Large tank experiment on nitrate fate and transport: the role of permeability distribution

A long-term elution experiment to study the saturated transport of pre-accumulated fertilizers by-products, was conducted within a large tank (4 × 8 × 1.4 m) equipped with 26 standard piezometers. Sandy sediments (35 m³), used to fill the tank, were excavated from an unconfined alluvial aquifer near Ferrara (Northern Italy); the field site was connected to a pit lake located in a former agricultural field. To evaluate spatial heterogeneity, the tank’s filling material was characterized via slug tests and grain-size distribution analysis. The investigated sediments were characterized by a large spectrum of textures and a heterogeneous hydraulic conductivity (k) field. Initial tank pore water composition exhibited high concentration of nitrate (NO₃ ⁻) sulfate (SO₄ ²⁻) calcium (Ca²⁺), and magnesium (Mg²⁺), due to fertilizer leaching from the top soil in the field site. The initial spatial distribution of NO₃ ⁻ and SO₄ ²⁻ was heterogeneous and not related to the finer grain-size content (<63 μm). The tank’s material was flushed with purified tap water for 800 days in steady-state conditions; out flowing water was regularly sampled to monitor the migration rate of fertilizer by-products. Complete removal of NO₃ ⁻ and SO₄ ²⁻ took 500 and 600 days, respectively. Results emphasized organic substrate availability and spatial heterogeneities as the most important constraints to denitrification and nitrogen removal, which increase the time required to achieve remediation targets. Finally, the obtained clean-up time was compared with a previous column experiment filled with the same sediments.     

Groundwater contamination with arsenic in Sherajdikhan,Bangladesh: geochemical and hydrological implications

An integrated study has been carried out to elucidate the distribution and occurrence of arsenic in selected groundwater samples in the area of Sherajdikhan, Bangladesh. Arsenic and other parameters (T, pH, EC, Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, NO₃ ⁻, SO₄ ²⁻, HCO₃ ⁻, PO₄ ³⁻, Fe, Mn and DOC) have been measured in groundwater samples collected from shallow/deep tube wells at different depths. Hydrogeochemical data suggest that the groundwaters are generally Ca–Mg–HCO₃ and Mg–Ca–HCO₃ types with bicarbonate (HCO₃ ⁻) as the dominant anion, though the other type of water has also been observed. Dissolved arsenic in groundwater ranged from 0.006 to 0.461 mg/l, with 69% groundwater samples exceeded the Bangladesh limit for safe drinking water (0.05 mg/l). Correlation and principal component analysis have been performed to find out possible relationships among the examined parameters in groundwater. Low concentrations of NO₃ ⁻ and SO₄ ²⁻, and high concentrations of DOC, HCO₃ ⁻ and PO₄ ³⁻ indicate the reducing condition of subsurface aquifer where sediments are deposited with abundant organic matter. Distinct relationship of As with Fe and Mn, and strong correlation with DOC suggests that the biodegradation of organic matter along with reductive dissolution of Fe–Mn oxyhydroxides has being considered the dominant process to release As in the aquifers studied herein.     

Bottom sediments of Kandalaksha Bay in the White Sea: The phenomenon of Mn

The redox stratification of bottom sediments in Kandalaksha Bay, White Sea, is characterized by elevated concentrations of Mn (3–5%) and Fe (7.5%) in the uppermost layer, which is two orders of magnitude and one and a half times, respectively, higher than the average concentrations of these elements in the Earth’s crust. The high concentrations of organic matter (C_org = 1–2%) in these sediments cannot maintain (because of its low reaction activity) the sulfate-reducing process (the concentration of sulfide Fe is no higher than 0.6%). The clearest manifestation of diagenesis is the extremely high Mn²⁺ concentration in the silt water (>500 μM), which causes its flux into the bottom water, oxidation in contact with oxygen, and the synthesis of MnO₂ oxyhydroxide enriching the surface layer of the sediments. Such migrations are much less typical of Fe. Upon oxygen exhaustion in the uppermost layer of the sediments, the synthesized oxyhydroxides (MnO₂ and FeOOH) serve as oxidizers of organic matter during anaerobic diagenesis. The calculated diffusion-driven Mn flux from the sediments (280 μmM/m² day) and corresponding amount of forming Mn oxyhydrate as compared to opposite oxygen flux to sediments (1–10 mM/m² day) indicates that >10% organic matter in the surface layer of the sediments can be oxidized with the participation of MnO₂. The roles of other oxidizers of organic matter (FeOOH and SO₄²⁻) becomes discernible at deeper levels of the sediments. The detailed calculation of the balance of reducing processes testifies to the higher consumption of organic matter during the diagenesis of surface sediments than it follows from the direct determination of C_org. The most active diagenetic redox processes terminate at depths of 25–50 cm. Layers enriched in Mn at deeper levels are metastable relicts of its surface accumulation and are prone to gradual dissemination     

