A. Butcher

Using boreholes as windows into groundwater ecosystems.

Groundwater ecosystems remain poorly understood yet may provide ecosystem services, make a unique contribution to biodiversity and contain useful bio-indicators of water quality. Little is known about ecosystem variability, the distribution of invertebrates within aquifers, or how representative boreholes are of aquifers. We addressed these issues using borehole imaging and single borehole dilution tests to identify three potential aquifer habitats (fractures, fissures or conduits) intercepted by two Chalk boreholes at different depths beneath the surface (34 to 98 m). These habitats were characterised by sampling the invertebrates, microbiology and hydrochemistry using a packer system to isolate them. Samples were taken with progressively increasing pumped volume to assess differences between borehole and aquifer communities. The study provides a new conceptual framework to infer the origin of water, invertebrates and microbes sampled from boreholes. It demonstrates that pumping 5 m3 at 0.4–1.8 l/sec was sufficient to entrain invertebrates from five to tens of metres into the aquifer during these packer tests. Invertebrates and bacteria were more abundant in the boreholes than in the aquifer, with associated water chemistry variations indicating that boreholes act as sites of enhanced biogeochemical cycling. There was some variability in invertebrate abundance and bacterial community structure between habitats, indicating ecological heterogeneity within the aquifer. However, invertebrates were captured in all aquifer samples, and bacterial abundance, major ion chemistry and dissolved oxygen remained similar. Therefore the study demonstrates that in the Chalk, ecosystems comprising bacteria and invertebrates extend from around the water table to 70 m below it. Hydrogeological techniques provide excellent scope for tackling outstanding questions in groundwater ecology, provided an appropriate conceptual hydrogeological understanding is applied.

Investigating rising nitrate concentrations in groundwater in the Permo-Triassic aquifer, Eden Valley, Cumbria, UK

Abstract Groundwater nitrate concentrations in the Permo-Triassic aquifer of the Eden Valley vary from less than 4 mg l−1 to in excess of 100 mg l−1 (as NO3). A significant number of boreholes exhibit rising trends in nitrate concentration that either approach or exceed the CEC Directive 80/778 Maximum Admissible Concentration (MAC) of 50 mg l−1. The main source of the nitrate is believed to be the nitrogen applied to grassland, both as slurry and as inorganic fertilizers. The variability in groundwater nitrate concentrations is thought to be due in part to land use, particularly where low-yielding boreholes derive their water from a limited/localized area, and in part due to the variability in the travel times for water and solutes to migrate from the soil to the water table and then to the borehole. This variability in travel times is a function of surficial geology, depth to water table, depth of borehole and superficial deposit thickness, amongst other factors. It is surprising, given the considerable storage within the saturated zone of the aquifer and the slow groundwater movement, that some relatively deep boreholes pump groundwater with nitrate concentrations in excess of 20 mgl−1. Simple numerical modelling suggests that the fraction of modern water pumped is sensitive to the presence of fissures close to the abstraction boreholes and the location of the boreholes relative to superficial deposits. For some scenarios, using realistic superficial deposit geometries and aquifer hydraulic parameters, the proportion of modern water (water that is derived from infiltration that reached the water table since pumping started) could exceed 40% within 15 years of pumping.

Modification to the flow properties of repository cement as a result of carbonation

Abstract A UK repository concept currently under consideration for the disposal of intermediate-level radioactive waste and some low-level waste not suitable for surface disposal involves using large quantities of cementitious materials for construction, grouting, waste containers, waste isolation matrix and buffer/backfill. CO2 generated from the degradation of organic material in the waste will result in cement carbonation and associated mineralogical changes. Hydraulic and gas permeability tests were performed on Nirex Reference Vault Backfill (NRVB) cement at 40 °C and either 4 or 8 MPa. Carbonation reactions using CO2 gas halved the permeability of the NRVB under simulated repository conditions. A greater decrease in permeability (by three orders of magnitude) was found during carbonation using dissolved CO2. Mineralogical changes were found to occur throughout the cement as a result of the reaction with CO2. However, a narrow zone along the leading edge of a migrating reaction front was associated with the greatest decrease in porosity. Fluid pressures increased slightly due to permeability reductions but fluid flow still continued (albeit at a lower rate) preventing the build-up of overly high pressures. Overall, the observed reductions in permeability could be beneficial in that they may help reduce the potential for fluid flow and radionuclide migration. However, continued carbonation could lead to potential issues with regards to gas pressure build-up.