Daniels and Orndorff, 2003 ICARD Acid Rock Drainage

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W L DANIELS and Z W ORNDORFF Upon exposure, sulfide oxidation causes pH to decrease rapidly. Weathered materials at the surface of roadcuts through the Chesapeake Group and Lower Tertiary deposits typically appear yellowish-brown, have pH values between 2.5 - 3.5, have PPA values between 10 - 20 Mg CaCO 3/1000 Mg material, and retain less than one per cent S. Less oxidised, underlying grey sediments have slightly higher pH values and much higher PPA values, ranging from about 30 - 50 Mg CaCO 3/1000 Mg material. Acid sulfate weathering problems were less severe at a few sites that are surficially mapped as the Sedgefield member of the Pleistocene aged Tabb formation. The Sedgefield member consists of fossiliferous brackish-bay sand, beach and near-shore marine clayey sand, and lagoonal and marsh clay and clayey sand. In our experience, materials with PPA values below 10 Mg CaCO 3/1000 Mg are readily reclaimed with proper management, while materials with PPA values between 10 - 60 require intense reclamation management (Daniels, Li and Stewart, 2000). Considering these guidelines, and the widespread occurrence of S through the Chesapeake Group and Lower Tertiary deposits, exposure of Tertiary marine sediments may be considered highly likely to produce problematic roadside management conditions which require intense reclamation efforts. Exposure of the Sedgefield member of the Tabb formation may be considered likely to produce moderately problematic roadside vegetation management conditions, which could require special reclamation efforts. Piedmont Province Acid roadcuts in Stafford County in Northern Virginia occur in pyritic phyllite and slate of the Quantico Formation. Reflected light microscopy of polished sections revealed the presence of pyrite as corroded subhedral and euhedral grains, along with chalcopyrite and covellite. Microcrystalline forms, such as those described for Coastal Plain sediments, were not observed. The PPA values for surface samples ranged from 6 - 22 Mg CaCO 3/ 1000 Mg material, and S values ranged from 0.24 - 1.00 per cent. One sample from relatively unweathered underlying material had a significantly higher PPA value, 99 Mg CaCO 3/1000 Mg material, and contained over 3.8 per cent S. Previous analysis of six phyllite samples collected by VDOT in Stafford revealed PPA values ranging from 1 - 85 Mg CaCO 3/1000 Mg material. These values indicate sulfides are unevenly distributed throughout the roadcut; however, more detailed sampling would be necessary to characterise this spatial variability. Compared to sulfidic sediments of the Coastal Plain, sulfide levels in the Quantico Formation appear to be more variable and occur over a much larger range of S values. With one exception, drainage from this site had lower EC, and higher acidity and metal concentrations, than any other evaluated roadcut. Exposure of the Quantico Formation may be considered highly likely to produce severely problematic roadside management conditions, which require intense reclamation efforts. Roadcut surfaces of the Quantico Formation may be quite steep and generally consist of shallow, rocky, weathered material over bedrock, and rock outcrops, which are less suited for standard soil remediation methods than the unconsolidated sediments of the Coastal Plain. Similar detailed information on acid-forming materials and associated soil, water quality and engineered materials affects in the other regions of Virginia can be found in Orndorff (2001) and Orndorff and Daniels (2002). These reports also contain a full list of all cited studies of sulfidic geologic and mine waste materials in all five geologic provinces of Virginia. Final compilation of a statewide sulfide hazard rating map The impact of acid drainage resulting from the exposure of sulfidic materials during road construction depends on many variables, including the relative volume of ARD moving to surface stream flow, the flow rate of local surface waters, and the neutralising capacity of surrounding geologic materials. Although materials may be rated based on characteristics related to S content, PPA, and rock drainage quality, the true risk of environmental impact will depend on site-specific conditions. With this in mind, the following scheme was developed to assess geologic materials with general ratings in terms of sulfide hazard. Materials were placed into four risk screening classes based on PPA and total-S values: 1. materials for which 90 per cent of samples tested less than 10 Mg CaCO3/1000 