INVESTIGATION OF THIRTY SUBSURFACE FLOW CONSTRUCTED WETLANDS IN AKUMAL, MEXICO

Wastewater management is major concern in the Yucatan Peninsula, as sewage pollution has been linked to a rise in human health concerns as well as the decline of the Meso-American coral reef. Subsurface flow constructed wetlands (SFCWs) are engineered ecosystems which involve the use of hydrological, biological and mineralogical processes to reduce wastewater volume and decrease or ideally eliminate the polluting components. Geotechnical properties, plant cover and water quality characteristics of 30 SFCWs in Akumal, Quintana Roo were investigated to determine efficacy of the systems and assess causes for variability of performance.

The number of cells, shape and size varied mostly according to the space available for construction. Of the 35 plant species found in these wetlands, Alocasia spp., Cyperus spp. and Acrostichum danaeifolium were common. Reduction in ammonia and phosphate varied widely from 96 and 92% reductions to 350 and 250% gains respectively. The coefficient of permeability (k) for aggregate varied from 0.019 cm/s to 0.06 cm/s with an average of 0.041 cm/s. The average porosity was 43.89% and the minimum and maximum porosity was found to be 26% and 52% respectively.

The limestone aggregate is angular to subangular with an average uniformity coefficient of 1.52 and average aspect ratio of 2.3 indicating elongated aggregate particles. X-ray diffraction of powdered aggregate from the wetlands indicated four types i.e. (1) calcite (2) calcite and aragonite (3) calcite, quartz and boehmite (4) calcite, quartz, boehmite and aragonite. SEM analysis of samples indicates that the aggregate was highly porous with distinct dissolution-reprecipitation textures. Rare occurrences of Cassiterite (SnO₂) were found in few samples however no other heavy metal bearing minerals were observed. The bio-films on the aggregate have a distinct chemical composition with high phosphate content.

Many of the systems in Akumal seem to be undersized, overloaded and poorly maintained and planted with vegetation that might not be optimal for expected results. Aggregate dissolution might pose a risk of releasing pollutants into the environment. This study identifies several problems with these SFCWs and indicates a more efficient design and maintenance regime is needed.