Update on USGS Santa Rosa Plain Cooperative Groundwater

Update on USGS Santa Rosa Plain Cooperative Groundwater Study State of Laguna Conference and Science Symposium October 14, 2009 Marcus Trotta, PG, CHg Sonoma County Water Agency

Alexander Valley Study Area Completed 2006 Santa Rosa Plain Study Area Initiated 2005 Sonoma Valley Study Area Completed 2006 Petaluma Valley Study Area

Overview of Study and Cooperating Agencies Ø Santa Rosa Plain Basin Study – 5 Yr Study Began December 2005 Ø Complete in December 2010 Ø $2. 2 Million ($1. 5 M local/$0. 7 M USGS) Ø Cooperating and Funding Agencies: Sonoma County Water Agency § Cities of Santa Rosa, Rohnert Park, Cotati, Sebastopol, Town of Windsor § County of Sonoma § Cal American Water Co. §

Study Objectives • Develop an updated assessment of the hydrogeology & geochemistry of the Santa Rosa Plain (SRP). • Develop a fully-coupled surface water/groundwater flow model for SRP. • Evaluate the hydrologic impacts of alternative groundwater management strategies on the basin. • What are efficient strategies for surface-water/groundwater management that will assure the long-term viability of water supply in the SRP?

Santa Rosa Plain – Groundwater Study Tasks Ø Data Collection, Compilation & Data Management (GIS) § Water Level & Quality, Geologic, Geophysical, & Land Use Ø Estimate Groundwater Demand Over Time Ø Data Interpretation and Hydrogeologic Characterization § Geologic Framework Model § Conceptual Model § Water Balance Ø Fully Coupled Surface Water/Groundwater Flow Model

Land Use Maps & Water Use Estimates - Santa Rosa Plain Historical Land Use Ø 1959, 1974, 1979, 1986, 1999 Water Use Estimates Ø Water use records - Urban Ø Per capita – Domestic Non-Urban Ø Crop Use Factors - Agriculture

Gravelly unit; south of Sebastopol Wilson Grove Formation Upper sand unit; west of Sebastopol Fine-grained unit; Whitaker Bluff Ø West of Sebastopol, lower very fine-grained sand Ø North of Sebastopol, massive fine-grained sand Ø Southeast of Sebastopol, gravelly, grades into Petaluma Formation

3 D framework built with well data

3 D Hydrogeologic Framework and Hydraulic Properties Compilation Construction of 3 D hydrogeologic framework model and determination of aquifer thickness Defining heterogeneity in the hydrogeologic units Ø Well lithology at 5 -m increments Ø Horizontal extrapolation away from each drill hole Ø Highly dependent of density of well data 3 D framework

Oblique View of Depth to Basement Surface from Geophysics • Nearly 2, 000 new gravity measurements in the northern San Francisco Bay area obtained to delineate to estimate thickness of the valley-fill and volcanic aquifers. • Santa Rosa Plain is underlain by two sedimentary basins (the Windsor and Cotati Basins) about 1 to 2 km deep separated by the Trenton Ridge, a shallow west-northwest-striking bedrock ridge west of Santa Rosa.

SRP Groundwater Quality Sampling Sites ØWater quality database– 5, 000 records ØInorganic, trace element, nutrient, & misc. parameters Ø 411 wells Ø 6 surface-water sites ØDatabase updated with Dept. Public Health data ØDatabase incorporates recent GAMA sampling program

Development of GSFLOW Flow Model Ø Coupled surface water (PRMS)-groundwater flow (MODFLOW) model Ø Capable of simulating land surface processes (runoff, evapotranspiration, infiltration) – coupled to groundwater flow Ø Provides spatially distributed climate, land surface processes (overland flow, retention storage, etc) Ø Upon completion, USGS will run several scenarios to assess potential future conditions (e. g. , climate change) & alternative water management strategies (e. g. , groundwater banking)

Watershed Model Physical characteristics of watersheds • topography • hydrography • vegetation • soils • geology • land use • streamflow gaging records • meteorological data

Ground-Water Flow Model • Inputs include boundary conditions, aquifer properties, streams and lakes, flow barriers, and pumpage • Parameter estimation

Stakeholder Assessment – Santa Rosa Plain Groundwater Study Ø In progress – Conducted by Center for Collaborative Policy, California State University, Sacramento. Ø Impartial assessments of issues and concerns related to groundwater management in the Santa Rosa Plain and to learn if and how stakeholders might want to address these issues. Ø Conducted confidential interviews with nearly 50 individuals representing over 30 organizations. Ø Preliminary findings indicate differing interpretations on the value and potential of groundwater management planning as well as a lack of technical understanding of both surface and groundwater resources within the SRP. Ø Completed assessment expected in late 2009

Acknowledgements/Questions? USGS Personnel • Database/GIS – Kathryn Koczot – Andy Morita – Donna Knifong • Data Collection/Interpretation – Loren Metzger – Chris Farrar • Geologic Modeling – Victoria Langenheim – Robert Mc. Laughlin – Robert Jachens • Lithologic Modeling – Don Sweetkind – Emily Taylor • Modeling – Joe Hevesi – Linda Woolfenden – Diane Rewis – Tracy Nishikawa – Eric Reichard http: //ca. water. usgs. gov/user_projects/sonoma/santarosaplain. html