New Mexico Geological Society Annual Spring Meeting — Abstracts

Rio Calaveras, located in the Jemez Mountains ofnorthern New Mexico, is a first order montane stream with a shallow alluvial aquifer system. The hydrogeology of the system has been well documented during the past six years allowing for an understanding of the seasonal dynamics. The aquifer is ideal for the examination of the seasonal response of redox processes in a dynamic near-surface environment because 1) the chemistry shows a seasonal response, 2) vadose zone and aquifer sediments are relatively high in organic carbon (an important consideration in biogeochemically mediated systems), 3) a large biogeochemical and hydrological data base has been assembled, and 4) it is a pristine environment.

In addition to surface water sampling at the site, there are also floodplain and transect wells. These three main sampling localities add up to a combined total of 54 sampling locations. Measurements performed in the field and in the lab include pH, oxidation reduction potential (ORP), dissolved oxygen, and major anion and cation analysis (for major and minor elements including iron and manganese).

This study compares hydrologic data (stream discharge, water table fluctuations) with geochemical parameters (anion and cation concentrations, pH, ORP, and dissolved oxygen content). The water table fluctuates up to 0.7 meter during an annual cycle with a
maximum reached during spring snow melt and minimum during winter base flow conditions. During periods of high discharge (spring snow melt), dissolved oxygen in the system is at a maximum (2 to 6 mg/L). Elements such as iron and manganese are predominantly present in their oxidized states and remain stationary within the aquifer system. During periods of base flow conditions, dissolved oxygen within the system is at a minimum and iron and manganese are reduced. Iron and manganese become mobile in the system with concentrations of iron and manganese to 15 mg/L and 2 mg/L respectively. Understanding the dominant hydrologic pulse (snowmelt) and how it affects the geochemistry of the system allows for the application of this knowledge to perturbed systems.