MINERALOGIC ASPECTS OF AGGREGATE USED IN CONSTRUCTED WETLANDS IN AKUMAL, MEXICO

Mineralogy of geologic media used for aggregate in subsurface flow constructed wetlands is often undetermined in the study and design of these systems. Constructed wetlands in Akumal, Quintana Roo, Mexico use limestone aggregate that potetnially have multiple carbonate, silicate, and oxide minerals present. Bulk mineral composition of aggregate from two subsurface flow constructed wetlands was determined to consist solely of calcite using bulk powder X-ray diffraction. Mineral composition from fresh aggregate control samples and local quarry aggregate source rock commonly is a mixture of calcite and aragonite. The occurrence of aragonite in control samples and local sources and the lack of aragonite in wetland aggregate, in combination with macroscale evidence of dissolution in wetland aggregate indicates that aragonite is preferentially dissolved in these systems. Limestone aggregate dissolution however neutralizes acids that would otherwise be introduced into the sensitive aquifer. Aggregate levels in wetlands drop at an estimated rate between 3 and 10 cm per year for overloaded wetlands. Some wetlands will be effectively depleted of their aggregate in 6 to 10 years and this indicates that the effective lifetime of subsurface flow constructed wetlands is on the order of 8 to 10 years before aggregate needs replacement. Although PET bottle derived aggregate may be an alternative to the current limestone aggregate there are potential problems in that it will not neutralize acids in wastewater and may react with alcohol and cleaning products and produce unwanted chemicals in the final effluent. Replacing aggregate with mixtures of PET and limestone, or dolomite may be an effective solution.