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DEVELOPMENT of THE REGULATORY FRAMEWORK FOR DENVER BASIN AQUIFERS about CDSS and the individual decision support systems can be found at the Decision Support System website at cdss.state.co.us. The prime contractor for the groundwater component of the SPDSS, Camp Dresser McKee (CDM) will develop at least two integrated water budget models of the South Platte River. One model will look at the South Platte River in that part of the Basin where the river alluvium is in con- tact with the outcrops of the bedrock aquifers, and the sec- ond model will look at the river downstream from the contact of the river alluvium with the outcrops of the bedrock aquifers. The CDM modeling effort will use data developed since the 1985 data set used in the original Den- ver Basin model (MODFLOW) described earlier. In addi- tion, CDM will produce a new set of maps for the bedrock aquifers, including structure on the tops and bases of the individual aquifers, and net sandstone /siltstone isoliths for each aquifer to be used to determine amounts of recover- able water. The final report may also recommend other changes to the Denver Basin Rules and Regulations. If changes to the Rules are recommended and accepted by the State Engineer, then a new rulemaking process under the Administrative Procedures Act will take place. In evaluating the need for, and impact of changing the Denver Basin Rules, several factors must be considered. Based on additional data acquired since 1985, much dis- cussion has centered around the need to adjust the statu- tory specific yields of the individual Denver Basin bedrock aquifers. Some of the discussion is based on the work done on the core taken from the Denver Museum of Nature and Science's Kiowa core hole (Lapey, 2001). This study indicates that the specific yields assigned to the indi- vidual aquifers are too high, perhaps by as much as 30 percent in some cases. Since the 1985 implementation of The Denver Basin Rules, a significant portion of the groundwater along the Front Range has been quantified and become the subject of decrees of the water court. Those decrees are now res judi- cata, or subject to existing court decisions, and cannot be changed. Any future changes to the rules would not affect these existing decrees to withdraw Denver Basin ground- water. Applications or claims for rights to this groundwater continue to be filed in court each month. Any reduction in specific yield which could be supported by Lapey's work and the work being done by CDM could only be applied to claims made after implementation of new Denver Basin Rules. Most of the land area in the Denver Basin to which any new or revised rules would apply is located on the plains east of the Front Range metropolitan areas. Another factor that should be considered is the actual physical limitation on how much water can be withdrawn from and individual well regardless of how much "paper water" a person or organization has a right to pump. As water levels decline, pumping rates decline in response to increased heads and costs increase because it takes more energy to lift the water higher. Eventually an additional well or wells will have to be constructed in order to pro- duce the same amount of water delivered by the original well. The cost of constructing additional wells to maintain a specific level of water production may make the water to expensive to develop. This limitation is particularly signifi- cant to municipal type water providers because of the high volumes of water that must be produced and the cost of constructing a well that must deliver several hundred gal- lons of water per minute (gpm). Individual residential well owners, who are limited by statute to a maximum pump- ing rate of 15 gpm will probably not be faced with the prospect of having to drill additional wells to maintain an adequate flow of water to serve their needs, but may be faced with increased energy costs as a result of having to lift the groundwater higher. Certainly, those individuals and entities with bedrock aquifer wells located in the outcrop areas will, or already have begun to feel the effects of the lowering of the arte- sian heads in the aquifers. In some cases, the remedy is to drill deeper into the aquifer to extend the useful life of their well. In some cases drilling deeper may not be an option, or may come at a cost in addition to well construc- tion costs because the groundwater in deeper aquifers had been severed from the land by a previous owner and does not belong to the owner of the surface. In some cases there is no deeper aquifer present. CONCLUSIONS While there may ultimately be sufficient new data to jus- tify reducing the specific yields of the individual Denver Basin bedrock aquifers, therefore reducing the calculated amount of recoverable groundwater beneath a specific par- cel of land, or the entire Denver Basin, it is our opinion that the economics of well construction and pumping costs will be of greater significance in controlling the actual develop- ment and production of groundwater, and will encourage the development and use of renewable sources of water. For example, use of treated surface water to recharge bedrock aquifers may be an attractive economic alternative to further development of groundwater in some locations. ACKNOWLEDGMENTS We thank the CDWR for access to its files and library. Much of the information presented in this paper is derived from internal memos and reports in the files of the CDWR, and from conversations with present and former (mostly former) staff of the CDWR and members of the water related consulting community. 159 The Rocky Mountain Association of Geologists