To: Board of Directors, Sonoma County Water Agency
Department or Agency Name(s): Sonoma County Water Agency
Staff Name and Phone Number: Todd Schram 707-524-1173
Vote Requirement: 4/5th
Supervisorial District(s): Countywide
Title:
Title
Water Resources Investigations to Evaluate Streambed Clogging Impacts on Riverbank Filtration
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Recommended Action:
Recommended action
A) Authorize Sonoma County Water Agency’s General Manager to execute an agreement with United States Geological Survey, United States Department of the Interior, in substantially the form as the draft presented to this Board, for water resources investigations for microbial controls on water availability during riverbank filtration through March 1, 2027, in the not-to-exceed amount of $112,042.
B) Authorize Sonoma County Water Agency’s General Manager to amend or terminate this agreement with approval of County Counsel.
C) Adopt a Resolution authorizing adjustments for the Board Adopted Budget for FY 2024/2025 for Water Transmission Fund in the amount of $112,042 for the Microbial Controls on Water Availability During Riverbank Filtration Collaborative Agreement.
(4/5th Vote Required)
end
Executive Summary:
Sonoma County Water Agency (Sonoma Water) uses riverbank filtration (RBF) as a sustainable alternative to traditional drinking-water treatment. RBF relies on maintaining the natural hydraulic connection between surface water and groundwater across the streambed of the Russian River. The seasonal erection of Sonoma Water’s inflatable dam near Wohler bridge fosters accumulation of fine sediments and growth of biofilms in the streambed, which can lead to streambed clogging and an associated reduction in the water-withdrawal efficiency of Sonoma Water’s RBF system. During the most recent drought, reductions in water-withdrawal efficiencies at Wohler were significantly greater than have been observed in the past. Sonoma Water and the United States Geological Survey (USGS) have a continued interest in collaborating on studies that provide data to better understand how drought conditions impact the magnitude and persistence of streambed clogging and associated reductions in water withdrawal efficiency of Sonoma Water’s RBF facilities. This item requests authorization for Sonoma Water’s General Manager to execute a collaborative agreement with USGS to continue with these studies.
Discussion:
HISTORY OF ITEM/BACKGROUND
Sonoma Water serves over 620,000 residents of Sonoma and Marin counties in California. Sonoma Water draws water from the Russian River through underlying bed sediments consisting of highly transmissive, poorly sorted alluvial sand and gravel using RBF for treatment. Six collector wells, extending roughly 80 feet below the streambed have horizontal lateral screened intakes arranged radially near the bottom in the aquifer. Under the seasonal low flow river conditions of the dry season, a rubber dam is inflated to augment water yields for the three upstream collector wells (numbers 1, 2, and 6). The dam increases the river stage and the infiltration area behind the dam, thereby increasing groundwater levels and production capacity.
RBF operations are affected more directly by changes in environmental conditions (e.g., droughts, floods, nutrients) than conventional systems. Thus, careful and ongoing study is needed to address any uncertainties that may exist from potential changes in surface-water quality and quantity to carefully plan for and mitigate operational risks and costs (e.g., pumping strategies). Changes within the streambed’s hyporheic zone could result in reduced permeability and consequent impacts on water production. Riverbed clogging (decreased riverbed permeability) has been attributed to changes in physical (precipitation, infiltration of suspended particles), mechanical (gas entrapment), biological (bacterial growth and reproduction), and chemical (precipitation, complexation reactions) processes. A better understanding of the controls on these types of disruptions to RBF performance would help to inform sustainable mitigation strategies.
Research suggests that low summer flows and associated higher water temperatures accelerate biomass growth, while the absence of rainy-season high-flow events, which typically scour and redistribute riverbed sediments, likely increases the duration and magnitude of streambed clogging. However, more research is needed to understand the roles of reduced river velocities, temperatures, water quality, microbial activity, and riverbed scouring have on the extent of streambed clogging, observed water-level declines, and associated reductions in water-withdrawal efficiencies.
