Atlantic Richfield Company
Butte, Montana

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Wet Weather

What you should know

Wet weather events exist when runoff occurs due to precipitation or snowmelt.

Wet weather events exist when runoff occurs due to precipitation or snowmelt. Typically wet weather flow has been defined as flow greater than 50 cfs at station SS-07 or greater than 35 cfs at SS-04. Additional factors such as low base flows, volume, duration, intensity of precipitation and visible flow in streets are taken into consideration when identifying wet weather events.

Within the BPSOU, wet weather data is used to evaluate whether Blacktail Creek and Silver Bow Creek are meeting applicable or relevant and appropriate requirements (ARARs) for certain Contaminants of Concern (COCs). COCs within the site are arsenic, cadmium, copper, mercury, lead and zinc.

Typically, concentrations of COCs are expressed in micrograms per liter (ug/L). Concentrations are also measured for both the dissolved (Dis) and total recoverable (TR) forms of the sample.

Below are links to several storm water charts of stations at Blacktail Creek and Silver Bow Creek. These charts show compliance ratios for each COC based on each storm event. A compliance ratio is the ratio of the concentration of the COC to the applicable compliance standard. The compliance standards are functions of total hardness (mg/l, CaCO3) at the time of sampling which are defined by Circular DEQ-7.

For further information, visit our Reports and Publications page to download related documents.