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MEMORANDUM <br />TO: Gordon McCurry -Camp, Dresser and McKee <br />FROM: Jon Ford <br />DATE: January 25, 2006 <br />~~ Peer Review of SPDSS Phase 2 TM's 42.2, 42.3, 43.2, 43.3, 44.2 and 44.4 <br />As part of the SPDSS Phase 2 Peer Review process, we have reviewed the above-referenced TM's <br />along with the associated figures and appendices. You requested comments be redlined of the <br />documents; however, our comments do not lend themselves to that format. Thus, they are provided <br />in this Memorandum. <br />With regard to TM 42.2, we provide the following comments for your consideration: <br />1) In many places, the report refers to net sand thickness in the aquifers. Our belief is that <br />the mapped thickness is sand plus silt as was done for the SB-5 Rules. This distinction is <br />important to us because we have found that the net sand decreases from west to east <br />across the basin in both the Denver and Arapahoe Aquifers while the sand plus silt <br />thickness remain roughly constant across the basin. We have also found that well yields <br />are proportional to net sand thickness, not sand plus silt. Ultimately, our concern is that <br />future ground water modeling needs to consider this distinction, as do readers of this TM. <br />2) It is not clear to us when we read the TM if all of the picks were done by SEO staff. If <br />some were done by your staff and you truly used net sand, the resulting maps are a mix of <br />different interpretations. If that is the case, some rework seems necessary. Regardless, it <br />seems appropriate to be clear about who made the picks. <br />3) We feel that we could do a more complete peer review if the aquifer thickness maps were <br />provided and not just the net sand thickness maps. We suspect that those maps would <br />show anomalous thickness changes particularly in the Denver and Arapahoe Aquifers in <br />the southwest portion of the study area. We have long believed that there is a correlation <br />problem in that area. <br />4) When we compare Figure 8, Net Sand Thickness of the Upper Dawson Aquifer to Figure <br />9, Net Sand Thickness of the Lower Dawson Aquifer, we observe a thicker interval in the <br />Upper Dawson is coincident with a very thin interval in the Lower Dawson. This might <br />suggest a correlation bust in that area. <br />5) It would be helpful if a comment column were added to Appendix C that explained what <br />type of outlier was encountered for each data point. <br />6) The approach used to gather and interpret the data is consistent with how we would have <br />undertaken the project. We agree with all of the conclusions except #5. With regard to <br />conclusion #5, we disagree with the portion of the conclusion relating to the statement <br />that the net sand thickness is sufficient to commence regional scale modeling (see <br />comments above). We agree with all of the recommendations except #5. This <br />recommendation is part recommendation and part conclusion regarding the extent and <br />thickness of the confining layers. This study does not evaluate the degree that the <br />confining layers actually separate the aquifers. For the Dawson and Arapahoe Aquifers, <br />we believe that the confining layers are only an administrative reality and that the <br />correlation of these units basin wide is suspect. <br />LEONARD RICE ENGINEERS, INC. <br />2000 CLAY STREET, SUITE 300, DENVER, COLORADO 8021 1-51 1 9 PtiONe (303) 455-9589 FAx (303) 455-01 1 5 <br />