USING A MULTI-TRACER APPROACH TO DETERMINE MUNICIPAL DRINKING WATER AND WASTEWATER INPUTS TO URBAN STREAMS

With over half of the world’s population living in cities, releases of municipal waters, including drinking water, treated wastewater, and untreated wastewater, can considerably alter the chemistry of urban streams. Municipal waters can be released into proximal waterbodies through 1) over irrigation of lawns, 2) discharges of treated wastewater, 3) discharges from sewer overflows, and 4) leaks from water infrastructure. Releases of drinking water can be wasteful, but releases of wastewater may threaten human health if people are in contact with the receiving waters. Thus, this work aims to quantify drinking water and wastewater inputs to streams in the highly urbanized City of Saint Louis, Missouri. Our study area features high application rates of drinking water for lawn irrigation, over 350 sewer overflows, and a drinking water source with unique chemical characteristics. We sampled streams along a gradient of land use from rural to urban, ranging from 1.4% to 62.6% impervious surface area in the watersheds. Stream samples were analyzed for fluoride (F) and boron (B) via ion chromatography and inductively coupled plasma optical emission spectrometry, respectively, to determine municipal inputs. F is added to drinking water in known quantities (520 ± 90 ppb) for dental health, B levels are naturally high (125 ± 25 ppb) in the Missouri River-derived drinking water, and B concentrations are even higher (201 ± 154 ppb) in municipal wastewaters due to additions from detergents during use. We observed F and B values as low as 60 ppb and 14 ppb, respectively, in rural streams, but concentrations increased with urbanization to ~450 ppb for F and ~225 ppb for B. We used a mass balance approach with our F and B data to quantify municipal water inputs to streams. We estimate that in the most densely urbanized areas of the city, municipal waters may account for 60-80% of the total stream flow. This is likely due to increases in 1) outdoor drinking water use for lawn irrigation, 2) the density of water-related infrastructure, and 3) the age of water-related infrastructure in more developed areas. Understanding the type and amount of municipal water contributions to streams is crucial in guiding water conservation efforts and infrastructure improvements to conserve drinking water and reduce the release of potentially harmful wastewaters.