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1.
We report measurements of pH, total dissolved inorganic carbon (DIC), total or titration alkalinity (TAlk), Ca2+, Mg2+, sulfate, and sulfide data at the seawater-freshwater interface in a shallow groundwater aquifer in North Inlet, South Carolina. These measurements and a diagenetic modeling analysis indicate that the groundwaters at North Inlet are mixtures of seawater and freshwater end-members and are seriously modified by carbon dioxide inputs from organic carbon degradation via SO42− reduction across the entire salinity range and fermentation and CaCO3 dissolution in the low-salinity region. DIC and TAlk are several times higher than the theoretical dilution line, whereas Ca2+ is slightly higher and SO42− is somewhat lower than the dilution line. Partial pressure of CO2 in the groundwater is extremely high (0.05 to 0.12 atm). These deviations are consistent with theoretical predictions from known diagenetic reactions. Estimated groundwater DIC fluxes to the South Atlantic Bight from either the surficial aquifer (via salt marshes) or the Upper Floridan Aquifer (direct input) are significant when compared to riverine flux in this area.  相似文献   

2.
Monthly sampling for 1 year at the Konza Prairie LTER (Long-Term Ecological Research) Site in northeastern Kansas shows a connection between the annual cycles of CO2 in soil air and shallow groundwater DIC (dissolved inorganic C). Soil air CO2 reached 6–7% in July to mid-August, when moisture was not limiting to soil respiration. Following the annual maximum there was a sequential decrease in CO2 in three soil horizons to less than 0.5% because of moisture deficiency in the late summer and temperature decline in the fall and winter. Groundwater pCO2 reached its maximum of 5% in October; the lag-time 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 and DIC production when CO2 was high.  相似文献   

3.
Isotopic and chemical composition of groundwater from wells and springs, and surface water from the basalt-dominated Axum area (northern Ethiopia) provides evidence for the origin of water and dissolved species. Shallow (depth < 40 m) and deep groundwater are distinguished by both chemical and isotopic composition. Deep groundwater is significantly enriched in dissolved inorganic carbon up to 40 mmol l−1 and in concentrations of Ca2+, Mg2+, Na+ and Si(OH)4 compared to the shallow type.The δ2H and δ18O values of all solutions clearly indicate meteoric origin. Shifts from the local meteoric water line are attributed to evaporation of surface and spring water, and to strong water–rock interaction. The δ13CDIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO2 from local soil horizons, whereas δ13CDIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO2, δ13CDIC = +1‰ of the deep groundwater with highest PCO2 = 10−0.9 atm yields δ13CCO2(gas) ≈ −5‰, which is close to the stable carbon isotopic composition of magmatic CO2. Accordingly, stable carbon isotope ratios within the above range are referred to individual proportions of CO2 from soil and magmatic origin. The uptake of magmatic CO2 results in elevated cations and Si(OH)4 concentrations. Weathering of local basalts is documented by 87Sr/86Sr ratios of the groundwater from 0.7038 to 0.7059. Highest values indicate Sr release from the basement rocks. Besides weathering of silicates, neoformation of solids has to be considered, which results in the formation of, e.g., kaolinite and montmorillonite. In several solutions supersaturation with respect to calcite is reached by outgassing of CO2 from the solution leading to secondary calcite formation.  相似文献   

