TO: Technical Advisory Group (TAG) for the Napa County Groundwater Sustainability Agency
FROM: Brian D. Bordona, Director of Planning, Building and Environmental Services
REPORT BY: Jamison Crosby, Natural Resources Conservation Manager
SUBJECT: Napa County groundwater monitoring networks and associated data gaps
RECOMMENDATION
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The TAG will receive an overview of Napa County groundwater monitoring networks and associated data gaps.
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BACKGROUND AND DISCUSSION
Napa County has long identified the importance of groundwater, surface water, and their interactions. Monitoring of these important natural resources has been conducted by multiple entities, including the United States Geological Survey (USGS), California Department of Water Resources (DWR), and Napa County. In 2013, Napa County reviewed and updated their groundwater monitoring plan and implemented numerous improvements. Annual reporting of groundwater resources for the entirety of Napa County began in 2014. Building on decades of historical monitoring, Napa County formalized nine (9) monitoring networks within the Napa Valley Subbasin Groundwater Sustainability Plan (GSP; 2022). As part of the GSP, data gaps associated with each monitoring network were identified with an associated plan to address the data gaps. During the past two and a half years of GSP implementation activities, many data gaps have been addressed.
The GSP established nine (9) monitoring networks with different measurement types.
• Groundwater Level Monitoring Network
o Groundwater levels
• Groundwater Storage Monitoring Network
o Groundwater levels
o Napa Valley Integrated Hydrologic Model (NVIHM)
• Land Subsidence Monitoring Network
o Groundwater levels
o Benchmark monitoring
o InSAR data
• Stream Stage and Stream Discharge Monitoring Network
o Stream stage and discharge measurements
o Stream Watch program
o Napa Flood Control Monitoring system
• Interconnected Surface Water-Groundwater
o Groundwater levels
o NVIHM
• Groundwater Dependent Ecosystems (GDE) Monitoring Network
o Groundwater levels
o Stream habitat monitoring
o Remote sensing vegetation health
• Groundwater Quality Monitoring Network
o Groundwater quality measurements
• Seawater Intrusion Monitoring Network
o Water quality and chloride measurements
• Surface Water Quality Monitoring Network
o Surface water quality measurements
The groundwater level monitoring network was established to improve the understanding of the occurrence and movement of groundwater; monitor local and regional groundwater levels including seasonal and long‐term trends; and identify vertical hydraulic head differences in the aquifer system and aquifer‐specific groundwater conditions. Data gaps assessed during GSP development included the need to review well construction information and improve monitoring in six key areas of the Subbasin. The Napa GSA has improved the quality of the monitoring data through recruitment of voluntary wells with known construction details and the installation of additional dedicated dual-completion monitoring wells; the interconnected surface water and groundwater monitoring sites were expanded from five (5) existing locations to thirteen (13) monitoring locations. The Napa County groundwater level monitoring network now includes 116 wells with 74 wells located within the Napa Valley Subbasin. This is approximately one well per 650 acres within the Subbasin.
Groundwater storage is evaluated as a subset of wells within the groundwater level monitoring network. The monitoring wells used to evaluate the change in groundwater storage must be screened within the alluvial aquifer and represent stable groundwater level conditions, i.e., no recent pumping at or near the well. The change in groundwater storage monitoring network has grown to include 32 wells, as of the Spring 2023 evaluation.
Interconnected surface water-groundwater (ISW) is also evaluated based on a subset of groundwater level monitoring wells. This monitoring network includes wells that are very shallow and located near a river and/or dedicated dual-completion monitoring wells. During GSP development, data gaps associated with this monitoring network included the need for four (4) additional dual-completion monitoring locations. The installation of new dual-completion monitoring wells, discussed above, addressed this data gap. Additional refinements to the ISW monitoring network have been identified, including refinements to the stream gage and discharge monitoring network; this would facilitate improved quantification of the contributions of groundwater during low-flow conditions.
