indicators of seawater intrusion - Seasidebasinwatermaster.org

FINAL DRAFTintrusion may occur at a given monitored location, there is no assumption thatthose data follow a specific distribution while seawater mixes with the ambientgroundwater to produce changes in the overall geochemistry.The test involves computing a statistic S, which is the difference between thenumber of pairwise differences that are positive minus the number that arenegative. If S is a large positive value, then there is evidence of an increasingtrend in the data. If S is a large negative value, then there is evidence of adecreasing trend in the data. The Mann‐Kendall statistical approach is basedsolely on calculations of the sign and does not take into account the magnitudesof the differences. It is a robust statistical test as it is insensitive to outliers. Thetest should be conducted on wells where data were collected at equal timeintervals, such as quarterly or semiannually monitored data.It is important to note that there are different calculation approaches for theMann‐Kendall Test dependent on sample size. The following provides anexample of the Mann‐Kendall Trend test for a population of up to 10 data points.Table C‐2: Example Mann‐Kendall Statistical Test for FO‐9 ShallowTime 1 2 3 4 5 6 7 No. of No. of[Cl ‐ ]1 = [Cl ‐ [Cl ‐ [Cl ‐ [Cl ‐ [Cl ‐ [Cl ‐ increase decreaseData56 ]2=56 ]3=50 ]4=50 ]5=53 ]6=50 ]7=49 s s[Cl ‐ ]1 =56 nc ‐ ‐ ‐ ‐ ‐ 0 5[Cl ‐ ]2=56 ‐ ‐ ‐ ‐ ‐ 0 5[Cl ‐ ]3=50 nc + nc ‐ 1 1[Cl ‐ ]4=50 + nc ‐ 1 1[Cl ‐ ]5=53 ‐ ‐ 0 2[Cl ‐ ]6=50 ‐ 0 1[Cl ‐ ]7=492 15Statistical Test:1) S = 2‐15 = ‐132) Look up critical values for the sample size of 7 at 0.10 and 0.20 significancelevels.a. At 0.10 significance level, the critical value is 11.b. At 0.20 significance level, the critical value is 7.3) Look up the probabilities for the sample size of 7.a. The probability for a sample size of 7 is 0.035.Seawater Intrusion Response PlanNovember 19, 2008 C-6