Future challenges of coastal landfills exacerbated by sea level rise.

In England and Wales, there are at least 1700 coastal landfills in the coastal flood plain and at least 60 threatened by erosion, illustrating a global problem. These landfills are a major issue in shoreline management planning (SMP) which aims to manage the risks associated with flooding and coastal erosion. Where landfills exist, "hold the line" (requiring the building or upgrading of artificial defences to maintain the current shoreline) is often selected as the preferred SMP option, although government funding is not available at present. To investigate these issues in detail, three case-study landfills are used to examine the risks of future flooding and erosion together with potential mitigation options. These cases represent a contrasting range of coastal landfill settings. The study includes consideration of sea-level rise and climate change which exacerbates risks of erosion and flooding of landfills. It is fundamental to recognise that the release of solid waste in coastal zones is a problem with a geological timescale and these problems will not go away if ignored. Future erosion and release of solid waste is found to be more of a threat than flooding and leachate release from landfills. However, while leachate release can be assessed, there is presently a lack of methods to assess the risks from the release of solid waste. Hence, a lack of science constrains the design of remediation options.

Doublet tracer tests to determine the contaminant flushing properties of a municipal solid waste landfill.

This paper describes a programme of research investigating horizontal fluid flow and solute transport through saturated municipal solid waste (MSW) landfill. The purpose is to inform engineering strategies for future contaminant flushing. Solute transport between injection/abstraction well pairs (doublets) is investigated using three tracers over five separate tests at well separations between 5m and 20m. Two inorganic tracers (lithium and bromide) were used, plus the fluorescent dye tracer, rhodamine-WT. There was no evidence for persistent preferential horizons or pathways at the inter-well scale. The time for tracer movement to the abstraction wells varied with well spacing as predicted for a homogeneous isotropic continuum. The time for tracer movement to remote observation wells was also as expected. Mobile porosity was estimated as ~0.02 (~4% of total porosity). Good fits to the tracer breakthrough data were achieved using a dual-porosity model, with immobile regions characterised by block diffusion timescales in the range of about one to ten years. This implies that diffusional exchanges are likely to be very significant for engineering of whole-site contaminant flushing and possibly rate-limiting.

Sediment-associated organic matter sources and sediment oxygen demand in a Special Area of Conservation (SAC): A case study of the River Axe, UK.

Oxygen demand in river substrates providing important habitats for the early life stages of aquatic ecology, including lithophilous fish, can arise due to the oxidation of sediment-associated organic matter. Oxygen depletion associated with this component of river biogeochemical cycling, will, in part, depend on the sources of such material. A reconnaissance survey was therefore undertaken to assess the relative contributions from bed sediment-associated organic matter sources potentially impacting on the River Axe Special Area of Conservation (SAC), in SW England. Source fingerprinting, including Monte Carlo uncertainty analysis, suggested that the relative frequency-weighted average median source contributions ranged between 19% (uncertainty range 0-82%) and 64% (uncertainty range 0-99%) for farmyard manures or slurries, 4% (uncertainty range 0-49%) and 35% (uncertainty range 0-100%) for damaged road verges, 2% (uncertainty range 0-100%) and 68% (uncertainty range 0-100%) for decaying instream vegetation, and 2% (full uncertainty range 0-15%) and 6% (uncertainty range 0-48%) for human septic waste. A reconnaissance survey of sediment oxygen demand (SOD) along the channel designated as a SAC yielded a mean SOD5 of 4 mg O2 g-1 dry sediment and a corresponding SOD20 of 7 mg O2 g-1 dry sediment, compared with respective ranges of 1-15 and 2-30 mg O2 g-1 dry sediment, measured by the authors for a range of river types across the UK. The findings of the reconnaissance survey were used in an agency (SW region) catchment appraisal exercise for informing targeted management to help protect the SAC.

Multiple-tracer tests for contaminant transport process identification in saturated municipal solid waste.

Two column tests were performed in conditions emulating vertical flow beneath the leachate table in a biologically active landfill to determine dominant transport mechanisms occurring in landfills. An improved understanding of contaminant transport process in wastes is required for developing better predictions about potential length of the long term aftercare of landfills, currently measured in timescales of centuries. Three tracers (lithium, bromide and deuterium) were used. Lithium did not behave conservatively. Given that lithium has been used extensively for tracing in landfill wastes, the tracer itself and the findings of previous tests which assume that it has behaved conservatively may need revisiting. The smaller column test could not be fitted with continuum models, probably because the volume of waste was below a representative elemental volume. Modelling compared advection-dispersion (AD), dual porosity (DP) and hybrid AD-DP models. Of these models, the DP model was found to be the most suitable. Although there is good evidence to suggest that diffusion is an important transport mechanism, the breakthrough curves of the different tracers did not differ from each other as would be predicted based on the free-water diffusion coefficients. This suggested that solute diffusion in wastes requires further study.

Investigating the effect of compression on solute transport through degrading municipal solid waste.

