania Bureau of Abandoned Mine Reclamation estimates that the state has about 250,000 acres of abandoned surface mines in 45 of its 67 counties, and about 24,000 miles of streams in the state do not meet water quality standards due to drainage from abandoned mines. The state calls acid mine drainage its biggest water pollution problem.
|The first sediment pond in the newly constructed system is lined with larger limestone than in the previous system, in order to reduce total surface area and help prevent system clogging with iron hydroxide precipitate.|
This water-pollution phenomenon is what led the watershed association to have an acid mine drainage system constructed in 1999 to treat runoff from a large mine just north of State Route 711/381, which is located a few hundred yards from Indian Creek. But by 2005, says David Lopes, agricultural engineer for the U.S. Department of Agriculture Natural Resources Conservation Service (NRCS), the original system had been rendered ineffective by a major design flaw. Berner Construction of Gap, PA, was awarded a contract to fix the system and began work in May 2008 with the help of grading technology that saved the watershed association significant cost.
Lopes, who inspected the remediated system for the federal government as the site abuts federally protected wetlands, says he thinks he knows why the original drainage system failed. That system was constructed with a large-diameter underground pipe near the mine opening that diverted runoff into a network of four drainage treatment ponds: three sediment ponds for separating the iron from the effluent and another lined with compost and limestone for bacterial treatment. The first sediment pond was lined with large limestone that had a high calcium content.
As effluent flowed over the limestone, the calcium raised the pH of the acidic effluent, raising its alkalinity and causing iron hydroxide particles to settle to the bottom. The problem, Lopes speculates, was that effluent gravity-flowed through a channel until it reached the first sediment pond, then contacted the open air and flowed from the top of the limestone to the bottom before flowing into the next sediment pond. Lopes believes that the immediate contact with oxygen actually raised the pH too abruptly and separated the iron hydroxide particles from water molecules, eventually clogging up the gaps between the limestone and the inlet end of the pond. Lopes explains that the pipe from the vertical shaft got clogged up and the pressure caused it to rupture, creating an upward flow of water and forming a puddle and a stream of untreated acid mine runoff that bypassed the drainage system and eventually made its way to Indian Creek.
By February 2008, the watershed association realized that the acid mine drainage system was not working as planned, and the NRCS requested bids for reconstruction of a redesigned system. Of seven bids, Berner's was one of only two that came in under NRCS' estimate that included reasonable profit. Work began in May 2008 on the project, which was sponsored by the watershed association with assistance from the Fayette County Conservation District, the Fayette County Commissioners and the Pennsylvania Department of Environmental Protection.
Like the original system, the one designed by the NRCS and constructed by Berner is passive; i.e., it utilizes gravity to allow mine-contaminated storm water to flow from a higher to lower elevation without the use of electric pumps. The system is also passive in that the acidity of the effluent is treated as it flows across naturally occurring materials such as limestone and compost; no man-made mechanical processes are used other than diverting the storm water through the system. However, the new system has a couple of fundamental differences from the original one.
The first sediment pond is designed as a vertical upflow pond — that is, storm water flows through 120 feet of 12-inch-diameter solid drain pipe from a manhole to the first sediment pond and contacts the limestone by flowing from the bottom up. The pond's lower elevation compared with the inlet end of the drain pipe near the mine opening, combined with the position of the pond's inlet pipe at the bottom of the limestone layer, allows gravity flow to create enough head pressure to force the storm water upward through the limestone. In contrast, the original system used an open channel as an inlet to the first sediment pond, exposing the water to open air immediately.
In fact, the original system was entirely downward flowing; the upward effluent flow in the new system's first sediment pond minimizes contact between the effluent and air until the effluent has flowed to a second sediment pond. The redesigned system also prevents the first sediment pond from clogging through the use of a network of intersecting perforated pipe that was constructed beneath the limestone, which evenly distributes effluent throughout the entire first pond.
Lopes explains that the system is designed to be anoxic — characterized by a lack of oxygen — at the first treatment stage. The existing system was supposed to be anoxic, but water coming out of the mine was immediately diverted into an open channel, which exposed it to the atmosphere and caused the iron hydroxide to precipitate right away.
