losing reaches of tributary streams, or the Clark Fork 

 itself) , and most discharge along relatively short flowpaths 

 back to the surface environment. Aquifers respond to new 

 ground water withdrawals (wells) with potentiometric 

 adjustments that either increase inflow to the aquifer or 

 decrease discharge to the surface environment or both. Some 

 part of this response may involve increased inflows from 

 other aquifers with more remote relationships to the basin's 

 surface water environment. More often, the major hydrologic 

 response is likely to be an eventual adjustment of surface 

 water flows in some other part of the system. 



The fact that DNRC's ground water permitting has not 

 always reflected these physical realities can be attributed 

 to two factors. First is the information requirement for 

 realistically assessing the overall hydrologic consequences 

 of a given level and manner of ground water development. 

 This level of understanding is only achieved for a given 

 aquifer system through an intensive research program. Often, 

 complex aquifer responses are only predictable through the 

 creation of computer simulations, which in turn rest heavily 

 on an adequate base of regional field information. Because 

 DNRC does not collect much of this type of data itself 

 (viewing it as a research function appropriately left to 

 other agencies and the university system) , the opportunities 

 for the ground water permitting process to meaningfully 

 consider integrated hydrologic implications are limited by 

 others' research priorities and DNRC's ability to direct 

 those priorities. 



The second factor is the comparative scale of existing 

 ground water withdrawals with respect to surface water use in 

 the major hydrologic basins. In the Missoula Aquifer, for 

 instance, annual withdrawals for all purposes average about 

 60,700 acre-feet (Missoula City-County Health Department 

 1987) , some of which returns to the aquifer as water main 

 leakage, septic system discharge, and other recharge 

 flowpaths. This appears minor in relation to the discharge 

 of the Clark Fork, which averages 2.2 million acre-feet/year 

 at a point upstream of the Missoula Aquifer's recharge area. 

 However, the generous hydraulic characteristics of the 

 Missoula Aquifer present the possibility of substantially 

 increasing ground water withdrawals on a sustainable basis. 

 Ground water withdrawals amounting to several percent of the 

 raainstem Clark Fork's flows seem significant where consump- 

 tive and instream priorities, including surface water 

 quality, compete for available flows. Similar arguments 

 could be made regarding other aquifers in the basin that are 

 capable of supplying high yields to wells, as most have 

 significant recharge/discharge relationships with the basin's 

 streams. 



3-11 



