PARENT SESSION
46 - Surface Water Pollution
8:00 AM to 6:30 PM, Tuesday, 14 May 2002
Exhibition Area

(46-45) Iron Amendments as Adjuncts to Constrain Arsenic Solubility in a Marsh Environment.

Davis, Andy^*,1, ¹ Geomega, 2995 Baseline Road, Suite 202, Boulder, Colorado

ABSTRACT- Arsenic (As) behavior in a wetland environment is complicated by redox reactions that control both valence and solubility, and hence an understanding of remediation from a risk-based perspective. This paper investigated the forms of As in a west coast marsh/creek system (containing up to 4740 mg/kg As) and evaluated the use of zero valent iron as a potential passive remedial alternative to reduce As mobility in conjunction with a risk-based excavation plan. In the pre-excavation marsh, As levels in marsh pore water (shallow groundwater) were directly related to bulk As in the adjacent soil samples. High pore water arsenic (2390 g/L) was associated with high soil As (3980 mg/kg), and low pore water arsenic (3.8 g/L) with low soil arsenic (65 mg/kg). Surface water samples collected from a swale in the marsh ranged from 52 g/L to 234 g/L, with concentrations varying during local tidal patterns. Because potential risk from As was greatest for ecological receptors in the adjacent creek system (due to aqueous-phase transport), the soils within 40 feet to the north of the creek and 30 feet to the south of the creek were targeted for removal to a uniform depth of 3 feet. This soil block contained 46% of the total arsenic in the marsh and represented 23 percent of the total soil volume. As an adjunct to soil removal, an amendment was developed to mitigate any remnant soluble arsenic that may possibly migrate to Castro Creek. One set of experiments using Zero Valent Iron (ZVI) combined site groundwater (~3 mg/L arsenic), marsh soil (2000 - 3000 mg/kg arsenic), clean top soil, and/or DI water, with and without the addition of peat, iron, and/or gypsum as soil amendments. Experiments with ZVI, site water, and clean fill only, resulted in removal of >99% of soluble arsenic from the system. These data demonstrate that even in the presence of high As concentrations, hot-spot removal combined with passive remediation would effectively treat soluble As concentrations up to 3 mg/l.

Key words: arsenic, marsh, remediation, zero valent iron