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. Coastal groundwater supplies are vulnerable to chloride contamination. Validate linear relationship of specific conductance (SC) to chloride concentration. Validate linear relationship of specific conductance (SC) to chloride concentration. Advantages of using conductivity sensor for long-term field deployments. Advantages of using conductivity sensor for long-term field deployments.

Goals of This Study Satisfy customer requests for a viable field technique to estimate chloride. Satisfy customer requests for a viable field technique to estimate chloride. Provide a laboratory procedure that a field hydrologist can perform. Provide a laboratory procedure that a field hydrologist can perform. Minimize the amount of analytical costs and equipment needed to develop data. Minimize the amount of analytical costs and equipment needed to develop data.

Saltwater Intrusion – Coastal

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

Chemically Conservative Parameters SC and chloride are chemically conservative or stable water quality indicators and tracers. 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. Chloride is least affected by movement away from the source and provides a true representation of contamination.

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

Current Chloride Measurement Technologies Chloride ISEs – Sensitive to drift, fouling, and not designed for field deployments. Chloride ISEs – Sensitive to drift, fouling, and not designed for field deployments. Titrimetric methods – Less precise and may use hazardous chemicals. Titrimetric methods – Less precise and may use hazardous chemicals. Ion chromatography – Very accurate but potentially expensive laboratory technique. Ion chromatography – Very accurate but potentially expensive laboratory technique.

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

Methodology – Correlation Testing OSIL Atlantic Seawater Standard (35.0 PSU) was diluted to 10 additional concentrations. 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. 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. Total of 66 samples stabilized in thermal bath for a minimum of 1 hour.

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

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

Methodology – Drift Testing SC values compared to chloride ISE values during a 7-day continuous test. 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. 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. Secondary NIST-calibrated conductivity and Cl - sensors used to monitor test solution.

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. 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 Conductivity sensor: ±0.5% of reading

SC and Chloride Relationship

Low chloride concentrations: Chloride concentration and SC values showed strong linearity (R 2 = ) Low chloride concentrations: Chloride concentration and SC values showed strong linearity (R 2 = ) Low to high chloride concentrations: Chloride concentration (x) and SC values (y) showed strong linearity (R 2 = ) Low to high chloride concentrations: Chloride concentration (x) and SC values (y) showed strong linearity (R 2 = )

Drift Results

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

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

Conclusions Conductivity Sensors Proven, stable method for measuring SC. Proven, stable method for measuring SC. Much less susceptible to drift than ISEs. Much less susceptible to drift than ISEs. Require less maintenance than ISEs. Require less maintenance than ISEs. Saves on analytical testing costs. Saves on analytical testing costs. Recalibrate every 3 to 6 months depending on matrix vs. daily recalibration for ISE. Recalibrate every 3 to 6 months depending on matrix vs. daily recalibration for ISE.

Conclusions Conductivity Sensors Ideal for field deployments and long-term monitoring to generate real-time data. Ideal for field deployments and long-term monitoring to generate real-time data. Develop more robust data sets. Develop more robust data sets. Matrix-specific linear correlation to chloride. Matrix-specific linear correlation to chloride. Use correlation data to estimate chloride. Use correlation data to estimate chloride.

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

Additional Resources Application and Technical Notes: Conductivity Measurement Methodology Conductivity Measurement Methodology Controlling Saltwater Intrusion in CA Controlling Saltwater Intrusion in CA Hurricane Surge and Inland Saltwater Impacts Hurricane Surge and Inland Saltwater Impacts Tracking Saltwater Intrusion in Coastal Aquifers Tracking Saltwater Intrusion in Coastal Aquifers Three-Point, Bi-Thermal Calibration of ISEs Three-Point, Bi-Thermal Calibration of ISEs White paper: Verifying SC as a Surrogate for Chloride Verifying SC as a Surrogate for Chloride

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