Evaluating long-term patterns of decreasing groundwater discharge through a lake-bottom permeable reactive barrier

• Published in: Journal of environmental management 2018-08, Vol.220 (C), p.233-245
• Main Authors: McCobb, Timothy D., Briggs, Martin A., LeBlanc, Denis R., Day-Lewis, Frederick D., Johnson, Carole D.
• Format: Article
• Language: English
• Subjects: Environmental Sciences & Ecology; Fiber-optic distributed temperature sensing (FO-DTS); Groundwater/surface-water interaction; Instruments and techniques: modeling; Instruments and techniques: monitoring; Permeable reactive barrier (PRB); Temperature
• ISSN: 0301-4797; 1095-8630
• DOI: 10.1016/j.jenvman.2018.02.083

Identifying and quantifying groundwater exchange is critical when considering contaminant fate and transport at the groundwater/surface-water interface. In this paper, areally distributed temperature and point seepage measurements are used to efficiently assess spatial and temporal groundwater discharge patterns through a glacial-kettle lakebed area containing a zero-valent iron permeable reactive barrier (PRB). Concern was that the PRB was becoming less permeable with time owing to biogeochemical processes within the PRB. Patterns of groundwater discharge over an 8-year period were examined using fiber-optic distributed temperature sensing (FO-DTS) and snapshot-in-time point measurements of temperature. The resulting thermal maps show complex and uneven distributions of temperatures across the lakebed and highlight zones of rapid seepage near the shoreline and along the outer boundaries of the PRB. Repeated thermal mapping indicates an increase in lakebed temperatures over time at periods of similar stage and surface-water temperature. Flux rates in six seepage meters permanently installed on the lakebed in the PRB area decreased on average by 0.021 md−1 (or about 4.5 percent) annually between 2004 and 2015. Modeling of diurnal temperature signals from shallow vertical profiles yielded mean flux values ranging from 0.39 to 1.15 md−1, with stronger fluxes generally related to colder lakebed temperatures. The combination of an increase in lakebed temperatures, declines in direct seepage, and observations of increased cementation of the lakebed surface provide in situ evidence that the permeability of the PRB is declining. The presence of temporally persistent rapid seepage zones is also discussed. •Seepage and temperature measurements indicate declining discharge though lake-bottom PRB.•Uneven spatial patterns in groundwater discharge reflect heterogeneity.•Field evidence of cementation of lakebed sediment corroborates clogging over time.•Dynamic nature of exchange reflected in short-term vertical fluid flux modeling.•Persistent, high-flux discharge zones can be targeted to improve PRB effectiveness.