Groundwater contributions of flow and nitrogen in a headwater agricultural watershed

Nonpoint sources of nitrogen (N) and other nutrients are a major source of water pollution within the Chesapeake Bay watershed and other basins around the world. Human activities associated with agricultural practices can account for a large percentage of N loadings delivered to streams and rivers. This work aims to improve understanding of N transport from groundwater to surface waters, quantifying the principal hydrological processes driving water and N fluxes into and out of a headwater agricultural stream reach. The study site is a 175-m stream reach in a heavily cultivated 40-ha watershed in east-central Pennsylvania. This subwatershed is underlain by fractured shale bedrock, and receives most of its baseflow from groundwater, either by diffuse matrix discharge through the streambed or by localized discharge through riparian seeps. Samples of stream, seep, and shallow groundwater were collected approximately monthly under steady hydrologic conditions in 2017. Calculated matrix flow from hydraulic head and conductivity measurements paired with differential stream gauging was used to solve for the riparian seep flux using a mass balance approach. Riparian seep fluxes ranged from 45 to 217 m³/d, transporting 0.6–4.2 kg N d⁻¹ of nitrate-N from the fractured bedrock aquifer to the stream. Hydrochemical data suggest that the stream is mainly disconnected from the underlying aquifer and that seeps supply essentially all water and N to the system. Seeps are likely sourced with N in nearby agricultural fields and accelerated through the system with shorter residence times than shallow groundwater. Water isotope data reinforced this notion. This study underscores the importance of agriculture as a source of N to ground and surface waters. Identifying source areas that are causing groundwater enrichment of N and seep areas where N discharges to streams is beneficial for developing N pollution mitigation strategies and implementing management practices that aim to reduce nutrient loads to the Chesapeake Bay.     

Sampling Method for the Determination of Total Inorganic Carbon (TIC) in Strongly Acidic Waters

Samples from extremely acidic waters should be taken using the introduced syringe technique avoiding atmospheric contact to minimize CO2 evaporation from the samples. The syringe technique inclusive manual injection enables the determination of TIC in acid waters with high precision and accuracy. If a TOC analyser with a small sample volume is used, the vials should be filled directly inside the water sampler without air bubbles, covered with aluminium foil and transported at ambient water temperatures. If an autosampler is used, partial selection of the vials increases the stability of the results. Care should be taken that samples can be analysed within a short time.     

Seasonal and long-term trends in the water quality of Florida Bay (1989–1997)

Analysis of 6 yr of monthly water quality data was performed on three distinct zones of Florida Bay: the eastern bay, central bay, and western bay. Each zone was analyzed for trends at intra-annual (seasonal), interannual (oscillation), and long-term (monotonic) scales. the variables TON, TOC, temperature, and TN∶TP ratio had seasonal maxima in the summer rainy season; APA and Chla, indicators of the size and activity of the microplankton tended to have maxima in the fall. In contrast, NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, turbidity, and DO_sat, were highest in the winter dry season. There were large changes in some of the water quality variables of Florida Bay over the study period. Salinity and TP concentrations declined baywide while turbidity increased dramatically. Salinity declined in the eastern, central, and western Florida Bay by 13.6‰, 11.6‰, and 5.6‰, respectively. Some of the decrease in the eastern bay could be accounted for by increased freshwater flows from the Everglades. In contrast to most other estuarine systems, increased runoff may have been partially responsible for the decrease in TP concentrations as input concentrations were 0.3–0.5 μM. Turbidity in the eastern bay increased twofold from 1991 to 1996, while in the central and western bays it increased by factors of 20 and 4, respectively. Chla concentrations were particularly dynamic and spatially heterogeneous. In the eastern bay, which makes up roughly half of the surface area of Florida Bay, Chla declined by 0.9 μg l⁻¹ (63%). The hydrographically isolated central bay zone underwent a fivefold increase in phytoplankton biomass from 1989 to 1994, then rapidly declined to previous levels by 1996. In western Florida Bay there was a significant increase in Chla, yet median concentrations of Chla in the water column remained modest (∼2 μg l⁻¹) by most estuarine standards. Only in the central bay did the DIN pool increase substantially (threefold to sixfold). Notably, these changes in turbidity and phytoplankton biomass occurred after the poorly-understood seagrass die-off in 1987. It is likely the death and decomposition of large amounts of seagrass biomass can at least partially explain some of the changes in water quality of Florida Bay, but the connections are temporally disjoint and the process indirect and not well understood.     

Comparison of the raman microprobe spectra of (Mg,Fe)2SiO4 and Mg2GeO4 with olivine and spinel structures

Raman microprobe (RMP) spectra were produced for each of the olivine and spinel structured phases of Mg₂GeO₄ and (Mg, Fe)₂SiO₄. The assembled data show that bands due to the tetrahedra in silicate and germanate olivines shift in a way that indicates a dominant mass effect. This correspondence is difficult to make in spinels due to differences in structural type. Differences in Fe/Mg content of olivine shift the tetrahedral vibration bands only slightly, but their linear shifts could be used to indicate the composition of the phase.     

Study of a few palaeozoic ignimbrites in the armorican massif and in the central Massif,France

A detailed study of the petrographical characteristics of the acid palaeovolcanic formations in the Armorican Massif and in part of the Central Massif (France) reveals important ignimbritic provinces ranging from Precambrian to Carboniferous age. Close comparisons show that the ignimbrites, though varied in the details, form a whole which has constant characteristics. They can thus be related or opposed to ignimbrites of more recent origin. Field data, essential for the identification of recent ignimbrites, are of little practical help when dealing with old ones. But the study of macroscopic samples proves highly effective: heterogeneity, pseudo-fluidality, spindle-shaped structures, etc…, are among the relevant features. Only a microscopic study, however, can show vitroclastic structures and reveal the striking similarity between spindle-shaped structures and the « fiammae » of more recent ignimbrites.     

