United States Department of Transportation
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Runoff Water Quality From Highway Cut Slopes in Pyritic Rock Formations: Characterization and Potential Treatment
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2018-06-30
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Edition:Final Report June 1, 2013-May 31, 2018
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Abstract:Road construction through pyritic (sulfidic) rock formations has the potential to cause episodic runoff acidification and impairment to aquatic biota when acid producing materials (APM) are exposed to precipitation. State highways departments (DOTs) across eastern United States in the Appalachia region have recognized the need to handle APM during road construction projects, including Pennsylvania, Virginia, and Tennessee. These state DOTs have developed testing, material handling, and disposal guidance. Although much is known about acid material drainage (AMD) chemistry, treatment options, and potential harmful impacts from it, little is known about the water quality generated from post-construction road cuts through APM, and what hydrogeological conditions may produce harmful levels of acidic waters containing sulfates and dissolved metals. The project objectives were to: 1) characterize the water quality from APM highway cut slopes to assess whether long-term environmental issues occur to receiving streams; 2) assess the potential effectiveness of onsite treatment applications at road cuts through a physical model experiment; and 3) by a review of AMD treatment literature assess the possibility of using passive treatment solutions for the level of acid pollution generated from road cuts in Tennessee. Characterization of runoff water quality from highway cut slopes through pyrite geology was conducted at ten monitoring stations in Middle and East Tennessee. The ten stations were located among five different surficial geologic formations; they were the Chattanooga Shale, Fentress Formation, Sandsuck Formation, Great Smoky Group (Anakeesta Formation), and the Snowbird Group: Roaring Fork Sandstone. The influence of various environmental factors was investigated on their effect on runoff water quality, which included: rock formation type and age of cut, cut slope and aspect, vegetation, and rainfall magnitude and intensity. Runoff pH and other chemical parameters varied widely between sites and within sites among different runoff events. Site mean pH ranged from 4.29 to 6.27, and the full sample event range was between 2.97 and 7.70. Site means for acid neutralizing capacity (ANC) ranged from -0.82 to 29.85 meq/L, and did not correlated per site with mean pH values. The fact that pH and ANC did not correlate suggests complex and unique biogeochemical processes are occurring among the study sites. Availability of base cations from the pyritic formation or an adjacent over- or under-burden sedimentary formation that contribute to the runoff greatly controls the ANC. Site means for sum of base cations (Ca2+, Mg2+, Na+ , K+ ) ranged from 1.15 to 40.24 meq/L. Decreased pH can result from the displacement of base cations from the rock with Al complexes, where the base cations act as counter ions to electrochemically balance leached sulfate and other acid anions. Overall, rock formation type was the dominant controlling factor for water chemistry of road cut runoff. The findings suggested that there are minimal effects from seasons, storm event magnitude, and road cut characteristics of slope and aspect. Also, it suggests the older the road cut the likelihood of acidic runoff is small and that planting vegetative cover at runoffs can be used as an effective management strategy to minimize acidic runoff. Useful information was obtained from an outdoor experimental study which consisted of constructing four container panels to place pyrite rock, and for three of the four panels install treatments to limit or prevent exposure to the rock, and subjected to both simulated and natural rainfall. The treatment types were: 1) soil/vegetation cover, 2) shotcrete, and 3) a geosynthetic membrane liner. One panel with exposed pyrite rock (no treatment) served as the experimental design control, and provided unique data on runoff chemistry and ion export from the bare rock surface. For the simulated rainfall experiment, pH increased from approximately 3.6 to above 6.0 within four hours. However, rapid oxidation occurred where the next day the initial pH was near 3.6 however reached pH values above 6.0 within two hours. Runoff ANC from the pyrite rock remained low during the simulations due to the lack of base cations to neutralize the acid anions. Mineral ions from the rock surface are quickly washed off as observed by the rapid decline in conductivity within the first hour of rainfall and basically diminished of ion source within two hours of rainfall. Shotcrete and a geosynthetic membrane liner were found to be effective to protect runoff water quality. The treatment consisting of soil/vegetation was more complex, and runoff quality needs further investigation. From the experiments, several roadside treatment options are available; they include: limestone beds and oxic/open channels, aerobic wetlands, settling ponds, bio-reactors, and pebble quicklime. Design of these treatment options are described in the guidance document “Guidelines for Acid Producing Rock Investigation, Testing, Monitoring and Mitigation” by Golder Associates.
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