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DEVELOPMENT of THE REGULATORY FRAMEWORK FOR DENVER BASIN AQUIFERS • Some development and use of nontributary groundwa- ter is desirable. • Nontributary groundwater must be allocated with spe- cial care because it is essentially a nonrenewable resource. • Any nontributary groundwater allocation scheme must recognize that no groundwater is completely nontribu- tary. • Withdrawals of nontributary groundwater that affect the stream system should require compensation to the stream system. • Administration in the priority system is a concept gener- ally inapplicable to nontributary groundwater. • The interests of overlying landowners with respect to nontributary groundwater should be clarified. • Changes in the law should minimize complication and expense. The groundwater legislation committee proposed legis- lation that resulted in the 1985 act known as Senate Bill 5 (SB 85 -5). This act directed the State Engineer to promul- gate rules and regulations for the administration of ground- water in the Denver Basin bedrock aquifers. The official title of these rules as adopted is Rules and Regulations Applying Exclusively to the Withdrawal of Ground Water from the Dawson, Denver, Arapahoe and Laramie -Fox Hills Aquifers in the Denver Basin. The short title for these rules and regulations is The Denver Basin Rules. Along with The Denver Basin Rules, the State Engineer promulgated The Statewide Nontributary Groundwater Rules. These rules applied to applications for claims to nontributary groundwater outside the Denver Basin. Provi- sions of these rules, including the requirement for obtain- ing geophysical well logs, banking of groundwater, the 100 year aquifer life, and the definitions of "well field" and "additional well" also apply to wells taking water from the Denver Basin bedrock aquifers. SB 85 -5 established a technical definition of "nontribu- tary groundwater ", coined the term "not nontributary" and provided for special treatment of the development of groundwater in these "not nontributary" areas. Nontribu- tary groundwater is now defined as " —that groundwater - -the withdrawal of which will not, within 100 years, deplete the flow of a natural stream, including a natural stream as defined in sections 37 -82 -101 (2) and 37 -92 -102 (1) (b), at an annual rate greater than one -tenth of one percent of the annual rate of withdrawal (from the well)." (CRS 37 -90 -103 (10.5))• This is a very difficult standard to meet. SB 85 -5 also required some of the nontributary groundwater developed to be relinquished to the stream System. A first step in the creation of the Denver Basin Rules was a review of over 3,000 geophysical logs from oil and gas wells and water wells within the Denver Basin. Using about 250 of these logs, 15 cross sections were con- structed, four in a north -south direction and eleven in an east -west direction. These cross sections confirmed the hypothesis of the State Engineer that there were four aquifer intervals separated by significant, laterally extensive confining layers of claystone and other fine - grained materi- als. The cross sections further established that in some areas, the Dawson and Arapahoe aquifers could each be subdivided into an upper and lower hydrogeologic unit. Additional data gathered from the available geophysical logs allowed the mapping of the tops and bases of the individual aquifer intervals. Sand counts provided informa- tion about the net thickness of aquifer material. Literature review and field investigations led to refined outcrop /subcrop maps of the aquifers and alluviums of natural streams. All of the cross sections and aquifer maps were reviewed by a geological advisory committee consisting of Stan Robson of the USGS; Curt Wells, consulting geologist in the water community; and Robert Kirkham, geologist on the staff of the Colorado Mined Land Reclamation Board. These independent experts reviewed the work of the CDWR staff for consistency and accuracy. Knowledge of a final component, specific yield, was necessary to calculate how much water was actually avail- able for withdrawal. Specific yield is the aquifer property that describes the amount of groundwater that will drain by gravity from a bulk volume of aquifer material. Some outcrop samples were collected and analyzed for specific yield. After lengthy discussions between the CDWR and the consulting community, the average specific yields for the Denver Basin aquifers were determined to be 20% for the Dawson, 17% for the Denver and the Arapahoe, and 15% for the Laramie -Fox Hills aquifers. Knowing both the thickness of saturated aquifer material and the specific yield of the aquifer, a theoretical volume of drainable water could be calculated for any specific parcel of land within the Denver Basin. To comply with the 100 -year aquifer life provision, only one percent of the amount of water determined to be available could be withdrawn in any year. However, that water can be banked from the date of issuance of a permit or court decree. The final step in evaluating the resource was to identify the area within each aquifer that contained nontributary groundwater. The State made that determination by using MODFLOW (McDonald and Harbaugh, 1983)• Data files containing parameters for each aquifer were created using data gathered by the CDWR and the USGS. A model grid system of 120 rows and 84 columns was selected. Each individual cell represented one square mile, and was assumed to represent a legal section of land. Values for the 157 The Rocky Mountain Association of Geologists