Gareth Farr

Mapping shallow urban groundwater temperatures, a case study from Cardiff, UK

Low-enthalpy ground source heating systems can help to reduce our dependence on fossil fuels, in turn reducing greenhouse gas emissions and increasing energy security. To de-risk and support the sustainable development, regulation and management of ground source heating systems in urban areas, detailed baseline mapping of groundwater temperatures is required. Groundwater temperatures were measured in 168 monitoring boreholes primarily within a Quaternary sand and gravel aquifer in the city of Cardiff, UK. The data have been used to create the first city-wide map of shallow groundwater temperatures in the UK. This map can be used both to support development of ground source heating and to act as a detailed baseline from which to measure change. Shallow groundwater temperatures under the city were found to be 2°C warmer than the UK average groundwater temperature and this additional heat is attributed to the urban heat island. The zone of seasonal fluctuation varies from 7.1 and 15.5 m below ground level (mbgl) within the shallow Quaternary aquifer, averaging 9.5 mbgl. Deeper groundwater temperature profiles incorporating both the Quaternary and bedrock aquifers suggest that a ‘zone of anthropogenic influence’ exists down to about 70 mbgl.

Airborne geophysics: a novel approach to assist hydrogeological investigations at groundwater-dependent wetlands

This paper provides an assessment of existing airborne baseline geophysical data in relation to the hydrogeological characterization of protected groundwater-dependent terrestrial ecosystems (wetlands) in Anglesey, Wales. The attenuation of the radiometric data identifies the main areas of water saturation in the very near surface. The radiometric data have the potential to identify additional areas where similar degrees of saturation exist. The data may therefore be used to help define extensions to existing site boundaries and to provide information in the assessment of new wetland areas. The radiometric data also show regional-scale transition from the Carboniferous Limestone in the east, important for water supply to the wetlands, to older more impermeable strata to the west. The conductivity data appear capable of mapping the lateral extent of clay accumulations, concealed below peat cover which can act as a confining layer to the bedrock aquifer. At the regional scale the data reveal the extent of a former Holocene lake system and the hydrogeological control of a Carboniferous limestone–sandstone contact. The data therefore provide a non-invasive spatially continuous, characterization of the sites, improving the understanding of their dynamic water balances and, potentially, guiding further ground assessments and invasive investigations.

Airborne Geophysical Data Used to Characterise Groundwater Dependant Wetlands

This paper provides an assessment of existing airborne baseline geophysical data in relation to the hydrogeological characterisation of protected groundwater dependant wetland areas found on Anglesey, Wales. Radiometric data appear to identify the main areas of water saturation together with the degree of saturation in the very near surface. The radiometric data also show regional scale transition from limestone in the east, important for water supply to the wetlands, to older more impermeable strata to the west. The conductivity data appear capable of mapping the lateral extent of clay accumulations, concealed below peat cover and acting as a confining layer. The data therefore provide a non-invasive spatially-continuous, characterisation of the sites, improving the understanding of their dynamic water balances and, potentially, guiding further invasive investigations.

The Influence of Flow and Bed Slope on Gas Transfer in Steep Streams and Their Implications for Evasion of CO2

The evasion of greenhouse gases (including CO2, CH4 and N2O) from streams and rivers to the atmosphere is an important process in global biogeochemical cycles, but our understanding of gas transfer in steep (> 10%) streams, and under varying flows is limited. We investigated gas transfer using combined tracer injections of SF6 and salt. We used a novel experimental design in which we compared four very steep (18.4-29.4%) and four moderately steep (3.7-7.6%) streams, and conducted tests in each stream under low flow conditions and during a high discharge event. Most dissolved gas evaded over short distances (~100 and ~200-400 m respectively), so accurate estimates of evasion fluxes will require sampling of dissolved gases at these scales to account for local sources. We calculated CO2 gas transfer coefficients (KCO2) and found statistically significant differences between larger KCO2 values for steeper (mean 0.465 min-1) streams compared to those with shallower slopes (mean 0.109 min-1). Variations in flow had an even greater influence. KCO2 was substantially larger under high (mean 0.497 min-1) compared to low flow conditions (mean 0.077 min-1). We developed a statistical model to predict KCO2 using values of streambed slope x discharge which accounted for 94 % of the variation. We show that two models using slope and velocity developed by Raymond et al. [2012] for streams and rivers with shallower slopes, also provide reasonable estimates of our CO2 gas transfer velocities (kCO2; m d-1). We developed a robust field protocol which could be applied in future studies.

Ecohydrological studies of dune slack vegetation at Kenfig dunes, South Wales, UK

Dune slacks are important coastal sand dune habitats and seasonal changes in water levels within dune aquifers control both their formation and the specific hydrological conditions which then govern the floristic composition of their characteristic plant communities. Kenfig Dunes National Nature Reserve is one of the largest dune sites in South Wales and Southern Britain. It supports an exceptional range of dune slack communities, including most of those recognised in the British National Vegetation Classification scheme. Detailed studies of the vegetation ecology and hydrology of dune slacks reveal the important influence of hydrological variables in controlling the composition of dune slack vegetation and also valuable information on water table profile and the key factors governing the annual hydrological budget of the dune system aquifer.

The unusual inter-tidal niche of the rare moss Bryum marratii Wilson

This is the first detailed ecological investigation of the rare moss Bryum marratii Wilson. Results show occupancy of a niche at the two study sites that is unusual amongst bryophytes, within the upper edge of saltmarsh. All colonies (n = 120), which comprise the largest populations in Wales, occur in an elevation zone of just 57 cm and are subject to frequent inundation by higher spring tides. In total, the average location of a B. marratii colony was submerged by seawater 3.6 day-1 yr-1. Sea level rise, projected in the study region to be 30–46 cm during the current century, is a major threat to the long-term survival of these populations.

Hydrological assessment and monitoring of wetlands

The physical and chemical characteristics which favour wetland plant communities, primarily high soil water levels and anaerobic soil chemistry, are related directly to the hydrology/hydrogeology of the wetland and often its surrounding catchment. Appreciation and successful management of a wetland therefore almost always requires an understanding of its hydrological functioning, including the influences on hydrological functioning which often lie beyond the designated boundary of the site. This section introduces ecohydrological conceptual models as a repository for knowledge about the combined ecological and hydrological functioning of a wetland and then provides a starting point (or initial framework) for the development of such a model. Also introduced are hydro-environmental supporting conditions (HSCs) that allow us to describe specific hydrological conditions required to support wetland plant communities. A suite of techniques for ecohydrological investigation and characterization of wetlands are described, the results from which can be used to develop and refine the ecohydrological conceptual model.