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~ N e e~ The Department of Geophysics at the Colorado School of Mines carried out various electrical geo- physical surveys of the local area around Glenwood Springs. One survey was done using the dipole- dipole surveying method capable of measuring the electrical resistivity of rocks to depths of a few hundred feet. A contour map of observed resistivity along the Colorado River Valley at Glenwood Springs is shown in Fig. 6. An area of anomalously low resistivity, less than 35 ohm-meters, coincides with the extent of the open aquifer in the Colorado River Valley. The geophysical data indicate an average thickness of approximately 100 feet for the 35-ohm-meter block. Alluvial material such as forms the open aquifer typically has a formation factor (ratio of rock resistivity to the resistivity of the contained water) of 8 to 15. Assuming a rough estimate of 10 for the formation factor, The observed resistivity value of 35 ohm-meters for the open aquifer corresponds to a water resistivity of 3.5 ohm-meters (conductiv- ityof 0.28 S/m). This, in turn, corresponds to a water salinity of roughly 20,000 parts per million, at surface tern perature Assuming the 35-ohm-meter volume of rock corresponds to the alluvial aquifer and that it has a water-filled porosity of 25%, the volume of water present instantaneously in the open aquifer is 2.2 million cubic feet. 4. Interpretation: The question to be answered is as follows. If a pound of salt is removed from water produced from a well penetrating into a rock saturated with saline thermal water beneath Glenwood Springs, how much will be the reduction in salt entering the Colorado River as it flows through Glenwood Springs? Perhaps the reduction in pressure caused by removal of the water from the open reservoir will permit saline water to enter the system to make up some of the removed salt. If this were to be a significant fraction of the salt removed, this approach to reduction of salt added to the River might be inefficient and not economically warranted. On the other hand, if the salt removed by production from a well is not replaced, the economics characterizing this approach to desalination are straightforward and can be evaluated. Available evidence indicates that the rocks through which saline waters move from the thermal aquifer to the surface are well interconnected, and that there is no significant compartmentation of the rocks through which the saline waters rise to the surface. Reduction of the head on the aquifer beneath Glenwood Springs by one foot would increase the total pressure drive from the areas of aquifer recharge by only 0.05 to 0.1 % and the flow by the same amount, after equilibrium would be reached. In actual operations, drawdowns as great as several tens of feet might be used, resulting in a steady state addition of salt of 1 to 2% (of the present supply). Initially during transient establishment of the new pressure regime through the local part of the carbonate aquifer where it has high hydraulic conductivity, production of salt water will be accelerated. Well test data indicate thet this transient should have a time constant of a few weeks to few months, but the added salt production will be salt that would normally be added to the River in due time. In essence, it is to be expected that the efficiency in reducing salt added to the River is 98% or belter. Considering the information available, which presents a convincing, coherent picture of the hydrogeologic scheme of the thermal springs, there appears to be no way in which production from the Redstone well could increase either the total amount of saline water produced from the aquifer or tap new sources of salinity not now draining into the River. George V. Keller, Consultant February 7, 1994 3 LJ