C150094 Red Mesa Aquifer Groundwater Study 2002

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Red Mesa Aquifer Groundwater Study — Interim Report 2002 particles and is not available for withdrawal. Specific yield is the amount of water available for withdrawal and also represents the amount of water in transient storage. 5.0 GROUNDWATER TRAVEL TIMES Preliminary travel time calculations were made for all of the flow paths on the May 2001 map (see Figure 5). Uniform porosity and hydraulic conductivity values were assumed. The results of the calculations are represented on Table 1A. The travel times should reflect the approximate time it takes for groundwater that originates at each well to resurface downgradient as surface water. Similarly, travel time calculations were made for all of the flow paths on the December 2001 map (see Figure 6) and are shown on Table 1B. The travel time calculations are dependent upon the assigned values of porosity and hydraulic conductivity, as well as gradient and distance derived from computer-generated flow path mapping. An estimated porosity of 30 percent was selected from published findings in Bureau of Reclamation studies and published values for material types observed in well borings. A hydraulic conductivity value of 500 gpd/sf-' was assigned to the terrace aquifer for the calculation. This value was derived from published values for specific material types as well as limited information obtained from one hour of recovery data gathered from a pumping test on the Delaney well on May 21, 2002. Because of the high uncertainty of assigned values of porosity and hydraulic conductivity, these calculations are preliminary and approximate. The primary cause for this uncertainty is that hydraulic conductivity is not known. To improve upon these inaccuracies, it is recommended that additional tests be performed (see Findings and Recommendations). 6.0 FINDINGS AND RECOMMENDATIONS (1) Hydrographs for 34 wells monitored in the study area indicate that water table fluctuations in the Red Mesa Aquifer can be correlated to irrigation practices. Groundwater elevations rise shortly after irrigation begins and continue to rise 991-077/031 Wright Water Engineers, Inc. Page 12 Interim Report