DEEP GROUNDWATER UPWELLING IN THE SOCORRO BASIN, NEW MEXICO; INFLUENCE ON WATER QUALITY AND MICROBIAL COMMUNITY STRUCTURE

Chemical analyses of surface water and shallow groundwater samples from the Socorro basin of the Rio Grande rift suggest deep groundwater upwelling has a considerable influence on water quality and subsurface microbial community structure. The upwelling increases the salinity of the shallow groundwater, as previous researchers have noted. We hypothesize it also contributes elevated levels of arsenic and selenium and high concentrations of reduced iron and manganese, molecular hydrogen, and methane to the groundwater. These constituents provide the potential for diverse microbial activity in the shallow alluvial aquifer.

Our primary study site is a well transect crossing the Rio Grande at San Acacia, the terminus of the Albuquerque Basin. The New Mexico Tech Rio Grande Project maintains the site and has identified it as a region of groundwater upwelling. We collected surface water samples from 2 locations and groundwater samples from 7 well nests, each containing a well completed at the water table and a well completed 25 feet below the water table. One nest also contained a deep well completed 75 feet below the water table. In addition, we collected baseline samples from two downstream transects having no evidence of groundwater upwelling.

There is a sharp contrast between the composition of the samples taken in the zone of upwelling and the baseline samples. The total dissolved solids content exceeds 6500 mg/L in some samples from the upwelling zone compared to 950 mg/L in the baseline samples. Iron, manganese, arsenic, and selenium levels are as high as 8 mg/L, 7.5 mg/L, 70 µg/L, and 200 µg/L, respectively, in the zone of upwelling compared to 1.6 mg/L, 1.3 mg/L, <40 µg/L, and <20 µg/L, respectively, in the baseline samples. Hydrogen and methane content is up to 42 nM and 320 nM, respectively, in the samples from the upwelling zone; analyses of the baseline samples are pending.

During sampling, we observed oxidized iron in water from wells completed at the water table and particulate sulfide in water from wells completed below the water table. These observations, along with the dissolved chemical analyses, suggest our molecular characterization of the microbial communities, which is presently underway, will reflect metal and sulfide oxidizing communities near the water table and sulfate reducing communities below.