indicators of seawater intrusion - Seasidebasinwatermaster.org

FINAL DRAFTINDICATORS OF SEAWATER INTRUSIONSeawater intrusion is generally identified through chemical analyses ofgroundwater. Low groundwater levels indicate an opportunity for seawaterintrusion, but the actual seawater intrusion is indicated by various geochemicalchanges in groundwater.No single analysis definitively identifies seawater intrusion, however by lookingat various analyses we can ascertain when fresh groundwater mixes withseawater. At low chloride concentrations, it is often difficult to identify incipientseawater intrusion. Mixing trends between groundwater and seawater are moreeasily defined when chloride concentrations exceed 1,000 milligrams per liter(mg/L). This is due to the dominance of natural variation in fresh waterchemistry at chloride concentrations below 1,000 mg/L (Richter and Kreitler,1993). Chloride concentrations greater than 1,000 mg/L are clearly indicative ofseawater intrusion in the local aquifers.Common geochemical indicators of seawater intrusion are discussed, andexample analyses are presented, in the following sections.CATION/ANION RATIOSMolar ratios of cations and anions can prove distinctive for various groundwatersystems. Seawater intrusion is often indicated by graphically analyzing shifts inthese molar ratios. Two common graphical techniques for these analyses arePiper diagrams and Stiff diagrams.PIPER DIAGRAMSExample Piper diagrams are shown for data from the Pajaro Valley and SalinasValley in Figure 4 and Figure 5, respectively. These figures are included todemonstrate the utility of Piper diagrams, and show how they have been used innearby basins. These figures are not provided for directly comparing databetween basins; groundwater quality trends in one basin will not necessarilycorrelate with trends in other basins.On these Piper diagrams, the relative abundances of individual cations andanions are plotted in the left and right triangles, respectively, and their combineddistribution is plotted in the central diamond. Waters from similar or relatedsources will generally plot together. The mixture of two waters will generallySeawater Intrusion Response PlanNovember 19, 2008 A-6