Preliminary Data Evaluation Findings Sample Clauses

Preliminary Data Evaluation Findings. The harborwide stormwater concentration bar graphs presented in Attachment C1 were evaluated to assess whether City outfall basins that might require additional source tracing activities could be readily identified. The following general observations were made, based on these bar graphs:  About half of the sampled outfall basins have low concentrations falling within a narrow range of values; this outcome is attributable in part to the convention of assigning all non-detected results a value of zero.  The higher mean values are distributed along curves with gradual or steeper increases in slope; however, the graphs do not exhibit breaks in the slopes that are sufficiently abrupt to define a “natural” boundary of “high” stormwater analyte concentrations relative to the harborwide range of average values.  Stormwater data from individual outfall basins are generally positively skewed (asymmetric tail extends to the right of the mean).  Plots are useful for: o Presenting the variability of concentrations (range of concentration values) between individual stormwater sampling events at each location. o Providing context of the mean stormwater concentration for an individual outfall basin against harborwide outfall basin data. o Identifying a few outfalls with concentrations that are clearly elevated compared to general harborwide concentrations  The graphs do not readily lend themselves to the primary objective of the City’s stormwater data evaluation (i.e., identifying City basins that might warrant additional source tracing activities). The preliminary data evaluation did not clearly identify individual or groups of City outfall basins that might be considered for additional source tracing activities, based on individual event or mean stormwater concentrations. Therefore, it was determined that more robust evaluation of the stormwater data was needed to identify City outfall basins with higher analyte concentrations within the Harborwide data set that could indicate the presence of significant upland sources and the need for additional source tracing. The statistical evaluation methodology is described in the following sections of this appendix. C3 Selection of Stormwater Analytes for Statistical Evaluation Steps in Selection of Stormwater Analytes for Statistical Evaluation