INVESTIGATIONS OF GEOLOGIC MATERIALS IN CONSTRUCTED WETLANDS AKUMAL, QUINTANA ROO, MEXICO

Constructed wetlands are an extremely important technology for treating domestic wastewater in the Yucatan Peninsula. Geologic properties of aggregate used in constructed wetlands in the developing world are often undetermined. An understanding of these parameters is critical for long term maintenance of these systems.

Mineralogy of aggregate used in subsurface flow constructed wetlands in the village of Akumal, Quintana Roo, Mexico was investigated to determine if this was a parameter that could be improved for wetland operation. Bulk mineral composition of aggregate from two subsurface flow constructed wetlands was determined to consist solely of calcite using bulk powder X-ray diffraction. Aggregate levels in wetlands drop at an estimated rate of several centimeters per year for overloaded wetlands. Aragonite is preferentially dissolved in the wetland systems. Mineral compositions from fresh aggregate samples consist of a mixture of calcite and aragonite. Coefficients of hydraulic conductivity of fresh crushed aggregate samples investigated vary from 0.002 cm/s to 0.027 cm/s. Grain size analysis indicates that two types of aggregate are present, one group has a strong mode between 2.0 and 4.0 mm and others have a wide range of multimodal distributions.

Investigations of other geologic media from the Reforma region indicate that a dolomitic limestone with minor amounts of Fe-oxide and palygorskite may be a better aggregate source that the current materials used. Dolomite has a lower solubility and Fe-oxides and palygorskite may be more reactive with organic pollutants than calcium carbonate polymorphs. Dolomitic material is comparatively porous and has uniform, very fine grained dolomite rhombs commonly 3 to 15 micrometers in diameter. A Ca-montmorillonite-rich bed was also identified in the Reforma region. The unit has no sulfides or other potentially environmentally damaging minerals. The Ca-montmorillonite-rich unit may serve as material for clay liners in new and existing wetland systems to prevent seepage leaks.

This investigation indicates the importance of mineralogy in constructed wetlands studies and illustrates that local alternative geologic media can be found to improve efficiency and environmental quality.