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

Rob MooneyRob Mooney

Technical Marketing ManagerTechnical Marketing Manager

In-SituIn-Situ®® Inc. Inc.

Abstract SummaryAbstract Summary

• Coastal groundwater supplies are vulnerable Coastal groundwater supplies are vulnerable to chloride contamination.to chloride contamination.

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

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

Goals of This StudyGoals of This Study

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

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

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

Saltwater Intrusion – CoastalSaltwater Intrusion – Coastal

Saltwater Intrusion – InlandSaltwater Intrusion – Inland

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

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

Chemically Chemically Conservative ParametersConservative Parameters

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

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

Chloride RetardationChloride Retardation

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

Current Chloride Current Chloride Measurement TechnologiesMeasurement Technologies

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

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

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

Validating Use of SC as a Validating Use of SC as a Surrogate for ChlorideSurrogate for Chloride

• SC can be directly correlated to chloride SC can be directly correlated to chloride concentration.concentration.

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

Methodology – Correlation Methodology – Correlation TestingTesting

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

• 11 concentrations brought to temperatures of 11 concentrations brought to temperatures of 0, 10, 20, 30, 40, and 500, 10, 20, 30, 40, and 50°° C. C.

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

Methodology – Correlation Methodology – Correlation TestingTesting

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

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

Methodology – Correlation Methodology – Correlation TestingTesting

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

• Results were plotted to compare chloride and Results were plotted to compare chloride and SC values.SC values.

Methodology – Drift TestingMethodology – Drift Testing

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

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

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

Technology ComparisonTechnology Comparison

AccuracyAccuracy• Chloride ISE: ± 15% of reading or 5 mg/L, Chloride ISE: ± 15% of reading or 5 mg/L,

whichever is greater. Accuracy can be whichever is greater. Accuracy can be maximized by performing a three-point,maximized by performing a three-point,bi-thermal calibration.bi-thermal calibration.

• Conductivity sensor: ±0.5% of readingConductivity sensor: ±0.5% of reading

SC and Chloride RelationshipSC and Chloride Relationship

SC and Chloride RelationshipSC and Chloride Relationship

SC and Chloride RelationshipSC and Chloride Relationship

• Low chloride concentrations: Chloride Low chloride concentrations: Chloride concentration and SC values showed strong concentration and SC values showed strong linearity (Rlinearity (R22 = 0.9887) = 0.9887)

• Low to high chloride concentrations: Low to high chloride concentrations: Chloride concentration (x) and SC values (y) Chloride concentration (x) and SC values (y) showed strong linearity (Rshowed strong linearity (R22 = 0.9845) = 0.9845)

Drift ResultsDrift Results

Drift ResultsDrift Results

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

• Conductivity sensor drift over 7 days: Conductivity sensor drift over 7 days: 25 µS/cm or 0.08% of the reading.25 µS/cm or 0.08% of the reading.– Equates to a drift of ≈15 mg/L chloride at this range.Equates to a drift of ≈15 mg/L chloride at this range.

ConclusionsConclusions

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

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

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

ConclusionsConclusionsConductivity SensorsConductivity Sensors

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

ApplicationsApplications

• Saltwater intrusion monitoringSaltwater intrusion monitoring

• Salt marsh and coastal wetlands Salt marsh and coastal wetlands researchresearch

• Aquifer storage and recovery Aquifer storage and recovery systemssystems

Additional ResourcesAdditional Resources

Application and Technical Notes:Application and Technical Notes:

• Conductivity Measurement MethodologyConductivity Measurement Methodology

• Controlling Saltwater Intrusion in CAControlling Saltwater Intrusion in CA

• Hurricane Surge and Inland Saltwater ImpactsHurricane Surge and Inland Saltwater Impacts

• Tracking Saltwater Intrusion in Coastal AquifersTracking Saltwater Intrusion in Coastal Aquifers

• Three-Point, Bi-Thermal Calibration of ISEsThree-Point, Bi-Thermal Calibration of ISEs

White paper:White paper:

• Verifying SC as a Surrogate for ChlorideVerifying SC as a Surrogate for Chloride

www.in-situ.comwww.in-situ.com

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

Rob MooneyRob MooneyTechnical Marketing ManagerTechnical Marketing Manager

In-SituIn-Situ®® Inc. Inc.970-498-1655970-498-1655

rmooney@in-situ.comrmooney@in-situ.com