ADMINISTRATIVE REVISION OF THE DENVER BASIN RULES

Aquifer parameters currently used to estimate recoverable water in the Denver Basin bedrock aquifers were codified by Senate Bill 5 (1985) after an exhaustive review of the literature, examination of thousands of geophysical logs from both water wells and oil and gas test holes, and laboratory analysis of a limited number of outcrop samples.

Four discrete hydrogeologic intervals were identified from the construction of 15 cross-sections using about 250 geophysical logs. Outcrop samples were evaluated for specific yield, or relative volume of water drainable by gravity, and geophysical logs were analyzed to determine net saturated aquifer thickness for each of the four-aquifer intervals identified. A peer review process and public meetings resulted in a set of maps for each aquifer interval showing the structural surface on the top and base of each interval and an isolith map of the net saturated aquifer material. An average specific yield was also assigned to each aquifer interval on the basis of the limited laboratory analysis and peer review process.

In the 17 years since implementation of the Denver Basin Rules, several thousand additional geophysical logs have become available, and 2 core holes have been drilled allowing laboratory analysis for specific yield and hydraulic conductivity on unweatherd samples from each of the aquifer intervals. Comparison of the specific yield data from analysis of samples from the 2 core holes with the original specific yield data from analysis of outcrop samples indicates that the specific yield values assigned to the individual aquifers may be too high resulting in an overestimation of the amount of ground water recoverable from the Denver Basin aquifers. The additional geophysical logs now available indicate that the net saturated aquifer isopach maps generated in 1985 show greater saturated thickness in some areas than current data would support.

Based on data obtained since 1985, and an executive order by the governor to review existing rules and regulations at least every 5 years, the Colorado Division of Water Resources is preparing to start the approximately yearlong process to revise the Denver Basin Rules and Regulations.