Seawater Intrusion Protection

1
Seawater Intrusion Protection
San Juan County, Washington November 29, 2005
2
Saltwater Intrusion

• The movement of saline (salty) water into a
  freshwater aquifer
• If source is marine water, the process is known
  as Seawater Intrusion
• Can result in loss of aquifer / resource
• Concerns have resulted in adoption of current San
  Juan County Seawater Intrusion Policy

3
(No Transcript)
4
(No Transcript)
5
(No Transcript)
6
San Juan CountySeawater Intrusion Policy

• Creates Susceptibility Ranking Categories
• Based on Chloride Concentration
• Also Includes a 1000 ft buffer on Lopez Island
  Shoreline

• Utilizes a 1000 ft Radius Around Impacted Wells
• Currently Only Chloride Concentrations in
  Applicants Well are Reviewed
• Current Review Process does not Evaluate Impacts
  to Other Users of the Groundwater Resource

7
(No Transcript)
8
Seawater Intrusion Protection
Three Principle Components

• Triggering Mechanism
• Where Should Policy Apply?
• e.g. Chloride, Water Level Elevation, Shoreline
  Buffer
• Applicability
• What Type of Projects are Reviewed?
• e.g. New Water Systems, Land Subdivision
• Implications
• Resulting Actions for Triggered Applicable
  Projects
• e.g. Testing, Monitoring, Hydrogeologic
  Analysis, Require Mitigation of Impacts or
  Project Denial

9
Chloride Based Trigger

• Ease of Implementation
• Testing Inexpensive
• Substantial Existing Data
• Other Sources of Chloride (False Positives)
• Very Hard Groundwater Septic Systems
• Connate (Old) Waters
• Windblown Sea Spray
• Irrigation Recharge
• Well Disinfection
• Transient Response of Interface (False Negatives)
• Lateral Intrusion May Take Years to Establish a
  New Equilibrium Position
• Lack of Prediction
• Lack of Confidence

10
Shoreline Buffer Trigger

• Conceptually Sensible
• Intrusion begins at shoreline
• If intrusion problems are extensive may be
  effective
• Ease of Implementation
• Areas are easy to identify
• Virtually no Cost unless
• Potentially Overly Conservative
• If intrusion problems are not widespread will
  likely result in review of projects that are not
  at risk.
• For most of the county, current chloride
  distributions do not indicate a need for broad
  shoreline buffering.

11
Water Level Elevation Trigger

• Adequate pressure can prevent seawater intrusion
• The primary factor underlying seawater intrusion
• Provides confidence in resource (if appropriate)
• High water level elevations prevent intrusion
• Provides a predictive (rather than reactive) tool
• Low water level elevations indicate intrusion
  susceptibility prior to onset of intrusion
• Data Availability
• Little data currently available
• Cost of data is significantly higher than other
  methods

12
Island County Phase II Assessment

• Data Collected from Nearly 400 Wells / 2 per
  Mile2
• Depth to Water Measurements
• Computerized Data Loggers

• Water Sample ? Laboratory
• Major Ion Analysis

• Survey Grade GPS to Determine Measuring Point
  Elevations

13
Phase II Data Analysis

• Evaluate the use of Water Level Elevation
• Water Level Elevation vs. Chemistry.
• Complicated by Non-Intrusion Chemistry Problems

14
Conceptual Model
15
Countywide Data

• Low Water Level Elevation Near Shorelines
• Low WLE Clustered

• False Negatives

• High WLE

• False Interface / Confidence

16
Triggering Mechanism Concepts
?

Small Projects Allowed Unlikely to Cause
Significant Problems Large Projects Reviewed
All Projects Reviewed
Select Trigger Values?
17
Chloride Trigger Selection
Current SJC Triggers 100 and 160 mg/l
to Avoid False Positives WLE as Initial
Screen Greatly Reduce False Positives Any
defined trigger level will in the long run be
met Additional withdrawals will continue (at some
rate)
18
WLE Trigger Selection

• Utilize Statistical Analysis
• of Intruded Wells
• Intruded Based on All 2514 Analysis
• Screened Below Sea Level
• Not in Central Whidbey

of Intruded Wells
19
Seawater Intrusion Protection
Science or Policy?
Cost of Protection Applicants Regulators
Seawater Intrusion Loss of Resource

• Although Seawater Intrusion is Analyzed /
  Predicted by Science
• Protection Strategies Must Find a Balance Between
  Risks of Failure (Loss of Resource) and Costs of
  Protection

Science or Policy ? Both
20
Seawater Intrusion Protection
Where do we go from here?

• Seawater intrusion policy is intended to provide
  triggering for project review, not a final
  diagnostic on intrusion potential
• Once review is triggered, a projects intrusion
  potential is evaluated after collection of
  site-specific data
• San Juan County Water Resources Management
  Committee to review options
• Provide recommendations for modifications of
  triggers, applicability and implications

21
Questions?