Why is Pasco Basin groundwater important?
Many rely on groundwater in the Pasco Basin, including landowners, farmers, groundwater permit holders and those living in the area. A portion of the groundwater that exists in the Pasco Basin area is the result of return flows infiltrating from surface water irrigation as part of the Federal Columbia Basin Project.
For over a half-century, however, the groundwater has been managed by interim rules. By adopting a permanent rule, longstanding uncertainty will be resolved and new groundwater supplies may become available.
Working with the U.S. Bureau of Reclamation
In 2021, the Washington Legislature revised Ecology’s authority to enter into an agreement with the U.S. Bureau of Reclamation. This allows the two agencies to develop a groundwater co-management strategy. That legislative action was necessary, because the Pasco Basin has naturally occurring groundwater that mixes with irrigation water return flows -- referred to as “commingled groundwater.”
Differentiating between these water types is important because all naturally occurring groundwater in Washington is a state resource managed by Ecology. Return flows coming from irrigation water used in the Columbia Basin Project, however, may be managed through agreement and licensed by Reclamation to be provided to irrigators to be used on crops at a per acre fee.
A Memorandum of Understanding (MOU) between Reclamation and Ecology was signed in 2022. The MOU commits Ecology and Reclamation to develop a groundwater co-management strategy. A subsequent Memorandum of Agreement will establish the details of the co-management program and will be developed concurrently with Ecology rulemaking.
Pasco Basin boundary designation
A critical first step in the development of the co-management program is defining the boundaries of the Pasco Basin, where groundwater will be managed. We are currently working on the development of a process for the boundary designation, including a timeline and opportunities for input from Tribes, stakeholders, interested parties and the public. We anticipate the boundary designation will be finalized through an administrative order.
USGS Study and Groundwater Model
To better understand the water balance in the Pasco Basin, the United States Geological Survey (USGS) prepared a numerical groundwater model to estimate the amount and locations of commingled groundwater in the Pasco Basin area. The USGS report and model improved the understanding of the hydrogeology and provided estimates of increased groundwater storage in the Pasco Basin area since the development of the Columbia Basin Project.
The 2016 USGS model is currently being revised to refine existing conditions and to better support modeling of different water management scenarios in the Pasco Basin. A third-party review of this revised modeling effort will be completed prior to finalizing the model and its results.
The USGS study, along with additional technical work that may be identified as needed, is intended to help inform estimated volumes and location of shallow commingled groundwater in the Pasco Basin that may be available for allocation under the groundwater co-management program.