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• Probability is estimated from available tables for the test of S = 0 (no trend), based on the calculated statistic and the number of data points • A "no-trend" interpretation is given for data within a 95% confidence interval of the S = 0 hypothesis • If a trend is evident, negative values of S indicate a decreasing trend, while positive values of S indicate an increasing trend • Sen's test is applied(Sen, 1968, Gibbons and Coleman, 2001) • This test is applied to a set of slope estimates (yi -y,)/(xa-x,) with xj > x, and i and j are drawn from the N data points (x,y) • The median value of the distribution of slopes is taken as the best estimate of slope A two-sided confidence interval on slope is estimated from a calculated variance, as is a one-sided confidence inter-vat for zero or decreasing slope • A best-estimate slope within a 95% confidence interval of zero slope is interpreted as "no trend" • If a trend is evident, a negative slope is a decreasing trend, and a positive slope is an increasing trend • Consistencies of results from the two non-parametric tests are used m interpreting the data trend Data for the time period from January 2000 to the latest available (either 2nd or 3rd Quarterly monitoring data for 2003) is included in the trend analysis Results of the statistical analysis for the three sites are included in Table 3 2 Plots of time versus concentration for the monitoring well data are included in Appendix 1 30011 Crown Valley Parkway site The site stratigraphy consists of silt to a depth of 25 ft, and fine- to medium-grained,.silty sand from 25 to 35 ft below ground surface Siltstone is also present at 25 ft below ground surface MW-1 through MW-6 are screened at a depth of approximately 15 to 18 ft in perched water Unconfined groundwater is present at approximately 24 ft depth in MW-7, MW-8, MW-10 through MW-13, which are screened from 10 to 30 ft in native sediments Groundwater flow varies from east to northeast with a gradient of 0 01 to 0 02 ft/ft Hydraulic conductivity was measured at 5 1 x 10 5 ft/sec (4 4 ft/day) based on slug tests, consistent with literature and site lithology Over the time period from March 2000 to September 2003, MTBE is evidently either stable or decreasing at this site in every monitoring well that met the statistical evaluation criteria (at least four sampling events with detected concentrations) TBA is apparently decreasing or stable, except for wells MW-05, MW-07, MW-08, and MW-11 MW-05 is screened within a shallow perched zone adjacent to the tank pit, where the highest on-site concentrations have been detected The other identified increasing wells are screened in shallow groundwater either adjacent to the tank pit (MW-08, weak confidence in increasing trend) or within approximately 30 feet of the tank pit(MW-07,MW-11) 16969 Brookhurst site. Depth from surface to first encountered groundwater on the site is 7 to 12 ft The water table gradient is from west to east at approximately 0 023 ft/ft 4