Delineating landfill leachate discharge to an arsenic contaminated waterway

Abstract

Discharge of contaminated ground water may serve as a primary and on-going source of contamination to surface water. A field investigation was conducted at a Superfund site in Massachusetts, USA to define the locus of contaminant flux and support source identification for arsenic contamination in a pond abutting a closed landfill. Subsurface hydrology and ground-water chemistry were evaluated in the aquifer between the landfill and the pond during the period 2005–2009 employing a network of wells to delineate the spatial and temporal variability in subsurface conditions. These observations were compared with concurrent measures of ground-water seepage and surface water chemistry within a shallow cove that had a historical visual record of hydrous ferric oxide precipitation along with elevated arsenic concentrations in shallow sediments. Barium, presumably derived from materials disposed in the landfill, served as an indicator of leachate-impacted ground water discharging into the cove. Evaluation of the spatial distributions of seepage flux and the concentrations of barium, calcium, and ammonium-nitrogen indicated that the identified plume primarily discharged into the central portion of the cove. Comparison of the spatial distribution of chemical signatures at depth within the water column demonstrated that direct discharge of leachate-impacted ground water was the source of highest arsenic concentrations observed within the cove. These observations demonstrate that restoration of the impacted surface water body will necessitate control of leachate-impacted ground water that continues to discharge into the cove.

Introduction

Ground-water discharge can serve as a conduit for the transport of nutrients and contaminants to sediments within surface water systems (e.g., Hayashi and Rosenberry, 2002, McCobb et al., 2003, Hagerthey and Kerfoot, 2005, Reichard and Brown, 2009). The degree to which inorganic contaminants may accumulate in sediments or transport unhindered into the overlying surface water will be governed by the rate and magnitude of contaminant flux, the aqueous geochemistry within the ground-water/surface-water interface, and the adsorptive properties of existing or newly-formed solid phases within the sediment layer. Successful restoration of a contaminated portion of the surface water body depends on reliable assessment of the source(s) of contaminant flux and the processes controlling contaminant fate within the boundaries of the impacted water body. This poses a significant challenge for sites impacted by inorganic contaminants such as arsenic where contaminant mass is conserved and physicochemical speciation may change dramatically within a given site (e.g., Keimowitz et al., 2005b, Ford et al., 2006, Gan et al., 2006, Wilkin and Ford, 2006, Root et al., 2009, Johnston et al., 2011).

A common source of ground-water contamination to shallow aquifers in the northeastern United States are legacy landfills that were installed and operated prior to the inception of permitting requirements for leachate interception, collection and treatment. In many instances, these legacy landfills were constructed in topographic areas with lower elevations than the surrounding land surface to maximize available disposal capacity (Lisk, 1991). This design has the unintended consequence of placing waste materials near or in contact with the former natural drainage system for the local watershed. A potential outcome from these legacy landfill designs was direct contact between buried waste materials and the ground-water table and/or the introduction of a reduced water source (leachate) into the shallow ground-water system downgradient of the disposal unit (e.g., Gobler and Boneillo, 2003, Atekwana and Krishnamurthy, 2004, Jorstad et al., 2004, Canton et al., 2010, Cozzarelli et al., 2011). Recent studies have documented the occurrence of elevated arsenic concentrations in shallow ground-water systems underlying or adjacent to legacy landfills (Keimowitz et al., 2005a, deLemos et al., 2006, Gan et al., 2006, Parisio et al., 2006). For these studies, the primary source for elevated arsenic concentrations was attributed to the mobilization of naturally occurring arsenic from aquifer solids due to reducing conditions established by leachate migrating beyond the landfill boundary. The maximum concentration of arsenic in filtered ground-water samples reported in these studies was ∼300 μg L⁻¹. The range of arsenic concentrations reported for municipal solid waste (MSW) landfill leachates at several locations throughout the world is 10–1000 μg L⁻¹ (Christensen et al., 2001). Comparison of these arsenic concentration data reveals the difficulty of attributing elevated arsenic in ground water directly to a landfill-derived source.

Defining the impact of historical and current landfill leachate releases to shallow ground-water systems that are hydraulically connected to surface water bodies is an important step to assessing appropriate management responses to mitigate current or future impairments to sediments and/or surface water. Linking observed contamination in sediments or surface water to a ground-water plume derived from landfill leachate may pose a challenge for contaminants like arsenic that have potential anthropogenic and natural sources. Although unique chemical arsenic species may exist in disposed waste materials and/or the generated leachate (e.g., Ponthieu et al., 2007, Li et al., 2010, Li et al., 2011), biotic processes active within the landfill or along a ground-water flow path may cause inter-conversion of arsenic between organic and inorganic species that masks the original form migrating from a contaminant source area (e.g., Davis et al., 1994, Blodau et al., 2008, Hempel et al., 2009). In addition, there is only one stable isotope of arsenic (Shore et al., 2010), which prevents use of forensic measures such as the ratio of stable isotopes to distinguish and track leaching from a waste-derived source with a unique isotopic signature. Ultimately, assessing the source(s) and transport of arsenic via ground water to surface water will rely on multiple lines of evidence to define the potential hydrologic pathway and indirect geochemical indicators that either serve as a tracer for buried waste releases and/or establish the impact of leachate on mobilization of naturally-occurring arsenic in the aquifer.

The current study was undertaken to confirm that ground-water discharge is a contemporary source of arsenic contamination observed in the sediments and surface water of a pond located adjacent to a closed landfill at a Superfund site in Massachusetts. The work was conducted to address the uncertainty for the need to address ground-water impacts in addition to contaminated sediment removal in an effort to restore the surface water body. The results of this site investigation supported decisions on best approaches to mitigate historical and on-going contamination within the pond. The objectives of the work were to: (1) define the dimensions of the presumed contaminant plume discharging to a shallow cove within the pond, (2) determine the existence of chemical indicators for waste constituents to constrain identification of the plume origin, and (3) evaluate whether the flux of arsenic and other constituents transported by the plume were consistent with constituent concentrations measured in surface water within the discharge zone.