Water quality and spatio‐temporal hot spots in an effluent‐dominated urban river

Abstract

In arid and semi-arid regions, many rivers experience extremely low flow conditions during seasonal dry periods. During these times, effluent from wastewater treatment plants can make up the majority of flow in the river. However, water quality in urban systems can also be strongly influenced by the natural or human-influenced flow regime and discharge from other anthropogenic sources such as industrial operations and runoff from impervious surfaces. In this study, we aimed to determine whether water quality was controlled primarily by wastewater discharge in an effluentdominated river. Between May 2016-May 2019, we systematically measured water temperature, pH, dissolved oxygen, biochemical oxygen demand, and the concentrations of nitrate-N, ammonia-N, and orthophosphate in the South Platte River in the Denver metropolitan area, Colorado, USA. We found that, despite being an effluentdominated river, wastewater treatment plant discharge was not the principal factor controlling water quality in many of the sampled areas. Non-point source pollution from impervious surfaces, delivered to the river through storm drains and minor tributary streams, also contributed to the high nutrient conditions in several locations. We also noted a strong seasonality in water quality, with higher concentrations of nutrients and higher biochemical oxygen demand in the winter months when wastewater effluent can make up more than 90% of the flow in the river. Thus, the interaction of discharge location and reduced seasonal flow produced spatio-temporal hot spots of diminished water quality. More stringent enforcement of water quality regulations may improve water quality in this system. However, a large portion of the pollution seems to be from non-point sources, which are very difficult to control.

Ungaged inflow and loss patterns in urban and agricultural sub‐reaches of the Logan River Observatory

“…If a sub-reach had insufficient solutions (i.e., less than 100 solutions) for Q gw , Q sw , and Q out , boxplots are not shown. Urban and agricultural areas are a complex system of human-built systems that interact and interfere with underlying natural surface water and groundwater flowpaths (Kendy & Bredehoeft, 2006;Schliemann et al, 2021). As discussed in Cook (2013), the accuracy of the mass balance methods applied here are dependent on the ability to distinguish concentration ranges for different source categories and as discussed in Aubert et al (2013), the observed concentrations of different ions can be time and flow dependent for a given sampling location.…”

Urban Drool Water Quality in Denver, Colorado: Pollutant Occurrences and Sources in Dry-Weather Flows

“…Dry-weather flows, or urban drool, derives primarily from "nuisance" flows (e.g., overirrigation of ornamental landscaping and turf, car washes, street cleaning, irrigation return flows), permitted and illicit point sources of discharge (e.g., discharges from industrial, construction, and dewatering operations), domestic wastewater and drinking water infras-tructure leaks or overflow, "natural" non-urban water flowing from outlying areas, and groundwater inflows [8,9,11,12,16,17,24,33,34]. The most predominant sources of nonpoint dry-weather flow in semi-arid climates include irrigation return flow and shallow groundwater contributions.…”

“…The South Platte River flows through Denver, Colorado and represents a source of recreation for people, habitat for aquatic life, and a supply of drinking water for Denver and downstream communities [23]. Similar to many urban waterways, urban processes and drainage within the city severely impair the river's water quality [24]. This metropolitan area is situated in a semi-arid climate and, similar to many urban waterways in the western United States, the South Platte River is dominated by wastewater effluent during periods of dry-weather [24].…”

“…Similar to many urban waterways, urban processes and drainage within the city severely impair the river's water quality [24]. This metropolitan area is situated in a semi-arid climate and, similar to many urban waterways in the western United States, the South Platte River is dominated by wastewater effluent during periods of dry-weather [24]. However, wastewater treatment plants and other point sources have largely been characterized and are regulated under varying legislature such as the Clean Water Act (CWA) and surface water quality standards set by the Colorado Department of Public Health and Environment (CDPHE) [24][25][26].…”

“…This metropolitan area is situated in a semi-arid climate and, similar to many urban waterways in the western United States, the South Platte River is dominated by wastewater effluent during periods of dry-weather [24]. However, wastewater treatment plants and other point sources have largely been characterized and are regulated under varying legislature such as the Clean Water Act (CWA) and surface water quality standards set by the Colorado Department of Public Health and Environment (CDPHE) [24][25][26]. Though these point sources of pollution certainly contribute to stream impairment, nonpoint sources have been identified as significant and uncontrollable sources of contamination and have the potential to produce pollutant loads at magnitudes equal to or greater than point sources [4,24,[27][28][29].…”