INVESTIGATION OF AN UNCONSOLIDATED PLEISTOCENE LIMESTONE UNIT FOR USE IN SUBSURFACE FLOW CONSTRUCTED WETLANDS IN THE YUCATAN

One critical area of need for sewage treatment is in the Yucatan peninsula where rural communities and thriving tourist areas alike have sanitation problems due to the lack of effective sewage treatment systems. Subsurface flow constructed wetlands (SFCWs) are promising systems used for sewage treatment in the developing world where traditional wastewater treatment is either too expensive or not technically accessible. One example of implementation of these systems in the Yucatan is in Akumal, Quintana Roo where SFCWs have been in use since the mid 1990’s. Although these systems have been effective in many measures historically in reducing pollution, there is much room for improvement in regard to the geotechnical and mineralogical design of these wetlands. One specific problem with these systems is that the limestone aggregate used is often too coarse and flow through systems is too fast. In addition, these systems commonly use aragonite-rich aggregate which is extremely soluble. Accordingly, new sources of aggregate are being sought for systems in Akumal and regions to the south where tourism-related development is occurring.

Eleven partially unconsolidated Pleistocene limestone samples were taken from a quarry just outside of X-Hazil, Mexico. X-ray diffraction indicates that the mineralogy of samples is very uniform and is composed of calcite with lesser amounts of quartz. Scanning electron microscopy shows that quartz occurs in the form of chert and also minor impurities of aluminum oxyhydroxides occur. No minerals of environmental concern were identified. Coefficients of permeability (K values) for most samples vary from 0.0025 cm/s to 0.0393 cm/s, with an average value being 0.0093 cm/s. One sample had an anomalously low value of 0.0001 cm/s. Grain size analysis indicates that all samples have multimodal distributions. Modes vary from 8.67 to 28.93 mm with an average mode of approximately 14.36 mm. Coefficients of uniformity (Cu) vary from 3 to 5.35 with an average of approximately 3.53. Particle size is much smaller and K values are lower than existing aggregate in SFCWs. Nine out of the eleven samples had a Cu value, of <4.0, a property deemed essential to prevent clogging of pores. This new locality shows promise as an alternative aggregate source for future implementation of SFCWs.