Stormwater Impacts to an Urban River in the Intermountain West: Use of Continuous Monitoring Datasets

摘要

In 2012 the Utah Department of Environmental Quality completed a total maximum daily load (TMDL) study on the Jordan River in Salt Lake City and concluded that the lower reaches of the river were dissolved oxygen (DO) impaired. This impairment was initially associated with course particulate organic matter loading to and produced within the lower Jordan River. A high intensity rainstorm in the Salt Lake Valley on July 4, 2013, resulted in anaerobic conditions in sections of the lower Jordan River below large stormwater discharge points for more than a 10-hour period and suggested potentially significant contributions of oxygen demanding materials from urban stormwater inputs to the river.Subsequent laboratory studies investigating the nature of natural organic material (NOM) produced within the urban stormwater drainage areas of the lower Jordan River revealed that this NOM was rapidly leached from urban vegetation, was readily biodegradable, and could be easily and rapidly be transported to the lower Jordan River via the existing stormwater conveyance system throughout the Salt Lake Valley.To determine the baseline and episodic contribution of stormwater to DO impairment in the lower Jordan River, a continuous aquatic data monitoring station was installed in 2015 at a major storm drain discharge site on the Jordan through the NSF-funded iUTAH project. Parameters available for continuous monitoring (15-minute intervals) included flow measurement, as well as fluorescent dissolved organic matter (fDOM) and specific conductivity, both of which dramatically spike during storm events. An ISCO autosampler was installed at the site and programmed to begin sampling in response to storm events. Grab samples from the stormwater discharge and in the Jordan River upstream of the monitoring site were analyzed for BODu, and DOC so correlations could be established between the continuous parameter, fDOM, and lab generated measurements of corresponding oxygen demand and oxygen uptake rates. These correlations were used to quantify both baseline and storm generated loading of BODu using continuous fDOM measurements from both the storm drain and the Jordan River.Results generated from 19,008 paired fDOM data and 35,003 paired flow data indicated that despite baseline storm drain flows being only 6% of Jordan River flows, dry weather BODu loadings adjusted for relative biodegradability of measured fDOM represented 26% of baseline Jordan River BODu load. Peak loading during storm events were documented to reach 1200% of upstream Jordan River loads.This paper presents results of the fDOM degradability studies, as well as methods and detailed results of continuous monitoring data correlations with laboratory-based oxygen uptake rates that demonstrate the significance of urban stormwater organic loading, even during dry weather conditions, to oxygen impairment to the Jordan River. Finally, the significance of these findings to future stormwater management and green infrastructure implementation within the Salt Lake Valley will be addressed.