Mark O. Cuthbert

Towards best practice for assessing the impacts of climate change on groundwater

Groundwater is vital to human well-being, providing 2 billion people with drinking water (Morris et al. 2003), supporting

A field and modeling study of fractured rock permeability reduction using microbially induced calcite precipitation.

210–

Understanding process dynamics at aquifer‐surface water interfaces: An introduction to the special section on new modeling approaches and novel experimental technologies

230 billion of annual global output of irrigated agricultural produce (Shah et al. 2007), and controlling the flows of water through the world’s biomes (Alley et al. 2002). Given this importance, it is all the more disappointing that the Fourth Report of the Intergovernmental Panel on Climate Change (IPCC) still reports that there “has been very little research on the impact of climate change on groundwater” and that “the few studies of climate impacts on groundwater for various aquifers show very site-specific results” (Kundzewicz et al. 2007). To contribute to addressing these perceived shortcomings and to maximize future study value, methodological recommendations are provided here for hydrogeologists to consider in groundwater-related climate change impact and adaptation studies.

The use of electrical resistivity tomography in deriving local-scale models of recharge through superficial deposits

Microbially induced calcite precipitation (MICP) offers an attractive alternative to traditional grouting technologies for creating barriers to groundwater flow and containing subsurface contamination, but has only thus far been successfully demonstrated at the laboratory scale and predominantly in porous media. We present results of the first field experiments applying MICP to reduce fractured rock permeability in the subsurface. Initially, the ureolytic bacterium, Sporosarcina pasteurii, was fixed in the fractured rock. Subsequent injection of cementing fluid comprising calcium chloride and urea resulted in precipitation of large quantities (approximately 750 g) of calcite; significant reduction in the transmissivity of a single fracture over an area of several m(2) was achieved in around 17 h of treatment. A novel numerical model is also presented which simulates the field data well by coupling flow and bacterial and solute reactive transport processes including feedback due to aperture reduction via calcite precipitation. The results show that MICP can be successfully manipulated under field conditions to reduce the permeability of fractured rock and suggest that an MICP-based technique, informed by numerical models, may form the basis of viable solutions to aid pollution mitigation.

Assessing the accuracy of 1-D analytical heat tracing for estimating near-surface sediment thermal diffusivity and water flux under transient conditions

[1] This paper introduces the special section on “new modeling approaches and novel experimental technologies for improved understanding of process dynamics at aquifer-surface water interfaces.” It is contextualizing the framework for the 27 research papers of the special section by firth identifying research gaps and imminent challenges for ecohydrological research at aquifer-surface water interfaces and then discussing the specific paper contributions on (i) new developments in temperature/heat tracing at GW-SW interfaces, (ii) new methods to capture the temporal and spatial variability of groundwater—surface water exchange, (iii) new approaches in modeling aquifer-river exchange flow, and (iv) new concepts and advanced theory of groundwater—surface water exchange.

Straight thinking about groundwater recession

Abstract The way in which superficial deposits affect groundwater recharge is often a significant source of uncertainty in groundwater resources and vulnerability assessments. A study of a small catchment in Shropshire, UK, shows how electrical resistivity tomography (ERT), with a degree of borehole control, can be an effective tool for defining the geometry of superficial deposits for purposes of inferring the hydraulic processes controlling groundwater recharge. Major lithological units were mapped to within c. 0.5 m vertically and 5 m horizontally using ERT surveys with a minimum electrode spacing of 2 m. Interpretation was aided by the strong contrast in resistivity between till and glaciolacustrine deposits (20–40 Ω m) and glaciofluvial deposits (generally >100 Ω m) that overlie the Permo-Triassic sandstone aquifer (saturated resistivity 60–145 Ω m) in the study area. A range of local-scale (tens to hundreds of metres) recharge models are presented, based on the findings of the field surveys, and it is shown how existing mapping misses key features of the superficial geology that may be very significant in enhancing or restricting aquifer recharge.

