PH15 Filling Critical Data Gaps in Ecological Risk Assessment
Exhibit Hall
8:00 AM - Thursday
(PH180) Seasonal study of groundwater-surface water interactions at a mining site in the southwestern United States.
Sciera, K1, Greenberg, M2, 3, Rauscher, J1, Purcell, M1, Henry, R4, Forsythe, B4, Wagner, A5, 1 U.S. EPA Region VI, Dallas, TX, USA2 U.S. EPA Environmental Response Team, Edison, NJ, USA3 Wright State University, Dayton, OH, USA4 U.S. Fish and Wildlife Service5 Molycorp, Inc., Questa, NM, USA
ABSTRACT- The quantitative evaluation of groundwater-surface water interactions (GSI) is becoming increasingly important in ecological risk assessments on streams and rivers. Since GSI dynamics are not routinely incorporated into site assessments, there is a significant data gap limiting our ability to address contaminated groundwater discharges to surface water when it is a potentially complete exposure pathway for aquatic organisms. We used mini-piezometers equipped with differential pressure gauges for direct measurement of vertical hydrologic flow conditions (e.g., upwelling, downwelling) and to sample groundwater at 30 and 60 cm within the stream bed of the Red River, northern New Mexico. Sampling was conducted in October 2003 and March 2004 to assess seasonal differences in hydrologic and physicochemical conditions of the transition zone. The groundwater samples were analyzed for metals contamination and standard water quality parameters were measured daily. Numerous metals exceeded screening benchmarks in groundwater sampled from 30 and 60 cm depth in the streambed. For some metals (e.g., Al, Cd), the concentrations in samples from mini-piezometers were greater than those measured in surface water by up to two orders of magnitude. The vertical hydraulic gradients were similar between sampling events and ranged from -0.4 to +1.2 cm/cm and -0.7 to +1.2 cm/cm in October 2004 and March 2004, respectively. However, groundwater discharge was more significant in March 2004 as the proportion of upwelling measurements was 15% higher than the previous fall. The groundwater metals chemistry and VHG data are important for determining chronic exposure risks from discharging contaminated groundwater. The results of the seasonal GSI study are discussed in relation to the results of the site characterization, benthic macroinvertebrate community studies, and laboratory and in situtoxicity studies.
Key words: ecological risk assessment, groundwater-surface water interactions, mini-piezometers, mining site