Transient States in Diagenesis Following the Deposition of a Gravity Layer: Dynamics of O<Subscript>2</Subscript>, Mn,Fe and N-Species in Experimental Units

Biogeochemical processes induced by the deposition of gravity layer in marine sediment were studied in a 295-day experiment. Combining voltammetric microelectrode measurements and conventional analytical techniques, the concentrations of C, O₂, N-species, Mn and Fe have been determined in porewaters and sediments of experimental units. Dynamics of the major diagenetic species following the sudden sediment deposition of few cm-thick layer was explained by alternative diagenetic pathways whose relative importance in marine sediments is still a matter of debate. Time-series results indicated that the diffusion of O₂ from overlying waters to sediments was efficient after the deposition event: anoxic conditions prevailed during the sedimentation. After a few days, a permanent oxic horizon was formed in the top few millimetres. At the same time, the oxidation of Mn²⁺ and then Fe²⁺, which diffused from anoxic sediments, contributed to the surficial enrichment of fresh Mn(III/IV)- and Fe(III)-oxides. Vertical diffusive fluxes and mass balance calculations indicated that a steady-state model described the dynamic of Mn despite the transitory nature of the system. This model was not adequate to describe Fe dynamics because of the multiple sources and phases of Fe²⁺. No significant transfer of Mn and Fe was observed between the underlying sediment and the new deposit: Mn- and Fe-oxides buried at the original interface acted as an oxidative barrier to reduced species that diffused from below. Nitrification processes led to the formation of a NO₃⁻/NO₂⁻ rich horizon at the new oxic horizon. Over the experiment period, NO₃⁻ concentrations were also measured in the anoxic sediment suggesting anaerobic nitrate production.     

Interrelationship of TOC,As, Fe,Mn, Al and Si in shallow alluvial aquifers in Chapai-Nawabganj,Northwestern Bangladesh: implication for potential source of organic carbon

Two boreholes and ten piezometers in the Ganges flood plain were drilled and installed for collecting As-rich sediments and groundwater. Groundwater samples from the Ganges flood plain were collected for the analysis of cations (Ca²⁺, Mg²⁺, K⁺, Na⁺), anions (Cl⁻, NO₃ ⁻, SO₄ ²⁻), total organic carbon (TOC), and trace elements (As, Mn, Fe, Sr, Se, Ni, Co, Cu, Mo, Sb, Pb). X-ray powder diffraction was performed to characterize the major mineral contents of aquifer sediments and X-ray fluorescence (XRF) to analyze the major chemical composition of alluvial sediments. Results of XRF analysis clearly show that fine-grained sediments contain higher amounts of trace element because of their high surface area for adsorption. Relative fluorescence index (15–38 QSU) of humic substance in groundwater was measured using spectrofluorometer, the results revealed that groundwater in the Ganges flood plain contains less organic matter (OM). Arsenic concentration in water ranges from 2.8 to 170 μg/L (mean 50 μg/L) in the Ganges flood plain. Arsenic content in sediments ranges from 2.1 to 14 mg/kg (mean 4.58 mg/kg) in the flood plains. TOC ranges from 0.49 to 3.53 g/kg (mean 1.64 g/kg) in the Ganges flood plain. Arsenic is positively correlated with TOC (R ² = 0.55) in sediments of this plain. Humic substances were extracted from the sediments from the Ganges flood plain. Fourier transform infrared analysis of the sediments revealed that the plain contains less humic substances. The source of organic carbon was assigned from δ¹³C values obtained using elemental analysis-isotope ratio mass spectrometry (EA-IRMS); the values (−10 to −29.44‰) strongly support the hypothesis that the OM of the Ganges flood plain is of terrestrial origin.     

