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1. What is a GSP?

A Groundwater Sustainability Plan (GSP) is the state-mandated plan to sustainably manage groundwater. Each GSP will contain an assessment of each basin, measurable objectives that are specific, and quantifiable goals for the maintenance or improvement of specified groundwater conditions that lead to achieving the sustainability goal for the basin. A GSP must also include any necessary monitoring, management, enforcement, and other requirements to achieve and maintain sustainability.

2. What is the goal of the GSP?

The goal of SGMA is to ensure that local or regional agencies have the necessary support and authority to properly manage groundwater resources through the use of information and evaluations to protect communities, farms, and the environment against prolonged dry periods and climate change, and to preserve water supplies for existing and potential beneficial uses. The goal is eliminating all undesirable results within 20 years of GSP approval and continue to sustainably manage the subbasin over the next 50 years and beyond.

3. What are required elements of a GSP?

Required elements of a GSP are:

A description of the physical setting and characteristics of the aquifer system underlying the basin, including:

• Historical data
• Groundwater levels, groundwater quality, subsidence, and groundwater-surface water interaction
• Historical and projected water demands and supplies
• A map of the basin area and the boundaries of the groundwater sustainability agencies that overlie the basin
• A map identifying existing and potential recharge areas for the basin

Measurable objectives to achieve the sustainability goal in the basin within 20 years of the implementation of the plan, including interim milestones.

• How the plan helps meet each objective
• How each objective is intended to achieve the sustainability goal

A planning and implementation horizon

Information about the following, as applicable:

• The monitoring and management of groundwater levels within the basin.
• The monitoring and management of groundwater quality, groundwater quality degradation, inelastic land surface subsidence, and changes in surface flow and surface water quality that directly affect groundwater levels or quality or are caused by groundwater extraction in the basin.
• Mitigation of overdraft.
• How recharge areas identified in the plan substantially contribute to the replenishment of the basin.
• A description of surface water supply used or available for use for groundwater recharge or in-lieu use.
• A summary of existing monitoring efforts, including site types, measured data, and monitoring frequency. These include all locations monitoring groundwater levels, groundwater quality, subsidence, streamflow, precipitation, evaporation, and/or tidal influence.
• Monitoring protocols for detecting changes in groundwater levels, groundwater quality, and flow and quality of surface water that are linked to groundwater levels, quality, or extraction.
• An assessment of how the GSP may affect applicable county and city general plans, as well as the various adopted water resources-related plans and programs within the basin.

More detailed requirements are identified in Section 10727 of the California Water Code.

4. How do the GSP and the Napa Valley Integrated Hydrologic Model account for future development?

Section 8 of the GSP addresses future development and population change in the context of the water budget analysis, using best-available data. Projections of future development in the Subbasin are incorporated to provide consistency with local land use agency assumptions and projections. Projected water budget results were made available in Summer 2021. For more information, see Section 8 of the GSP.

5. Does the water budget account for extractors of groundwater and surface water outside of the Subbasin?

SGMA and DWR GSP Regulations require that GSPs include a water budget that quantifies inflows to, outflows from, and changes in storage for the relevant groundwater basin or subbasin. For this reason, the water budget presented in the GSP will address the Napa Valley Subbasin. However, the County and technical team recognize the potential for land uses adjacent to the Subbasin to affect the Subbasin water budget and have expanded the Napa Valley Integrated Hydrologic Model (MODFLOW-OWHM) domain to include areas adjacent to the Napa Valley Subbasin including the Milliken-Sarco-Tulucay and Carneros Subareas as well as other areas of developed land use that are contiguous with the Subbasin. These additional areas include areas served by the City of Napa potable water system west of the Subbasin, the Yountville Veterans Home, and agricultural lands contiguous with the Subbasin boundary.

6. What is the timeline for GSP submittal?

The implementation process of the GSP is continuous and will be ongoing over at least the next 50 years. The GSA adopted the Groundwater Sustainability Plan on January 11, 2022 and submitted the GSP to DWR on January 31, 2022. DWR has 20 days to complete their initial review prior to making the GSP public on their SGMA portal. DWR will then offer a 75-day public comment period. DWR has two years to determine whether the GSP meets the requirements of SGMA.

The GSP must be updated every five years. Annual reporting is required to demonstrate implementation actions and progress towards either achieving or maintaining groundwater sustainability. SGMA Annual Reports are due to DWR every April 1; the next Annual Report is due April 1, 2022. The next GSP update will be due in 2027.

7. How is the GSP related to the Napa Valley Drought Contingency Plan?

Both planning efforts will propose projects and management actions to achieve their respective goals. While the two efforts are on slightly different timelines, there is opportunity for collaborating on potential joint projects that address both groundwater and drought resiliency. The Draft Napa Valley DCP is expected to be available in Spring 2022 while the adopted GSP is currently available.

8. What is the study of Hennessey and Milliken watersheds and how does it relate to the GSP?

In 2017, the City and County of Napa entered into an agreement to study surface water quality within the municipal watersheds of the reservoirs that provide the City with the majority of its drinking water. The Study provides detailed and comprehensive data that can be used by both the City and the County to improve reservoir and land use management to better protect the public’s water supply.

The Study calls for collecting water quality samples from 20 locations along streams that flow into Milliken and Hennessey reservoirs. Samples are collected during storm events throughout the winter months and analyzed for nearly 30 different constituents. The voluntary 2017 agreement commits both the City and County to each spend up to $200,000 annually over three years to conduct this Study, for a total cost of $1.2 million.

Samples were collected over three years, although the last two years were very dry and there were not many storms where measurements could be collected, so data was limited. However, no significant contamination has been detected to date.

The study does not directly relate to the GSP, as it analyzes water quality in the upper watersheds, outside of the Napa Valley Subbasin. But it does complement the GSP, as it provides valuable information on the water quality of surface flows in the tributaries that flow into the subbasin.

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