Peat porewater chloride concentration profiles in the Everglades during wet/dry cycles from January 1996 to June 1998: field measurements and theoretical analysis

Water quality is a key aspect of the Everglades Restoration Project, the largest water reclamation and ecosystem management project proposed in the United States. Movement of nutrients and contaminants to and from Everglades peat porewater could have important consequences for Everglades water quality and ecosystem restoration activities. In a study of Everglades porewater, we observed complex, seasonally variable peat porewater chloride concentration profiles at several locations. Analyses and interpretation of these changing peat porewater chloride concentration profiles identifies processes controlling conservative solute movement at the peat–surface water interface, that is, solutes whose transport is minimally affected by chemical and biological reactions. We examine, with an advection–diffusion model, how alternating wet and dry climatic conditions in the Florida Everglades mediate movement of chloride between peat porewater and marsh surface water. Changing surface water–chloride concentrations alter gradients at the interface between peat and overlying water and hence alter chloride flux across that interface. Surface water chloride concentrations at two frequently monitored sites vary with marsh water depth, and a transfer function was developed to describe daily marsh surface water chloride concentration as a function of marsh water depth. Model results demonstrate that porewater chloride concentrations are driven by changing surface water chloride concentrations, and a sensitivity analysis suggests that inclusion of advective transport in the model improves the agreement between the calculated and the observed chloride concentration profiles. Published in 2007 by John Wiley & Sons, Ltd.     

Hypothesis testing methods to detect spoofing attacks: a test against the TEXBAT datasets

GPS Solutions - The hazardous effects of spoofing attacks on the global navigation satellite system (GNSS) receiver are well known. Technologies and algorithms to increase the awareness of GNSS...     

Microanalysis of the iron oxidation state in (Mg,Fe)O and application to the study of microscale processes

We report application of the flank method using the electron microprobe to a suite of twelve (Mg,Fe)O samples with composition 2–47 wt% Fe and Fe³⁺/ΣFe = 1 to 11%, where Fe³⁺/ΣFe was determined independently using Mössbauer spectroscopy on the same grains used for the flank method measurements. A calibration curve of the form Fe²⁺ = A + B × (ΣFe)² + C × (Lβ/Lα) was fit to the data and gave excellent agreement between Fe³⁺/ΣFe calculated from the flank method and Fe³⁺/ΣFe determined using Mössbauer spectroscopy. We found the method to be sufficiently sensitive to determine meaningful variations in Fe³⁺/ΣFe for geophysically relevant compositions of (Mg,Fe)O (<25 wt% Fe), and calibration parameters remained constant within experimental uncertainty over the course of the entire study (20 months). Flank method measurements on an inhomogeneous sample of synthetic (Mg,Fe)O showed evidence of diffusion processes resulting from rupture of the capsule during the high-pressure experiment and the possibility to measure Lβ/Lα variations with a spatial resolution of a few microns. We detected the presence of exsolved magnesioferrite in a suite of (Mg,Fe)O single crystals using transmission electron microscopy and Mössbauer spectroscopy. Flank method measurements on the same suite of single crystals showed enhanced Fe³⁺/ΣFe values, consistent with the presence of magnesioferrite even though the grains were too small to be resolved by conventional electron microprobe measurements.     

Application of electrical resistivity to map the stratigraphy and salinity of fluvio-deltaic aquifers: case studies from Bangladesh that reveal benefits and pitfalls

Fluvio-deltaic aquifers are the primary source of drinking water for the people of Bangladesh. Such aquifers, which comprise the Ganges-Brahmaputra-Meghna Delta, are hydrogeologically heterogeneous. Because of widespread groundwater quality issues in Bangladesh, it is crucial to know the hydrostratigraphic architecture and hydrochemistry, as some aquifer units are contaminated, whereas others are safe. Geophysical methods provide a potentially effective and noninvasive method for extensive characterization of these aquifers. This study applies and investigates the limitations of using electrical resistivity imaging (ERI) for mapping the hydrostratigraphy and salinity of an aquifer-aquitard system adjacent to the Meghna River. Some electrical resistivity (ER) sections showed excellent correlation between resistivity and grain size. These suggest that ERI is a powerful tool for mapping internal aquifer architecture and their boundaries with finer-grained aquitards which clearly appear as low-ER zones. However, in parts of some ER sections, variations in electrical properties were determined by porewater resistivity. In these cases, low ER was indicative of brine and did not indicate the presence of finer-grained materials such as silt or clay. Accordingly, the following hydrostratigraphic zones with different resistivities were detected: (1) aquifers saturated with fresh groundwater, (2) a regional silt/clay aquitard, and (3) a deeper brine-saturated formation. In addition, shallow silt/clay pockets were detected close to the river and below the vadose zone. ERI is thus a promising technique for mapping aquifers versus aquitards; however, the observations are easily confounded by porewater salinity. In such cases, borehole information and groundwater salinity measurements are necessary for ground-truthing.

