Stochastic Empirical Loading and Dilution Model (SELDM) model archive and instructions for the Siskiyou Pass, Oregon

Chloride deicers have been applied by the Oregon Department of Transportation (ODOT) to Interstate-5 (I-5) from the Oregon-California border north to mile marker 10 for several years in the high-elevation area known as the Siskiyou Pass. Magnesium chloride (MgCl2) and sodium chloride (NaCl) are applied to keep the interstate safe for drivers and allow for efficient transport of goods and people through adverse weather conditions, particularly snow and ice. The USGS entered into a cooperative agreement with ODOT to research the effects of the application of these chloride deicers in the Carter Creek and Wall Creek watersheds within the Siskiyou Pass. Hydrologic and meteorological data were collected within the study area, and water-quality samples were collected from the Bear Creek watershed, which includes Carter and Wall Creeks. Results indicate a moderate range of natural chloride (Cl), magnesium (Mg) and sodium (Na) concentrations within the Bear Creek watershed, but at Carter and Wall Creeks downstream of I-5, measured constituent concentrations were lower that what was recorded from ODOT’s 2012-2017 pilot project. The Stochastic Empirical Loading and Dilution Model (SELDM) uses a stochastic mass-balance approach to estimate combinations of prestorm streamflow, stormflow, highway runoff, event mean concentrations (EMCs) and loads of stormwater constituents from a site of interest. SELDM was used to evaluate the effects of roadway application of chloride deicers on downstream and highway runoff conditions, particularly EMCs, exceedance rates of hypothetical criteria maximum concentrations (CMCs), and concurrent runoff loads of stormwater constituents from a site of interest. SELDM was also used to evaluate the efficiency of hydrograph extension best-management practices (BMPs) to reduce peak constituent concentrations. In addition, several SELDM scenarios were developed as sensitivity analyses to evaluate the model benefit of collecting specific local sets of data, such as streamflow, precipitation, highway runoff and riverine water-quality samples and volumetric runoff coefficient statistics. These analyses are meant to serve as templates and illustrative examples for ODOT. ODOT is interested in using SELDM for impact analysis and to identify locations and streams that could be vulnerable to excessive deicer loading if chlorides are used. This data release serves as a model archive for the SELDM simulations performed for Stonewall, A.J., Yates, M.C., and Granato, G.E., 2022, Assessing the impact of chloride deicer application in the Siskiyou Pass, southern Oregon: U.S. Geological Survey Scientific Investigations Report 2022–5091, 94 p., https://doi.org/10.3133/sir20225091. Also included in this data release is a text document that is meant to serve as informal guidance to the Oregon Department of Transportation (ODOT) on how to assess the potential effects of chloride on highway runoff, receiving water and groundwater. This guidance is not meant to be comprehensive nor universal, but rather a compendium of guidance and resources that may help investigators.