Cold Springs water-balance model identifies data gaps in north-south connectivity; stakeholders to help scope scenarios

Washoe County and consultant Stantec on May 7 presented findings from an ongoing Cold Springs Valley water-balance analysis and groundwater–surface water model and identified key data gaps, including uncertainty about hydraulic connectivity across White Lake and the extent of clay layers that affect groundwater flow.

Dwayne Smith of Washoe County introduced the project and said it was conceived “to understand the relationship between the ground and surface water hydrology within a closed basin area,” particularly to assess how new development and other stresses affect basin hydrology. Claire Archer, a hydrogeologist with Stantec, walked commissioners through the model construction, calibration and initial results.

Stantec’s modeling work used a linked surface-water model (USGS PRMS) and a groundwater model (MODFLOW-NWT). The transient simulation period ran from water years 1999 through 2023. The groundwater model domain covers unconsolidated basin sediments on a 200-foot-by-200-foot grid and includes five model layers. The team calibrated groundwater levels using 21 monitoring wells and simulated boundary conditions including municipal and domestic pumping, mountain-front recharge, underflow from the Long Valley groundwater basin, infiltration from the Cold Springs wastewater reclamation facility, and a lake package representation for White Lake.

The consultants reported a primary data gap: limited knowledge of whether the north and south parts of the basin are hydraulically connected beneath White Lake, a question controlled by the depth and continuity of fine-grained (clay) deposits and fault-related permeability. Archer said the available borehole data show clay layers that are not continuous across the basin, producing locally confined conditions.

On recharge estimates, Archer compared the study’s results to prior work, including Huntington (2020), which the consultant noted is recognized by the state engineer. Archer said Huntington estimated about 250 acre-feet per year of mountain-front runoff that infiltrates to groundwater, while Stantec’s study estimated about 650 acre-feet per year of the mountain-front infiltration component, using a different multi-year period (1999–2023); she characterized the two estimates as within method uncertainty.

Commissioners asked about lake-stage data at White Lake and potential additional datasets. Archer said she had not used numeric lake-stage data but had employed satellite imagery to determine presence/absence of surface water; commissioners and staff noted that former commissioner Michael DeMartini and other local stakeholders may have longer-term stage datasets that could be shared to improve calibration. Stantec also plans to incorporate deeper boring logs, Lumos data from south of White Lake and other monitoring to refine the southern model area.

On potential aquifer storage and recovery (ASR), Archer said the feasibility question is “very location dependent” because of heterogeneous clay layers and variable permeabilities; she suggested it “almost deserves its own analysis.”

Next steps the team outlined include adding monitoring records (Lumos and others), incorporating recent borehole and test-well data, engaging stakeholders to define scenario runs (e.g., future development, changes in effluent management, importation and pumping), and reporting scenario results to the commission. Dwayne Smith said the project is about 45% into its budget and staff would continue to coordinate stakeholder involvement and data sharing.

No formal action was required; staff directed consultants to continue model refinements and to convene stakeholders for scenario development.