Performance of a Sequential Reactive Barrier for Bioremediation of Coal Tar Contaminated Groundwater

Following a thorough site investigation, a biological Sequential Reactive Barrier (SEREBAR), designed to remove Polycyclic Aromatic Hydrocarbons (PAHs) and BTEX compounds, was installed at a Former Manufactured Gas Plant (FMGP) site. The novel design of the barrier comprises, in series, an interceptor and six reactive chambers. The first four chambers (2 nonaerated-2 aerated) were filled with sand to encourage microbial colonization. Sorbant Granular Activated Carbon (GAC) was present in the final two chambers in order to remove any recalcitrant compounds. The SEREBAR has been in continuous operation for 2 years at different operational flow rates (ranging from 320 L/d to 4000 L/d, with corresponding residence times in each chamber of 19 days and 1.5 days, respectively). Under low flow rate conditions (320-520 L/d) the majority of contaminant removal (>93%) occurred biotically within the interceptor and the aerated chambers. Under high flow rates (1000-4000 L/d) and following the installation of a new interceptor to prevent passive aeration, the majority of contaminant removal (>80%) again occurred biotically within the aerated chambers. The sorption zone (GAC) proved to be an effective polishing step, removing any remaining contaminants to acceptable concentrations before discharge down-gradient of the SEREBAR (overall removals >95%).

Haematite in Lateritic Soils Aids Groundwater Disinfection

Microbiologically contaminated water severely impacts public health in low-income countries, where treated water supplies are often inaccessible to much of the population. Groundwater represents a water source that commonly has better microbiological quality than surface water. A 2-month intensive flow and quality monitoring programme of a spring in a densely settled, unsewered parish of Kampala, Uganda, revealed the persistent presence of high chloride and nitrate concentrations that reflect intense loading of sewage in the spring’s catchment. Conversely, thermotolerant coliform bacteria counts in spring water samples remained very low outside of periods of intense rainfall. Laboratory investigations of mechanisms responsible for this behavior, achieved by injecting a pulse of H40/1 bacteriophage tracer into a column packed with locally derived granular laterite, resulted in near-total tracer adsorption. X-ray diffraction (XRD) analysis showed the laterite to consist predominantly of quartz and kaolinite, with minor amounts (<5%) of haematite. Batch studies comparing laterite adsorption capacity with a soil having comparable mineralogy, but with amorphous iron oxide rather than haematite, showed the laterite to have a significantly greater capacity to adsorb bacteriophage. Batch study results using pure haematite confirmed that its occurrence in laterite contributes substantially to micro-organism attenuation observed and serves to protect underlying groundwater.

Clara Bog: An ombotrophic groundwater dependant terrestrial ecosystem (Submitted)

Comparison of flow duration curves for a weir draining an undrained raised peat with those generated 20 years previously reveal that more recent curves reflect to be flatter with a lower Q95/Q5 ratio. Comparison of the bog topography for the same period revealed that although marginal drainage/peat reclamation had resulted in desiccation of peat around the bog margin and more frequent intense runoff, the central part of the bog had subsided to form an enclosed basin ,resulting in the creation of newly formed lakes that gave the central part of the bog an improved capacity to store, and more slowly discharge, water. Interrogation of groundwater monitoring data revealed a net decline in groundwater levels of up to three metres in the glacial tills underlying the bog associated with deepening and expansion of a marginal drain network which penetrated the base of the peat. Comparing organic carbon levels in peat the central part of the bog over a ten year period revealed an overall increase, with changes being most marked in deeper fen peat layers. These findings suggest that the decline in groundwater levels in the peat substrate resulted in an increase in effective stress in the peat causing greater subsidence in the central part of the bog due to greater overall thickness. Study results highlight how the hydrology of apparently isolated obotrophic raised bog ecosystems may be influenced by groundwater pressures in deeper deposits, and how marginal drains may have the capacity to impact areas at significant distances.

Analysis of groundwater-level response to rainfall and recharge estimates in fractured hard rock aquifers, NW Ireland

Despite fractured hard rock aquifers underlying over 65% of Ireland, knowledge of key processes controlling groundwater recharge in these bedrock systems is inadequately constrained. In this study, we examined 19 groundwater-level hydrographs from two Irish hillslope sites underlain by hard rock aquifers. Water-level time-series in clustered monitoring wells completed at the subsoil, soil/bedrock interface, shallow and deep bedrocks were continuously monitored hourly over two hydrological years. Correlation methods were applied to investigate groundwater-level response to rainfall, as well as its seasonal variations. The results reveal that the direct groundwater recharge to the shallow and deep bedrocks on hillslope is very limited. Water-level variations within these geological units are likely dominated by slow flow rock matrix storage. The rapid responses to rainfall (⩽2 h) with little seasonal variations were observed to the monitoring wells installed at the subsoil and soil/bedrock interface, as well as those in the shallow or deep bedrocks at the base of the hillslope. This suggests that the direct recharge takes place within these units. An automated time-series procedure using the water-table fluctuation method was developed to estimate groundwater recharge from the water-level and rainfall data. Results show the annual recharge rates of 42–197 mm/yr in the subsoil and soil/bedrock interface, which represent 4–19% of the annual rainfall. Statistical analysis of the relationship between the rainfall intensity and water-table rise reveal that the low rainfall intensity group (⩽1 mm/h) has greater impact on the groundwater recharge rate than other groups (>1 mm/h). This study shows that the combination of the time-series analysis and the water-table fluctuation method could be an useful approach to investigate groundwater recharge in fractured hard rock aquifers in Ireland.