Environmental pollution impact on water and sediments of Kumaun lakes, Lesser Himalaya, India: a comparative study

A study of the water and sediment chemistry of the Nainital, Bhimtal, Naukuchiyatal and Sattal Lakes of Kumaun, has shown that the water of these lakes are alkaline and that electrical conductivity, total dissolved solid and bicarbonate HCO ₃ ⁻ are much higher in Nainital than in the other three lakes. The weathering of limestone lithology and anthropogenic pollution, the latter due to the very high density of population in the Nainital valley, are the primary sources of enhanced parameters. The low pH of Nainital Lake water is due to low photosynthesis and enhanced respiration, increasing CO₂ in the water and the consequent enhancement of Ca²⁺ and HCO ₃ ⁻ . The dissolved oxygen in Nainital Lake is less compared to other lakes, indicating anoxic conditions developing at the mud–water interface at depth. The PO ₄ ³⁻ content in Nainital is higher (124 μg/l), showing an increasing trend over time leading to eutrophic conditions. The trace metals (Cu, Co, Zn, Ni, Mn, and Sr) are present in greater amounts in the water of Nainital Lake than in the other three lakes, though Fe and Cr are high in Bhimtal and Fe in Naukuchiyatal. The higher abundance is derived from the leaching of Fe–Mg from metavolcanic and metabasic rocks. Most of the heavy metals (Cr, Ni, Cu, Mn, Fe, Sr, and Zn) significantly enrich the suspended sediments of the lakes compared to the bed sediments which due to their adsorption on finer particles and owing to multiple hydroxide coating and organic content, except for Fe, which is enriched in the bed sediments. The high rate of sedimentation, 11.5 mm/year in Nainital, compared to Bhimtal with 4.70 mm/year, Naukuchiyatal with 3.72 mm/year, and Sattal with 2.99 mm/year, has resulted in shorter residence time, poor sorting of grains, and lesser adsorption of heavy metals, leading consequently, their depletion in the bed sediments of Nainital Lake.     

Denitrification and the stoichiometry of nutrient regeneration in Waquoit Bay, Massachusetts

To determine the removal of regenerated nitrogen by estuarine sediments, we compared sediment N₂ fluxes to the stoichiometry of nutrient and O₂ fluxes in cores collected in the Childs River, Cape Cod, Massachusetts. The difference between the annual PO₄ ³⁻ (0.2 mol P m⁻² yr⁻¹) and NH₄ ⁺ (1.6 mol N m⁻² yr⁻¹) flux and the Redfield N∶P ratio of 16 suggested an annual deficit of 1.5 mol N m⁻² yr⁻¹. Denitrification predicted from O₂∶NH₄ ⁺ flux ratios and measured as N₂ flux suggested a nitrogen sink of roughly the same magnitude (1.4 mol N m⁻² yr⁻¹). Denitrification accounted for low N∶P ratios of benthic flux and removed 32–37% of nitrogen inputs entering the relatively highly nutrient loaded Childs River, despite a relatively brief residence time for freshwater in this system. Uptake of bottom water nitrate could only supply a fraction of the observed N₂ flux. Removal of regenerated nitrogen by denitrification in this system appears to vary seasonally. Denitrification efficiency was inversely correlated with oxygen and ammonium flux and was lowest in summer. We investigated the effect of organic matter on denitrification by simulating phytoplankton deposition to cores incubated in the lab and by deploying chambers on bare and macroaglae covered sediments in the field. Organic matter addition to sediments increased N₂ flux and did not alter denitrification efficiency. Increased N₂ flux co-varied with O₂ and NH₄ ⁺ fluxes. N₂ flux (261±60 μmol m⁻² h⁻¹) was lower in chambers deployed on macroalgal beds than deployed on bare sediments (458±70 μmol m⁻² h⁻¹), and O₂ uptake rate was higher in chambers deployed on macroalgal beds (14.6±2.2 mmol m⁻² h⁻¹) than on bare sediments (9.6±1.5 mmol m⁻² h⁻¹). Macroalgal cover, which can retain nitrogen in the system, is a link between nutrient loading and denitrification. Decreased denitrification due to increasing macroalgal cover could create a positive feedback because decreasing denitrification would increase nitrogen availability and could increase macroalgae cover.     