"This system here actually had [the effluent] come out into the air first and then try to make its way underground," says Lopes. "There used to be a section of compost in the pond that the effluent would work its way through before going in. As soon as [the effluent] hit the compost and the air, it already started to change pH and the iron started to drop out, so we wanted to eliminate that. Now there is some air contact on the surface, but it's pretty much running underground through a distribution pipe that runs underneath the stone."
By early May, the first sediment pond had been reconstructed. While using a pump to dewater the pond, Berner Construction had removed about a 3-foot-deep layer of topsoil and the limestone. The limestone was then stockpiled, and rain washed away the iron hydroxide precipitate over time.
Next, the perforated pipe network was constructed on the bottom of the first pond. More than 5,000 tons of new limestone from Keystone Lime Co., Springs, PA, was trucked in and placed on the bottom of the first pond. Lopes notes that the new limestone has a relatively large 6- to 8-inch average size so as to provide effluent with less stone surface area and provide a further safeguard against clogging.
From the manhole, storm water flows through the 120-foot-long 12-inch pipe and into a pipe network at the bottom of the upflow pond. After flowing into an intersecting 12-inch pipe, the storm water flows through more intersecting 8-inch perforated pipe sections. There are three more intersecting 12-inch pipe sections in the first sediment pond positioned transverse to the inlet pipe; these converge on the side of the pond nearest the creek to form a system bypass for use if and when the system undergoes future maintenance. Three valves were installed near the point of convergence, which will allow access for future maintenance. "We have an [operations and maintenance] plan for this so that once in a while we flush the system by building up the head and flushing it out to try and pull as much of the precipitate out," says Lopes
The first stage of the system was where Jim Irey, vice president, Berner Construction, first relied on Global Navigation Satellite System (GNSS) technology on this project. In 2006, he had purchased a Topcon Positioning Systems HiPer Lite + GNSS from Boyd Instruments of Horsham, PA. On some projects, the company mounts the rover unit on a dozer blade. Because this project was a system redesign and relatively little dirt moving was necessary, Irey moved the rover around the site by foot to check grades.
Using the receiver, Irey recorded the position of the pipe intersections and the location of the pipe for an as-built survey. He also used the unit to check grading throughout the site to ensure that effluent would gravity-flow from one pond to the next at a lower elevation.
"We've used [GNSS] for everything," Irey says. "We're laying this channel out, we've used it to set grade, we've used it for pre- and post-construction topographic surveys, to locate all the valves — there are nine valves in here — we've located all of the valves so that they have them and they have the data points that they need to go back to in time. The valves give them flushing capabilities. One of the concerns with all of these ponds is that even though it's supposed to be an anoxic system, they still get iron precipitating, so what they want to be able to do is open these valves, have these things full and flush the iron out so they don't get clogged.
"This is a passive system; there is no electricity or telecommunications out here," adds Irey. "Pipe alignment, grades and slopes are critical since everything flows by gravity as the system is on a down slope toward the creek. That is where the [GNSS] was invaluable. And Boyd Instruments has been invaluable in helping us set the system up. No two of our jobs are the same. They've come to most of our sites to help set up the system and provide the best coverage."
Berner Construction excavated a 100-foot-long channel between the first two sediment ponds. Effluent fills the first pond and overflows a bank on one end, filling up the limestone-lined channel. The second pond is effectively divided into thirds by two limestone baffles constructed on two concrete weirs. As the pond fills up, effluent flows over the baffles and is treated for iron hydroxide contamination and acidity by the pond's limestone a second time.
With most of the iron hydroxide removed and the pH raised after the second stage, the effluent flows into a third pond containing a 1-foot layer of mushroom compost sitting on another 4-foot layer of limestone. This pond provides the last intensive treatment of the effluent, as the compost removes oxygen and the limestone further raises the effluent's pH.
