Groundwater chemistry and groundwater-surface water interactions in the Soos Creek watershed Active

Problem: 

The Soos Creek watershed is located in the Puget Sound lowlands, in western Washington State. The watershed drains approximately 66 square miles and is inside Water Resource Inventory Area (WRIA) 9. Four main tributaries drain the watershed: Little Soos, Soosette, Jenkins, and Covington Creeks. All four tributaries drain into the mainstem Big Soos Creek, which then drains into the Middle Green River near Auburn at River Mile (RM) 33.7. Historically, the Soos Creek watershed has supported all five species of North American Pacific salmon (chinook, coho, chum, pink, and sockeye) as well as steelhead and cutthroat trout. Chinook salmon and steelhead are listed as threatened on the Endangered Species List. Portions of the watershed have been listed as impaired by Ecology for temperature, DO, and bacteria, which necessitates the development of a TMDL in the watershed for these parameters.  

Objectives: 

The objective of this study is to evaluate surface water-groundwater interactions in the Soos Creek watershed to characterize the quality and quantity of groundwater discharging to Big Soos, Jenkins, Covington, Little Soos, and Soosette Creeks. In addition, the USGS will conduct water level monitoring in multiple locations in the upper watershed to inform hydrologic modeling. This work will provide opportunities to provide training to Ecology staff on the methods employed for this study to expand this work to other impaired watersheds in the state.   

Relevance and Benefits: 

The study serves a local and regional need to better support the development of a technically sound TMDL for temperature and dissolved oxygen in the Soos Creek watershed.  The USGS will help the TMDL development process by providing Ecology with seasonal information on shallow groundwater chemistry and patterns of groundwater-surface water interactions that will be incorporated into a watershed model. The USGS study team will work directly with Ecology team members to provide field and office technology transfer that will improve the current modeling effort in the Soos Creek watershed to improve temperature and oxygen conditions within the watershed known to support threatened species of salmonids.  

Approach: 

Scientists will sample shallow groundwater chemistry and assess groundwater-surface water interactions at multiple locations in the Soos Creek watershed. The shallow groundwater sampling will consist of two phases.  

Phase 1: 

Temporary piezometers will be installed at approximately 15-20 sites across the watershed to investigate gaining stream reaches. From these piezometers, measurements of field parameters of shallow groundwater will be made for temperature, DO, pH, and specific conductance. In addition to measuring field parameters, an estimate of the potential for groundwater discharge into the creek will be made by determining the vertical hydraulic gradient (VHG) at each location.  

Phase 2: 

A subset of Phase 1 locations will be used for a more detailed assessment of groundwater chemistry and groundwater exchange. Monthly sampling of VHG and water quality will be performed from June 2023 through June 2024 in at least five locations with groundwater piezometers. Water quality will also be sampled monthly in up to five groundwater wells to assess shallow groundwater. Water quality sampling will include: 

• Field parameters (temperature, DO, pH, specific conductance)

• Chloride 

• Dissolved organic carbon (DOC) 

• Total organic carbon (TOC) 

• Total phosphorus, 

• Ammonia 

• Nitrate + nitrite 

• Total nitrogen 

• Orthophosphate 

• Iron 

In addition, four water level sites at the upper reaches of the tributaries in the basin will be established for one year of continuous monitoring to provide details on perennial flow in headwater reaches in the basin to inform the hydrologic models being developed for the TMDL.  

Groundwater and surface-water interactions will also be assessed at the piezometer sites using temperature rods to measure streambed temperature profiles over one year. These profiles will be modeled using a heat balance equation to estimate groundwater exchange during the year.