Acid mine drainage (AMD) is a serious problem in many watersheds where coal is mined. Passive treatments, such as wetlands and anoxic limestone drains (ALDs), have been developed, but these technologies show varying treatment efficiencies. A new passive treatment technique is a vertical flow wetland or successive alkalinity producing system (SAPS). Four SAPS in Pennsylvania were studied to determine changes in water chemistry from inflow to outflow. The Howe Bridge SAPS removed about 130 mg l 1 (40%) of the inflow acidity concentration and about 100 mg l 1 (60%) iron (Fe). The Filson 1 SAPS removed 68 mg l 1 (26%) acidity, 20 mg l 1 (83%) Fe and 6 mg l 1 (35%) aluminium (Al). The Sommerville SAPS removed 112 mg l 1 (31%) acidity, exported Fe, and removed 13 mg l 1 (30%) Al. The McKinley SAPS removed 54 mg l 1 (91%) acidity and 5 mg l 1 (90%) Fe. Acid removal rates at our four sites were 17 (HB), 52 (Filson1), 18 (Sommerville) and 11 (McKinley) g of acid per m 2 of surface wetland area per day (g/m 2 d 1 ). Calcium (Ca) concentrations in the SAPS effluents were increased between 8 and 57 mg l 1 at these sites. Equilibrators, which were inserted into compost layers to evaluate redox conditions at our sites, showed that reducing conditions were generally found at 60 cm compost depths and oxidized conditions were found at 30 cm compost depths. Deeply oxidized zones substantiated observations that channel flow was occurring through some parts of the compost. The Howe Bridge site has not declined in treatment efficiency over a six year treatment life. The SAPS construction costs were equal to about seven years of NaOH chemical treatment costs and 30 years of lime treatment costs. So, if the SAPS treatment longevity is seven years or greater and comparable effluent water quality was achieved, the SAPS construction was cost effective compared to NaOH chemical treatment. Construction recommendations for SAPS include a minimum of 50 cm of compost thickness, periodic replacement or addition of fresh compost material, and increasing the number of drainage pipes underlying the limestone.

Gypsum (CaSO 4. 2H 2 O) precipitates were observed in limestone beds at four vertical flow ponds (VFP) treating acid mine drainage, but the textures and localization show that it precipitated within other materials, not in contact with the open solution. At the Filson 1 VFP, gypsum was found within clay and organic matter left as residues on the surface of partly dissolved fragments of impure limestone. It was also found in the contact zones between fragments. At the Tangascootack 1 VFP, gypsum coated limestone and was overlain by amorphous Al precipitate. At the Jennings VFP, gypsum was present in limestone-bearing compost and within Al-rich gel occupying pores in limestone gravel. At Middle Branch, Kettle Creek, gypsum occurred in Al-rich gel and on limestone. At all sites, the effluent water was undersaturated with gypsum. Gypsum at these sites is interpreted to have formed in situations where limestone dissolution was characterized by diffusive transport of Ca away from the limestone surface. In this situation, the Ca concentration near the limestone surface is considerably higher than in the open solution, so that gypsum was oversaturated and precipitates, even though the open solution was undersaturated. The occurrence of gypsum as coatings or as a component of an impurity layer probably slows the dissolution of limestone, and makes the VFP less effective in neutralizing acidity.