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Geohydrologic Characterization, Water-Chemistry, and Hydrologic Model of the Petaluma Valley Watershed, Sonoma County, CA

Tracy Nishikawa, D.S. Sweetkind, N.F. Teague, and Jonathan Traum

This information is preliminary and is subject to revision. It is being provided to meet the need for timely best science. The information is provided on the condition that neither the U.S. Geological Survey nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.

U.S. Department of the Interior U.S. Geological Survey

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Petaluma Valley Watershed

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Approach: 4 Tasks

· Geohydrologic characterization · Data collection/interpretation: Primarily water

quality

· Hydrologic model · Report P r e l i m i n a r y S u b j e c t t

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Geology and Lithologic Framework Model

· Information from previous studies were

integrated with digital geologic map, borehole, and geophysical data to create a three- dimensional geologic framework model of the Petaluma Valley watershed (PVW) that defines the subsurface stratigraphic and structural architecture for the study area.

· This digital model provides the fundamental

geologic framework for the subsequent development of a transient hydrologic model of the PVW.

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Surface Geology

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Wells Used for Geologic Framework Model

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Cardwell, 1958 (Spring 1951 Water Levels)

Petaluma Valley

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Spring 2010s Water Levels

Petaluma Valley

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Petaluma Valley

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Water Quality Constituents

· Used previously collected data and new data · Field parameters: Temperature, pH, dissolved

• xygen, and specific conductance

· Total Dissolved Solids and Major ions: calcium,

magnesium, sodium, potassium, chloride, sulfate, carbonate, and bicarbonate

· Selected trace elements: nitrate, iron,

manganese, arsenic, and boron

· Stable isotope: Isotopes of hydrogen (deuterium)

and oxygen-18

· Age dating: hydrogen-3 (tritium) and carbon-14

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Water- Quality Sampling Locations

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A-A’

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B-B’

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Chlorides

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Nitrate

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Stable Isotopes

Franciscan/Tolay Volcanics

W22 W42

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Age Dates

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Is Seawater Intrusion Occurring?

Freshwater entering marine sediments Seawater entering freshwater sediments

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Petaluma Valley Integrated Hydrologic Model (PVIHM) Overview

Groundwater Data Climate Data Hydrogeologic Data Water Supply and Demand Data Streamflow Data Landscape Data Water Budgets

MODFLOW-OWHM

Fully coupled simulation of groundwater flow, surface-water flow, and landscape processes Streamflow

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Groundwater Levels

Major Features of PVIHM

· Simulates the groundwater flow, surface-water

flow, and landscape processes in the 99,000 acre Petaluma Valley watershed

· Simulates 56 years of historical hydrology from

1959 to 2015

· Utilizes data from local, state, and federal

sources

· Incorporates the updated hydrogeologic model

to represent the multi-layered aquifer system

· Calibrated using groundwater level from 41

groundwater monitoring wells and measured streamflow data from 3 USGS streamflow gages

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Model Domain and Grid

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Petaluma Valley

Average Groundwater Budget

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End of Simulation - Simulated Groundwater Levels

Model Uses

· Understand the hydrologic responses related

to conjunctive use of surface water, recycled water, and groundwater

· Simulate future conditions under different

hydrologic and water management conditions

· Changes in future climate · Changes in future land use · Different magnitudes, distributions, and timing of

city of Petaluma pumping

· Support the development of a Groundwater

Sustainability Plan (GSP)

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Future Climate: Wet vs Dry

End of Simulation Wet - Dry

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Future Land Use

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Summary and Conclusions

· Study area is a modified version of the Petaluma

River watershed: Petaluma Valley watershed (PVW)

· Geohydrology

· Principal aquifer units spatially constrained · Data confirms Petaluma Valley fault · Data indicate the Quaternary mixed unit located along

axis of valley

· Surface-water Hydrology

· Local surface water not important for supply but

affects groundwater quality and supply.

· Petaluma River tidally influenced north of downtown

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Summary and Conclusions

· Groundwater

· Found in Wilson Grove, Petaluma, Quaternary, Sonoma

Volcanics, and Bay Muds

· Primary sources: infiltration from precipitation, with

some stream leakage, boundary inflows, and irrigation- return flow.

· Primary sinks: pumping, evapotranspiration of shallow

groundwater, boundary outflow, and baseflow.

· Groundwater flows from hills toward Petaluma River.

Then flows southeast toward San Pablo Bay. Not much change over time.

· Hydrographs unchanged with local declines.

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Summary and Conclusions

· Geochemistry

· Confirms precipitation is primary source of

recharge

· Seawater intrusion is occurring near San Pablo

• Bay. Source is high-salinity water from tidally-

influenced Petaluma River and inflow from San Pablo Bay

· Age dating indicates that deep groundwater along

axis of valley represents groundwater at the end of long flowpaths and is old

· Nitrates in Wilson Grove formation now less than

MCL

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Summary and Conclusions

· Hydrologic model

· Simulates: groundwater, surface water, and

landscape processes

· Simulated 1959 to 2015 · Calibrated to measured groundwater levels and

streamflow

· Model uses

· Hydrologic response to conjunctive-use strategies · Hydrologic response to future: climate, land use, and/or

pumpage

· Support GSP

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SGMA Undesirable Results

· Storage decline: 1% removed from storage · Groundwater/surface water: Groundwater

providing baseflow

· Water-level decline: Mostly flat with some

localized declines

· Seawater intrusion: Primarily near bay,

although tidally influenced river may be affecting wells farther upstream

· Water-quality degradation: Nitrates improving · Subsidence: N/A P r e l i m i n a r y S u b j e c t t

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Suggested Future Work · Conduct a surface water/groundwater interaction study

along the tidally influenced reach of the river, focusing on the area near downtown Petaluma and the Lynch Creek confluence and the lower part of the river.

· Continue monitoring and collecting groundwater level,

groundwater quality, and streamflow data.

· Expand groundwater monitoring network to areas of

uncertainty such as the Wilson Grove and Santa Rosa Plain boundaries.

· Start collecting data on groundwater production for rural

and agricultural use and surface-water diversions from local sources used for irrigation.

· Collect crop-related data from local growers to reduce the

uncertainty in land-use parameters.

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Questions?

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