New Mexico Geological Society Annual Spring Meeting — Abstracts

Located in the Jemez Mountains of northern New Mexico, Rio Calaveras is a fIrst order montane stream with a shallow alluvial aquifer system. Over the past six years the hydrogeology has been well documented, including three years of detailed hydrochemistry work. The Rio Calaveras aquifer is ideal for the examination of the interannual response of redox processes in a dynamic near-surface environment because 1) the geochemistry shows an interannual response, 2) a large geochemical and hydrological data base has been assembled (over 60,000 data points), 3) it is a pristine stream environment.

Floodplain and transect wells, along with surface water collection, are the primary sampling sources at Rio Calaveras. These three main sampling areas add up to a combined total of 54 sampling locations. The various geochemical parameters gauged the fIeld and in the laboratory include major ion analysis (including iron and manganese), pH, dissolved oxygen, and oxidation-reduction potential (ORP). Geochemical parameters and hydrologic data (stream discharge, water table elevation, and water table temperature fluctuations) are compared in this study for sitewide interannual responses. The water table fluctuates up to 0.7 meter during an annual cycle with a maximum reached during spring snowmelt and a minimum during winter baseflow conditions. During the periods of high discharge, dissolved oxygen in the system is at a maximum (2-6 mg/L). Elements such as iron and manganese are predominantly present at this time in their oxidized states and are at minimum concentrations in the groundwater within the aquifer system. When baseflow conditions occur, iron and manganese are reduced because dissolved oxygen within the system is at a minimum. Concurrently with these conditions, these redox sensitive elements become mobile in the system with concentrations of iron and manganese up to 15 mg/L and 2 mg/L respectively. Understanding the dominant hydrologic pulse (snowmelt), in relation to the interannual variability and the geochemical effects allows the application of this knowledge to anthropologically perturbed systems.