Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Synthesizing binational data to characterize shared water resources is critical to informing binational management. This work uses binational hydrogeology and water resource data in the Mesilla/Conejos-Médanos Basin (Basin) to describe the hydrologic conceptual model and identify potential research that could help inform sustainable management. The Basin aquifer is primarily composed of continuous basin-fill Santa Fe Group sediments, allowing for transboundary throughflow. Groundwater flow, however, may be partially or fully restricted by intrabasin uplifts and limited recharge. The shallow groundwater in the Rio Grande alluvium receives recharge from the Rio Grande and responds to changes in water supply and demand. About 11% of Rio Grande alluvial groundwater volume is recharged annually, an amount that is less than recent withdrawals. Potentially recoverable fresh to slightly brackish groundwater was estimated at 82,600 cubic hectometers in the U.S. portion of the Basin and 69,100 cubic hectometers in the Mexican portion. Alluvial groundwater geochemistry is governed by the evaporative concentration of the Rio Grande and agricultural diversions, whereas deeper groundwater geochemistry is governed by mixing and geochemical processes. Continued refinements to storage estimates, the water budget, and deep groundwater extent and geochemistry can improve estimates of sustainable use and inform alternative water sources.

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The U.S. Geological Survey (USGS), in cooperation with San Miguel County, New Mexico, conducted a study to assess publicly available information regarding the hydrologic resources of San Miguel County and to identify data gaps in that information and hydrologic information that could aid in the management of available water resources. The USGS operates four continuous annual streamgages in San Miguel County. Monthly discharge at these streamgages is generally bimodally distributed, with most runoff corresponding to spring runoff and to summer monsoonal rains. Data compiled since 1951 on the geology and groundwater resources of San Miguel County are generally consistent with the original characterization of depth and availability of groundwater resources and of source aquifers. Subsequent exploratory drilling identified deep available groundwater in some locations. Most current (2011) development of groundwater resources is in western San Miguel County, particularly in the vicinity of El Creston hogback, the hogback ridge just west of Las Vegas, where USGS groundwater-monitoring wells indicate that groundwater levels are declining. Regarding future studies to address identified data gaps, the ability to evaluate and quantify surface-water resources, both as runoff and as potential groundwater recharge, could be enhanced by expanding the network of streamgages and groundwater-monitoring wells throughout the county. A series of seepage surveys along the lengths of the rivers could help to determine locations of surface-water losses to and gains from the local groundwater system and could help to quantify the component of streamflow attributable to irrigation return flow; associated synoptic water-quality sampling could help to identify potential effects to water quality attributable to irrigation return flow. Effects of groundwater withdrawals on streamflow could be assessed by constructing monitoring wells along transects between production wells and stream reaches of interest to monitor decline or recovery of the water table, to quantify the timing and extent of water-table response, and to identify the spatial extent of capture zones. Assessment of groundwater potential could be aided by a county-wide distribution of water-level information and by a series of maps of groundwater potential, compiled for each individual aquifer, including saline aquifers, for which the potential for municipal use through desalination could be explored. A county-wide geographic information system hydrologic geodatabase could provide a comprehensive picture of water use in San Miguel County and could be used by San Miguel County as a decision-support tool for future management decisions.

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