Urban drool water quality in Denver, Colorado: pollutant occurrences and sources in dry-weather flows

Abstract

Though stormwater is a significant contributing factor to urban surface water pollution, dry-weather flows in urban channels and streams, often termed “urban drool”, also represent an important source of urban surface water impairment, particularly in semi-arid environments. Urban drool is a combination of year-round flows in urban channels, natural streams, and storm-sewer systems (runoff from irrigation return flow, car washes, street cleaning, leakage of groundwater or wastewater into streams or storm sewers, etc.). Denver, Colorado represents an expanding urban area with a solid perennial stream network in a semi-arid climate, making it an ideal location to investigate urban dry-weather water quality. The purpose of this study was to better understand the extent and sources of urban drool pollution in Denver, Colorado by identifying relationships between urban catchment characteristics (e.g. land use and imperviousness) and pollutants. Water samples were taken throughout Denver at different drainage locations (e.g. gulches, outfalls, and other urban drainage that maintain year-round flows) that represent a variety of urban characteristics. Samples were assessed for various water quality parameters, including total suspended solids (TSS), coliforms, Escherichia coli (E.coli), nutrients (nitrogen, phosphorus, and potassium), dissolved and total organic carbon, and dissolved and total recoverable metals. Results indicated that nutrients, arsenic, and selenium were positively correlated with more impervious surfaces and urban land uses, suggesting that dense urban cover was a significant contributor to these pollutants under dry-weather conditions. However, the source of these contaminants was determined to likely be from shallow groundwater intrusion instead of direct urban inputs. Anthropogenic effects to groundwater, namely enhanced urban recharge and rising shallow groundwater tables, likely exacerbated groundwater contamination and contributions to dry-weather surface streams. E.coli was a contaminant present at exceeding concentrations regardless of urban characteristics. Most metals did not have significant correlations to urban characteristics and were found at concentrations similar to natural surface and groundwater concentrations. Though groundwater inputs appeared to be the predominant source of contaminants, nitrate, nickel, and manganese were evidenced to be partially loaded from industrial inputs. Medical marijuana cultivation sites were also identified as possible sources of nutrient and zinc pollution. Finally, erosion of contaminated urban soils, presumably from construction, was found to significantly increase concentrations of TSS, total phosphorus, and total metals, suggesting that contaminated urban soils and construction activities may contribute to dry-weather pollutant loads. Increasing urbanization and predicted drier climates suggest that dry-weather urban drainage will become more important to manage. Results from this study not only alerts the City and County of Denver about which catchment areas are out of compliance, but it also facilitates and informs dry-weather pollution mitigation decisions that would benefit and improve urban stream ecosystems.