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

1. Verifying the Use of Specific Conductance as a Surrogate for Chloride in Seawater Matrices Rob Mooney Technical Marketing Manager In-Situ® Inc.

2. Abstract Summary • 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.

3. Goals of This Study • 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.

4. Saltwater Intrusion – Coastal

5. Saltwater Intrusion – Inland • 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.

6. Chemically Conservative 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.

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

8. Current Chloride Measurement 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.

9. Validating Use of SC as a Surrogate for Chloride • SC can be directly correlated to chloride concentration. • Balanced cost of ISE lab technique vs. reduced accuracy compared to IC.

10. Methodology – Correlation Testing • 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.

11. Methodology – Correlation Testing • 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.

12. Methodology – Correlation Testing • Five readings were averaged to determine the final response value for each test point. • Results were plotted to compare chloride and SC values.

13. Methodology – Drift Testing • 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.

14. Technology Comparison 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

15. SC and Chloride Relationship

16. SC and Chloride Relationship

17. SC and Chloride Relationship • 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)

18. Drift Results

19. Drift Results • 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.

20. Conclusions • 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.

21. ConclusionsConductivity 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.

22. ConclusionsConductivity 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.

23. Applications • Saltwater intrusion monitoring • Salt marsh and coastal wetlands research • Aquifer storage and recovery systems

24. Additional Resources 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

25. 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