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small volume of water would be present in the pond at the time of lowest seasonal groundwater <br />levels prior to and after the slurry wall was installed and would be potentially subject to freezing <br />during the winter. <br />5.4.4 Comparison of Groundwater Model Predictions with Monitoring Data <br />SCIENTECH prepared asteady-state groundwater flow model of the Stegner Farm mine area as <br />part of the permit process. The model results suggested that the groundwater level decreases <br />immediately adjacent to the slurry wall would be about 2 feet and that the groundwater level <br />decreases in the area of Mr. Roe's property would be about 1 foot. The analysis presented in this <br />evaluation indicates that groundwater level decreases immediately adjacent to the slurry wall is <br />about 4 feet and in the area of Mr. Roe's property is about 1.8 feet. The difference between the <br />model prediction and groundwater level data results from the combined effects of the slurry wall, <br />two-years of drought, cessation of imgation at the Stegner Farm, and local groundwater <br />withdrawal. Given that the model did not consider drought conditions, cessation of irrigation at <br />the Stegner Farm, or local groundwater withdrawal, the predicted and observed groundwater <br />level decreases at Mr. Roe's property are in reasonable agreement. <br />5.5 Statistical Analysis of Groundwater Level Changes <br />Discussions above concerning pre- and post-slurry wall groundwater conditions in the vicinity of <br />the Stegner Farm gravel mine largely focused on qualitative groundwater level changes based on <br />visual comparison of the monitoring data collected to date. However, because of the various <br />influences on groundwater levels in the area (e.g, precipitation, temperature, evapotranspiration, <br />ditch irrigation, local well pumping, dewatering, and cessation of flood imgation at the Stegner <br />property) a more quantitative statistical approach, using nonparametric tolerance limits, was <br />chosen to describe the groundwater level conditions that existed prior to the slurry wall and to <br />determine whether a significant groundwater level change has occurred since the slurry wall was <br />installed. <br />The statistical procedure used involves defining two-sided (i.e., an upper and lower) <br />nonparametric tolerance limits for the pre-slung wall groundwater levels at each well and <br />subsequent comparison of the post-slurry wall groundwater levels with the tolerance limits to <br />determine if they are statistically different from the pre-slurry wall groundwater levels. Prior to <br />calculating the tolerance limits, the groundwater level data were corrected for monthly <br />precipitation variations which influence groundwater levels. The precipitation correction was <br />implemented by determining the ratio of the monthly precipitation to the historical monthly <br />precipitation mean which was used to correct the groundwater levels for abnormally "wet" or <br />14 <br />