Inorganic nitrogen dynamics in intertidal rocky biofilms and sediments of the Douro River estuary (Portugal)

In this study rates of oxygen, ammonium (NH₄ ⁺), nitrate (NO₃ ⁻), nitrite (NO₂ ⁻), and nitrous oxide (N₂O) fluxes, nitrogen (N) fixation, nitrification, and denitrification were compared between two intertidal sites for which there is an abundant global literature, muddy and sandy sediments, and two sites representing the rocky intertidal zone where biogeochemical processes have scarcely been investigated. In almost all sites oxygen production rates greatly exceeded oxygen consumption rates. During daylight, NH₄ ⁺ and NO₃ ⁻ uptake rates together with ammonification could supply the different N requirements of the primary producer communities at all four sites; N assimilation by benthic or epilithic primary producers was the major process of dissolved inorganic nitrogen (DIN) removal; N fixation, nitrification, and denitrification were minor processes in the overall light DIN cycle. At night, distinct DIN cycling processes took place in the four environments, denitrification rates ranged from 9 ± 2 to 360 ± 30 μmol N₂ m⁻² h⁻¹, accounting for 10–48% of the water column NO₃ ⁻ uptake; nitrification rates varied from 0 to 1712 ± 666 μmol NH₄ ⁺ m⁻² h⁻¹. A conceptual model of N cycle dynamics showed major differences between intertidal sediment and rocky sites in terms of the mean rates of DIN net fluxes and the processes involved, with rocky biofilm showing generally higher fluxes. Of particular significance, the intertidal rocky biofilms released 10 times the amount of N₂O produced in intertidal sediments (up to 17 ± 6 μmol N₂O m⁻² h⁻¹), representing the highest N₂O release rates ever recorded for marine systems. The biogeochemical contributions of intertidal rocky substrata to estuarine and coastal processes warrant future detailed investigation.     

Spatial and temporal patterns in sediment and water column nutrients in a eutrophic Southern California estuary

Quarterly field sampling was conducted to characterize variations in water column and sediment nutrients in a eutrophic southern California estuary with a history of frequent macroalgal blooms. Water column and sediment nutrient measures demonstrated that Upper Newport Bay (UNB) is a highly enriched estuary. High nitrate (NO₃ ⁻) loads from the river entered the estuary at all sampling times with a rainy season (winter) maximum estimated at 2,419 mol h⁻¹. This resulted in water NO₃ ⁻ concentration in the estuary near the river mouth at least one order of magnitude above all other sampling locations during every seasons; maximum mean water NO₃ ⁻ concentration was 800 μM during springer 1997. Phosphorus (P)-loading was high year round (5.7–90.4 mol h⁻¹) with no seasonal pattern. Sediment nitrogen (N)-content showed a seasonal pattern with a spring maximum declining through fall. sediment and water nutrients, as well as percent cover of three dominant macroalgae, varied between the main channel and tidal creeks. During all seasons, water column NO₃ ⁻ concentrations were higher in the main channel than in tidal creeks while tidal creeks had higher levels of sediment total Kjeldhal nitrogen (TKN) and P. During each of the four sampling periods, percent cover ofEntermorpha intestinalis andCeramium spp. was higher in tidal creeks than in the main channel, while percent cover ofUlva expansa was always higher in the main channel. Decreases in sediment N in both creek and channel habitats were concurrent with increases in macroalgal cover, possibly reflecting use of stored sediment TKN by macroalgae. Our data suggest a shift in primary nutrient sources for macroalgae in UNB from riverine input during winter and spring to recycling from sediments duirng summer and fall.     

Hydrogeochemical processes in the groundwater environment of Heihe River Basin,northwest China

The Heihe River Basin is a typical arid inland river basin for examining stress on groundwater resources in northwest China. The basin is composed of large volumes of unconsolidated Quaternary sediments of widely differing grain size, and during the past half century, rapid socio-economic development has created an increased demand for groundwater resources. Understanding the hydrogeochemical processes of groundwater and water quality is important for sustainable development and effective management of groundwater resources in the Heihe River basin. To this end, a total of 30 representative groundwater samples were collected from different wells to monitor the water chemistry of various ions and its quality for irrigation. Chemical analysis shows that water presents a large spatial variability of chemical facies (SO₄ ²⁻–HCO₃⁻, SO₄ ²⁻–Cl⁻, and Cl⁻–SO₄ ²⁻) as groundwater flow from recharge area to discharge area. The ionic ratio indicates positive correlation between the flowing pairs of parameters: Cl⁻ and Na⁺(r = 0.95), SO₄ ²⁻ and Na⁺ (r = 0.84), HCO₃ ⁻ and Mg²⁺(r = 0.86), and SO₄ ²⁻ and Ca²⁺ (r = 0.91). Dissolution of minerals, such as halite, gypsum, dolomite, silicate, and Mirabilite (Na₂SO₄·10H₂O) in the sediments results in the Cl⁻, SO₄ ²⁻, HCO₃ ⁻, Na⁺, Ca²⁺ and Mg²⁺ content in the groundwater. Other reactions, such as evaporation, ion exchange, and deposition also influence the water composition. The suitability of the groundwater for irrigation was assessed based on the US Salinity Laboratory salinity classification and the Wilcox diagram. The results show that most of the groundwater samples are suitable for irrigation uses barring a few locations in the dessert region in the northern sub-basin.     