Evaporative cooling of speleothem drip water

Amplitude decay and phase delay of oscillating temperature records measured at two vertical locations in near-surface sediments can be used to infer water fluxes, thermal diffusivity, and sediment scour/deposition. While methods that rely on the harmonics-based analytical heat transport solution assume a steady state water flux, many applications have reported transient fluxes but ignored the possible violation of this assumption in the method. Here we use natural heat tracing as an example to investigate the extent to which changes in the water flux, and associated temperature signal nonstationarity, can be separated from other influences. We systematically scrutinize the assumption of steady state flow in analytical heat tracing and test the capabilities of the method to detect the timing and magnitude of flux transients. A numerical model was used to synthesize the temperature response to different step and ramp changes in advective thermal velocity magnitude and direction for both a single-frequency and multifrequency temperature boundary. Time-variable temperature amplitude and phase information were extracted from the model output with different signal-processing methods. We show that a worst-case transient flux induces a temperature nonstationarity, the duration of which is less than 1 cycle for realistic sediment thermal diffusivities between 0.02 and 0.13 m2/d. However, common signal-processing methods introduce erroneous temporal spreading of advective thermal velocities and significant anomalies in thermal diffusivities or sensor spacing, which is used as an analogue for streambed scour/deposition. The most time-variant spectral filter can introduce errors of up to 57% in velocity and 33% in thermal diffusivity values with artifacts spanning ±2 days around the occurrence of rapid changes in flux. Further, our results show that analytical heat tracing is unable to accurately resolve highly time-variant fluxes and thermal diffusivities and does not allow for the inference of scour/depositional processes due to the limitations of signal processing in disentangling flux-related signal nonstationarities from those stemming from other sources. To prevent erroneous interpretations, hydrometric data should always be acquired in combination with temperature records.

Bacterially produced calcium phosphate nanobiominerals : sorption capacity, site preferences, and stability of captured radionuclides

While in catchment and hillslope hydrology a more nuanced approach is now taken to streamflow recession analysis, in the context of major aquifers it is commonly still assumed that the groundwater head recession rate will take exponential form, an idea originally proposed in the 19th Century. However it is shown here that, in early times, the groundwater head recession in a major aquifer should take an almost straight line form with a rate approximately equal to the long-term recharge rate divided by the aquifer storage coefficient. The length of this phase can be estimated from an analytical expression derived in the paper which depends on the aquifer diffusivity, length scale, and the position of the monitoring point. A transitional phase then leads to an exponential phase after some critical time which is independent of the position of the monitoring point. Major aquifers in a state of periodic quasi-steady state are expected to have rates of groundwater flux recession which deviate little from the average rate of groundwater recharge. Where quasi-exponential groundwater declines are observed in nature, their form may be diagnostic of particular types of aquifer properties and/or boundary effects, such as proximity to drainage boundaries, variations in transmissivity with hydraulic head, storage changes due to pumping, nonequilibrium flow at a range of spatial and temporal scales, and variations in specific yield with depth. Recession analysis has applicability to a range of groundwater problems and is powerful way of gaining insight into the hydrologic functioning of an aquifer.

The legacy of chlorinated solvents in the Birmingham aquifer, UK: Observations spanning three decades and the challenge of future urban groundwater development

This study describes the first use of concurrent high-precision temperature and drip rate monitoring to explore what controls the temperature of speleothem forming drip water. Two contrasting sites, one with fast transient and one with slow constant dripping, in a temperate semi-arid location (Wellington, NSW, Australia), exhibit drip water temperatures which deviate significantly from the cave air temperature. We confirm the hypothesis that evaporative cooling is the dominant, but so far unattributed, control causing significant disequilibrium between drip water and host rock/air temperatures. The amount of cooling is dependent on the drip rate, relative humidity and ventilation. Our results have implications for the interpretation of temperature-sensitive, speleothem climate proxies such as δ18O, cave microecology and the use of heat as a tracer in karst. Understanding the processes controlling the temperature of speleothem-forming cave drip waters is vital for assessing the reliability of such deposits as archives of climate change.

A Spring Forward for Hominin Evolution in East Africa

A Serratia sp. bacterium manufactures amorphous calcium phosphate nanominerals (BHAP); this material has shown increased sorption capacity for divalent radionuclide capture. When heat-treated (≥450 °C) the cell biomass is removed and the biominerals are transformed to hydroxyapatite (HAP). Using a multimethod approach, we have elucidated both the site preferences and stability of analogue radionuclide incorporation for Sr, Co, Eu, and U. Strontium incorporates within the bulk amorphous inorganic phase of BHAP; however, once temperature modified to crystalline HAP, bonding was consistent with Sr substitution at the Ca(1) and/or Ca(2) sites. Cobalt incorporation occurs within the bulk inorganic amorphous phase of BHAP and within the amorphous grain boundaries of HAP. Europium (an analogue for trivalent actinides) substituted at the Ca(2) and/or the Ca(3) position of tricalcium phosphate, a known component of HAP grain boundaries. Uranium was surface complexed with no secondary minerals detected. With multiple sites for targeted radionuclide incorporation, high loadings, and good stability against remobilization, BHAP is shown to be a potential material for the remediation of aqueous radionuclide in groundwater.