indicators of seawater intrusion - Seasidebasinwatermaster.org

FINAL DRAFTSTATISTICAL TREND ANALYSESDEVELOPMENT OF CHLORIDE THRESHOLD VALUESHistorical data were analyzed to develop representative backgroundconcentrations of chloride and to determine a reasonable chloride thresholdvalue. Background chloride concentrations are estimated as the average chlorideconcentration at each well, calculated from the historical data available. Thechloride threshold value is the background chloride concentration plus anacceptable tolerance interval multiplied by the standard deviation that ensuresthat 99% of the population of chloride measurements collected and evaluatedwould not exceed the chloride threshold value. This approach assures that thevariability observed over time is incorporated into the final chloride thresholdvalue used for identifying seawater intrusion at individual wells. The accuracyof this approach for determining a true average chloride concentration andupper confidence interval depends on the accuracy of the assumption that thedata display a normal distribution of chloride concentrations.STATISTICAL APPROACHThe most powerful test for normality is the Shapiro‐Wilk W test (US EPA, 2006).This test is difficult to compute by hand, but many statistical programs havebeen developed that calculate the statistical distribution such as the EPAprogram ProUCL (US EPA, 2007) which is available to download athttp://www.epa.gov/esd/tsc/software.htm.The formula to determine the average chloride concentrations is as follows:[ Cl−]average=n∑i=1[ Cln−]iThe standard deviation is calculated as:[ Cl−]SD=1n −1n∑i=1([ Cl−]i− [ Cl−]average)2Seawater Intrusion Response PlanNovember 19, 2008 C-2