Laserfiche WebLink
<br />DATE: May 3, 2001 <br />TO: Wallace Erickson <br />FROM: Allen Sorenson <br /> <br />RE: Analysis of Potential Off-Site Impacts from Pit Dewatering and River Flooding at <br />the Proposed Line Camp Pit, Four States Aggregates, LLC, File No. M-2001-001 <br />The Division of Minerals and Geology (DMG) and surrounding water well owners have expressed <br />concerns with the potential for well interference and water table depression resulting from the proposed <br />dewatering operations at the Line Camp Pit. The Applicant has provided theoretical analysis of the <br />potential for well interference to be caused by pit dewatering. This analysis is included in the revised <br />Exhibit G to the application received at DMG's office on March 20, 2001. The Applicant concludes that <br />pit dewatering will cause no more than, and probably significantly less than, 1.82 feet of water table <br />depression at the nearest potentially affected well, known as the Robinson Well. The Applicant further <br />concludes that anticipated worst-case draw down at the Robinson Well will have no significant impact <br />on the well yield. <br />Creation of a ground water model to simulate the behavior of the ground water system resulting from <br />outside stresses such as pumping a well or dewatering a pit is an effective way to predict the nature and <br />magnitude of changes in the system. In order to create a ground water model, certain simplifying <br />assumptions must be made as to the hydrogeologic character of the area to be modeled. Hydraulic <br />conductivity of a geologic media, such as sand and gravel, is a property of the media and also a function <br />of the type of fluid flowing through the material. Hydraulic conductivity, along with the hydraulic <br />gradient, governs the rate and quantity of ground water flow through a given ground water system. Few <br />physical parameters vary over as wide a range of values as does the hydraulic conductivity of geologic <br />media. The values for hydraulic conductivity, expressed in units of distance divided by time, range over <br />13 orders of magnitude. In general, hydraulic conductivity has high values for sand and gravel, and low <br />values for clay and most rocks. However, even small amounts of clay and silt within a predominantly <br />coarse grained material can significantly reduce hydraulic conductivity. The DMG routinely evaluates <br />the potential for well interference to be caused by pit dewatering and has found that a method of <br />parametric sensitivity analysis is an effective tool in predicting the range of possible impacts to the water <br />table. This method involves simply varying the value of physical parameters that influence ground water <br />behavior as input to the ground water model, and examination of the range of results in the model <br />output. The range of values for the parameters are taken from the ground water literature and from any <br />site-specific hydrogeologic information that is available. <br />