Bacteria Water Quality Data Analysis and Interpretation: Glen Canyon National Recreation Area
Technical Report NPS/NRWRD/NRTR-95/46
Glen Canyon National Recreation Area (GLCA) has been monitoring bacteria levels in shoreline waters of Lake Powell to assess impacts from recreational bathing and boating since 1988. The Water Resources Division (WRD) contributed to the development of the GLCA bacteria monitoring program via recommendations made in a report titled "Water quality alternatives for the Glen Canyon Natnional Recreation Area Water Resources Management Plan" (Flora and Wood 1986) and the park's "Water Resources Management Plan" (Wood and Kimball 1987). Recent data have shown an increase in bacteria levels at some beaches during the current drought. It is speculated that this trend may have resulted from less bacteria dilution at lower lake levels and/or changing recreational use patterns. GLCA posted advisory notices at several beaches during the last couple of years due to the exceedences of the Utah primary-contact recreation (swimming), and Arizona full body-contact, water quality standard for fecal coliform bacteria of 200 colony forming units per 100 milliliters (cfu/100mL). In 1991, high bacteria counts resulted in the closure of five beaches: Hansen Creek, Stanton Creek, Hobi Cat Beach, Moqui Canyon, and Farley Canyon. Eight canyons were posted with no swimming signs in 1992: Moqui Canyon, Farley Canyon, Government Housing, Llewellyn Gulch, Oak Canyon, Upper Bullfrog Bay, Hite Marina, and Forgotten Canyon. Subsequently, Llewellyn Gulch was posted as closed due to bacterial contamination. Llewellyn Gulch remained closed to public access until May, 1994. In 1993 and 1994, coliform counts were low and no new advisories or beach closures occurred. Water levels in Lake Powell rose over 50 feet in response to spring runoff during 1993. In response to these data and increasing concerns regarding interpretation of the results by park management, staff, and the public, the park requested that WRD analyze and interpret the bacteria data collected between 1988 and 1993, and help them develop a way to measure acute levels of bacterial contamination in time to warn recreational users. The purpose of this paper is twofold: (1) to present a clear and concise explanation of what we know, and (2) to recommend changes in field, laboratory, and data management procedures which result in the most effective and efficient use of this information in making management decisions.