Diel biogeochemical cycling in a hyperventilating shallow estuarine environment

A diel biogeochemical study was performed to assess the influence that periods of elevated biological activity have on the biogeochemical cycling of macronutrients and redox-sensitive elements in a natural estuarine environment. High-resolution data (15 min sampling) illustrates periodic extreme variations in dissolved oxygen (DO) in the shallow waters of Azevedo Pond, Elkhom Slough, California. During periods of low tidal flushing, DO values can range from highly oxic (>560 μM O₂: >250% saturation) during sunny days to suboxic conditions (<5 μM) at night. Nutrient cycling and redox-sensitive trace element biogeochemistry were evaluated in response to the extreme daily DO fluctuations. A diel sampling study was conducted over a 26-h period, where O₂ concentrations ranged from 346 μM to sustained non-detectable levels in the night hours. In concert with the DO fluctuations, diel phosphate cycling was on the order of 4 μM in response to tidal flushing events and biological assimilation and regeneration. The IO₃ ⁻/I⁻ redox couple quickly responded to suboxic conditions in the water column by a marked increase in I⁻ concentrations and corresponding depletion of IO₃ ⁻. The extreme fluctuations of the p∈ in the water column resulted in diel dissolved Mn²⁺ variations of nearly 5 μM, with observed dissolved Mn removal rates on the order of 1 μM h⁻¹. The elevated biogeochemical cycling of oxygen, nitrogen, phosphorus, iodine, manganese, and iron found in this shallow estuarine environment suggest that tidal restrictions and anthropogenic nutrient enrichments can amplify diel variations and potentially hinder the functional and ecological stability of these systems. These data suggest that accurate chemical monitoring of the health of an estuarine ecosystem must account for the diel variability inherent in these highly productive environments.     

Surface and groundwater quality characterization of Deoria District,Ganga Plain,India

A water quality investigation was carried out in the Deoria district, Ganga plain, to assess the suitability of surface and groundwaters for domestic, agricultural, and industrial purposes. As much as 50 representative samples from river and groundwater were collected from various stations to monitor the water chemistry of various ions, comprising Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃ ⁻, SO₄ ²⁻, NO₃ ⁻, Cl⁻, F⁻, and trace metals, such as Fe, Cu, Mn, Zn, Cd, and Pb. The results showed that electrical conductance (EC), total dissolved solids (TDS), HCO₃ ⁻, Mg²⁺, Na⁺, and total hardness (TH) are above the maximum desirable limit, and apart from Fe and Mn all other trace metals are within the maximum permissible limit for drinking water. The calculated values for sodium absorption ratio (SAR), salinity, residual sodium carbonate (RSC), and permeability index (PI) indicate well to permissible use of water for irrigation. High values of Na%, RSC, and Mg-hazard (MH) at some stations restrict its use for agricultural purpose. Anthropogenic activities affect the spatial variation of water quality. Economic and social developments of the study area is closely associated with the characteristics of the hydrological network.     

Environmental geochemistry and quality assessment of mine water of Jharia coalfield,India

