Research Problem and Background:
Increased groundwater pumping and reduced surface water diversions for irrigation in the eastern Snake River Basin (ESRB) of southern Idaho have caused ground water level declines and have reduced the magnitude and distribution of aquifer discharge to the Snake River. Water managers in the state are considering artificial aquifer recharge programs as a means of augmenting groundwater supplies in the upper basin and increasing base flow to the Snake River.

This modeling study identifies potential aquifer recharge sites within the ESRB, and analyzes the hydrologic impacts of artificial aquifer recharge. The two year investigation was conducted in partnership with the Idaho Department of Water Resources and the University of Idaho. Environmental and institutional issues associated with artificial aquifer recharge are also being addressed by this partnership.

Project Objectives:
1. Determine water availability for managed aquifer recharge in the ESRB.
2. Couple a Modflow based groundwater model of the ESRB with a decision support software interface.
3. Model four artificial aquifer recharge scenarios proposed for the ESRB. Describe impact of artificial recharge on groundwater levels and on aquifer discharge to the Snake River. Show potential for integration into basin-wide conjunctive water management plan.
4. Provide opportunities for stakeholder involvement in recharge modeling studies through public meetings, presentations, and ad hoc partnerships.
5. Document modeling results in final report. Transfer technology to other Reclamation offices through workshops and conferences.

Overall Outcome or Conclusions:
The hydrologic impact of artificial recharge depends (in large part) on aquifer recharge rate. Recharge rate is limited by availability of surplus flows in the upper Snake River Basin and by the capacity of recharge facilities. Surplus flow typically occurs during winter months, often as a result of flood releases made from upper basin reservoirs In a typical year, surplus flows during mid-winter average 4,000 cfs. Aquifer recharge rates were determined for various size recharge facilities by statistical analysis of the historical frequency and magnitude of surplus flows.

Recharge scenarios describe location, rate, and timing of potential recharge activity in the basin. In FY98, two of four proposed recharge scenarios were evaluated using the ESRB ground water model. Preliminary modeling results indicate that artificial recharge in the upper basin can augment base flow to the middle Snake River by as much as 700 cfs. Ground water levels in Reclamation’s Minidoka project area can be increased by as much as 20 feet through aquifer recharge. Achieving these results requires significant long-term commitment to planning and operation of artificial aquifer recharge facilities in the upper basin.

Partners:
A 50/50 cost sharing partnership exists with the Idaho Department of Water Resources. University of Idaho participates through its involvement in Reclamation’s Snake River Resources Review project. Other state agencies, including the Idaho Department of Environmental Quality and Idaho Fish and Game Department contribute significantly to the project on an ad hoc basis.

Publications and/or Other Deliverables
The ESRB Modflow ground water model, incorporating new spatial data and a decision support interface, is available to PN regional planners. Two public meetings were held in 1998 to present project goals and to solicit input regarding hydrologic, environmental, and institutional concerns of stakeholders. An interim report on water availability was completed and delivered to reviewers. Extracts from this report will be published in AWRA in an upcoming article co-authored with the University of Idaho. Two more recharge scenarios will be evaluated in FY 99 using the ESRB ground water model. The final report on recharge modeling investigations is scheduled to be delivered in July 1999, at the completion of this two year study. Specific publications include:

Idaho Department of Water Resources. Scheduled July 1999. Managed Aquifer Recharge in the Eastern Snake River Plain Aquifer. Research Technical Completion Report to the Idaho State Legislature.

Johnson G., W. Sullivan, D. Cosgrove, R. Schmidt. Accepted and scheduled for publication, June 1999. Recharge of the Snake River Plain Aquifer: Transitioning from Incidental to Intentional, Journal of American Water Resources Association.

Schmidt, R.D, J. Lefkoff, and P. Castelin. Modeling Managed Aquifer Recharge in the Eastern Snake River Plain. In Proceedings: National Groundwater Association, Pacific Northwest Groundwater Conference, Portland, Oregon, February 17-18, 1999.