THE EFFECTS OF NATURAL ACID ROCK DRAINAGE ON FOREST STANDS IN HANDCART GULCH, COLORADO

Acid rock drainage (ARD) is the formation and outflow of extremely acidic water that occurs upon geochemical reactions between oxygenated water and sulfide minerals. It is often initiated or exacerbated by mining, but ARD also happens naturally when sulfide-rich rocks are exposed and weathered. ARD threatens water resources by contaminating rivers with acidic water and toxic metals, disrupting growth and reproduction of aquatic plants and animals, and corroding infrastructure. Handcart Gulch, located in the Pike-San Isabel Forest, is an alpine glacial valley along the continental divide within the South Platte river watershed, which feeds Denver’s water supply. In Handcart Gulch, natural acid rock drainage has drastically affected the landscape; acidic streams and iron mineral precipitation are widespread. The introduction of metals and acidity to alpine forest systems is a crucial yet understudied piece of the larger picture of ARD impact. Past studies in Handcart Gulch aimed to record spatial variations in surface and groundwater chemistry. However, it is currently unknown what effects natural ARD has on soil quality and local forests. To assess the extent of ARD and its potential impacts on forest ecosystems, we compared stream water quality at several sites to soil quality and tree health in the immediate area. We measured levels of total iron, sulfate, phosphate and the pH, electrical conductivity (EC), and oxidation-reduction potential (ORP) of stream and soil water to gauge the extent of ARD contribution. Furthermore, we gathered tree measurements and health data such as diameter at breast height (DBH), dead fraction, trees per acre (TPA) and height. We predict that soil iron loading and acidity will strongly affect the health and species composition of tree stands. Understanding the effects of ARD on local aquatic and forest ecosystems is crucial to assessing watershed health and developing targeted remediation efforts.