The Napa Valley Integrated Hydrologic Model (NVIHM) is an integrated surface water-groundwater hydrologic model that aids Napa County in understanding how groundwater flows through the aquifer system and account for groundwater withdrawals. The NVIHM is part of the change in groundwater storage and ISW monitoring network. Identified hydrogeologic data gaps in the GSP include understanding the influence of highly heterogeneous properties of Quaternary deposits and the role of older geologic formations as aquifers. The NVIHM will be being utilized to address these data gaps once more refined geology, based on new well completion reports, and additional calibration with updated monitoring well observations have been completed.
Groundwater quality conditions are evaluated through the combination of GSP-specific monitoring, conducted by Napa County, in addition to compiling data from other monitoring entities, such as public water suppliers. Constituents of concern have been identified as arsenic, iron, manganese, and nitrate. Arsenic, iron, and manganese have historically been found in groundwater in Napa County as part of natural weathering of source rock as well as the presence of a hydrothermal system north of Calistoga. Nitrate, typically associated with agricultural applications, onsite septic systems, and other sources of nitrogen, has not historically been found in high concentrations within the Napa Valley Subbasin. Multiple factors, including low-application rates in vineyards as well as high rates of recharge in the winter, have contributed to low nitrate concentrations.
Seawater intrusion monitoring includes groundwater sampling for chloride, sodium, electrical conductivity, and/or total dissolved solids. The seawater intrusion monitoring network extends south of the Subbasin to better characterize the gradient from the San Pablo Bay through the Napa-Sonoma Lowlands Subbasin and into the tidally influenced sections of the Napa River, typically within the City of Napa. One (1) new dual-completion monitoring well has been included in the seawater intrusion monitoring network to provide long-term data to better evaluate temporal trends.
Monitoring groundwater dependent ecosystems in the GSP occurs through a combination of shallow monitoring wells and remotely sensed vegetation health. Data gaps associated with this monitoring included the need for additional shallow monitoring wells, which was conducted through the installation of the dual-completion ISW monitoring wells, and the development of a workplan that included biological field assessments, review of existing data, investigations of stream habitat conditions, and evaluation of listed species. The development of the Interconnected Surface Water and Groundwater Dependent Ecosystems Workplan (Workplan) was completed with implementation beginning in May 2024. The Workplan identified six intensive survey sites: four on the Napa River (Napa River at Calistoga, Napa River at St. Helena, Napa River at Yountville, Napa River at Oak Knoll) and two tributary sites (Sulphur Creek and Bale Slough). Access was available to four of the sites for summer 2024 (Napa River at Calistoga, Napa River at St. Helena, Napa River at Yountville, and Sulphur Creek). Access was recently obtained to the Napa River at Oak Knoll site, and access permissions are still in discussion for the Bale Slough site.
Surveys conducted in 2024 at each of the sites include:
• fish habitat and fish population;
• two surveys for aquatic wildlife (one in May and one in July);
• eDNA sampling for northwestern pond turtle and foothill yellow-legged frog;
• California freshwater shrimp in the Calistoga Reach of the Napa River;
• flow connectivity studies; and
• continuous water quality sampling for temperature and dissolved oxygen.
Foothill yellow-legged frog tadpoles, eggs, and subadults were observed at the Napa River at St. Helena and Sulphur Creek, while northwest pond turtle was observed at the Napa River at St. Helena and at Yountville. The water quality measurements are still being collected and will be processed this fall. The studies described above will be conducted again in 2025 and expanded to the Napa River at Oak Knoll and Bale Slough sites (pending access approval). Over the next year, we will begin our California Environmental Flows Framework assessment starting with the results of the 2024 surveys to determine the ecological management goals at each site.
Procedure
Staff introduces.
Questions and answers with the TAG.
Public comments.
ENVIRONMENTAL IMPACT
ENVIRONMENTAL DETERMINATION: The proposed action is not a project as defined by 14 California Code of Regulations 15378 (State CEQA Guidelines) and therefore CEQA is not applicable.
SUPPORTING DOCUMENTS
A. Napa County Groundwater Sustainability Agency, Monitoring Network, Data Gaps, and Future Implementation (LSCE, September 2024)