Groundwater sampling in karst terranes: passive sampling in comparison to event-driven sampling strategy
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PAPER
Groundwater sampling in karst terranes: passive sampling in comparison to event-driven sampling strategy Malcolm S. Field 1 Received: 27 April 2020 / Accepted: 8 September 2020 # This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2020
Abstract Karst aquifers are very easily contaminated because of the surficial features that commonly exist in karst terranes. Pollutant releases into sinkholes, sinking streams, and/or losing streams commonly result in concentrated solutes rapidly infiltrating and migrating through the subsurface to eventually discharge at downgradient springs unless intercepted by production wells, but slow percolation through soils also may result in serious contamination of karst aquifers. The unique features of karst terranes tend to cause significant problems in the interpretation of results obtained from water-quality grab samples of karst groundwater. To obtain more representative samples, event-driven sampling was proposed some decades ago, but event-driven sampling can be difficult and expensive to implement. In this paper, application of passive-sampling strategies is advocated as a means for effectively obtaining representative water-quality samples from karst aquifers. A passive-sampling methodology may be particularly useful for karst aquifers that may be found in complexly folded and faulted terranes. For example, a groundwater tracing investigation of a contaminated site in a karst terrane confirmed that several offsite springs and wells are connected to the contaminated site. Tracer recoveries suggested transport rates that were relatively slow for flow in a karstic aquifer (~0.02 m/s). Breakthrough curves were erratic and spiky. To obtain representative groundwater samples, a passive-sampling methodology is recommended. Keywords Karst . Contamination . Passive sampling . Groundwater monitoring
Introduction Karst aquifers are generally regarded as some of the most difficult aquifer types to be effectively investigated for groundwater contamination (Kresic 2013). The difficulties arise from the nature of how karst aquifers develop, specifically the dissolution of soluble rocks in which the aquifer is formed. As is well documented in numerous sources (e.g., Ford and Williams 2007; Palmer 2007; White 1988), the dissolution of soluble rocks results in discrete input points that direct surface water in a relatively unrestricted manner into the subsurface. Dreybrodt (2004, pp. 295–300) provides a very Published in the special issue “Five decades of advances in karst hydrogeology”. * Malcolm S. Field [email protected] 1
US Environmental Protection Agency, Center for Public Health and Environmental Assessment (8623R), 1200 Pennsylvania Ave. N.W., Washington, DC 20460, USA
brief overview of the dissolution of soluble rocks that emphasizes the complexities involved. Below the soil zone, bedrock fissures, bedding-plane partings, and vadose shafts within the vadose zone continue to direct and concentrate the inflowin
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