Sustainable Management Criteria: Seawater Intrusionsonomavalleygroundwater.org/wp-content/uploads/05... · Seawater Intrusion Overview. Representative Chloride Concentrations for

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Sustainable Management Criteria: Seawater IntrusionWhat we will cover today:• SGMA considerations• Background information• Significant and Unreasonable Conditions: Suggested approach• How to measure: Suggested approach• Minimum Thresholds and Measurable Objectives: Suggested approach• Undesirable Result: Suggested approach• Questions and feedback

5/8/2020 SONOMAVALLEYGROUNDWATER.ORG 1


Key Points/Considerations• Available data suggest some seawater intrusion

into the Baylands area of Sonoma Valley from San Pablo Bay, however characterizing the distribution and trends is limited by significant data gaps.

• The GSA may establish that seawater intrusion is acceptable in areas with minimal groundwater pumping, where seawater intrusion may not cause a significant or unreasonable condition.

• Control of high salinity connate waters (older water not associated with recent seawater) is not covered by this SMC

Seawater Intrusion OverviewRepresentative Chloride Concentrations for Different Water Bodies

Fresh Groundwater: <250 mg/L Brackish estuaries: 500 to 5,000 mg/L Seawater: > 19,400 mg/L Deep Connate water: Variable

• In Sonoma Valley, brackish water occurring beneath the Baylandsarea, which is connected to San Pablo Bay, represents the primary threat to beneficial users associated with this SMC

• For purposes of establishing the seawater intrusion SMC for Sonoma Valley, the definition for seawater intrusion includes the intrusion of this brackish water into the freshwater portions of the aquifer system


Significant & Unreasonable Conditions

Suggested approach: Any Seawater intrusion inland of areas of existing brackish groundwater is a significant and unreasonable condition.

Existing seawater intrusion is not considered unreasonable because there are currently few beneficial uses and users in the areas of existing brackish groundwater.


• DWR’s GSP Regulations defined metric for measuring subsidence is the chloride concentration isocontour

• Additional metrics can be applied to support monitoring of the isocontour, including:• Chloride concentrations at

representative monitoring points• Groundwater elevation proxies (if a

correlation can be established)

Establishing Metrics

Establishing Metrics


Suggested approach: Use isocontours as the seawater intrusion metric. Supplement the chloride isocontour by defining chloride concentrations at representative monitoring points. These representative monitoring points should exclude wells known to be affected by connate groundwater.


Available Data and Monitoring are Limited

• We can only infer the approximate location of the existing 250 mg/L chloride concentration isocontour due to the very limited available data (and no recent data from wells exceeding 250 mg/L)

• New Representative Monitoring Points will be needed to verify the location of the MT/MO is appropriate and monitor for compliance with the SMC in the future


Minimum Threshold and Measurable Objective

Suggested approach for MT: Establish the minimum threshold as a 250 mg/L* isocontour at, or inland from, the existing 250 mg/L isocontour.

Suggested approach for MO: Establish the measurable objective as a 150 mg/L isocontour at the same location.

* 250 mg/L is the secondary maximum contaminant level (MCL) for chloride and is based on the concentration where water can begin to taste salty.


Initial Recommended Location for MT/MO

• Set initial MT/MO between existing inferred area of brackish groundwater and beneficial users

• The initial MT/MO line will likely need to be adjusted as additional data is gathered during GSP implementation

• Initial MT/MO line reflects intentions to protect beneficial users from impacts of seawater intrusion


Priority Areas for Future Representative Monitoring Points (RMPs)

• Five potential areas for future RMPs are prioritized and shown here between inferred margins of brackish groundwater and areas with higher water well densities• Verify appropriateness of MT/MO

location• Establish monitoring network for

seawater intrusion SMC• Consider utilizing existing wells

where accessible and appropriately constructed

Undesirable Results


Suggested approach: An undesirable result occurs based on any exceedance of the minimum threshold chloride isocontour, or any exceedance of the minimum threshold chloride concentrations at a representative monitoring point.


Discussion and QuestionsSustainability Significant & Minimum Threshold Undesirable Result Measurable

Indicator Unreasonable ObjectiveCondition

Any Seawater Establish the Any exceedance of the Establish the intrusion inland of minimum threshold as minimum threshold measurable objective Sea Water areas of existing a 250 mg/L isocontour chloride isocontour, or as a 150 mg/L Intrusion brackish at, or inland from, the any exceedance of the isocontour at the groundwater is a existing 250 mg/L minimum threshold same location as the significant and isocontour. chloride concentrations minimum threshold unreasonable at a representative isocontour.condition. monitoring point.

1. Do you have concerns about the approach for setting the proposed seawater intrusion SMC?

2. Is there anything further you feel should be considered in the proposed seawater intrusion SMC?

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Sustainable Management Criteria: Seawater Intrusionsonomavalleygroundwater.org/wp-content/uploads/05... · Seawater Intrusion Overview. Representative Chloride Concentrations for