Open-file Report - 615
White Paper: A Summary of the Hydrogeology of the San Agustin Plains, New Mexico

Over the last several years, the New Mexico Bureau of Geology and Mineral Resources (NMBGMR), Aquifer Mapping Program, has been working on numerous research aspects of the geology and hydrology of the San Agustin Plains, New Mexico. This White Paper, Open-File Report 615, provides a detailed overview and summary of the results from this work, and will be followed by a comprehensive, peer-reviewed report on these results, with data, in the coming months. Major findings include:

• The San Agustin Plains consists of two distinct bedrock-floored, sediment-filled basins termed the East and West basins. Within each basin, groundwater flow is constrained by the underlying geologic structure. Of particular importance are areas where bedrock has been down-dropped along faults to form four distinct grabens: Horse Spring graben, North graben, C-N graben and White Lake graben.
• The surrounding mountains are made of layered volcanic rocks and sedimentary rocks made of eroded material from volcanoes. The type of rock controls how water moves through the mountain blocks. Some of the volcanic layers are highly fractured and transmit water more easily. Other volcanic layers and the majority of the sedimentary rocks are low permeability, thus not allowing water to move readily through the rock.
• The bedrock basin of the San Agustin Plains has been progressively filled with sediment over the past 25 million years. The sediment is mainly composed of sand and mud (silt-clay), with gravelly sand found near hills and mountains. The type of sediment deposited is a function of the paleo-environments of deposition. Piedmont deposits (including alluvial fans), near the edges of the basin, are composed of the coarsest sediment, whereas basin floor deposits (or alluvial flat) contain finer sediment, and sediment found in paleo-playa lake areas, typically near the middle of the basin floor, is the finest.
• Because the structural grabens have been continuously filling with sediment containing notable but variable proportions of mud, the San Agustin Plains is highly susceptible to groundwater-withdrawal-driven subsidence.
• Groundwater levels are nearly flat across most of the San Agustin Plains. The groundwater flow direction is generally west from the C-N graben through the West basin. At the southwestern end of the West basin, flow direction turns sharply south and is inferred to flow toward the Gila River watershed.
• Groundwater levels have been slowly declining (less than 2 in/year) throughout the central part of the San Agustin Plains over the last 30 years. This effect is best documented with the NMBGMR measurements from 2007 to 2017.
• The groundwater in the tributary valleys at the margins of the San Agustin Plains, and within the Plains, has low concentrations of total dissolved solids (TDS) in general, including low concentrations of arsenic and uranium (which are common naturally occurring water contaminants in New Mexico). Poorer water quality was found in some wells completed in tight bedrock and in wells under the playas in the western end of the basin.
• Groundwater recharge enters the basin at depth through fractured volcanic rocks called tuffs, and through shallow alluvial aquifers in tributary valleys at the margins of the basin.
• Recharge rate, defined as the ratio of water that enters the aquifer to amount of precipitation, is low (0.3 in/yr compared to 14.6 in/yr of precipitation), according to chloride mass-balance calculations. Recharge mainly originates in the mountain blocks and tributary valleys at the margins of the Plains.
• Mountain block recharge into the aquifers hosted in volcanic rocks is likely structurally controlled and likely occurs less than 5 miles from the margin of the basin. Focused recharge occurs along the length of the tributary valleys during flow events; focused recharge is a mixture of recent waters and older waters.
• Timescales for recharge are long. From the time that rain falls, and is focused through tributary valleys, it takes, on average, between 1,000 years and 4,000 years for that water to reach the aquifer. Water that percolates through the mountain-block fractured volcanic rocks (tuff) takes even longer to reach the aquifer, taking between 9,000 and 14,000 years. The majority of groundwater shows ages consistent with mountain block recharge, indicating that
  • Mountain block recharge is likely greater than focused recharge through tributary valleys.
  • It has taken 10,000 years for water to travel the five to ten miles from the mountain block into the basin aquifer.
• The current amount of water use slightly exceeds the recharge rate of the basin, based on the following points:
  • The long time needed for water to move through the basin, as indicated by low water table gradients.
  • The relatively low flow rates of recharge, as indicated by long travel times over short distances.
  • The small proportion of water that become groundwater in the recharge area.
  • The slow lowering of the water table.

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