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Memorandum to Tug Martin <br />May 13, 2002 <br />Page 2 <br />Pit. It is anticipated that monitoring wells MW-24 and MW-25 will be used to assess both pre- and <br />post-mining effects in the vicinity of the Sage Hil] Farm wetland. All 25 piezometers should be <br />read as follows: <br />• May through mid-July-biweekly <br />• Mid-July through October-monthly <br />It is recommended that the Last Chance and Side Hill ditches be observed when the piezometers aze <br />read to determine if the ditches aze flowing. Relative water level observations should also be made <br />of the slough barrow ditch east and north of the wetland, respectively. This information can assist <br />with determining the effects of irrigation on groundwater levels. <br />Proposed Wetland Mitigation Trigger <br />It is important that any trigger for wetland mitigation measures be able to distinguish effects on the <br />groundwater table due to the Nix operation from effects due to other factors. Other factors include: <br />drought conditions, changes in ditch diversions, pumping from high volume irrigation wells {e.g., <br />the Piatt well northeast of MW-4), and the Varra pit. All of these other factors could result in a <br />lowered groundwater table apart firom the Nix Pit. Therefore, it is recommended that any trigger <br />assess whether: <br />I) A drop in the groundwater table neaz the Nix Pit is due to the pit itself and not other factors, and <br />2) The drop is affecting or could affect the wetland on the Sage Hill Farm. <br />It is not possible at this time to define specific, numeric limits for the trigger because of the lack of <br />data to detemrine pre-mining groundwater conditions and the appazent variability in groundwater <br />levels. The hydrologic regime of the wetland requires further monitoring and evaluation. <br />Therefore, the following describes the recommended approach to collect additional data to define <br />specific trigger limits in the future. <br />It is recommended that the fast component be assessed by comparing readings from new <br />piezometer MW-25 that is unaffected by the Nix Pit and indicative of the regional goundwater <br />table with readings from MW-3 that will show the effects of the operation. These two piezometers <br />will be read this summer and a correlation established between readings. Future measurements that <br />deviate from this correlation will indicate an impact on the groundwater table from the Nix Pit. It is <br />anticipated that the correlation between the wells and the basis for a deviation will be established at <br />the end of this summer. <br />It is recommended that the second component of the trigger be evaluated by comparing readings in <br />MW-6 and MW- 24 (a new piezometer located in the middle of the wetland) with "pre-mining <br />conditions" established after another full growing season of monitoring. If groundwater levels fall <br />below the established pre-muting conditions and it is shown that the lowering is due to the Nix Pit <br />