GEOCHEMICAL PROCESSES AFFECTING STORAGE OF MAJOR AND TRACE ELEMENTS IN IRON BOG AND WETLAND SEDIMENTS, UPPER ANIMAS RIVER WATERSHED, COLORADO

Mineralized areas that have been extensively mined are found throughout the Rocky Mountain region. In many of these areas, depositional environments such as organic-rich wetlands and iron-rich wetlands (iron bogs) are subject to inputs of metal-bearing acid-rock or acid-mine drainage. The fate of dissolved major (Al, Fe, Ca) and trace (As, Cu, Pb, Zn) elements mobilized via acid drainage is controlled by several geochemical processes that affect the concentration of an element that is partitioned into dissolved or solid phases. Chemical constituents that become stored in local wetland or iron bog sediments depend upon host rock mineralogy and alteration, rock (including mine wastes) weathering, surface and ground water chemistry, biologically-mediated reactions, and depositional conditions.

In the upper Animas River watershed of southwestern Colorado, distinct geochemical processes occurring in each depositional environment were shown to exert significant control on element mobilities and to produce differing solid-phase concentrations. Thus, concentrations of a particular element stored in wetland or iron bog sediments encompass wide ranges.

Iron bogs are dominated by ground water input with elevated dissolved element concentrations, acidic pH (<4.0), and low organic matter content (<1 wt.%). Iron-bearing mineral formation is the major mechanism for controlling solid-phase partitioning in iron bogs. The maximum Fe and Al contents in solids are 46.0 and 8.8 wt.%, respectively. Generally, iron bog sediments lack consistently-high solid-phase concentrations of most trace elements even though inflow concentrations are high. However, As was an exception in one iron bog, having a maximum solid-phase concentration of 5000 ppm. This high-As iron bog formed downslope from an acid-sulfate mineral deposit.

In contrast, wetlands have ground and surface water inputs with generally lower dissolved element concentrations, higher pH (5-7), and abundant organic matter (10-90 wt.%). Formation of surficial metal-bearing iron oxyhydroxides, sorption onto organic matter or detrital minerals, and incorporation into diagenetic sulfide minerals are major controls on element concentrations in organic-rich sediments. The Fe and Al maxima in wetland solids are 9.8 and 12.0 wt.%, respectively.