The ancient Maya site of La Trinidad lies on the shores of lake Peten Itza in northern Guatemala. The central location of the site and the presence of an ancient boat dock suggest that marketplace activities may have occurred at the site. We sampled several courtyards and the boat dock to determine levels of phosphate and heavy metals that may represent chemical residues of ancient human activities. Higher phosphate levels in comparison to surrounding areas have been used to indicate areas of food preparation and consumption. Elevated concentrations of heavy metals often indicated ritual or workshop activities.