Dissolved inorganic carbon in soil and shallow groundwater, Konza Prairie LTER Site, NE Kanas, USA

Sources and seasonal trends of dissolved inorganic carbon (DIC) in a shallow limestone aquifer were studied for 1 year at the Konza Prairie LTER (Long-Term Ecological Research) Site in northeastern Kansas, from spring 2010 to spring 2011. Annual cycles of soil air CO2, groundwater DIC, and isotope characteristics showed a strong dependency on weather conditions and soil respiration. Soil air CO2 reached its annual maximum in the middle of the growing season, when moisture was not limiting to soil respiration. Following the maximum, the CO2 decreased because of moisture deficiency in the late summer and temperature decline in the fall and winter. The decrease began first in the shallowest part of the soil and last in the deepest part. Groundwater CO2 reached its annual maximum in October; this lag-time between the soil and groundwater CO2 maxima of 2-3 months may correspond to the travel time of soil-generated CO2 to the water table. The time-variable CO2 caused an annual carbonate-mineral saturation cycle, intensifying limestone dissolution, thus soil CO2 and carbonate minerals are the two main sources of DIC in soil and groundwater. The stable carbon isotope composition of soil air CO2 and DIC exhibited primarily C4 plant signature and were similar to that of soil organic matter, suggesting that both root and bacterial respiration are sources of CO2. DIC was enriched in 7-10 per mil PDB relative to the CO2 source due to isotope fractionation in a system open to soil CO2; the enrichment was smallest under highest pCO2. For this reason, d13CDIC was out of phase with DIC, the lightest in the late growing season. The carbon flux from the unsaturated zone to the unconfined aquifer during the year was variable depending on respiration and precipitation regimes, and had two main pathways. Transport of soil CO2 in the dissolved form with diffuse flow of recharge water was the most effective during the entire growing season. Downward movement of gaseous CO2 and equilibration with groundwater at the water table was possible in July to August. Storm rainfall events rapidly recharged the aquifer through preferential flow and stream-groundwater interaction. Rather than forcing soil gases downward because of water-saturated pores, the main effect of these events was dilution of groundwater. The calculated flux was about 0.3 M/m2/yr of C, which is less than 1% of the CO2 that is released by soil to the atmosphere via efflux. However, the climate prediction of increased respiration rates, temperature, and frequency of extreme rainfall events has the potential to cause higher carbon flux to the saturated zone, intensifying weathering and groundwater acidification

Modeling the influence of climate on groundwater flow and heat regime in Brandenburg (Germany)

This study investigates the decades-long evolution of groundwater dynamics and thermal field in the North German Basin beneath Brandenburg (NE Germany) by coupling a distributed hydrologic model with a 3D groundwater model. We found that hydraulic gradients, acting as the main driver of the groundwater flow in the studied basin, are not exclusively influenced by present-day topographic gradients. Instead, structural dip and stratification of rock units and the presence of permeability contrasts and anisotropy are important co-players affecting the flow in deep seated saline aquifers at depths >500 m. In contrast, recharge variability and anthropogenic activities contribute to groundwater dynamics in the shallow (<500 m) freshwater Quaternary aquifers. Recharge fluxes, as derived from the hydrologic model and assigned to the parametrized regional groundwater model, reproduce magnitudes of recorded seasonal groundwater level changes. Nonetheless, observed instances of inter-annual fluctuations and a gradual decline of groundwater levels highlight the need to consider damping of the recharge signal and additional sinks, like pumping, in the model, in order to reconcile long-term groundwater level trends. Seasonal changes in near-surface groundwater temperature and the continuous warming due to conductive heat exchange with the atmosphere are locally enhanced by forced advection, especially in areas of high hydraulic gradients. The main factors controlling the depth of temperature disturbance include the magnitude of surface temperature variations, the subsurface permeability field, and the rate of recharge. Our results demonstrate the maximum depth extent and the response times of the groundwater system subjected to non-linear interactions between local geological variability and climate conditions

Data_Sheet_1_Modeling the influence of climate on groundwater flow and heat regime in Brandenburg (Germany).pdf

This study investigates the decades-long evolution of groundwater dynamics and thermal field in the North German Basin beneath Brandenburg (NE Germany) by coupling a distributed hydrologic model with a 3D groundwater model. We found that hydraulic gradients, acting as the main driver of the groundwater flow in the studied basin, are not exclusively influenced by present-day topographic gradients. Instead, structural dip and stratification of rock units and the presence of permeability contrasts and anisotropy are important co-players affecting the flow in deep seated saline aquifers at depths >500 m. In contrast, recharge variability and anthropogenic activities contribute to groundwater dynamics in the shallow (<500 m) freshwater Quaternary aquifers. Recharge fluxes, as derived from the hydrologic model and assigned to the parametrized regional groundwater model, reproduce magnitudes of recorded seasonal groundwater level changes. Nonetheless, observed instances of inter-annual fluctuations and a gradual decline of groundwater levels highlight the need to consider damping of the recharge signal and additional sinks, like pumping, in the model, in order to reconcile long-term groundwater level trends. Seasonal changes in near-surface groundwater temperature and the continuous warming due to conductive heat exchange with the atmosphere are locally enhanced by forced advection, especially in areas of high hydraulic gradients. The main factors controlling the depth of temperature disturbance include the magnitude of surface temperature variations, the subsurface permeability field, and the rate of recharge. Our results demonstrate the maximum depth extent and the response times of the groundwater system subjected to non-linear interactions between local geological variability and climate conditions.</p

Semiconductor-to-Insulator Transition in Inter-Electrode Bridge-like Ensembles of Anatase Nanoparticles under a Long-Term Action of the Direct Current

The results of experimental studies of ohmic conductivity degradation in the ensembles of nanostructured anatase bridges under a long-term effect of direct current are presented. Stochastic sets of partially conducting inter-electrode bridges consisting of close-packed anatase nanoparticles were formed by means of the seeding particles from drying aqueous suspensions on the surfaces of silica substrates with interdigital platinum electrodes. Multiple-run experiments conducted at room temperature have shown that ohmic conductivity degradation in these systems is irreversible. It is presumably due to the accumulated capture of conduction electrons by deep traps in anatase nanoparticles. The scaling analysis of voltage drops across the samples at the final stage of degradation gives a critical exponent for ohmic conductivity as ≈1.597. This value satisfactorily agrees with the reported model data for percolation systems. At an early stage of degradation, the spectral density of conduction current fluctuations observed within the frequency range of 0.01–1 Hz decreases approximately as 1/ω, while near the percolation threshold, the decreasing trend changes to ≈1/ω2. This transition is interpreted in terms of the increasing contribution of blockages and subsequent avalanche-like breakdowns of part of the local conduction channels in the bridges into electron transport near the percolation threshold