Article pubs.acs.org/est
Contaminants at the Sediment−Water Interface: Implications for Environmental Impact Assessment and Effects Monitoring T. G. Milligan* and B. A. Law Coastal Ecosystem Science Division, Fisheries and Oceans Canada, Bedford Institute of Oceanography, PO Box 1006 Dartmouth, Nova Scotia, B2Y 4A2 S Supporting Information *
ABSTRACT: Many contaminants in aquatic environments are associated with loosely packed aggregates of particulate material called flocs. Flocculation allows contaminants to accumulate at the sediment−water interface and it packages them in a form that is readily available for ingestion by filter feeding organisms. Unfortunately, most samplers being used for environmental assessment and monitoring suspend this material on impact and fail to sample this critical component of the seabed. In this study we use a slo-corer to collect seabed samples with an undisturbed surface layer and a Gust microcosm erosion chamber to erode the surface of the cores at increasing shear stresses. Results from two different sites, one impacted by tailings from historic gold mining and the other by open-pen salmon aquaculture, showed the levels of metals suspended at stresses below 0.24 Pa were greater than in the underlying sediment. Sampling this highly mobile surface layer is critical for determining the total contaminant load in bottom sediments and, more importantly, this layer represents the most readily available material for suspension. The loss of this layer during sampling could lead to inaccurate measurements of contaminant levels during environmental assessment and effects monitoring. A re-evaluation of the ISO standard for bottom sediment sampling is recommended.
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even though the floc appeared intact as the grab closed. Blomquist14 examined the effect of a number of different bottom sediment samplers, including box cores. In addition to the loss of surficial sediments during sampling with gravity corers, he also demonstrated problems with subsampling from box cores. Box cores are often seen as the most effective method for retention of the sediment−water interface but issues with subsampling and drainage can still result in loss of this mobile layer. The degree of loss is a function of the rate of descent and the configuration of the sampler.14 Sediment corers that preserve the sediment−water interface have been developed and successfully used.15−20 By slowing the rate of a descent of an open core barrel and sealing it after penetration, it is possible to sample the mobile layer at the sediment−water interface. Results from Boston Harbor show that metals values in samples collected with a slow corer were significantly higher than those collected by grab.17,18,21 Similar results have been found using other corers (e.g., Multicorer) that preserve the sediment−water interface. In this study we examine in detail the mobility and contaminant load of the material at the sediment−water interface that can be lost during sampling with conventional grabs and corers. Cores with
INTRODUCTION Flocculation is the term used for the process in aquatic environments that brings together colloids, organic material, and small inorganic particles to form loosely packed aggregates of particulate material called flocs. Through flocculation, the settling velocity of fine grained sediment and organic material is increased by several orders of magnitude.1−3 Many contaminants associated with health risks are transported in aquatic environments as part of flocs.4−7 Flocs are also potential carriers of pathogens, both human7,8 and molluscan.9 Because flocs can contain large amounts of organic material they are a preferred source of food for suspension feeding organisms such as scallops, clams, mussels, and oysters.10−12 Flocculation serves two critical roles for increasing risk of environmental impact: (1) it allows contaminants to accumulate at the sediment− water interface and (2) packages them in a form that is readily available for ingestion by filter feeding organisms. Failure to characterize flocs adequately can lead to underestimation of contaminant and pathogen concentrations in the sediment. In addition, an important pathway for uptake of contaminants by organisms is missed. Flocs are notoriously difficult to sample as they are easily dispersed during sampling by standard equipment such as grabs and corers. Muschenheim and Milligan 13 made video observations of a thick layer of flocculated material resting on a sand bottom near an active offshore drilling rig, but attempts to sample it using a hydraulically activated video grab failed Published XXXX by the American Chemical Society
Received: November 28, 2012 Revised: May 2, 2013 Accepted: May 6, 2013
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dx.doi.org/10.1021/es3031352 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
drive a 10 cm diameter open polycarbonate core tube into the sediment. Once the corer is on the bottom, hydraulic damping is used to control the rate at which the core tube descends, ensuring that material at the sediment−water interface is not disturbed. The open structure of the corer and a poppet valve that remains open until the core barrel is imbedded in the sediment reduces the formation of a bow wave on descent. The need for a core cutter and catcher is eliminated by using a plate that seals the bottom of the core as it is withdrawn from the sediment, further reducing the disturbance of the sediment− water interface. The total weight of the mini slo-corer is 70 kg allowing it to be deployed from small vessels in shallow water. Gust Erosion Chamber. Duplicate cores were collected using a mini slo-corer for each study. The top 1 cm of the surface sediment of one of the cores was sampled for disaggregated inorganic grain size (DIGS) and metals analysis and the other core was used for erosion measurements. To determine the erosion rate and size specific mobility of the sediment surface, a Gust erosion chamber was fitted to the top of the second core22,23,28 (Figure S4, SI). The Gust chamber comprises a housing with a rotating stirring disk, a removable lid, and water input and output connections. It fits directly on top of a slo-core tube. By controlling both the rotation rate of the stirring disk and the rate at which background water collected at the corresponding station is pumped through the device, a uniform shear stress can be applied across the sediment surface. Following the method described by Stevens et al.29 and modified by Law et al.,28 the core surface was subjected to increasing shear stress at 0.01, 0.08, 0.16, 0.24, 0.32, and 0.40 Pa, with the first step of 0.01 Pa normally being used to flush the tubing and Gust chamber with background water collected above the core location. Improvements to the Gust erosion chamber stirrer motor and new calibration equations supplied by the manufacturer increased the maximum achievable stress for the Charlie Cove and Maces Bay samples. Stress steps for these locations were 0.01, 0.08, 0.16, 0.24, 0.32, 0.40, and 0.60 Pa. For each increasing shear stress the background water plus any material suspended from the surface of the core was pumped through an attached turbidity meter until recorded levels returned to background concentrations. The water pumped from the chamber was collected in acid washed flasks to determine the total mass, disaggregated inorganic grain size (DIGS), and metal concentration of the material suspended from the core surface. The concentration of suspended particulate matter (SPM) in the water collected for each stress step was determined using standard gravimetric analysis. Aliquots of the suspension were filtered onto preweighed 8 μm SCWP Millipore filters, dried at