4.
This study investigated the potential for the uranium mineral carnotite (K2(UO2)2(VO4)2·3H2O) to precipitate from evaporating groundwater in the Texas Panhandle region of the United States. The evolution of groundwater chemistry during evaporation was modeled with the USGS geochemical code PHREEQC using water-quality data from 100 groundwater wells downloaded from the USGS National Water Information System (NWIS) database. While most modeled groundwater compositions precipitated calcite upon evaporation, not all groundwater became saturated with respect to carnotite with the system open to CO2. Thus, the formation of calcite is not a necessary condition for carnotite to form. Rather, the determining factor in achieving carnotite saturation was the evolution of groundwater chemistry during evaporation following calcite precipitation. Modeling in this study showed that if the initial major-ion groundwater composition was dominated by calcium-magnesium-sulfate (>70 precent Ca + Mg and >50 percent SO4 + Cl) or calcium-magnesium-bicarbonate (>70 percent Ca + Mg and <70 percent HCO3 + CO3) and following the precipitation of calcite, the concentration of calcium was greater than the carbonate alkalinity (2mCa+2 > mHCO3− + 2mCO3−2) carnotite saturation was achieved. If, however, the initial major-ion groundwater composition is sodium-bicarbonate (varying amounts of Na, 40–100 percent Na), calcium-sodium-sulfate, or calcium-magnesium-bicarbonate composition (>70 percent HCO3 + CO3) and following the precipitation of calcite, the concentration of calcium was less than the carbonate alkalinity (2mCa+2 < mHCO3- + 2mCO3−2) carnotite saturation was not achieved. In systems open to CO2, carnotite saturation occurred in most samples in evaporation amounts ranging from 95 percent to 99 percent with the partial pressure of CO2 ranging from 10−3.5 to 10−2.5 atm. Carnotite saturation occurred in a few samples in evaporation amounts ranging from 98 percent to 99 percent with the partial pressure of CO2 equal to 10−2.0 atm. Carnotite saturation did not occur in any groundwater with the system closed to CO2.  相似文献   

5.
Groundwater may be highly enriched in dissolved carbon species, but its role as a source of carbon to coastal waters is still poorly constrained. Exports of deep and shallow groundwater-derived dissolved carbon species from a small subtropical estuary (Korogoro Creek, Australia, latitude ?31.0478°, longitude 153.0649°) were quantified using a radium isotope mass balance model (233Ra and 224Ra, natural groundwater tracers) under two hydrological conditions. In addition, air-water exchange of carbon dioxide and methane in the estuary was estimated. The highest carbon inputs to the estuary were from deep fresh groundwater in the wet season. Most of the dissolved carbon delivered by groundwater and exported from the estuary to the coastal ocean was in the form of dissolved inorganic carbon (DIC; 687 mmol m?2 estuary day?1; 20 mmol m?2 catchment day?1, respectively), with a large export of alkalinity (23 mmol m?2 catchment day?1). Average water to air flux of CO2 (869 mmol m?2 day?1) and CH4 (26 mmol m?2 day?1) were 5- and 43-fold higher, respectively, than the average global evasion in estuaries due to the large input of CO2- and CH4-enriched groundwater. The groundwater discharge contribution to carbon exports from the estuary for DIC, dissolved organic carbon (DOC), alkalinity, CO2, and CH4 was 22, 41, 3, 75, and 100 %, respectively. The results show that CO2 and CH4 evasion rates from small subtropical estuaries surrounded by wetlands can be extremely high and that groundwater discharge had a major role in carbon export and evasion from the estuary and therefore should be accounted for in coastal carbon budgets.  相似文献   