Identification of a magma chamber in the Ghoubbet-Asal rift (Djibouti) from a magnetotelluric experiment

A very broad band (10^?3 to 10⁴ Hz) magnetotelluric investigation of the axial zone of the Ghoubbet-Asal rift (Djibouti) has revealed a shallow (2–4 km) magma chamber which can be mapped in some detail. The suggested roof of the chamber is shallowest very close to the Ardoukoba volcano which was built during the November 1978 rifting episode.     

Sources of anomalous transient electric signals (ATESs) in the ULF band in the Lamia region (central Greece): electrochemical mechanisms for their generation

Anomalous transient electric signals (ATESs) in the ultra low frequency (ULF) band have been often observed during magnetotelluric (MT) investigations [Nature 319 (1986) 310; Phys. Earth Planet. Int. 114 (1999) 141; Geophys. J. Int. 142 (2000) 948], but their origin was unknown until now. They have the same characteristics as the so-called seismic electric signals (SES) claimed to be earthquake precursors by the VAN group (e.g. [Tectonophysics 110 (1984) 73] and later works by this group). Our analysis suggests that the so-called SES could be of anthropic origin. Following the devastating 7 September 1999 Athens earthquake, the VAN group claimed that a SES had been recorded at LAM station (Lamia, central Greece) some days prior to the main shock and that a second SES, which might correspond to an impending even larger earthquake, had been observed after the main shock. In the 2 years after the Athens main shock, no subsequent large earthquakes have occurred near Athens. We conducted a campaign of measurement in the Lamia region in May and June 2001. The results show that ATESs, which look like SES, have several different sources: pump-stations for ground-water, high power electric lines, and factories located to the SE of Lamia city. The ATESs can be generated by two electrochemical mechanisms of metallic electrode polarization: the “galvanic cell” and the “ac electrolytic cell” which are studied by simulated field experiments and discussed in detail in Appendix A. These two mechanisms can occur over a wide range of length scales in the field. Any isolation failure in buried metallic conductors, such as electrical and telecommunication networks, oil, water and gas pipes, railways, high power electric lines, factories and so on, can produce a galvanic cell or an ac electrolytic cell, or both, which could generate, under some circumstances, an “overvoltage”, the ATES. Finally, the absence of a magnetic signal has been observed during ATES and does not constitute a firm criterion for SES [Acta Geophys. Pol. 44 (1996b) 301]. Thus, great care must be taken when claiming the existence of electric precursors of seismic or volcanic events.     

Factors controlling nitrogen release from two forested catchments with contrasting hydrochemical responses

Quantifying biogeochemical cycles of nitrogen (N) and the associated fluxes to surface waters remains challenging, given the need to deal with spatial and temporal variability and to characterize complex and heterogeneous landscapes. We focused our study on catchments S14 and S15 located in the Adirondack Mountains of New York, USA, which have similar topographic and hydrologic characteristics but contrasting stream nitrate ( ) concentrations. We characterized the mechanisms by which reaches the streams during hydrological events in these catchments, aiming to reconcile our field data with our conceptual model of factors that regulate nutrient exports from forested catchments. Combined hydrometric, chemical and isotopic (δ ) data showed that the relative contributions of both soil and ground water sources were similar between the two catchments. Temporal patterns of stream chemistry were markedly different between S14 and S15, however, because the water sources in the two catchments have different solute concentrations. During late summer/fall, the largest source of in S14 was till groundwater, whereas shallow soil was the largest source in S15. concentrations in surface water decreased in S14, whereas they increased in S15 because an increasing proportion of stream flow was derived from shallow soil sources. During snowmelt, the largest sources of were in the near‐surface soil in both catchments. Concentrations of increased as stream discharge increased and usually peaked before peak discharge, when shallow soil water sources made the largest contribution to stream discharge. The timing of peaks in stream concentrations was affected by antecedent moisture conditions. By elucidating the factors that affect sources and transport of N, including differences in the soil nutrient cycling and hydrological characteristics of S14 and S15, this study contributes to the overall conceptualization of release from temperate forested catchments. Copyright © 2007 John Wiley & Sons, Ltd.     

New light on a dark subject: comment

In their article, “New light on a dark subject: On the use of fluorescence data to deduce redox states of natural organic matter,” Macalady and Walton-Day (2009) subjected natural organic matter (NOM) samples to oxidation, reduction, and photochemical transformation. Fluorescence spectra were obtained on samples, which were diluted “to bring maximum uvvisible absorbance values below 1.0.” The spectra were fit to the Cory and McKnight (2005) parallel factor analysis (PARAFAC) model, and consistent variation in the redox state of quinone-like moieties was not detected. Based on these results they concluded that fitting fluorescence spectra to the Cory and McKnight (2005) PARAFAC model to obtain information about the redox state of quinone-like moieties in NOM is problematic. Recognizing that collection and correction of fluorescence spectra requires consideration of many factors, we investigated the potential for inner-filter effects to obscure the ability of fluorescence spectroscopy to quantify the redox state of quinone-like moieties. We collected fluorescence spectra on Pony Lake and Suwannee River fulvic acid standards that were diluted to cover a range of absorbance wavelengths, and fit these spectra to the Cory and McKnight (2005) PARAFAC model. Our results suggest that, in order for the commonly used inner-filter correction to effectively remove inner-filter effects, samples should be diluted such that the absorbance at 254 nm is less than 0.3 prior to the collection of fluorescence spectra. This finding indicates that inner-filter effects may have obscured changes in the redox signature of fluorescence spectra of the highly absorbing samples studied by Macalady and Walton-Day (2009).