Examining an Urban Water Source: Groundwater Characteristics of San Francisco's Lobos Creek

Abstract

Lobos Creek is the potable water supply for people living and working in the Presidio of San Francisco, a population approaching 6,000. Its source is a groundwater basin lying predominantly beneath the densely developed Richmond District and is recharged through precipitation, surplus irrigation, and leaking pipes. Nitrates are the most pervasive water quality problem in San Francisco and have historically been reported as a constituent of concern in Lobos Creek. This study employed continuous-monitoring data loggers to examine three water quality parameters (water temperature, specific conductance, and groundwater level) from July 2008 to April 2009 at two upstream seeps where sampling has shown differing levels of nitrates over the years. Nitrogen isotope analysis of water samples was also conducted. Results of the latter indicated a source of nitrate consistent with sewage entering the creek near one of the study sites. Results of continuous monitoring revealed higher-than-expected water temperatures, contrasting water level patterns, and inconclusive specific conductance measurements. These baseline data illuminate conditions and provide context for future studies around nitrate identification and movement in the groundwater basin.