A long mining history and unscientific exploitation of Jharia coalfield caused many environmental problems including water resource depletion and contamination. A geochemical study of mine water in the Jharia coalfield has been undertaken to assess its quality and suitability for domestic, industrial and irrigation uses. For this purpose, 92 mine water samples collected from different mining areas of Jharia coalfield were analysed for pH, electrical conductivity (EC), major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺), anions (F⁻, Cl⁻, HCO₃ ⁻, SO₄ ²⁻, NO₃ ⁻), dissolved silica (H₄SiO₄) and trace metals. The pH of the analysed mine water samples varied from 6.2 to 8.6, indicating mildly acidic to alkaline nature. Concentration of TDS varied from 437 to 1,593 mg L⁻¹ and spatial differences in TDS values reflect the variation in lithology, surface activities and hydrological regime prevailing in the region. SO₄ ²⁻ and HCO₃ ⁻ are dominant in the anion and Mg²⁺ and Ca²⁺ in the cation chemistry of mine water. High concentrations of SO₄ ²⁻ in the mine water of the area are attributed to the oxidative weathering of pyrites. Ca–Mg–SO₄ and Ca–Mg–HCO₃ are the dominant hydrochemical facies. The drinking water quality assessment indicates that number of mine water samples have high TDS, total hardness and SO₄ ²⁻ concentrations and needs treatment before its utilization. Concentrations of some trace metals (Fe, Mn, Ni, Pb) were also found to be above the desirable levels recommended for drinking water. The mine water is good to permissible quality and suitable for irrigation in most cases. However, higher salinity, residual sodium carbonate and Mg-ratio restrict its suitability for irrigation at some sites.     

Microelectrode Study of Oxygen Uptake and Organic Matter Decomposition in The Sediments of Xiamen Western Bay

Sediment cores were sampled from Xiamen Western Bay at five sites during the summer and winter of 2006 and Hg–Au microelectrodes were used to make on board measurements of the concentration gradients of dissolved oxygen, Mn²⁺, and Fe²⁺ within the sediments. The O₂ concentrations decreased sharply from about 200 μmol L⁻¹ in the bottom seawater to zero within a depth of a few millimeters into the sediment. Dissolved Mn²⁺ was detected below the oxic zones with peak concentrations up to 600 μmol L⁻¹, whereas dissolved Fe²⁺ had peak concentrations up to 1,000 μmol L⁻¹ in deeper layers. The elemental contents of organic carbon and nitrogen within the sediments were analyzed and their C/N ratios were in the range of 9.0 to 10.1, indicative of heavy terrestrial origin. Sediments from two sites near municipal wastewater discharge outlets had higher organic contents than those from the other sites. These high organic contents corresponded to shallow O₂ penetration depths, high dissolved Mn²⁺ and Fe²⁺ concentrations, and negative redox potentials within the sediments. This indicated that the high organic matter content had promoted microbial respiration within the sediments. Overall, the organic content did not show any appreciable decrease with increasing sediment depths, so a quadratic polynomial function was used to fit the curve of O₂ profiles within the sediments. Based on the O₂ profiles, O₂ fluxes across the seawater and sediment interface were estimated to be in the range 6.07 to 14.9 mmol m⁻² day⁻¹, and organic carbon consumption rates within the surface sediments were estimated to be in the range 3.3 to 20.8 mgC cm⁻³ a⁻¹. The case demonstrated that biogeochemistry within the sediments of the bay was very sensitive to human activities such as sewage discharge.     

Groundwater chemistry and occurrence of arsenic in the Meghna floodplain aquifer,southeastern Bangladesh

Dissolved major ions and important heavy metals including total arsenic and iron were measured in groundwater from shallow (25–33 m) and deep (191–318 m) tube-wells in southeastern Bangladesh. These analyses are intended to help describe geochemical processes active in the aquifers and the source and release mechanism of arsenic in sediments for the Meghna Floodplain aquifer. The elevated Cl⁻ and higher proportions of Na⁺ relative to Ca²⁺, Mg²⁺, and K⁺ in groundwater suggest the influence by a source of Na⁺ and Cl⁻. Use of chemical fertilizers may cause higher concentrations of NH₄⁺ and PO₄³⁻ in shallow well samples. In general, most ions are positively correlated with Cl⁻, with Na⁺ showing an especially strong correlation with Cl⁻, indicating that these ions are derived from the same source of saline waters. The relationship between Cl⁻/HCO₃⁻ ratios and Cl⁻ also shows mixing of fresh groundwater and seawater. Concentrations of dissolved HCO₃⁻ reflect the degree of water–rock interaction in groundwater systems and integrated microbial degradation of organic matter. Mn and Fe-oxyhydroxides are prominent in the clayey subsurface sediment and well known to be strong adsorbents of heavy metals including arsenic. All five shallow well samples had high arsenic concentration that exceeded WHO recommended limit for drinking water. Very low concentrations of SO₄²⁻ and NO₃⁻ and high concentrations of dissolved Fe and PO₄³⁻ and NH₄⁺ ions support the reducing condition of subsurface aquifer. Arsenic concentrations demonstrate negative co-relation with the concentrations of SO₄²⁻ and NO₃⁻ but correlate weakly with Mo, Fe concentrations and positively with those of P, PO₄³⁻ and NH₄⁺ ions.     