The effect of applied compression on the nature of liquid flow and hence the movement of contaminants within municipal solid waste was examined by means of thirteen tracer tests conducted on five separate waste samples. The conservative nature of bromide, lithium and deuterium tracers was evaluated and linked to the presence of degradation in the sample. Lithium and deuterium tracers were non-conservative in the presence of degradation, whereas the bromide remained effectively conservative under all conditions. Solute diffusion times into and out of less mobile blocks of waste were compared for each test under the assumption of dominantly dual-porosity flow. Despite the fact that hydraulic conductivity changed strongly with applied stress, the block diffusion times were found to be much less sensitive to compression. A simple conceptual model, whereby flow is dominated by sub-parallel low permeability obstructions which define predominantly horizontally aligned less mobile zones, is able to explain this result. Compression tends to narrow the gap between the obstructions, but not significantly alter the horizontal length scale. Irrespective of knowledge of the true flow pattern, these results show that simple models of solute flushing from landfill which do not include depth dependent changes in solute transport parameters are justified.

Quantifying the effect of settlement and gas on solute flow and transport through treated municipal solid waste.

The effect of degradation and settlement on transport properties of mechanically and biologically treated (MBT) waste was examined by applying three different tracers to two waste columns (~0.5 m diameter) in a series of closed-loop experiments. One column was allowed to biodegrade and the other was bio-suppressed. Permeability and drainable porosity were reduced by settlement, in line with previous results. A dual-porosity model performed well against the data and suggested that more preferential flow occurred early on in the un-degraded column. Diffusion timescales were found to be between 0.8 and 6 days. Volumetric water contents of the mobile region were found to be small in the bio-suppressed cell (~0.01) and even smaller values were found in the degrading waste, possibly due to displacement by gas. Once either settlement or gas production had disrupted this pattern into a more even flow, subsequent compression made little difference to the diffusion time-scale. This may indicate that transport was thereafter dominated by other aspects of the waste structure such as the distribution of low-permeability objects. The presence of gas in the degrading waste reduced the volumetric water content through displacement. The model indicated that the gas was primarily located in the more mobile porosity fraction. Primary compression of the degrading waste tended to squeeze this gas out of the waste in preference to water.

Catchment source contributions to the sediment-bound organic matter degrading salmonid spawning gravels in a lowland river, southern England.

The ingress of particulate material into freshwater spawning substrates is thought to be contributing to the declining success of salmonids reported over recent years for many rivers. Accordingly, the need for reliable information on the key sources of the sediment problem has progressed up the management agenda. Whilst previous work has focussed on apportioning the sources of minerogenic fine sediment degrading spawning habitats, there remains a need to develop procedures for generating corresponding information for the potentially harmful sediment-bound organic matter that represents an overlooked component of interstitial sediment. A source tracing procedure based on composite signatures combining bulk stable (13)C and (15)N isotope values with organic molecular structures detected using near infrared (NIR) reflectance spectroscopy was therefore used to assess the primary sources of sediment-bound organic matter sampled from artificial spawning redds. Composite signatures were selected using a combination of the Kruskal-Wallis H-test, principal component analysis and GA-driven discriminant function analysis. Interstitial sediment samples were collected using time-integrating basket traps which were inserted at the start of the salmonid spawning season and extracted in conjunction with critical phases of fish development (eyeing, hatch, emergence, late spawning). Over the duration of these four basket extractions, the overall relative frequency-weighted average median (±95% confidence limits) source contributions to the interstitial sediment-bound organic matter were estimated to be in the order: instream decaying vegetation (39±<1%; full range 0-77%); damaged road verges (28±<1%; full range 0-77%); septic tanks (22±<1%; full range 0-50%), and; farm yard manures/slurries (11±<1%; full range 0-61%). The reported procedure provides a promising basis for understanding the key sources of interstitial sediment-bound organic matter and can be applied alongside apportionment for the minerogenic component of fine-grained sediment ingressing the benthos. The findings suggest that human septic waste contributes to the interstitial fines ingressing salmonid spawning habitat in the study area.

Making requests: illustrations of how right-hemisphere brain damage can affect discourse production.

This article examines several factors that influence the production of requests for behavior. Using a role-play methodology, we elicited request productions from well-recovered patients with right-hemisphere brain damage (RHD) and from non brain-damaged control participants. The stimulus items represented variation both on interpersonal factors based on characteristics of the people in the interaction and on situational factors based on what was being requested. A large corpus of responses was elicited from each patient. Responses were coded for request directness, amount of explanatory material over and above the request proper (a relatively demanding method for manipulating the tone of a request), and use of "please" (a relatively simple device for signaling a request). Case-by-case analysis of the patients' performances revealed some common areas of abnormality and also some idiosyncratic features. Some patients produced less explanatory supportive material than control participants, and they tended not to vary the amount of explanatory material as a function of the request scenario. Of interest is that some of the same patients overused "please," and varied their use of this simple device as a function of request scenarios. The discourse strategies observed were likely due to deficits both in pragmatic awareness and in planning utterances. One implication of these results concerns an apt description of the abnormal discourse of RHD patients. The relative lack of supportive explanatory material in their requests may result in patients' seeming rude or inappropriate.