Seawater Intrusion Protection

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					Seawater Intrusion Protection


       San Juan County, Washington
              November 29, 2005
       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
                  San Juan County
               Seawater Intrusion Policy
•   Creates “Susceptibility Ranking Categories”
•   Based on Chloride Concentration
•   Also Includes a 1000 ft buffer on Lopez Island Shoreline
     Susceptibility Category         Chloride (mg/L)
              Low                        < 100
             Medium                      100-160
                High                      > 160
•   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
Seawater Intrusion Protection
Three Principle Components
1. Triggering Mechanism
   Where Should Policy Apply?
   e.g. Chloride, Water Level Elevation, Shoreline Buffer
2. Applicability
   What Type of Projects are Reviewed?
   e.g. New Water Systems, Land Subdivision
3. Implications
   Resulting Actions for Triggered Applicable Projects
   e.g. Testing, Monitoring, Hydrogeologic Analysis,
   Require Mitigation of Impacts or Project Denial
               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
               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.
            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
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
                                    Phase II Data Analysis
         •               Evaluate the use of Water Level Elevation
         •               Water Level Elevation vs. Chemistry….
         •               Complicated by Non-Intrusion Chemistry Problems
                         400
                                                                                      Central Whidbey
                                                                                      Other Wells
                         300
Water Level Elevation
(feet above sea level)




                         200




                         100




                          0
                               0   200   400   600        800           1000   1200       1400      1600
                                                     Chloride (m g/l)
Conceptual Model
                 Countywide Data
•   Low Water Level Elevation
    Near Shorelines
•   Low WLE Clustered
•   False Negatives
•   High WLE
•   False Interface / Confidence
  Triggering Mechanism Concepts

   Low WLE · no           ?      Low WLE · with
  Chemistry Impacts       =     Chemistry Impacts

Small Projects Allowed         All Projects Reviewed
  Unlikely to Cause
 Significant Problems
Large Projects Reviewed       Trigger Combines
                              WLE & Chloride
                               Select Trigger Values?
       Chloride Trigger Selection
                                10000                       Chloride vs. Conductivity




Current SJC Triggers                     1000




                           Chloride (mg/l)
100 and 160 mg/l                             100




to Avoid False Positives                     10




                                              1
                                               100   1000                               10000   100000


WLE as Initial Screen
                                                                Conductivity (µs/cm)




Greatly Reduce False Positives

Any defined trigger level will – in the long run – be met
Additional withdrawals will continue (at some rate)
          WLE Trigger Selection
Utilize Statistical Analysis
     of Intruded Wells

•   Intruded Based on All 2514 Analysis
•   Screened Below Sea Level
•   Not in Central Whidbey




     Levels Below Threshold = Risk
    Seawater Intrusion Protection
               Science or Policy?

 Seawater Intrusion               Cost of Protection
  Loss of Resource                $ Applicants
                                  $ Regulators

• 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
     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
Questions?

				
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posted:11/24/2011
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