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18 . "August 1997. . COLORADO WATER ...... tions, and for some peslicides we did see dctections, they were generally much lower than the standard set by the EPA. For Diazinon, which occurrcd frequently, there is no standard, but concentrations we saw were very low. So again, some good news! One other thing we have done as part of examining water quality is to loole at the watcr quality effects of water operations in the basin, meaning storing water in reservoirs, releasing it for later use, exchanging water from one place to another in the basin, drying up agricultural land, and changing pumpage that might come about as a result of the Compact lawsuit. We have done some studies to loole at reductions in ground water pumpage and taleing agricultural land out of use. We did tbis in a small study area between La Junta and the county line downstream between La Junta and Las Animas, where we have done some modeling in the past to Joole at salinity. We developed a digital computer model of the groundwater system and the surface water system. We gOlthat model to fairly accurately reproduce the conditions we saw both in the river and in the groundwater system for the period from 1971 to about 1995. -Four sites sampled- Portland, Avondale, Catlin Dam, and Las Animas -Sampled during pesticide application and irrigation seasons Results . Nearly 97% of pesticides analyzed were not detected Then we played some games with that model. We looleed at how groundwater quality might change in that area with changes in pumpage from the ground- water system (see Figure 4). The first scenario was 25 percent reduction in pumpage, a second scenario was a 50 percent reduction of pumpage from historic patterns, and a third was complete cessation of pumpage from the groundwater systcm for irrigation in that area. -2,4-D detected at all sampled sites and about 75% of the time but well below EPA standards Fig. 3. Pestil:ides in the Arkansas River We asleed, If we had madc these changes in 1971, what would things be lilee in 19951 In these scenarios we had about a 200 mg per liler decrease in dissolved solids in this irrigated area, or this is what we projected would have happened if we had complelely done away with groundwaler pumpage. There were other interesting things. We saw that there wcre lesser effects with smaller de. creases in pumpage, but we also saw that ittoole about ten years for that groundwater systcm to destabilize. We also looleed at a situation similar to what has occurred under the Rocley Ford Canal, where pan of that land was laleen out of production and water was transferred to another location. ~ '00 to ::; ;; 0 '- ~ '-- --~---- m ----... m no .._- ... ..- .-- m m -------- . '. no_ Z ". ::J -lOa '-, '" ............ en .......... -....... -...-...-.... ...--.........-. a: ......... .~._..... ...--........ .200 _.... ....- hh'h_... .-". W >- '" ;: - 25% PUMPAGE REDUCTION 6 .JOO Z -.-.. 50% PUMP AGE REDUCTION => 0 a: .400 100% PUMPAGE REDUCTION "' ;; w 500 "' Z '" J: .600 () 0 W >- -700 '" ~ => :;; en -800 1971 1975 1980 1985 '990 1995 YEAR Fig. 4. Simulated Change in Groundwater Salinity Relative to Historic Conditions for Scenarios of Reduced Pumpoge. We looleed at various possibilities for drying up that land to see how changes in groundwater quality would have occurred (see Figure 5). With the same lcind of scenarios, we put this change into the model as if it had occurred in 1971 and looleed at how lhose changes would have occurred. For reference, these are shown as simulated change in groundwaler quality, but the average dissolved solids in that groundwatcr at the beginning of this period was about 2200 mg per liter. The maximum change we saw