Mangos shale‐associated alluvium

Abstract

AbstractInquiry into the dissolution kinetics of naturally occurring geologic materials, rather than individual mineral species, has been relatively neglected. This is especially true of surface processes, the realm of surface water hydrology and geomorphology. This paper focuses attention at a laboratory study of the rate of such complex reactions.Functions defining the dissolution rates of saline (0.1–20 per cent salt content) Mancos Shale‐associated alluvium in distilled water follow varying patterns. Dissolution is characterized by an initial (<5 min) high rate constant, by a following phase (20 min‐57 h) where rates are reduced drastically, and by a final period of encroachment to equilibrium. Initial dissolution rates increase with increase in salt content and sediment: water ratio. The time necessary to approach equilibrium is, however, found to be directly proportional to the sediment; water ratio. The concentration of Na+, Mg2+, Ca2+, SC2−4 and HCO−3, and the relative abundance of Ca2+ and HCO−3 increase continuously with contact time, indicating that the hydrated sodium and magnesium sulphate minerals provide most of the initial solutes, though not necessarily most of the total solute bulk.The results obtained in this study indicate that the high initial dissolution rate of soluble minerals from alluvium, and particularly from shales in contact with aqueous solutions, is too short‐lived to account for most of the solutes occurring in heavily sediment‐laden surface flow. Excluding input from slower, usually supersaturated subsurface flow, dissolution from sediment in transport should be a major source of solutes in originally undersaturated and kinetically unequilibrated surface water in semiarid and arid regions.