6.
Lakes worldwide are commonly oversaturated with CO2, however the source of this CO2 oversaturation is not well understood. To examine the magnitude of the C flux to the atmosphere and determine if an excess of respiration (R) over gross primary production (GPP) is sufficient to account for this C flux, metabolic parameters and stable isotopes of dissolved O2 and C were measured in 23 Québec lakes. All of the lakes sampled were oversaturated with CO2 over the sampling period, on average 221 ± 25%. However, little evidence was found to conclude that this CO2 oversaturation was the result of an excess of pelagic R over GPP. In lakes Croche and à l’Ours, where CO2 flux, R and GPP were measured weekly, the annual difference between pelagic GPP and R, or net primary production (NPP), was not sufficient to account for the size of the CO2 flux to the atmosphere. In Lac Croche average annual NPP was 14.4 mg C m−2 d−1 while the average annual flux of CO2 to the atmosphere was 34 mg C m−2 d−1. In Lac à l’Ours average annual NPP was −9.1 mg C m−2 d−1 while the average annual flux of CO2 to the atmosphere was 55 mg C m−2 d−1. In all of the lakes sampled, O2 saturation averaged 104.0 ± 1.7% during the ice-free season and the isotopic composition of dissolved O2 (δ18ODO) was 22.9 ± 0.3‰, lower than atmospheric values and indicative of net autotrophy. Carbon evasion was not a function of R, nor did the isotopic signature of dissolved CO2 in the lakes present evidence of excess R over GPP. External inputs of C must therefore subsidize the lake to explain the continued CO2 oversaturation. The isotopic composition of dissolved inorganic C (δ13CDIC) indicates that the CO2 oversaturation cannot be attributed to in situ aerobic respiration. δ13CDIC reveals a source of excess C enriched in 13C, which may be accounted for by anaerobic sediment respiration or groundwater inputs followed by kinetic isotope fractionation during degassing under open system conditions.  相似文献   

7.
Generation of dust particles from the Owens Lake playa creates a severe air pollution hazard in the western United States. Much of the dust produced from the dry lakebed is derived from salts formed by evaporation of saline groundwater that often contains high concentrations of dissolved arsenic (As). The objectives of this research were to study the spatial distribution of dissolved arsenic in the shallow groundwater, and to examine factors affecting arsenic solubility and speciation. Evapoconcentration, redox potential, pH, and mineral solubility were examined as factors regulating arsenic biogeochemistry. Dissolved arsenic concentrations ranged from 0.1 to 96 mg L−1 and showed a general increase from the shoreline to the center of the lakebed. Arsenic concentrations were strongly correlated to electrical conductivity (EC) and δD suggesting that evapoconcentration is an important process regulating total As concentrations. Arsenite [As(III)] was the dominant form of inorganic arsenic at Eh values less than about −170 mV while arsenate [As(V)] was predominant at higher Eh values. Organic arsenic was negligible (<0.21%) in all shallow groundwater samples. Dissolved arsenic concentrations do not appear to be strongly regulated by solid-phase reactions. Solid-phase arsenic concentrations generally ranged between 4.0 and 42.6 mg kg−1 and a maximum concentration range (20 to 40 mg kg−1) was reached as solution concentration increased up to 80 mg L−1, indicating minimal sorption and/or precipitation of arsenic. Chemical equilibrium modeling indicated that orpiment (As2S3) was the only solid phase with a positive saturation index (indicating over-saturation), but only at high arsenic and sulfide concentrations. The findings of this research are important for assessing the potential environmental impacts of elevated arsenic concentrations on dust mitigation efforts taking place at Owens Dry Lake.  相似文献   

8.
The spatial and temporal variations of the flux of CO2 were determined during 2007 in the Recife estuarine system (RES), a tropical estuary that receives anthropogenic loads from one of the most populated and industrialized areas of the Brazilian coast. The RES acts as a source of nutrients (N and P) for coastal waters. The calculated CO2 fluxes indicate that the upstream inputs of CO2 from the rivers are largely responsible for the net annual CO2 emission to the atmosphere of +30 to +48 mmol m?2 day?1, depending on the CO2 exchange calculation used, which mainly occurs during the late austral winter and early summer. The observed inverse relationship between the CO2 flux and the net ecosystem production (NEP) indicates the high heterotrophy of the system (except for the months of November and December). The NEP varies between ?33 mmol m?2 day?1 in summer and ?246 mmol m?2 day?1 in winter. The pCO2 values were permanently high during the study period (average ~4,700 μatm) showing a gradient between the inner (12,900 μatm) and lower (389 μatm) sections on a path of approximately 30 km. This reflects a state of permanent pollution in the basin due to the upstream loading of untreated domestic effluents (N/P?=?1,367:6 μmol kg?1 and pH?=?6.9 in the inner section), resulting in the continuous mineralization of organic material by heterotrophic organisms and thereby increasing the dissolved CO2 in estuarine waters.  相似文献   

