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Effectiveness of monitored natural attenuation (MNA) as a groundwater remedy for arsenic in phosphatic wastes

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doi: 10.1111/gwmr.12353
Authors:Bessinger, Brad A.; Hennet, Remy J.-C.
Author Affiliations:Primary:
S.S. Papadopulos and Associates, Camas, WA, United States
Volume Title:Ground Water Monitoring & Remediation
Source:Ground Water Monitoring & Remediation, 39(4), p.52-68. Publisher: Wiley-Blackwell on behalf of National Ground Water Association, Malden, MA, United States. ISSN: 1069-3629
Publication Date:2019
Note:In English. 48 refs.; illus., incl. 5 tables
Summary:A study was conducted to evaluate monitored natural attenuation (MNA) as a remedy for arsenic in groundwater at a former phosphate mining and manufacturing facility. The mineralogy, speciation, and lability of arsenic in phosphatic wastes present in soils were characterized using sequential extraction procedures, leaching experiments, batch adsorption tests, and microchemical speciation analysis. A PHREEQC-based reactive transport model was also parameterized using these laboratory results, and it was used to evaluate the importance of identified attenuation mechanisms on arsenic concentrations along a vertical flow path from a shallow, alluvial aquifer to the underlying Floridan aquifer. Arsenic was found to occur in several chemical forms in phosphatic wastes, including unstable sulfide minerals, adsorbed surface complexes, and relatively insoluble phosphate and oxide minerals. Most arsenic was associated with stable minerals. The reactive transport model predicted that historical leaching of solid-phase waste materials in soils would not have generated enough arsenic to explain the concentrations observed in downgradient groundwater; instead, the source of arsenic to groundwater was likely acidic and saline process water that infiltrated though unlined ponds and ditches during historical manufacturing operations. A key factor affecting the long-term effectiveness of natural attenuation of arsenic in groundwater is the occurrence and stability of iron oxyhydroxides in aquifer sediments. According to laboratory and reactive transport model results, sufficient levels were found to be present at the site to effectively limit arsenic migration at concentrations exceeding drinking water standards in the future in the Floridan aquifer. This study presents the geochemical evaluations that are needed to satisfy EPA guidelines on determining whether or not MNA is an acceptable remedy for a site. It specifically details the characterization and modeling that were used to demonstrate effectiveness at a site where MNA was ultimately selected as the remedy for arsenic in groundwater. Abstract Copyright (2019), , National Ground Water Association.
Subjects:Aquifers; Arsenic; Case studies; Chemical composition; Chemical fractionation; Ground water; Industrial waste; Infiltration; Metals; Mine waste; Mineral composition; Models; Monitoring; Natural attenuation; Oxides; Phosphate deposits; Phosphates; Pollution; Reactive transport; Remediation; Sequential extraction; Solid waste; Sulfides; Toxic materials; Transport; Uncertainty; Waste disposal; Florida; Floridan Aquifer; Hillsborough County Florida; United States; Iron oxyhydroxides
Coordinates:N273700 N280900 W0820200 W0823700
Record ID:869745-9
Copyright Information:GeoRef, Copyright 2021 American Geosciences Institute. Reference includes data from John Wiley & Sons, Chichester, United Kingdom
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