Sonoma Water has been coordinating with USGS to identify environmental drivers (winter storms, temperature, water quality) related to variability in abundance, structure, and persistence of microbial biomass within the hyporheic zone at Sonoma Water’s RBF sites. Additionally, this work will study the temporal variations in the source (i.e., surface-water infiltrate versus ambient groundwater) of water recovered by RBF facilities using isotope analyses. The work will relate changes to microbial processes within the hyporheic zone and the subsurface to reduced hydraulic streambed conductivities, the formation of an unsaturated zone, and ultimately reduced water production efficiency.
Sonoma Water and USGS began collaborating on this work under a prior agreement; however, additional time was required to complete the work. A new agreement is required by USGS because amendments to the prior agreement are no longer authorized due to internal USGS policy changes; therefore, a new collaborative agreement between Sonoma Water and USGS is required.
CALIFORNIA ENVIRONMENTAL QUALITY ACT (CEQA)
Sonoma Water’s General Manager has determined that the agreement is exempt from CEQA pursuant to Section 15306, Information Collection, because the data collection and research activities would not result in a serious or major disturbance to an environmental resource or result in a substantial adverse change to a historic resource. Sonoma Water staff has prepared a Notice of Exemption for the project in accordance with the CEQA, the State CEQA Guidelines, and Sonoma Water’s Procedures for the Implementation of CEQA.
SERVICES TO BE PERFORMED
Under the agreement, USGS will collect and analyze groundwater, surface water, and streambed sediment samples to identify various factors related to water quality and biological responses in the Russian River streambed (e.g. streambed clogging) to changing climate, surface, and groundwater conditions, and predict possible future outcomes for riverbed filtration efficiency. Collected data will be published and interpretive results will be provided to support further research activities.
The cost of services for the agreement will not exceed $112,042. This agreement covers services rendered from February 15, 2025, through March 1, 2027.
REQUEST FOR GENERAL MANAGER AMENDMENT AND TERMINATION AUTHORITY
Consistent with other agreements, staff recommend that the Board authorize Sonoma Water’s General Manager to amend or terminate the agreement with approval of County Counsel.
County of Sonoma Strategic Plan Alignment:
N/A
Sonoma Water Strategic Plan Alignment
Goal 2: Planning and Infrastructure - Implement comprehensive, integrated, and innovative infrastructure planning to strengthen existing services, minimize life cycle costs, and prepare for the future.
Strategy 2.1: Conduct planning that integrates and balances operational, maintenance, and infrastructure priorities.
Action 2.1.2: Continue to engage in planning efforts and partnerships to protect and enhance our water supply.
This research project improves our understanding of the operational conditions and limitations of our water supply production facilities.
Racial Equity:
Was this item identified as an opportunity to apply the Racial Equity Toolkit?
No
Prior Board Actions:
12/13/2022 Board authorized Sonoma County Water Agency’s General Manager to execute an agreement with USGS; cost $375,000, and term end December 31, 2024.
Fiscal Summary
|
Expenditures |
FY24-25 Adopted |
FY25-26 Projected |
FY26-27 Projected |
|
Budgeted Expenses |
|
|
|
|
Additional Appropriation Requested |
$112,042 |
|
|
|
Total Expenditures |
$112,042 |
|
|
|
Funding Sources |
|
|
|
|
General Fund/WA GF |
|
|
|
|
State/Federal |
|
|
|
|
Fees/Other |
|
|
|
|
Use of Fund Balance |
$112,042 |
|
|
|
General Fund Contingencies |
|
|
|
|
Total Sources |
$112,042 |
|
|
Narrative Explanation of Fiscal Impacts:
Additional appropriations are required to process this expense. With Board approval of the attached budgetary resolution, FY 2024/2025 appropriations of $112,042 will be made in the Water Transmission Fund.
Narrative Explanation of Staffing Impacts (If Required):
N/A
Attachments:
Attachment 1: Agreement with USGS
Attachment 2: Resolution
Related Items “On File” with the Clerk of the Board:
None.