9.
The partial pressure of CO2 (pCO2) and concentration of dissolved CH4 in surface waters have been studied in three coastal systems connected to Cadiz Bay (southwestern coast of Spain) over different time scales. The concentration of CH4 varied from 1 to 4200 nmol kg?1 (192.1 ± 463.6 nmol kg?1) and the saturation percent from 19 to 159,577% (6645 ± 16,921%), and pCO2 from 315 to 3240 μatm (841.9 ± 466.3 μatm), with saturation percent values varying between 72 and 981% (220 ± 133%). The seasonal variation of pCO2 mainly depends on the temperature. On the contrary, the annual distribution of dissolved CH4 is associated with the precipitation regime. In addition, pCO2 and dissolved CH4 showed spatial variation. pCO2 increased toward the inner part of the systems, with the proximity to the discharge points from human activities. Dissolved CH4 is influenced by both anthropogenic inputs and natural processes such as benthic supply and exchange with the adjacent salt marshes. pCO2 and dissolved CH4 also varied with the tides: The highest concentrations were measured during the ebb, which suggests that the systems export CO2 and CH4 to the Bay and adjacent Atlantic Ocean.  相似文献   

10.
Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50-150 mg dm−3) may promote the dissolution of silicate phases through complexation of Al3+ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4-99.2 mmol CO2 m−2 year−1) are lower than the marine weathering rates calculated from the solid phase data (198-245 mmol CO2 m−2 year−1) suggesting a decrease in marine weathering over time. The production of CO2 through reverse weathering in surface sediments (4.22-15.0 mmol CO2 m−2 year−1) is about one order of magnitude smaller than the weathering-induced CO2 consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO2 consumption through marine silicate weathering estimated here as 5-20 Tmol CO2 year−1 is as high as the global rate of continental silicate weathering.  相似文献   

11.
Groundwaters, river and lake waters have been sampled from the semi-arid Siberian Republic of Khakassia. Despite the relatively sparse data set, from a diversity of hydrological environments, clear salinity-related trends emerge that indicate the main hydrochemical evolutionary processes active in the region. Furthermore, the major ion chemistry of the evolution of groundwater baseflow, via rivers, to terminal saline lake water, can be adequately and simply modelled (using PHREEQCI) by invoking: (i) degassing of CO2 from groundwater as it emerges as baseflow in rivers (rise in pH); (ii) progressive evapoconcentration of waters (parallel accumulation of Cl−, Na+, SO42−, and increase in pH due to common ion effect); and (iii) precipitation of calcite (depletion of Ca from waters, reduced rate of accumulation of alkalinity). Dolomite precipitation is ineffective at constraining Mg accumulation, due to kinetic factors. Silica saturation appears to control dissolved Si in low salinity waters and groundwaters, while sepiolite saturation and precipitation depletes Si from the more saline surface waters. Gypsum and sodium sulphate saturation are only approached in the most saline environments. Halite remains unsaturated in all waters. Sulphate reduction processes are important in the lower part of lakes.  相似文献   

