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South Indian Bend Wash | Site Hydrogeology

At South Indian Bend Wash (SIBW), groundwater occurs in three aquifer units: upper, middle, and lower alluvial units. The materials are primarily a thick, basin-fill sequence of alluvial sediments derived from surrounding mountains. Igneous rocks intrude in places, and crystalline bedrock exists in juxtaposition to the alluvial units as a result of block faulting.

The upper alluvial unit (UAU) is distributed across the entire SIBW study area, and generally has a uniform thickness. The UAU typically is found near or at the ground surface and extends to approximately 110 to 170 feet below ground surface (bgs). The estimated transmissivity values varied widely from a low of 1,900 square feet per day (ft2/day) to a high of 73,000 ft2/day. Groundwater flow directions in the UAU are south to southwest during non-river flow conditions in the Salt River. These flow directions shift to south and southeast during river flow conditions in the Salt River when recharge influences groundwater flow directions. Groundwater flow through the UAU originates mainly from Salt River recharge (during flow events) and lateral inflow moves vertically downward, eventually entering the middle alluvial unit (MAU).

The MAU lies below the UAU and consists primarily of clay and sandy silt with significant interbedded layers of sand and gravel mixtures. These coarser-grained interbedded layers generally represent the zones with higher hydraulic conductivity in the MAU. Weak to strong calcium carbonate cementation is also present in the MAU.  The interbedded stratigraphy encountered within the MAU is subdivided into subunits A, B, and C. MAU Subunit A is very thin and discontinuous; consequently, no wells are screened in this subunit. MAU Subunit A ranges in thickness from 5 to 20 feet and is typically found between 170 and 200 feet bgs. MAU Subunit B ranges in thickness from 20 to 70 feet and is typically found between 250 and 300 feet bgs. The groundwater flow direction in MAU sub-unit B is generally west to east, but insufficient data exist to fully characterize the flow direction. MAU Subunit C ranges in thickness from 70 to 150 feet and is typically found between 380 and 550 feet bgs. The groundwater flow direction in MAU sub-unit C varies from due north to east, with northeast appearing to be the predominant flow direction.

The lower alluvial unit (LAU) underlies the MAU and is usually encountered at 500 feet bgs. Observations of the LAU indicate that the composition of the LAU is a conglomerate, dominated by weakly cemented gravel, sand, silt, and rock fragments. Limited data exist to estimate groundwater flow directions in the LAU. The general flow direction is to the east or northeast, similar to the MAU.