2005-01-24_REVISION - M1999120 - Laserfiche WebLink

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RE: L. G. Everist, Inc„ Fort Lupton Sand and Gravel Mine, DMG #M-1999-120; Second Adequacy Response Letter 12. Wetlands east of CR25 The "wetland" on the East of CR25 appears to presently be connected to the East Lateral, and presumably, occasionally receives water from the Lateral. The "wetland" may also at one time have received surface water from the west -there used to be another Lupton Bottom Ditch lateral located to the south and west. Therefore, this "wetland" is sustained by surface water from two possible sources -the East Lateral, and any remaining surface connections to the west, not by groundwater from the south. LGE did some additional field work prior to the January 19`" meeting and in the meeting noted that there is a large drainage ditch -which conveys water from Sand Hills Reservoir and possibly also seepage from the land around it -that travels along the north side of CR18, is about 10 feet deep, and will intercept shallow groundwater from the south side of CR18. The possible buffering effects of this ditch were not included in the model. Although factors would indicate that this "wetland" is fed by surface water and not by groundwater from the south, LGE already has a monitoring well in place on the south side of this wetland, and will be monitoring on a monthly basis. If the groundwater levels change enough to reach the trigger point, then LGE will start an assessment, and if responsible, will implement necessary mitigation measures. In addition to the monthly well monitoring, LGE will observe the "wetland" (or cottonwoods, see #13 and #14) to document pre-mining conditions, and then after slurry wall construction begins to assess any impacts. This pre-mining monitoring will include numeration of wetland species, percent cover of wetland species, and extent of upland weeds (or numeration of numeration of dead cottonwood trees, and observations of leaf desiccation, see #13 and #14). The "wetland" (or cottonwoods, see #13 and #14) will be observed in early July, and also early September (if slurry wall construction has not begun) for pre-mining conditions. Photographs will be taken from established photo points. Also, as discussed in the meeting, the "wetland" (or cottonwoods, see #13 and #14) will be visually monitored, and the monitoring well checked, on a weekly basis from the start of construction of a slurry wall until 1 month after completion of a slurry wall, in order to observe if there are any impacts. If observations indicate impacts from LGE operations, LGE will make an assessment, and if found responsible, will implement appropriate mitigation measures with the permission of any affected property owners. Please see #4 for a list of mitigation measures. If observations indicate no impacts, monitoring will go back to monthly monitoring of the well, and vegetation observations before and after the installation of each new slurry wall. 13. Cottonwoods north of CR18 along the South Platte River WWE and LGE believe the groundwater level beneath the cottonwood grove northeast of the expansion will be maintained by shallow groundwater from the South Platte River, which is immediately adjacent to the trees, and by water in the large drainage ditch (noted above in #12) that conveys water from Sand Hills Reservoir and runoff through the grove. Aerial photos show that water from the river backs up into the ditch as it bisects the grove (this can be seen on Drawing 5, included in the Groundwater Monitoring report from the first adequacy response). We are familiar with the USGS study on effects of alluvial water table declines on cottonwoods that DGM cited. This study found mortality to increase in a threshold response to water table declines of > 1 meter. The modeling completed by WWE showed a maximum water table decline of around one meter in a portion of the grove. However, the model did not include LM/FL-Resp2ndAtlegRev-dmgKP-012105.doc 6