Formatted Title:

High-Resolution Porewater Profiling with Equilibrium Passive Samplers to Evaluate Treatment Options in Mercury-Contaminated Sediments

Background/Objectives:

In aquatic sediments, mercury (Hg) is microbially transformed into the more toxic and bioaccumulative methylmercury (MeHg). Sediments are often a source of MeHg to the aquatic food web and, thus, a target in remedial practice. In remediation, the goal is to limit both Hg and MeHg transport into the water column and accumulation by benthic organisms. Commonly, a thick layer of sand or non-reactive material is applied to provide a physical barrier between the Hg-laden sediment and aquatic organisms. Treating sediments with activated carbon (AC) is being explored as it increases MeHg partitioning towards the sediment phase, thereby limiting aqueous transport and decreasing bioavailability. When evaluating these treatments, porewater spatial profiling is critical to assess porewater reductions and cap breakthrough. However, traditional porewater sampling methods are challenging due to the redox-sensitive nature of MeHg and difficulty extracting porewater in the fine-scale vertical sections often needed to estimate efflux. Our equilibrium passive sampler (EqPS), a 1-mm thick agarose gel polymer embedded with AC, offers an alternative sampling means. When the EqPS is placed in the sediment, MeHg in the porewater diffuses through the agarose gel medium and adsorbs to the embedded AC, eventually reaching equilibrium, after which MeHg in the EqPS can be measured. The AC in the EqPS stabilizes the sampled MeHg so that it is resistant to transformation upon retrieval. MeHg accumulation in the EqPS allows analysis with less sensitive detection limits than direct porewater analysis. The polymeric design allows for porewater profiling at a fine (roughly 0.5 cm) scale. With these profiles, we located the depth where porewater MeHg reached its peak and chemically related it to where the microbial activity responsible to MeHg production took place, through corresponding sulfide/sulfate profiles. In this remediation study, porewater profiles facilitated by this EqPS were used to compare the efficacy of several treatments consisting sand and/or AC to reduce porewater MeHg and transport into surface water.

Approach/Activities:

Microcosms were constructed using sediments containing a mixture of low-salinity sediments collected from the Berry’s Creek Study Area and freshwater sediments collected from a coastal stream at the Smithsonian Environmental Research Center (SERC). Sediment, resulting in a depth of 5 cm, were topped with natural, pristine stream water from SERC. The overlying water was constantly aerated and periodically rejuvenated to mimic natural conditions. Four sediment treatment methods were compared in this study employing some combination of a 2-cm sand cap and/or coconut shell PAC to make up 5 wt% of the total sediment: sand cap, sand cap atop an AC cap, AC cap alone, and AC fully mixed into the sediment. The EqPS was vertically inserted to traverse the length of the sediment, allowing at least 1 cm exposed to the surface water. After allowing treated sediments to stabilize for 8 weeks, EqPS were deployed in sediments for 4 weeks. EqPS were then collected and sectioned (0.5-cm) alongside sediment cores (1-cm sections) and surface water grab samples.

Results/Lessons Learned:

All four treatments resulted in reduced surface water MeHg. The highest surface water reductions were observed where AC was added as a cap either alone or beneath the 2-cm sand cap. The EqPS-derived porewater profile shows that the 2-cm sand cap significantly reduced porewater MeHg in the top 1 to 2 cm, although the AC treatments reduced porewater MeHg across the entire sediment column. Sediment profiles show lower MeHg in the top 1 to 2 cm in microcosms treated with a sand cap. However, deeper sediment MeHg levels were similar to the unamended control in sediments treated with an AC cap and/or sand cap. Fully mixing AC into the sediment resulted in higher sediment MeHg compared to the unamended control. This study gives implications about how a treatment will impact Hg fate and transport within sediments and highlights how porewater profiling, facilitated by the EqPS, is crucial in evaluating treatment efficacy.

Cynthia Gilmour

Affiliation

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(Group 2, Poster Board #34) High-Resolution Porewater Profiling with Equilibrium Passive Samplers to Evaluate Treatment Options in Mercury-Contaminated Sediments