Sulfide Inhibition of Nitrate Removal in Coastal Sediments

Microbial nitrate (NO₃⁻) removal via denitrification (DNF) at high sulfide (H₂S) concentrations was compared in sediment from a coastal freshwater pond in a developed area that receives salt-water influx during storm events, and a saline pond proximal to an undeveloped estuary. Sediments were incubated with added SO₄²⁻ (1,000 μg per gram dry weight basis (gdw)) to determine whether acid volatile sulfides (AVS) were formed. DNF in the sediments was measured with NO₃–N (300 μg gdw⁻¹) alone, and with NO₃–N and H₂S (1,000 μg S²⁻ gdw⁻¹). SO₄²⁻ addition to the freshwater sediments resulted in AVS formation (970 ± 307 μg S gdw⁻¹) similar to the wetland with no added SO₄²⁻ (986 ± 156 μg S gdw⁻¹). DNF rates measured with no added H₂S were greater in the freshwater than the wetland site (10.6 ± 0.6 vs. 6.4 ± 0.1 μg N₂O–N gdw⁻¹ h⁻¹, respectively). High H₂S concentrations retained NH₄–N in the undeveloped wetland and retained NO₃–N in the developed freshwater site, suggesting that potential salt-water influx may reduce the ability of the freshwater sediments to remove NO₃–N.     

Concentration levels of some inorganic contaminants in streams and sediments in areas of pyrometallurgical and hydrometallurgical activities at the Obuasi gold mine,Ghana

This study assessed the levels of selected inorganic contaminants in streams and stream sediments in the effluent areas relating to the pyrometallurgical and hydrometallurgical treatment of gold ores in the Obuasi gold mine, Ghana. Water and stream sediment samples were taken from specific locations during the consecutive rainy and dry seasons, and concentrations of phosphate (PO₄ ³⁻), nitrate (NO₃ ⁻), chloride (Cl⁻), sulphate (SO₄ ²⁻), sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), arsenic (As), copper (Cu), iron (Fe), zinc (Zn) and lead (Pb), were determined. Alkalinity, pH, temperature and specific electrical conductivity were also measured. In the water samples, the average pH range for both the seasons is 6.9–7.4, most anions and metals have relatively higher concentrations in the wet season than in the dry season at both the metallurgical sites. Trace metals concentrations were comparatively low (<0.01–5.00 mg/l), higher in the dry season at the pyrometallurgical sites. Irrespective of seasons, SO₄ ²⁻ (0.80–949.50 mg/l) and PO₄ ³⁻ (<0.01–6.30 mg/l) were pronounced at the pyrometallurgical sites, while NO₃ ⁻ (0.01–98.45 mg/l) and Cl⁻ (1.88-49.05 mg/l) were higher at the hydrometallurgical sites. In water samples, Ca²⁺ and SO₄ ²⁺ were the dominant cation and anion, respectively. In the stream sediments, except pH, NO₃ ⁻, Cl⁻, Na⁺ and Mg²⁺, all other parameter values were relatively higher at the hydrometallurgical areas. The average concentrations of Ca²⁺, Mg²⁺, As and Fe are remarkably high at both metallurgical sites (3,217–46,026 mg/kg). Overall, the level of parameters in the water samples are pronounced at pyrometallurgical sites, whereas the levels in sediments are higher at the hydrometallurgical sites.     

Benthic biogeochemistry beneath the Mississippi River plume

Biogeochemical processes occurring near the sediment-water interface of shallow (≈20 m) water sediments lying beneath the Mississippi River plume on the Louisiana shelf were studied using benthic chambers and sediment cores. Three sites were chosen with distinctly different characteristics. One was overlain by oxic water where aerobic respiration dominated organic matter remineralization. The second site was overlain by oxic water but organic matter remineralization was dominated by sulfate reduction. The third site was overlain by hypoxic water and aerobic remineralization was of minor significance. Major differences were observed in the fluxes of CO₂(17–56 mmol m⁻² d⁻¹), O₂(2–56 mmol m⁻² d⁻¹) and nutrients (e.g., NH₄ ⁺, 2.6–4.2 mmol m⁻² d⁻¹) across the sediment-water interface, and the relative importance of different electron acceptors, even though the sites were in close proximity and at nearly the same water depth. Large variations in the efficiency of organic-C burial (3%–51%) were also calculated based on a simplified model of the relationships between the fraction of organic matter remineralized by sulfate reduction and the fraction of sulfide produced that is buried as pyrite. These observations demonstrate the high degree of spatial heterogeneity of benthic biogeochemistry in this important near-deltaic environment.     

