MULTIPLE AQUIFER TYPES IN THE SOUTHERN SACRAMENTO MOUNTAINS, NEW MEXICO, AS REVEALED BY WATER LEVEL MONITORING IN WELLS

As part of a regional hydrogeologic characterization of the southern Sacramento Mountains, water levels in 53 wells have been measured continuously, bimonthly, or quarterly since November 2005. Significant monsoon precipitation in summer and fall 2006 provided an opportunity to monitor the response of shallow and deep aquifers to seasonal precipitation and storm events.

Water bearing zones in the majority of the wells are fractured limestone beds of the Permian Yeso and San Andres Formations, which form a shallowly east-dipping homocline. Several aquifer types can be identified based on trends in water levels, well depth, and hydrogeologic setting.

1. Shallow stream-connected aquifers occur in drainages, are unconfined, and perhaps perched. Wells are less than 100 feet deep. Water levels rose in August-September 2006, probably in response to streamflow, and decreased again within 2-4 months.

2. The fractured high-altitude aquifer is tapped by all wells above 8500 feet, and some wells at lower elevations to the east. It is characterized by a quick rise of water levels in August 2006 followed by a decline in two or more months.

3. Water levels in the intermediate to deep, interconnected regional aquifer began to rise at least one month later than the high-altitude wells and had not dropped as of April 2007. These wells are all located below 8500 feet.

4. Some wells 490 to 740 feet deep tap a deep regional confined aquifer. They show little or no water level changes.

The majority of the wells monitor aquifer types 2 or 3. Our preliminary interpretation is that the fractured high-altitude aquifer is recharged by local precipitation and soon begins to drain as groundwater flows downslope through zones of interconnected fractures. Fractured aquifers below 8500 feet are recharged by precipitation originating both locally and upslope, as indicated by isotope geochemistry of springs. Water level rises are lagged a month or more, on average, in wells tapping the intermediate to deep, interconnected regional aquifer. The recharge probably was not local precipitation and flow in this regional aquifer may not be associated with well-developed interconnected fracture networks. Ongoing monitoring and geochemical analyses of well waters will refine this interpretation of the aquifers in the Sacramento Mountains.