     

Bacterial Community Dynamics in a Seagrass (<Emphasis Type="Italic">Posidonia oceanica</Emphasis>) Meadow Sediment

Some traits of the bacterial community dynamics associated to the rhizosphere of the Mediterranean seagrass Posidonia oceanica, growing in carbonate sediments, were analyzed during a 2-year period in an enclosed bay of the Balearic Islands. The diversity of the bacterial community was studied by the construction of 16S rDNA clone library. For testing temporal and vertical differences in the abundance of total cells and active Bacteria, we used 4′,6-diamidino-2-phenylindole (DAPI) staining and fluorescence in situ hybridization (FISH). Moreover, some relevant groups of sulfate-reducing bacteria (SRB) were occasionally assessed by FISH. Despite the observed decrease in the total DAPI-stained cells, bacterial counts, and sulfate reduction rates throughout the sampling time, we found an increase in both the pore-water sulfide concentration and the proportion of SRB. Overall, the results revealed a very high bacterial diversity and indicated shifts in bacterial dynamics that could not be related to temperature-dependent factors, suggesting a link between the documented regression of the seagrass meadow and the decline of the microbial community, likely due to large organic matter inputs to the bay.     

Characterization of surface and ground water δ18O seasonal variation and its use for estimating groundwater residence times

¹⁸O is an ideal tracer for characterizing hydrological processes because it can be reliably measured in several watershed hydrological compartments. Here, we present multiyear isotopic data, i.e. ¹⁸O variations (δ¹⁸O), for precipitation inputs, surface water and groundwater in the Shingobee River Headwaters Area (SRHA), a well‐instrumented research catchment in north‐central Minnesota. SRHA surface waters exhibit δ¹⁸O seasonal variations similar to those of groundwaters, and seasonal δ¹⁸O variations plotted versus time fit seasonal sine functions. These seasonal δ¹⁸O variations were interpreted to estimate surface water and groundwater mean residence times (MRTs) at sampling locations near topographically closed‐basin lakes. MRT variations of about 1 to 16 years have been estimated over an area covering about 9 km² from the basin boundary to the most downgradient well. Estimated MRT error (±0·3 to ±0·7 years) is small for short MRTs and is much larger (±10 years) for a well with an MRT (16 years) near the limit of the method. Groundwater transit time estimates based on Darcy's law, tritium content, and the seasonal δ¹⁸O amplitude approach appear to be consistent within the limits of each method. The results from this study suggest that use of the δ¹⁸O seasonal variation method to determine MRTs can help assess groundwater recharge areas in small headwaters catchments. Copyright © 2005 John Wiley & Sons, Ltd.     

Assessment of chemical water types and their spatial variation using multi-stage cluster analysis, Queensland, Australia

A multivariate assessment has been adapted to the classification of a large, irregular dataset of approximately 34,000 surface water samples accumulated over more than 30 years. A two-stage K-means clustering method was designed to analyse chemical data in the form of percentages of major ions (Na, Mg, Ca, Cl, HCO₃ and SO₄); the first stage of clustering produced 347 groups, which were then re-clustered to generate the final nine water types. The analysis enabled the definition of provinces of water composition and highlighted natural processes influencing the surface water chemistry. On a statewide basis, sodium is the dominant cation and around 50% at all stream flows, while proportions of calcium and magnesium are about equal. Chloride and bicarbonate constitute the bulk of anions present, while sulfate occurs occasionally and tends to be localised. On a global basis, Queensland surface waters are relatively high in sodium, chloride and magnesium, and low in calcium and sulfate. It was also found that the geographical location has a greater impact on major ion ratios than does the stage of stream flow.

The regional chemical trends are consistent with geology and climate. Streams in northeast Queensland, with short, steep catchments and high rainfall, yield low salinity, sodium-dominated water; this is also the case for sandy southern coastal catchments. Both also reflect an oceanic influence. The proportions of sodium and chloride decrease westward; streams draining the western side of the Great Dividing Range or flowing into the Gulf of Carpentaria have low salinity but relatively hard water. Streams in western Queensland are higher in calcium and bicarbonate. In the large catchments flowing from Queensland into central Australia, the water composition is highly variable, commonly with elevated sulfate. Also in Queensland, there are several other clearly definable water provinces such as the high magnesium waters of basaltic areas.

The findings of this study confirm that the application of such analytical methods can provide a useful assessment of controls over water composition and support management at regional level; the approach used is shown and are applicable to large, disparate datasets.     

Using new remote sensing satellites for assessing water quality in a reservoir

Water quality monitoring could benefit from information derived from the newest generation of medium-resolution Earth observation satellites. The main objective of our study was to assess the suitability of both Landsat 8 and Sentinel-2A satellites for estimating and mapping of Secchi disk transparency (SDT), a common measurement of water clarity, in Cassaffousth Reservoir (Córdoba, Argentina). Ground observations and a dataset of four Landsat 8 and four Sentinel-2A images were used to create and validate models to estimate SDT in the reservoir. The selected algorithms were used to obtain graphic representations of water clarity. Slight differences were found between Landsat 8 and Sentinel-2 estimations. Thus, we demonstrated the suitability of both satellites for estimating and mapping water quality. This study highlights the importance of free and readily available satellite datasets in monitoring water quality, especially in countries where conventional monitoring programmes are either lacking or unsatisfactory.