"The thing we see with using the [GNSS] is that if you had a third-party survey company come in, unless it's the same guys coming out all the time, you've got to bring those guys up to speed and let them know what's going on, especially in terms of safety," Irey says. "That's a big issue for us, with all the heavy equipment and potential slip and trip falls. With [GNSS], you fire it up at 7 o'clock in the morning and it's running until 4:30, 5 o'clock until you shut it down, and we're getting the same continuity all the time."
Irey adds that the HiPer Lite + unit improves productivity in other ways. "Invariably, you get a conflict on a drawing. I'm sure there are lots and lots of projects out there where there's a conflict on a drawing and that section stops until that conflict is resolved." He relates a story of a design change that was needed on the system, which he handled in minutes by reading location coordinates to Lopes over the phone and getting approval. "There's a big net savings to the owner," Irey says. "We resolve the conflict and keep the job moving."
Using the HiPer Lite + unit significantly reduces overall site preparation and grading costs for the contractor, and ultimately, the owner, Irey points out. "On most of our projects, we don't carry surveying costs," he says. "That's a cost; not to take anything away from the surveyors, but if another company that we bid against has to carry survey costs, well, that's an asset to us. I called [Lopes] before the project started and I asked him if the as-builts were available and he said the CAD drawings were available and we uploaded them. I don't know what a surveyor would cost to come out and lay this out. To come out and do a layout, they might say, it's $31,000 or $42,000 and every time they come out they're maybe going to charge you a half-day rate for two guys. Those are costs that we don't carry."
The use of GNSS can also reduce maintenance costs later, Irey adds. He notes that if future maintenance on the pipe networks is necessary, it would be possible to use GNSS to pinpoint the location of every 90- or 45-degree bend, for example, even after the project is completed. "The valves you can see, but the bends you can't see," Irey says. "There are some 45s in there, and you can't see them. They might be doing a cleanout and hit some resistance. Dave can say, 'I know I've got a 45 and it's right here.' He doesn't have to dig up the whole thing."
It's just days before the new drainage system will go online and Lowe walks throughout the site, visualizing how the ponds will treat the acid mine runoff. Beyond the system boundaries, he points out the end of a 12-inch pipe sticking out of the ground a few hundred yards directly south of the manhole. The pipe has been installed in the ground temporarily so that the runoff bypasses the existing system while reconstruction is underway. The resulting outflow has formed a stream that flows downhill to a wetland buffer area that lies to the south of the drainage system. A bright orange tint to the water reveals the extent to which the mine runoff is contaminated with iron hydroxide and thus highly acidic.
Then Lowe points out a final sediment pond, the next treatment stage after the compost pond. The effluent flows across a final limestone baffle in the compost pond, and the final sediment pond is where more iron hydroxide settles onto a muddy surface, which also has an orange tint from more iron hydroxide settling. "There's compost on top and that's to take out some more of the oxygen, and it goes down to limestone again and the pH raises even more and the water flows into that final pond. The iron should mostly should drop out there," Lowe says.
Finally, the effluent flows out of the final sediment pond via a 12-inch PVC pipe onto a final apron of limestone before flowing onto a 200-foot-long wetland buffer that further treats the effluent before it makes its way into Indian Creek. Standing near the wetland buffer outflow pipe, Lowe notes, "If there's any residual iron, it's going to come out onto the natural wetland buffer before it gets into Indian Creek. Any remaining iron in there should be very little; theoretically it should be zero."
The target pH of the treated effluent is 7.0. Lab tests of effluent treated by the existing system ranged from 7.1 to 8.0, versus lab readings of untreated water that ranged between 3.8 and 5.1 from 1997—2000.
Lowe is very confident that this new system will keep the iron and acidity out of Indian Creek and the Youghiogheny River. Having worked with the National Mine Land Reclamation Center at West Virginia University's West Virginia Water Research Institute to design numerous similar systems during the last decade of his Army Corps career, Lowe is impressed with the new grading technology that Berner Construction is using. "All of the elevations are critical," he says. "Because it's all gravity flow, the elevation is critical — that's where the Topcon unit really comes in handy."