HEAVY METAL CONTAMINATION IN SEDIMENTS AND LAND USE/LAND COVER CHANGE IN THE GUÁNICA WATERSHED, PUERTO RICO FROM 1936-2016

Rivers transport sediments and their associated pollutants (i.e. heavy metals) from nearby land to the coast, causing stress to vulnerable coastal ecosystems, such as coral reefs. Yet, the inability to trace sediment contamination to non-point sources within a watershed, can hinder the initiation of meaningful conservation measures. In Southwest Puerto Rico, the coral reefs of Guánica Bay have been designated a “management priority area” by the National Oceanic and Atmospheric Association, due to the sediment runoff from the adjacent Rio Loco watershed. To date, limited research has focused on fingerprinting the source of soil contamination in relation to historical land use/land cover (LULC) changes and environmental threshold values for aquatic ecosystems. This study examines the changes in this region that occurred over 80 years. Additionally, this research addresses these knowledge gaps by comparing historical LULC changes within the Rio Loco watershed with downcore metal concentrations in two sediment cores from the lower watershed. Historical aerial and topographic photos from 1930 to 2016 were analyzed with ArcGIS to determine changes in National Land Cover Database categories to develop a historical timeline of key watershed events. Exchangeable and acid soluble metal concentrations were determined downcore and individual layers were age-dated using Pb-210. Preliminary LULC analysis shows relative changes in LULC over time, with an 81% increase in high/medium density development between 1930 to 2016. Key watershed events, like the construction of the Southwest Water Project and draining of the Lajas Lagoon in the 1950s, likely caused more contaminants to enter the local waterways. Several downcore metal concentrations (e.g. Cr, Ni, and Pb) were in excess of applicable guidelines for impacts to freshwater aquatic ecosystems. Finally, core layers at a depth of 0-8 cm spanned ~100 years, suggesting minimal sediment deposition within the lower watershed. The results suggest an increase in sediment erosion within the watershed over time, likely resulting in increased metal contamination reaching the coral reefs of Guánica Bay from terrestrial sediment. Study findings can be used by watershed managers to enact targeted measures for reducing sediment generation within the Rio Loco watershed.