The threat of groundwater pollution for petrifying springs; defining nutrient threshold values for an endangered bryophyte dominated habitat.

Eutrophication by human activities is increasingly affecting ecosystem functioning and plant community composition. So far, studies mainly focus on the effects of atmospheric nitrogen deposition, surface water eutrophication or soil nutrient accumulation. Groundwater pollution of spring habitats, however, has received much less attention, although numerous papers report groundwater nutrient enrichment worldwide. This study presents a survey on groundwater pollution (with emphasis on nitrate and phosphate) and bryophyte composition in 51 ambient petrifying springs in 5 NW European countries, which were compared to published data from 173 other sites in 11 European countries. The reviewed dataset covers a broad range of unpolluted to heavily polluted springs with nitrate concentrations between 0.7 and 3227 µmol l-1. Most petrifying springs in the rural lowlands of NW Europe were found to have elevated concentrations of nitrate and phosphate with the most polluted springs occurring in The Netherlands. The cover of individual characteristic bryophyte species significantly correlates with groundwater nutrient concentrations indicating that nutrient pollution of spring waters affects bryophyte composition. Palustriella commutata, Eucladium verticillatum and Brachythecium rivulare prefer unpolluted petrifying springs whereas Cratoneuron filicinum and Pellia endiviifolia show a much broader tolerance to groundwater pollution. In order to sustain at least the basic conditions for the typical bryophyte composition of petrifying springs habitats, threshold values of 288 µmol (18 mg l-1) NO3- l-1 and 0.42 µmol (0.04 mg l-1) ortho-PO43- l-1 were defined. Data analysis of the spring water composition indicates that the main source for nutrient and nutrient induced base cation enrichment are nitrate losses from intensively used agricultural fields. The anthropogenically induced but regionally different chemical processes in subsoil and aquifers can result in different levels of nutrient pollution in springs. Further regulations for nitrate and phosphate application are required to conserve and restore groundwater fed ecosystems in Europe.

Groundwater Microbiology of an Urban Open-Loop Ground Source Heat Pump with High Methane.

Ground source heat pumps (GSHPs) are low-carbon alternatives to gas boilers for decarbonizing heating. Open-loop GSHP systems abstract groundwater, pass it through a heat exchanger, and return it to ground or surface water. Groundwater samples from the top and base of an abstraction and a recharge borehole of an open-loop GSHP system in Cardiff, UK were assessed, and compared to two local boreholes in the same aquifer. Groundwater samples were taken when the GSHP system was active (once) and inactive (twice) and analyzed for changes in geochemistry, viable cell counts, and microbial community (16S rRNA gene sequencing). The GSHP had a distinct geochemistry and microbial community compared to the control boreholes, and the abstraction borehole showed greater variability than the recharge borehole. The microbial community of the GSHP system showed an increase in relative abundance of genera involved in oxidation of methane and methylated compounds, of which Methylotenera was the most abundant (up to 83.9% of 16S rRNA gene sequences). There were also changes in genera associated with nitrification (Nitrospira, Nitrosomonas) and those with potential for sulfur and iron cycling (Rhodoferax). Methane concentration was analyzed after identification of methylotrophs and found that methane concentrations were up to 2855 µg L-1 , thus likely having had a significant impact on the bacterial communities present. Understanding the microbiology and biogeochemistry of GSHP systems provides insight into potential issues with local infrastructure and long-term system performance, and supports modeling to maximize efficient and sustainable use of the subsurface.

Groundwater heat pump feasibility in shallow urban aquifers: Experience from Cardiff, UK.

Ground source heat pumps have the potential to decarbonise heating and cooling in many urban areas. The impact of using shallow groundwater from unconsolidated sedimentary aquifers for heating in urban areas is often modelled, but rarely validated from field measurements. This study presents findings from the 'Cardiff Urban Geo-Observatory' project. This study focuses on an experimental open loop ground source heat pump scheme retrofitted to a school building. Field monitoring for three years between 2015 and 2018 provided data on the environmental impact of the scheme on aquifer conditions. Average aquifer thermal degradation in the first three years was kept below 2 °C, with a maximum change of 4 °C measured during the heating season. The numerically modelled predictions of thermal degradation around the production and injection wells are compared with long-term field monitoring data, providing new insights into both aquifer, and user, behaviour. The Seasonal Performance Factor (SPFH4) of the pilot installation was 4.5 (W13/W50) in the monitoring period. An initial thermal resource estimation of the wider aquifer volume suggests that lowering the temperature of the aquifer by 8 °C could generate equivalent to 26% of the city's 2020 heating demand, but achievable heat extraction would in reality, be less. The study concludes that large parts of the aquifer can sustain shallow open loop ground source heat pump systems, as long as the local ground conditions support the required groundwater abstraction and re-injection rates. Future schemes can be de-risked and better managed by introduction of a registration of all GSHP schemes, with open sharing of investigation, design and performance monitoring data, and by managing thermal interference between systems using spatial planning tools.