The interaction of natural organic matter with iron in a wetland (Tennessee Park, Colorado) receiving acid mine drainage

Pore water from a wetland receiving acid mine drainage was studied for its iron and natural organic matter (NOM) geochemistry on three different sampl ing dates during summer 1994. Samples were obtained using a new sampling te chnique that is based on screened pipes of varying length (several centimet ers), into which dialysis vessels can be placed and that can be screwed tog ether to allow for vertical pore-water sampling. The iron concentration inc reased with time (through the summer) and had distinct peaks in the subsurf ace. Iron was mainly in the ferrous form; however, close to the surface, si gnificant amounts of ferric iron (up to 40% of 2 mmol L-1 total iron concen tration) were observed. In all samples studied, iron was strongly associate d with NOM. Results from laboratory experiments indicate that the NOM stabi lizes the ferric iron as small iron oxide colloids (able to pass a 0.45-mu m dialysis membrane). We hypothesize that, in the pore water of the wetland , the high NOM concentrations (>100 mg C L-1) allow formation of such collo ids at the redoxcline close to the surface and at the contact zone to the a djacent oxic aquifer. Therefore, particle transport along flow paths and re sultant export of ferric iron from the wetland into ground water might be p ossible.