HSI performed a subsurface hydrogeologic investigation and constructed a regional groundwater flow model of the Fountain Hills Sub-Basin of the Phoenix Active Management Area for the Fountain Hills Sanitary District.  The investigation and model provided an analytical tool to evaluate artificial recharge at the Fountain Hills Recharge Facility, including recovery alternatives for water stored at the recharge facility, as well as future groundwater management planning.

HSI developed a numerical groundwater flow model to be used as a quantitative tool to assess the potential impacts of groundwater development on the Red Gap Ranch, a property approximately 40 miles east of Flagstaff, Arizona.  The modeled area extended beyond the borders of Red Gap Ranch, including an area of approximately 1,300 square miles on the Colorado Plateau, and incorporated portions of the City of Winslow and the Navajo Indian Reservation, the Little Colorado River, Chevelon Creek, and Clear Creek.  Drillers logs were analyzed and correlated to surface geophysical surveys of the area which used Controlled Source Audio Frequency Magnetotellurics (CSAMT) in order to define the surface and subsurface geology.  Extensive aquifer testing and analysis was performed on wells all over the property and this information was correlated to geological structures in the area in order to better understand the influence of faulting and fracturing to the hydrogeologic system beneath the property.  USGS stream gage records were used to estimate the base flow contribution of groundwater in the area and Landsat imagery and land cover data sets were used to define the floodplain of the Little Colorado River and determine vegetation types and potential evapotranspiration along the floodplain.

The Red Gap Ranch numerical groundwater flow modeling analysis uses MODFLOW 2000 and simulates pumping, underflow, ephemeral and perennial stream recharge, and evapotranspiration from the floodplain of the Little Colorado River.  The hydrogeologic system was defined as steady state for current conditions and was used to simulate multiple water use scenarios.  The model was calibrated to observed groundwater elevation data collected from 166 wells throughout the area, and baseflow calculated for a perennial reach of Clear Creek.  The resulting model and report were used to support the sale of the property to the City of Flagstaff (See news article).

     Groundwater Modeling

All of this information was used to develop a simple groundwater flow model to determine the physical availability of existing and future groundwater supplies and for evaluating the impact of withdrawals and artificial recharge.  One simple transient modeling scenario was developed to determine the maximum amount of water that can be withdrawn from existing Rainbow Valley Wells without lowering the water levels more than 1,000 feet below land surface.

This project included performance of several aquifer tests and analysis of the test data, collection and analysis of lithologic information that was correlated to gravity and magnetic geophysical surveys of the basin, and collection of all historical hydrologic data available for the valley, which included water levels, rain gauge data, stream gauge information for the Salt and Verde Rivers and Sycamore Creek, historical agricultural diversions, well pumping records, GIS coverage of vegetation type, and municipal water use records from the Arizona Corporation Commission.  The numerical groundwater flow model was constructed by infusing GIS-based conceptual model data into the USGS MODFLOW software using Groundwater Modeling System (GMS) software.  The steady-state component of the groundwater model was pre-1970, and the transient component was from 1970 to 2000.  The model simulates domestic and municipal pumping, agricultural diversion of stream flow, underflow into the Salt River Valley, mountain-front recharge, stream infiltration, and evapotranspiration from riparian vegetation.  The model was calibrated to the available groundwater elvation data from wells throughout the basin.  The results of the model supported the application for a full-scale Underground Storage Facility operated by the FHSD.

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E-mail: info@hydrosystems-inc.com

SunChase Estrella Limited Partnership hired HSI to perform a hydrologic analysis and physical availability demonstration (PAD) to support the Master Development Plan for Estrella Mountain Ranch in Goodyear, Arizona.  This work included reviewing all available geophysical and hydrogeologic data for Rainbow Valley, including two exploratory wells drilled on the property in which drill cuttings were logged, the boreholes were geophysically logged, and zonal water quality samples were collected.  Additional geophysical work utilized several Controlled Source Audio Frequency Magnetotellurics (CSAMT) lines run across Rainbow Valley and existing gravity maps of the area.  The geophysical data was applied to the model to delineate the basin structure and two model layers roughly representing two aquifer units in the valley.