Daniels and Orndorff, 2003 ICARD Acid Rock Drainage

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W L DANIELS and Z W ORNDORFF occurrence of active acid-sulfate soil conditions on site. The slopes were barren of vegetation and prominent acid rock drainage was present. Concrete lined drainage ditches and culverts were coated in iron and significant etching and degradation of the cement components were noted. Galvanised steel standpipes in water control structures in stormwater basins below the site had also been completely degraded by the drainage over time, releasing large volumes of sulfidic sediments into the receiving floodplain. As discussed below, the acid drainage from this site had seriously degraded surface water both on- and off-site. In December 2001, the existing surface soils were composite sampled from 42 different locations across the site in association with soil mapping requirements for remedial treatment. These samples were analysed for soil:water pH and PPA as described earlier. Soil pH ranged from a low of 1.80 to high of 5.28 with an average of pH 3.05. Potential acidities ranged from -0.6 to -41.8 Mg calcium carbonate equivalence (CCE) per thousand Mg material, with an average presumed lime requirement of 9.6 parts per thousand or 21.5 Mg CCE per ha to an incorporation depth of 15 cm. It was clear from site mapping and detailed field inspection that a few fill cells and surfaces at the airport site received only minimal inputs of sulfidic materials. These areas generally supported vegetation, were reddish to yellowish brown in soil colour, and had soil pH values >3.8. However, the vast majority of the site was completely barren of vegetation, was characterised by grey (low chroma) soil colours, and soil pH
Location Date pH EC (uS/cm) SW 4 (Above site) SW 1 (In site) SW 6 (Below site) Due to the naturally acidic nature of the soils within this watershed, background surface water pH was typically less than 5.5, with moderate levels of dissolved Fe (Table 3). In general, water quality discharging from the airport and from the NRCS impoundment in early-2002 was highly acidified (pH 3.3 to 3.5) and high in dissolved Fe, Mn, Al and S. Based upon comparison with data from SW 7 and other control locations (data not shown), there is no doubt that the airport construction had significant negative water quality effects on Potomac Creek, and upon an undetermined reach of the stream below the dam discharge point due to the acidity and metals released over time. Water samples taken subsequent to application of lime-stabilised biosolids indicated that the pH of water discharging from the on-site stormwater detention basins (eg SW 6) and the NRCS floodwater structure sequentially increased into the 6’s and low 7’s, but then declined again somewhat by November, 2002. Dissolved Fe in discharge waters ranged from 10 to 40 mg/L, and ranged from 4 to 9 mg/L in early-2002 samples from the NRCS Dam. By early June, however, the pH at the NRCS Dam was 7.3 with much lowered levels of metals. Sulfur levels remained elevated, however, presumably due to the long-term release of sulfate accumulated from the pyrite weathering reactions associated with the site. However, our past experience in coalfield acid mine drainage dynamics (Daniels, Li and Stewart, 2000) has indicated that seasonal (fall/winter) flushes of acid reaction products from acid forming materials are possible. Therefore, we cannot reach any firm conclusions regarding the long-term effects of the lime-stabilised biosolids on site run-off acidity and metal levels at this time. Nitrate-N in all internal and discharge surface water samples was low through August of 2002, ranging from
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Page 3 and 4: ACID ROCK DRAINAGE FROM HIGHWAY AND
Page 5: Stafford airport case study The Sta
Page 9: Daniels, W L and Stewart, B R, 2000

Daniels and Orndorff, 2003 ICARD Acid Rock Drainage

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