Verifying the Use of Specific Conductance as a Surrogate for Chloride in Seawater Matrices
ABSTRACT
Coastal groundwater supplies, when overused, are particularly vulnerable to chloride contamination due to their close proximity to saltwater. Bevated chloride concentration in groundwater isthe most commonly used indicator of saltwater intrusion in coastal aquifers. Father well-defined relationships of specific conductance (SC) to chloride exist (Hem 1992: Christensen et al. 1999). To validate the relationship between SDand chloride. 33measurements by electrochemical conductivity cells and chloride concentration measurements by ion-selective electrode (ISE) were determined for 35 P9J OSL Atlantic Seawater Sandard and 10 dilutionsat six different temperatures (66 unique samples). Sfeawater has an average chloride concentration of 19.000 mg Li Hem 1992). A strong linear relationship was established, thus demonstrating the validity of using SCas a surrogate for chloride estimation. This study also compared inherent measurement drift of a chloride l££and a conductivity sensor under controlled laboratory conditions over a seven-day period. Minimal drift of the conductivity sensor coupled with a large drift ofthe chloride ^demonstrates a significant advantage of the conductivity sensor for long-term field deployments
APPLICATION OVERVIEW
Coastal groundwater supplies, when overused, are particularly vulnerable to chloride contamination due to their close proximity to saltwater. Bevated chloride concentration in groundwater isthe most commonly used indicator of saltwater intrusion in coastal aquifers Saltwater intrusion decreases freshwater levels in an aquifer and can lead to abandonment of supply wells Some prominent areas facing saltwater intrusion includethe Mediterranean, the Yucatan Fteninsula in Mexico, the Middle East California and the Atlantic Coast of the United Sates, especially in densely populated areas.
Inland areas also can experience saltwater intrusion. Many deeper aquifers in the central part ofthe United Sates contain saline waters Saltwater intrusion has occurred along the Mississippi Rver alluvial plain in Arkansas where groundwater withdrawalsfor irrigation from an alluvial aquifer have caused upward movement of saline water from the underlying Sfciarta Aquifer (Morriset al. 1986). VWhdrawing water from overlying aquifers increasesthe potential for saltwater intrusion from below. In addition, road saltsordeicingagentscan impact shallow unconsolidated aquifers and surface waters near roads (Kunze et al. 2004).
Chloride was selected as a reliable reference or a chemically conservative constituent to characterize salinity in water samples. Unlike sodium, the ground does not contain or contribute to chloride values Measuring sodium is not a reliable parameter because the ground retains sodium. Chloride values are preserved even when collecting data some distance away from the source. Chloride is least affected by travel from the source to points further downstream, thus it provides a true representation of contamination levels
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