Influence of Seasonal Redox Variations on Mobility of Trace Metals in a Shallow Alluvial Aquifer, Jemez Mountains, NM

Seasonal variations in surface water infiltration and microbial activity drive redox shifts (temporal and spatial) within the upper 2 m of an alluvial aquifer at Rio Calaveras, a first order stream in the Jemez Mountains of northern NM. These shifts have been shown to influence Fe, Mn, and SO₄ concentrations in groundwater at the site (Groffman and Crossey, 1999). The present study expands this work to examine: (1) the influence of redox variation on the mobility of V, Co, Cr, Ni, and Cu, and (2) the partitioning of these elements between water and associated sediment (extracted with 1N HNO₃). Two sets of water samples were collected using dialysis cells that were installed at 10 cm intervals and allowed to equilibrate for 4-6 weeks. One sample set (31 May) represents more reducing conditions characteristic of drier seasons; the other set (6 April) was collected during snowmelt infiltration and represents more oxidizing conditions. In the absence of surface water infiltration (31 May data) D.O. levels are < 1 ppm and pH is nearly constant (6.6-6.7) with depth. Under these conditions Cu levels are uniformly low (< 1 ppb) while Ni, V, and Co generally increase upsection (particularly within the uppermost 30 cm). The Ni variation closely follows that of Fe and TOC, suggesting that Ni mobility is influenced by the stability of Fe oxyhydroxides and/or by the availability of organic ligands. Conversely, Co and V appear to correlate more closely with Mn. Element profiles are markedly different in the 6 April data set, where the infiltration of oxygen rich snowmelt resulted in sulfide breakdown and formation of a low-pH (< 4), slightly more oxygenated (1-2 ppm) zone at the top of the aquifer. Within this zone Cu is elevated (up to 17 ppb) but Fe, Mn, and the other trace metals occur at their lowest levels (4-40 x lower than 31 May data). These results suggest that sulfide stability exerts an important influence on Cu mobility and that the other elements are primarily influenced by oxyhydroxides of Fe (Ni, Cr) and/or Mn (Co, V). Low element levels in the low-pH zone indicate that sorption is not the dominant process regulating their mobility, a finding supported by the generally poor correlation between trace metal concentrations in solution and trace metal concentrations in the sediment. The results of this study suggest that seasonal and interannual precipitation variations can influence the trace element geochemistry of shallow groundwater.