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Hubberson Gulch AVF impact evaluation with the Dry Creek AVF impact evaluation which follows. On page 129 and 130, there is an evaluation of salt loading impacts on the productivity of subirrigated grass hay fields on Dry Creek (the same fields referenced in the page 53 paragraph excerpt above). The narrative states that "...impacts to downstream subirrigated fields from Seneca II-W South may best be characterized using surface water chemistry downstream of the mining area". (This would seem to directly contradict the statement on page 53, that the subirrigated fields would 'not be affected by surface flow in Dry Creek). Further, the narrative states that "the postmining TDS concentration (1289 mg/p in Dry Creek, at Station WSD5 (which is representative of water in Dry Creek as it flows past the Olsen fields...)may represent aworst-case scenario for subsurface impacts to downgradient hay fields." (The TDS value of 1289 mg/I is taken from Table 17-42a, and represents projected salt loading impact to Dry Creek at Station WSD5, resu/ting from //-W South Area Spoi/ Discharge. Table 17-42b presents projected salt loading impacts resulting from combined spoil discharge from the //-W original permit area and the South Area extension. The WSD5 projected TDS value in this table is 2451 mg/I. Since this cumulative projection represents the water that would actually flow past the subirrigated fields in question, it would seem that the value of 2451 mg/I should be used in the impact evaluation on page 129, rather than the value of 1289 mg/I, which was used.) The Dry Creek AVF evaluation includes Table 17-43c (Projected Impacts to Downstream subirrigated Crops). The basis for projections regarding species composition and crop salt tolerance threshold and response slope values in Table 17-43c and related narrative for Dry Creek AVF evaluation is not well documented. Salinity threshold value and response slope (percent yield decrease per unit salinity increase) for the one species that occurs on both the Hubberson AVF and Dry Creek AVF (smooth brome) are not consistent. In Table 17- 43b (Hubberson AVF), irrigation water salinity threshold of 1.3 and response slope value of 13.2°k are used and source is cited. Table 17-43c values are 1.5 and 7.0, respectively, and source for the values is not cited. It appears that the response slope value may be erroneously based on projected soil extract values rather than projected irrigation water values, in Table 17-43c. If the Table 17-43b threshold and response slope values for smooth brome are substituted into Table 17-43c, the result is that the over-all crop yield reduction projection for the Dry Creek AVF field increases from 2.6% to 4.5%. Since the projected yield decrease would exceed 3%, the impact would no longer be considered a "negligible impact" as defined by the Divisiori's Material Damage Guidance Document, for a small ranching operation. If the apparently correct TDS value of 2451 mg/I were used as the basis of the evaluation rather than 1289 mg/I, the projected crop reduction would obviously be significantly greater. The operator will need to address and resolve the discrepancies and apparent errors discussed above. Dry Creek Water quality-projections used in the page 129 evaluation will need to be amended as appropriate, and field species composition and salinity response projections of Table 17-43c on page 130 will also need to be documented and updated as appropriate (following the process and assumptions used in Table 17-43a and b). If properly amended crop yield decline projections for the Dry Creek subirrigated fields indicate that yield declines would exceed 3% of the hay production for the subject ranching operation, potential material damage would be indicated. As such, the operator may be obligated to conduct intensive site specific surface water, alluvial water, soil; and vegetation field investigations to demonstrate that continued'mining will not result in material damage. Another line of investigation that the operator will need to explore, is how the fields in question fit into the landowner's farm/ranch operation. The only information given on this