Mg material and contained less than 0.5 per cent S; 2. materials for which 90 per cent of samples tested less than 10 Mg CaCO3/1000 Mg material and more than ten per cent of the samples tested greater than 0.5 per cent S; 3. materials for which more than ten per cent of samples tested greater than 10 Mg CaCO3/1000 Mg material and less than ten per cent of samples tested greater than 60 Mg CaCO3/1000 Mg material; and 4. materials for which more than ten per cent of the samples tested greater than 60 Mg CaCO3/1000 Mg material. These class boundaries were determined with consideration of standard remediation methods and the observed properties of a wide range of sulfidic materials. Application of these ratings to the range of geologic materials evaluated in this study is shown in Table 2. Again it should be emphasised that these ratings are based strictly on the acid-producing potential of a particular material, whereas actual acid production and severity of impact will depend on site conditions. The overall distribution of all scientifically documented acid-producing strata in the eastern portion of Virginia is presented in Figure 2, which is a detailed portion of the full statewide risk map produced by this study. TABLE 2 Sulfide hazard rating for evaluated geologic materials. Geologic map unit Sulfide hazard rating † Tabb formation – Sedgefield Member (Coastal Plain) 1 Wise, Kanawha, Norton, New River, Lee and Pocahontas Formations (Appalachian Plateau) 1 Ashe Formation of the Lynchburg Group (Blue Ridge) 2 Chesapeake Group (Coastal Plain) 3 Lower Tertiary deposits (Coastal Plain) 3 Marcellus shale and Needmore Formation (Ridge and Valley) 3 Millboro shale and Needmore Formation (Ridge and Valley) 3 Quantico Formation (Piedmont) 4 Chattanooga Shale (Ridge and Valley) 4 † 1 = least severe, 4 = most severe. 482 Cairns, QLD, 12 - 18 July 2003 6th ICARD
Stafford airport case study The Stafford regional airport site (see Figure 3) occurs directly between two of the locations that were sampled and documented in the VDOT corridor study described above. However, we were unaware of its existence until 2001 due to a lack of direct VDOT involvement in the construction and monitoring of the project. This particular location was only reported to us after multiple conventional revegetation efforts failed, and we were referred by Virginia regulatory agency personnel. To our knowledge, this is the largest single exposure of acid forming materials in the eastern USA to date, other than that which occurred in the unregulated era of Appalachian coal mining (pre-1977). Construction activities at the site between 1998 and 2001 disturbed over 150 ha of lower Tertiary Coastal Plain materials as the airport runway was constructed through a deeply dissected landscape. As construction proceeded, long spur ridges were excavated to depths ≥25 m, exposing significant volumes of grey, reduced, sulfidic (0.6 to 1.2 per cent pyritic-S) silty sediments which were subsequently filled into intervening valley fills to ACID ROCK DRAINAGE FROM HIGHWAY AND CONSTRUCTION ACTIVITIES IN VIRGINIA, USA FIG 2 - Geographic extent of sulfide-bearing geologic materials in the Coastal Plain of Virginia, USA. support the >1500 m runway. Excavated sulfidic materials exceeded the capacity of the valley fills and were also placed into several large, steeply sloping excess spoil fills along a first-order stream draining the eastern section of the site (see Figure 3). Due to the fact that the sulfidic nature of these materials was not recognised until well after all final grading was completed, the acid-forming materials were not isolated away from drainage, and in fact were essentially scattered randomly, and thoroughly, throughout the site. Soil acidity and associated site conditions Inspection of the 10 to 15 m deep cut faces left uncovered along the northern margin of the site in November 2001 revealed that the upper 5 to 8 m of the soil-geologic column was pre-oxidised by long term natural weathering processes, supported a soil pH of 4.1 to 4.5, and was well-vegetated. However, below this pre-oxidised depth, the soil pH in the weathered cut faces ranged from 3.5 to 1.8, with prominent white salt efflorescences. Fanning, Coppock and Rabenhorst (2002) also confirmed 6th ICARD Cairns, QLD, 12 - 18 July 2003 483
Page 1 and 2: Acid Rock Drainage From Highway and
Page 3: ACID ROCK DRAINAGE FROM HIGHWAY AND
Page 7 and 8: Location Date pH EC (uS/cm) SW 4 (A
Page 9: Daniels, W L and Stewart, B R, 2000

Daniels and Orndorff, 2003 ICARD Acid Rock Drainage

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