Tohono O’odham Nation, Arizona

Client: Tohono O'odham Nation
Project: Tohono O'odham Nation, Arizona

NRCE was hired by the Tohono O’odham Nation to conduct an appraisal-level study to quantify the historic, present, and future agricultural and non-agricultural water uses and water rights for the Sif Oidak, Gu-Achi, Hickiwan, Sells, and Schuck Toak Districts.

This study included surface and groundwater supply analysis, land classification, economics, and engineering feasibility study for water development. Surface water and groundwater supplies were evaluated, and two main sources of surface water were identified. The groundwater was determined to be the most important resource for sustained irrigated agricultural development, and its availability largely determined the amount, location, and field design of future irrigation lands. The present and historic groundwater characteristics were established, and the computer model MODFLOW was used to locate proposed future wells optimally and to simulate effects of pumping.

The agricultural water use quantification took into consideration the land base, the water supply from groundwater and surface water, the climate of the Districts relative to the adaptability of various crops in the region, the cost of facilities, and the associated economic return. The non-agricultural water use quantification included water requirements for stock ponds and domestic, municipal, mining, religious, industrial, and recreational purposes.

Groundwater Recharge and Irrigation Feasibility Studies for the San Xavier District

NRCE investigated the potential areas for recharging 40,000 acre-feet per year into groundwater aquifers within the San Xavier District of the Tohono O’odham Nation. NRCE performed the engineering design and cost estimation of the conveyance and recharge facilities for several alternatives. Alternative recharge scenarios were evaluated using optimization techniques. Hydraulics of groundwater recharge were modeled to help predict the potential success of alternatives. This also included a comprehensive survey of pertinent reports and data on geology, groundwater resources, and groundwater recharge of irrigation water.

NRCE also conducted two feasibility-level alternative designs for a 9,000-acre irrigation project for the San Xavier District. In addition, NRCE evaluated the soil data from the project area, determining which lands are arable and identified factors limiting various types of irrigation. In addition, NRCE designed conveyance, on-farm distribution, and drainage systems and prepared cost estimates.

Flood and Groundwater Withdrawal Damage Assessment

NRCE evaluated the hydrologic impacts of regional water development and urbanization on surface and groundwater resources of the Districts. NRCE evaluated the effects of upstream basin modification on the frequency and magnitude of flooding downstream and the effects of groundwater withdrawal by non-Indian pumping outside the District. The flood damage studies included an analysis of the effects of urbanization and levee construction on flood levels and the impact of a flood control reservoir on flood characteristics. These flood studies and surface and groundwater supply studies required statistical analysis to determine flood frequencies, characteristic flows, and the establishment of cumulative impacts of basin changes on resources.

Water Supply and Irrigation Potential of a Proposed Diversion of the Santa Cruz River

To support feasibility studies for a proposed Santa Cruz River diversion, NRCE developed a hydrologic model to analyze total available flows to the Santa Cruz River. The U.S. Environmental Protection Agency’s (EPA) Hydrologic Simulation Program – FORTRAN (HSPF) was used. After the model was calibrated and verified to account for seepage and evapotranspiration, NRCE staff was able to estimate the total annual additional waters that would be available to the Nation if this diversion were constructed. A flood frequency analysis was completed as a part of this report using Log Pearson Type III analysis. The results of this analysis were used to further calibrate the hydrology model. Finally, the reservoir operation was modeled, and analyses of groundwater recharge with and without the proposed diversion, as well as under different irrigation plans, were analyzed.

100-year Floodplain Analysis

NRCE developed a hydraulic model utilizing HEC-RAS software to establish the 100-year floodplain for a reach of Greene Wash in the Sif Oidak District of the Tohono O’odham Nation. Historically, flooding from Greene Wash has caused significant damage to the Village of Chuichi and has contributed to the abandonment of agricultural projects near the Village. NRCE staff surveyed the reach using total station and GPS equipment. Flows were established for various return periods by using data from the previous report (“Water Supply and Irrigation Potential…”) and a land use/area weighted approach. The output of this model is currently being analyzed and will be used to make recommendations for flood protection measures in this reach, possibly including additional constructed levees, floodwalls, and culvert improvements.

Evaluation of Mining Related Impacts to Water Supply

NRCE conducted an evaluation, review, and analysis of known and potential impacts to the Tohono O’odham Nation’s water resources resulting from previous, on-going, and proposed future mining activities as well as proposed reclamation efforts at the Cyprus Tohono copper mine. The mine is located in the Slate Mountains about 30 miles south of Casa Grande on Tohono O’odham Nation lands. The mine is not presently in operation due to a decline in copper prices but has a long history of operation that has included both open-pit and in-situ mining methods. The mine presents numerous potential water resource impacts, some of which are described below.

  • There has been a degradation in the water quality at the supply well for the community of North Komelik. Sulfate levels have increased from 250 ppm to over 500 ppm over the last several years and the Total Dissolved Solids level has increased to over 1,300 ppm.
  • Much of the old in-situ leaching solutions remain in the groundwater surrounding the open-pit. These fluids were to be pumped-out concurrent with dewatering of the open-pit as part of on-going mining activities. With the current suspension in mining activities, whose short and long-term disposition remains uncertain.
  • There is known groundwater contamination in the vicinity of the unlined evaporation and tailing ponds attributable to the dumping of roaster effluent and other process plant solutions into these facilities.
  • In 1999 and 2000 a series of inadvertent spills of over 20 million gallons of pregnant leachate solution into the open pit occurred. These fluids have not yet been removed as required by the current Mine Plan of Operations, and alternative remedial activities are currently being studied.

With the uncertain future of mining activities at the facility, reclamation issues were also pertinent. Reclamation issues include reclamation of the heap leach piles and vat leach tailings, ultimate disposition of the pit and in-situ mining fluids, and remediation/reclamation efforts at the tailing and evaporation pond, among others. Technical analysis and review of proposed remediation and reclamation plans and bond assessments was conducted.

To address the on-going environmental impact concerns, a technical committee has been formed to review monitoring, remediation, and reclamation activities. NRCE regularly attends these meetings with the Tribe and assists in the review of technical reports, computer models, monitoring plans, remediation strategies, and reclamation proposals to ensure Tribal water resources are protected.

NRCE assisted in the development and review of a groundwater model of the area near the mine. The model was developed to assist in the evaluation of operations, remediation, and reclamation proposals.

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