Recharge in a semi-arid basin aquifer : Ryan Flat and Lobo Flat, Trans-Pecos, Texas




Van Broekhoven, Norman G.

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In arid and semi-arid settings a key hydrological question is: does significant recharge occur and, if so, where, how much, and by what mechanisms? Ryan Flat and Lobo Flat are underlain by a bolson aquifer in Trans-Pecos Texas where the groundwater recharge is generally accepted to be slight. Previous studies suggest that recharge by direct infiltration into the basin fill and typical ephemeral streams in the basins is, at most, a few mm/yr. Evapotranspiration and the soil texture restricts infiltration and recharge. Nor were alluvial fans near the study aquifer usually found to be the sites of recent recharge. Infiltration into these fans appears to be impeded by low permeability layers deposited by sheet flow. Yet recharge occurs in portions of the aquifer underlying Ryan Flat and Lobo Flat. Recent recharge is suggested by groundwater potentiometric mounds centered about VH Canyon to the west and along the front of the Davis Mountains to the east. Groundwater from the mountains and these mounds have a different chemical facies than groundwater down gradient and are hypothesized to be younger members of a continuous chemical evolutionary trend. This infers that the central basin water originated in the mountains and basin margins. Groundwater isotopic data indicate that recharge is not subject to extensive evapotranspiration as occurs in the thick unsaturated zone of the basin fill. This supports the hypothesis that recharge occurs by rapid infiltration at select locations such as mountain fractures, basin margins, or ephemeral streams. Geophysical methods were used to investigate infiltration in the ephemeral stream channel near VH Canyon. Direct observations of rain events, stream flow, and infiltration provide supporting evidence that this is a site of preferential infiltration and recharge. Field mapping of fractures in mountain canyons, analysis of digital elevation models and aerial photographs indicate that the position of the mountain canyons and streams are controlled by fractures. The mechanisms of recharge seem to be a combination of fracture infiltration and flow in the mountains, infiltration into the bottoms of ephemeral streams in mountain canyons that are located along fracture zones, and infiltration of ephemeral streams near the basin margin. Digital elevation models (DEMs) were proven to be useful for identifying topographic linears caused by fractures in the mountains and under the basin fill. DEMs were able to enhance topographic trends that were less evident in aerial photographs having a much higher resolution.