Understanding Geochemical Recovery in Anthropogenically Disturbed Landscapes.

摘要

Environmental regulations have greatly reduced the output of anthropogenic chemicals to the environment from the mid 1970's to the present. However, lake sediment cores collected in Michigan show temporal concentration profiles of chemicals considered toxic to humans and wildlife to decrease at variable rates during the decades following the 1970s. This disparity may be due factors affecting recovery such as watershed or regional land use, population, and chemical production/consumption trends. For this study, sediment chemical chronologies collected from inland lakes across Michigan were compared to determine temporal and spatial trends of two organic chemicals: PCBs and DDT. These chemicals were selected for study as they both represent banned chemicals with different utilization and dispersion patterns, as well as, different chemical characteristics. Surface water chemistry was also studied in the Saginaw Bay Watershed attempting to relate differences in the dissolved constituent make-up with land use as a means of determining changes to the chemical pathways through the changing seasons.;Results for DDT show US production and usage to be the overriding determinants of the general sediment profiles. However, results show significant variation amongst the individual lakes in terms of temporal peak values and recovery since the ban of these chemicals. This is thought to be the result of both independent watershed-scale and regional sources of DDT recorded in the lake sediment. Some correlation to watershed %urban land-use is apparent but land-use characteristics overall were poorly correlated to DDT concentrations, peak and onset dates. DDT and its metabolites found in recent sediment (top 4 cm), as well as, sediment accumulation rates of these chemicals in each sediment core are well correlated to latitudes of Michigan with higher populations, even if the lake watershed itself wasn't especially populace. This suggests a significant regional component to DDT both historically and since its ban.;PCBs were elevated in the Lower Peninsula compared to the Upper Peninsula and locations of high PCB inventories near urban areas were found. However, concentration and accumulation peak dates increased with latitude of the lake possibly due to secondary mobility and deposition away from sources. During high PCB production years, congener clusters show clear localized patterns, but more recently, congener distributions suggest a more regional signal. However, state-wide regional patterns appear to serve as a component to both recent and historical data, despite the prominence of local and sub-regional trends during the peak PCB production/consumption years.;The SBW study documented strong local fluctuations of solutes attributed primarily to local proximity of urban centers and agricultural land use. However, the geochemical fingerprint of a given land use varied in intensity throughout the year: agricultural land use on surface water chemistry was most evident during a summer low flow event and a spring high flow event, which may be a reflection of seasonal variations in the intensity of agricultural practices. Urban land use exhibited its strongest fingerprint during winter high flow, likely due to road deicing salt application.;Overall, we conclude that studies attempting to link chemical concentrations to a particular land use should give consideration to coordinating surface water analytes to the hydrologic regime and time of year the sample collection is to be undertaken. Further, the regional component of banned chemicals continues to be represented in modern sediment, despite several decades since cessation of production.