Verifying the use of specific conductance as a surrogate for chloride in seawater matrices
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Verifying the Use of Specific Conductance as a Surrogate for Chloride in Seawater Matrices. Rob Mooney Technical Marketing Manager In-Situ ® Inc. Abstract Summary. Coastal groundwater supplies are vulnerable to chloride contamination.

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Verifying the use of specific conductance as a surrogate for chloride in seawater matrices l.jpg

Verifying the Use of Specific Conductance as a Surrogate for Chloride in Seawater Matrices

Rob Mooney

Technical Marketing Manager

In-Situ® Inc.


Abstract summary l.jpg
Abstract Summary Chloride in Seawater Matrices

  • Coastal groundwater supplies are vulnerable to chloride contamination.

  • Validate linear relationship of specific conductance (SC) to chloride concentration.

  • Advantages of using conductivity sensor for long-term field deployments.


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Goals of This Study Chloride in Seawater Matrices

  • Satisfy customer requests for a viable field technique to estimate chloride.

  • Provide a laboratory procedure that a field hydrologist can perform.

  • Minimize the amount of analytical costs and equipment needed to develop data.


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Saltwater Intrusion – Coastal Chloride in Seawater Matrices


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Saltwater Intrusion – Inland Chloride in Seawater Matrices

  • Interior of U.S. – Deep saline water underlies fresh water. Withdrawing water from overlying aquifers increases potential for saltwater intrusion from below.

  • Road salt impacts – Shallow aquifers and surface waters near roads may be impacted.


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Chemically Chloride in Seawater MatricesConservative Parameters

  • SC and chloride are chemically conservative or stable water quality indicators and tracers.

  • Chloride is least affected by movement away from the source and provides a true representation of contamination.


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Chloride Retardation Chloride in Seawater Matrices

Chloride shows little to no retardation effect in various aquifer matrices.


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Current Chloride Chloride in Seawater MatricesMeasurement Technologies

  • Chloride ISEs – Sensitive to drift, fouling, and not designed for field deployments.

  • Titrimetric methods – Less precise and may use hazardous chemicals.

  • Ion chromatography – Very accurate but potentially expensive laboratory technique.


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Validating Use of SC as a Surrogate for Chloride Chloride in Seawater Matrices

  • SC can be directly correlated to chloride concentration.

  • Balanced cost of ISE lab technique vs. reduced accuracy compared to IC.


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Methodology – Correlation Testing Chloride in Seawater Matrices

  • OSIL Atlantic Seawater Standard (35.0 PSU) was diluted to 10 additional concentrations.

  • 11 concentrations brought to temperatures of 0, 10, 20, 30, 40, and 50° C.

  • Total of 66 samples stabilized in thermal bath for a minimum of 1 hour.


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Methodology – Correlation Testing Chloride in Seawater Matrices

  • Prior to sample analysis, chloride ISE was calibrated using a 3-point, bi-thermal calibration with NIST-traceable chloride standards and validated throughout testing.

  • Five replicate readings were taken at each of the 66 chloride/temperature test points.


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Methodology – Correlation Testing Chloride in Seawater Matrices

  • Five readings were averaged to determine the final response value for each test point.

  • Results were plotted to compare chloride and SC values.


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Methodology – Drift Testing Chloride in Seawater Matrices

  • SC values compared to chloride ISE values during a 7-day continuous test.

  • Hourly readings taken in a 17 PSU dilution of OSIL Atlantic Seawater Standard.

  • Secondary NIST-calibrated conductivity and Cl- sensors used to monitor test solution.


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Technology Comparison Chloride in Seawater Matrices

Accuracy

  • Chloride ISE: ± 15% of reading or 5 mg/L, whichever is greater. Accuracy can be maximized by performing a three-point,bi-thermal calibration.

  • Conductivity sensor: ±0.5% of reading


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SC and Chloride Relationship Chloride in Seawater Matrices


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SC and Chloride Relationship Chloride in Seawater Matrices


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SC and Chloride Relationship Chloride in Seawater Matrices

  • Low chloride concentrations: Chloride concentration and SC values showed strong linearity (R2 = 0.9887)

  • Low to high chloride concentrations: Chloride concentration (x) and SC values (y) showed strong linearity (R2 = 0.9845)


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Drift Results Chloride in Seawater Matrices


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Drift Results Chloride in Seawater Matrices

  • Chloride ISE drift over 7 days: 1,036 mg/L or 8.4% of the reading.

  • Conductivity sensor drift over 7 days: 25 µS/cm or 0.08% of the reading.

    • Equates to a drift of ≈15 mg/L chloride at this range.


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Conclusions Chloride in Seawater Matrices

  • Strong correlation validates use of SC as a surrogate for chloride in this study.

  • Stability of conductivity sensor and strong linear correlation indicate advantage for using SC as a surrogate for chloride in situations that require real-time monitoring.


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Conclusions Chloride in Seawater MatricesConductivity Sensors

  • Proven, stable method for measuring SC.

  • Much less susceptible to drift than ISEs.

  • Require less maintenance than ISEs.

  • Saves on analytical testing costs.

  • Recalibrate every 3 to 6 months depending on matrix vs. daily recalibration for ISE.


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Conclusions Chloride in Seawater MatricesConductivity Sensors

  • Ideal for field deployments and long-term monitoring to generate real-time data.

  • Develop more robust data sets.

  • Matrix-specific linear correlation to chloride.

  • Use correlation data to estimate chloride.


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Applications Chloride in Seawater Matrices

  • Saltwater intrusion monitoring

  • Salt marsh and coastal wetlands research

  • Aquifer storage and recovery systems


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Additional Resources Chloride in Seawater Matrices

Application and Technical Notes:

  • Conductivity Measurement Methodology

  • Controlling Saltwater Intrusion in CA

  • Hurricane Surge and Inland Saltwater Impacts

  • Tracking Saltwater Intrusion in Coastal Aquifers

  • Three-Point, Bi-Thermal Calibration of ISEs

    White paper:

  • Verifying SC as a Surrogate for Chloride

    www.in-situ.com


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Verifying the Use of Specific Conductance as a Surrogate for Chloride in Seawater Matrices

Rob Mooney

Technical Marketing Manager

In-Situ® Inc.

970-498-1655

rmooney@in-situ.com