The primary goals of this study are to understand the impact of reduced traffic on urban stream water quality in the Denver Colorado metro area, which should provide insight to impacts in any city in an arid or semi-arid climate.  Another important objective is to provide information to urban planners and decision-makers regarding how more sustainable living behaviors that involve less personal transportation may enhance protection and preservation of the water quality of urban rivers and aquatic ecosystems.  Such practices include working, socializing, shopping, and attending classes from home, which became prevalent during the pandemic, a tendency that is likely to remain after the pandemic.  The restrictions put in place during the COVID-19 pandemic offered a unique opportunity embark on this research.   

Colorado shifted from ?stay-at-home? to ?safer-at-home? orders in late April 2020.  Our project funding became available in early June, too late to obtain data during the most rigorous shutdown period, but traffic was significantly reduced compared to normal traffic through March 2021 (see figure).   Our hypothesis is that urban streams would see significantly smaller loading of traffic-related pollutants during COVID restrictions.  

Water quality samples and flow measurements were collected at three study sites in the Denver metro area: Lakewood Gulch (LG) and Weir Gulch (WG), both at the junction with the South Platte River in downtown Denver, and South Golden Gulch (SG) in Golden, about 15  mile west of Denver in the urban corridor.  LG and WG are located in downtown Denver with significant traffic and urban land use.  SG is in a smaller town and drains an area of shopping centers and the main traffic-way through Golden.  

Storm runoff is the main actor that transports pollutants from impervious areas (e.g., roofs, roads, and parking lots) to urban streams.  71 wet weather (WW) samples were collected during storm events to understand the temporal distribution of contaminants within different stages of each storm hydrograph. A total of 28 samples were collected during dry-weather flows (DW), also called urban drool.

Streamflow discharge information was obtained from USGS streamflow gages located in LG and WG, a few miles upstream from sampling sites (see Figure 1). Streamflow at the SG was obtained using a flow meter or float, depending on flow conditions.  Water quality samples to enable estimation of pollutant loading were collected during using standard best practices, and analyzed for pH, electric conductivity (EC), temperature, heavy metals and cations, polycyclic aromatic hydrocarbons (PAHs), and BTEX (benzene, toluene, ethylbenzene and xylenes).  

Vehicular flow data was collected using CDOT traffic counters located on roadways within Denver and Golden. Traffic counters were chosen based on two criteria: (1) counters that contained the largest continuous records; and (2) close proximity to the study sites.  When the pandemic began Colorado applied severe ?stay-at-home? orders to keep the virus propagation under control and to flatten the curve of daily cases. Under the effect of these orders, traffic flows have been at their lowest levels in decades, gradually increasing since early 2020 to current days (see figure). Once the pandemic began, the flow decreased sharply at rush hours. The month of April showed the lowest level of traffic flow, slowly returning to ?normal? levels (based on 2018 and 2019 data). 

Traffic-related BTEX pollutants were rare during storm events, while traffic-related PAHs and metals were consistently detected.  The majority of PAHs and metals were present for most sampling events.  When combined with traffic data, our results suggest that reductions in traffic resulted in reduced urban stream pollutant loads, although pre-COVID data was sparse such that statistical significance was difficult to demonstrate.  We are continuing to collect data using other funds and will continue analysis of water quality behavior as post-covid traffic returns to normal (and potentially during another shutdown if COVID variants become important in the future).  In addition, we learned we can time our sampling to incorporate daily traffic and storm patterns as an additional means to evaluate the impact of traffic on urban stream pollutant loads, which will be a topic of continuing research.  Traffic-related BTEX pollutants were absent at all sites during dry weather (DW) periods, while PAHs and metals were infrequently detected. Different from storm events, vehicular PAHs and metals demonstrated less occurrence in dry weather. 

Several students received valuable professional training during this project.  One student will receive a Master?s graduate degree. One PhD student should get one dissertation chapter from this work.  Two non-thesis MS students received research experience and 3-credit Independent Study course credit.  The work is being presented to the Denver, Golden, Fort Collins, Colorado Springs, and Sonoma County Water Authority.

Last Modified: 09/01/2021
Modified by: John E Mccray