1. The study area Using Environmental Tracers to Help Calibrate a Ground-Water Model of The Middle Rio Grande Basin Ward Sanford and Niel Plummer USGS National Research Program, Reston, VA Doug McAda, Laura Bexfield & Scott Anderholm USGS Water Science Center, Albuquerque, NM

2. Information Used to Construct and Calibrate the Model Geologic Information Hydrologic Information Geochemical Information MODFLOW, MODPATH and UCODE

3. Geologic Information

4. Geologic Studies were used to Develop the Model Geometry and Zonation Geologic model of the Middle Rio Grande Basin constrained by gravity data (Grauch)

5. Inner Valley Alluvium River Gravels Ceja Gravels Medium to Coarse Sand Silts Medium Sand Fine to Medium Sand Eolian Sand Volcanic Sand Piedmont Slope Deposits Volcanic Flows Volcanic Intrusions Layer 1Layer 2

6. Layer 6Layer 5 Inner Valley Alluvium River Gravels Ceja Gravels Medium to Coarse Sand Silts Medium Sand Fine to Medium Sand Eolian Sand Volcanic Sand Piedmont Slope Deposits Volcanic Flows Volcanic Intrusions

8. Hydrologic Information

9. Albuquerque Middle Rio-Grande Basin Water Table in 1960 N

10. Rio Grande Eastern Mountain - Front Recharge Basin Underflow Boundary Conditions Rio Puerco Rio Salado Jemez River Tijeras Arroyo Abo Arroyo Jemez Mountain-Front Recharge and Underflow Southwestern Basin Underflow Middle Rio-Grande Basin Boundary Western Basin Underflow

11. Geochemical Information

12. Carbon-14 Deuterium 100 80 60 40 20 0 pmC -100 -90 -80 -70 -60 -50 -110 -120 per mil Middle Rio-Grande Basin Boundary

14. Local northwest recharge Northern MF Recharge Hagan basin underflow Jemez underflow Rio Puerco Eastern Mountain Front Rio Grande Ladron Peak Abo Arroyo Basin Discharge

15. Results

17. 40 km Creating a “Trough” with Low Recharge Western Recharge Northwestern Recharge River Water Northeastern Recharge Eastern Recharge

18. Water Levels For Layer 2

19. Estimating Paleorecharge Rates

20. Recharge Multiplier

22. Summary and Conclusions Environmental Tracers are an important source of data to use in addition to geology and water levels to calibrate regional groundwater models Recharge in the Middle Rio Grande Basin is much less than previously estimated, but was higher in the past. The lower recharge rates can explain spatial features in both the water table and geochemical zones