The effects of winter street treatment on Chester Creek Water Quality During Snow Melt Events

Abstract

Water quality parameters for temperature, electrical conductivity, dissolved oxygen, pH, and turbidity and ion activity for calcium, chloride, and magnesium were measured during a period of five winters at eight locations along Chester Creek that flows from the Chugach Mountains east of the Municipality of Anchorage through the Municipality to the west into Knik Arm. The goal of the project was to identify the impact of meltwater on water quality due to common winter street treatments.

Each year had its unique conditions. Melt events occurred when air temperature reached above freezing which were sometimes accompanied by rain events that enhanced snowmelt. Melt events throughout the season varied between four and nine with no considerable trend over the duration of the project. Maximum snowpack due to record snowfall occurred in winter 2022/2023 and 2023/2024.

Electrical conductivity, which measured the total ion concentration in the water, and turbidity, which measured the concentration of particles, showed the major impact on water quality during melt events. Both parameters can be related to street treatments with salt/deicer and sand/gravel which are used on streets, parking lots, and walkways to reduce hazardous conditions, and both parameters generally increase from the east to the west as the creek runs through the Municipality. The highest electrical conductivity and turbidity were measured in the Middle Fork of Chester Creek at the Northern Lights location and directly in storm drainages at the Seward Highway location. The electrical conductivity correlated well with chloride concentration--a major compound of deicing agents and salts. This supports the assumption that the increase in electrical conductivity is due to street treatments.

The total amount of ions, derived from electrical conductivity, is related to the area of each sub basin upstream from each sampling location, and indicates that the sub basin of the Seward Highway sampling location contributes fewer total ions to the creek per sub basin area than other sub basins.

Comparing the measured values to water quality standards for aquatic life and propagation (18 AAC 70 Water Quality Standards, March 2020), the electrical conductivity should not exceed 1,500 µS/cm, and this value was only exceeded once on March 26, 2021 in the Middle Fork. Turbidity should not exceed background values by more than 10%. Background values for the sampling locations derived from times before freezing and in the absence of rain range from 0.7 to 5.17 NTU from east to west. Values measured during melt events and summer rain events frequently exceeded this water quality standard.