Utilizing tropical tree rings to trace the spatial and temporal distribution of mercury levels in Cebu, Philippines

Tree rings are used for biomonitoring environmental conditions, providing insight into chemical constituents present during a tree's growth. Here, we report the utilization of tree rings and aerial parts (i.e., leaves, shoots, and fruits) of Muntingia calabura Linn. (locally known as mansanitas or aratiles), as well as soil near the trees, to monitor spatial distribution of the presence of mercury (Hg) in 18 cities and municipalities of Cebu, Philippines. While uncommon, tropical tree rings from core samples also provide insight into the chronology of mercury uptake in M. calabura trees between 2003 and 2016. Samples from a total of 65 sites are collected and analyzed for Hg using a Milestone Direct Mercury Analyzer DMA-80 following US EPA Method 7473 and validated using standard reference materials, i.e., NIST SRM 1515 (apple leaves), NIST SRM 1575a (pine needles), and NIST SRM 2687 (soil). Soil parameters are analyzed using standard US EPA methodologies. Mercury concentrations are found to range from 1.52-20.4 parts per billion (ppb, tree cores), 8.75-105 ppb (leaves), 3.82-166 ppb (shoots), 1.40-8.45 ppb (fruits), and 6.29-247 ppb (soil). Spatial and temporal distribution maps of Hg concentrations in M. calabura generated from the collected data, while not conclusive in terms of source attribution, indicate aerial transport and clearly show significant increases in Hg concentrations near heavily industrialized areas or landfills, as well as a general increase in Hg levels from south to north. Analysis of the Hg concentrations in tree cores shows a decreasing trend from 2003 to 2016, suggesting that Hg levels in the environment have been steadily declining, although lateral translocation of the metal from bark to pith remains a factor that cannot be dismissed. This study demonstrates the viability of utilizing aerial parts and tree rings of M. calabura to detect patterns of spatial and temporal distribution of mercury and potentially other metals. Local governments can take advantage of this knowledge to help formulate policies regarding the control of gaseous emissions and disposal of effluents and sludge from industrial facilities, power plants, and sanitary landfills.