UV ATTENUATION IN STREAMS: STORM RESPONSE OF GROUNDWATER, RUNOFF, AND CANOPY THROUGHFALL

Our objective is to develop a predictive model for variations in the concentration and optical qualities of dissolved organic carbon (DOC) and suspended sediments, the major contributors to attenuation of ultraviolet radiation (UVR) in streams. Previous studies in lakes have shown that DOC concentration and quality explain most of the seasonal and between-lake variation in UVR penetration in non-turbid water. Our recent work in streams has shown that variations in both DOC and turbidity are important factors for UVR attenuation during storm discharge, and that the relative proportion of soil-derived and algae-derived DOC is surprisingly variable among stream sites.

Our approach is to use paired headwater streams in eastern Pennsylvania chosen so that they differed in land cover (forest versus agriculture). These two streams are being used to investigate the contributions of storm overland runoff, forest canopy throughfall, and groundwater to stream flow. We measured changes in water quality with automated equipment (ISCO samplers triggered by rain and YSI datasondes). Samples of streamwater were collected before, during, and after storm events. Precipitation was collected in funnel collectors placed in the open and under forest canopy to infer the contribution of rainwater and canopy throughfall. Preliminary data include pH, specific conductance, stream stage, DOC, and turbidity.

Results for several storms ranging from small rain events to 50-year flood events indicated significant differences in the time of peak discharge and temporal variation in specific conductance. Turbidity was correlated with peak discharge. DOC was inversely correlated with specific conductance, a pattern we have seen in storm runoff monitor at two other sites in the Lehigh River watershed. Substantial DOC was present in canopy throughfall collected during each storm. Understanding the sources and hydrologic dynamics of UVR absorbing DOC will help us understand the transparency of lotic systems and the impact of damaging radiation on freshwater ecosystems.