New Mexico Geological Society Annual Spring Meeting — Abstracts

The Tularosa Basin Hydrogeology Study focuses on how recharge enters the basin from the east bounding mountains between Carrizozo in the north and Alamogordo in the south.  To understand the mechanisms controlling recharge to the area a broad range of analysis were carried out.  First, detailed geologic mapping at a scale of 1:24,000 was conducted, which provided us with an accurate understanding of the subsurface geology. Next, bi-monthly and continuous water-level measurements were collected from a network of appxomiately 100 wells for 2 years to provide insight on daily, monthly and seasonal fluctuations that occur in the aquifer system.  Additionally, these water-levels were used to generate a regional water table map.  Numerous geochemical analyses were conducted to help interpret the flow path of groundwater, identify the recharge areas, and evaluate the groundwater residence time.  Lastly, to quantify how much groundwater is recharging the basin we performed a basic Darcy flow estimation.  Darcy’s Law requires knowledge of the saturated thickness of the aquifers present, which were able to estimate from the detailed geologic mapping, the hydraulic conductivity of the units, which was estimated from previous studies in the area, and the gradient of the water table, which was determined using the regional water-table map that we created.

Direct infiltration of precipitation to the basin floor in arid climates is typically negligible, due to limited precipitation volumes, high evaporation rates, and a deep water table.  Instead, precipitation that falls on the neighboring mountains is typically the source of recharge for aquifers in the adjacent basins.  There are two mechanisms that facilitate recharge from mountain precipitation to these basin aquifers; mountain-front recharge (MFR), and mountain-block recharge (MBR).  MFR refers to water that recharges an aquifer as result of overland runoff that enters stream, and eventually infiltrates in the permeable alluvial fans the mouths of drainage basins on the valley floor.  A previous USGS study (Waltemeyer, 2001) suggests that this is the primary source of recharge to the basin.  Water-table mounding near the mouths of the drainage basins, as well as some geochemical results supports this.  MBR refers to water that infiltrates to the high mountain aquifer, slowly flows through the mountain-block, flowing down gradient through the aquifers present, before entering the basin fill aquifer.  There are several geochemical indications that some groundwater found in the basin was in prolonged contact with aquifers in the mountains, suggesting MBR.

To estimate the volume of recharge to the basin via MBR we looked at the difference between the Darcy flow calculations (67,900 AFY) and the USGS stream flow estimates (45,300 AFY).  If the USGS study (Waltemeyer, 2001) accounts for all surface runoff entering the basin via MFR, and the Darcy calculation accounts for infiltrated surface water (MFR) and deeper flow paths (MBR), then we estimate that 33% comes from deeper flow paths, or mountain-block recharge.

References:

1. Waltemeyer, S.D., 2001, Estimates of mountain-front streamflow available for potential recharge to the Tularosa Basin, New Mexico: U.S. Geological Survey Water-Resources Investigations Report 01-4013