Limestone Cuts Runoff Pollution

by Robert Whittaker, Watershed Specialist, Huntingdon Conservation District

If you would ask a resident of the Broad Top (an area on the border of Huntingdon and Bedford counties) where you could find a good trout stream in the area, they would probably say, “Not here!”

Little do they know, located within the heart of this old mine land, a native Brook Trout population is on the rebound.

Miller Run a tributary of Shoup’s Run, has undergone an “environmental makeover” over the last six years. With the cooperation of the Department of Environmental Protection, the U.S. Environmental Protection Agency, Game Commission, and the Huntingdon County Conservation District, the Shoup’s Run Watershed Association is seeing positive results from years of planning and construction within the watershed.

Over the last six years Miller Run has seen quite a bit of change: two limestone sand dosing areas, an acid mine land project complete with a large storm water retention basin, and one limestone bed with one more to be completed this summer. All of these practices are meant to treat the acid mine drainage occurring in that watershed.

However, the watershed group was looking to do more. That was when we started looking at the road surfaces. Continuing for over one mile alongside Miller Run is a state game lands access road which historically was built from and maintained with mine spoil. With every rain event, a little bit of that mine spoil and its acidic runoff would make its way into the stream, depressing the pH levels and making it harder for the trout to survive.

Finding a way to treat that runoff became the focus.

With the cooperation of the Penn State Dirt and Gravel Roads Program we had a plan developed to not only address runoff, but also address erosional problems along this 7,000 feet of road surface. The plan consisted of two phases.

The first phase addressed unstable road ditches and acidic seeps along this stretch of road. The second phase consisted of placing 2,600 tons of high calcium carbonate limestone on the road surface that years ago had been previously constructed of mine spoil

During the summer months, Miller Run has very little water so the year-round seeps, in spite of their acidity, are vital to the resident trout. Previous analyses have shown that the water from these acidic seeps fortunately is very low in metals. By simply providing high quality limestone and sufficient contact time in the roadside ditches, we’ve been able to treat the acidity before that water enters the stream.

Chemical analysis of the seep water before treatment shows zero alkalinity and very low pH. Water samples of the treated water taken at the end of the limestone ditches reveal that the system is indeed working, with recent data showing a remarkable alkalinity of 40 mg/L and a neutral pH of 7.0.

The ditches weren’t the only target. We also focused on treating the acidity at the outlet of the culvert pipes. In some cases there was just not enough space to overexcavate the ditches to fill them with limestone. In those cases, we designed infiltration boxes to be placed at the outlet of the culverts.

The infiltration box is essentially an excavated area containing two sizes of limestone. The larger limestone size provides erosion protection, while the smaller size limestone provides alkalinity to neutralize the acid water coming from the seeps. These structures are designed to allow water coming out of the culvert during low flows, to be stored in the infiltration box. Once the holding capacity is met, the water spills out of the end of the box and to the creek.

Although during high flow the water may fill the box and escape treatment but the box will still provide erosion protection benefits at the outlet of the culvert. The boxes are indeed functioning, producing the much-needed alkalinity or Miller Run while providing erosion control as the water from the ditch outlets to the stream.

This project was jointly funded through EPA/DEP 319 program and through the Penn State Dirt and Gravel Road Program.

(Reprinted from Abandoned Mine Posts, Western PA Coalition for Abandoned Mine Reclamation.)

9/14/2007