Benthic nutrient fluxes in the intertidal flat within the Changjiang （Yangtze River） Estuary

In an annual cycle from March 2005 to February 2006, benthic nutrient fluxes were measured monthly in the Dongtan intertidal flat within the Changjiang （Yangtze River） Estuary. Except for NH4^＋, there always showed high fluxes from overlying water into sediment for other four nutrients. Sediments in the high and middle marshes, covered with halophyte and consisting of macrofauna, demonstrated more capabilities of assimilating nutrients from overlying water than the low marsh. Sampling seasons and nutrient concentrations in the overlying water could both exert significant effects on these fluxes. Additionally, according to the model provided by previous study, denitrification rates, that utilizing NO3- transported from overlying water （Dw） in Dongtan sediments, were estimated to be from -16 to 193 μmol·h^-1·m^-2 with an average value of 63 μmol·h^-1·m^-2 （n=18）. These estimated values are still underestimates of the in-situ rates owing to the lack of consideration of DN, i.e., denitrification supported by the local NO3^- production via nitrification.     

Benthic metabolism and nutrient cycling along an estuarine salinity gradient

Benthic metabolism and nutrient exchange across the sediment-water interface were examined over an annual cycle at four sites along a freshwater to marine transect in the Parker River-Plum Island Sound estuary in northeastern Massachusetts, U.S. Sediment organic carbon content was highest at the freshwater site (10.3%) and decreased along the salinity gradient to 0.2% in the sandy sediments at the marine end of the estuary. C:N ratios were highest in the mid estuary (23:1) and lowest near the sea (11:1). Chlorophyll a in the surface sediments was high along the entire length of the estuary (39–57 mg chlorophyll a m⁻²) but especially so in the sandy marine sediments (172 mg chlorophyll a m⁻²). Chlorophyll a to phaeophytin ratios suggested most chlorophyll is detrital, except at the sandy marine site. Porewater sulfide values varied seasonally and between sites, reflecting both changes in sulfate availability as overlying water salinity changed and sediment metabolism. Patterns of sediment redox potential followed those of sulfide. Porewater profiles of inorganic N and P reflected strong seasonal patterns in remineralization, accumulation, and release. Highest porewater NH₄ ⁺ values were found in upper and mid estuarine sediments, occasionally exceeding 1 mM N. Porewater nitrate was frequently absent, except in the sandy marine sediments where concentrations of 8 μM were often observed. Annual average respiration was lowest at the marine site (13 mmol O₂ m⁻² d⁻¹ and 21 mmol TCO₂ m⁻² d⁻¹) and highest in the mid estuary (130 mmol O₂ m⁻² d⁻¹ and 170 mmol TCO₂ m⁻² d⁻¹) where clam densities were also high. N₂O and CH₄ fluxes were low at all stations throughout the year: Over the course, of a year, sediments varied from being sources to sinks of dissolved organic C and N, with the overall spatial pattern related closely to sediment organic content. There was little correlation between PO₄ ³⁻ flux and metabolism, which we attribute to geochemical processes. At the two sites having the lowest salinities, PO₄ ³⁻ flux was directed into the sediments. On average, between 22% and 32% of total system metabolism was attributable to the benthos. The mid estuary site was an exception, as benthic metabolism accounted for 95% of the total, which is attributable to high densities of filter-feeding clams. Benthic remineralization supplied from less than 1% to over 190% of the N requirements and 0% to 21% of the P requirements of primary producers in this system. Estimates of denitrification calculated from stoichiometry of C and N fluxes ranged from 0% for the upper and mid estuary site to 35% for the freshwater site to 100% of sediment organic N remineralization at the marine site. We hypothesize that low values in the upper and mid estuary are attributable to enhanced NH₄ ⁺ fluxes during summer due to desorption of exchangeable ammonium from rising porewater salinity. NH₄ ⁺ desorption during summer may be a mechanism that maintains high rates of pelagic primary production at a time of low inorganic N inputs from the watershed.