12.
Stable isotopes (H, O, C) were determined for ground and surface waters collected from two relatively undisturbed massive sulfide deposits (Halfmile Lake and Restigouche) in the Bathurst Mining Camp (BMC), New Brunswick, Canada. Additional waters from active and inactive mines in the BMC were also collected. Oxygen and hydrogen isotopes of surface and shallow groundwaters from both the Halfmile Lake and Restigouche deposits are remarkably uniform (− 13 to − 14‰ and − 85 to − 95‰ for δ18OVSMOW and δ2HVSMOW, respectively). These values are lighter than predicted for northern New Brunswick and, combined with elevated deuterium excess values, suggest that recharge waters are dominated by winter precipitation, recharged during spring melting. Deeper groundwaters from the Restigouche deposit, and from active and inactive mines have heavier δ18OVSMOW ratios (up to − 10.8‰) than shallow groundwaters suggesting recharge under warmer climate or mixing with Shield-type brines. Some of the co-variation in Cl concentrations and δ18OVSMOW ratios can be explained by mixing between saline and shallow recharge water end-members. Carbon isotopic compositions of dissolved inorganic carbon (DIC) are variable, ranging from − 15 to − 5‰ δ13CVPDB for most ground and surface waters. Much of the variation in the carbon isotopes is consistent with closed system groundwater evolution involving soil zone CO2 and fracture zone carbonate minerals (calcite, dolomite and siderite; average = − 6.5‰ δ13CVPDB). The DIC of saline Restigouche deposit groundwater is isotopically heavy (∼+ 12‰ δ13CVPDB), indicating carbon isotopic fractionation from methanogenesis via CO2 reduction, consistent with the lack of dissolved sulfate in these waters and the observation of CH4-degassing during sampling.  相似文献   

13.
Groundwater qualities of coastal aquifers in the Chennam-Pallippuram Panchayath of Alappuzha district, Kerala have been extensively monitored in summer from January to May, 2007 to assess its suitability in relation to domestic and agricultural uses. The water samples (n=36) were analyzed for various physico-chemical attributes like temperature, pH, electrical conductivity (EC), dissolved oxygen (DO), Na, K, Ca, Mg, alkalinity, hardness, silica, chloride, salinity, total dissolved solids (TDS) and sulphate (SO4 2?). Values of most of these parameters fall within permissible limits. Major ionic relationships indicate that weathering reactions have insignificant role in the hydrochemical processes of the shallow groundwater system. Hydrogeochemical processes controlling the water chemistry are precipitation rather than rock-water interaction. Various determinants such as Sodium Absorption Ratio (SAR), Percent Sodium (Na %), Residual Sodium Carbonate (RSC), and Kelley’s Ratio revealed that most of the samples are suitable for irrigation.  相似文献   

14.
《Applied Geochemistry》2001,16(7-8):745-758
The physical–chemical characteristics of the groundwater in the Baza–Caniles detrital aquifer system indicate that a wide diversity of hydrochemical conditions exists in this semiarid region, defining geochemical zones with distinct groundwater types. The least mineralized water is found closest to the main recharge zones, and the salinity of the water increases significantly with depth towards the center of the basin. Geochemical reaction models have been constructed using water chemistry data along flow paths that characterize the different sectors of the aquifer system, namely: Quaternary aquifer, unconfined sector and shallow and deep confined sectors of the Mio–Pliocene aquifer. Geochemical mass–balance calculations indicate that the dominant groundwater reaction throughout the detrital system is dedolomitisation (dolomite dissolution and calcite precipitation driven by gypsum dissolution); this process is highly developed in the central part of the basin due to the abundance of evaporites. Apart from this process, there are others which influence the geochemical zoning of the system. In the Quaternary aquifer, which behaves as a system open to gases and which receives inputs of CO2 gas derived from the intensive farming in the area, the interaction of the CO2 with the carbonate matrix of the aquifer produces an increase in the alkalinity of the water. In the shallow confined sector of the Mio–Pliocene aquifer, the process of dedolomitisation evolves in a system closed to CO2 gas. Ca2+/Na+ cation exchange and halite dissolution processes are locally important, which gives rise to a relatively saline water. Finally, in the deep confined sector, a strongly reducing environment exists, in which the presence of H2S and NH+4 in the highly mineralized groundwater can be detected. In this geochemical zone, the groundwater system is considered to be closed to CO2 gas proceeding from external sources, but open to CO2 from oxidation of organic matter. The geochemical modeling indicates that the chemical characteristics of this saline water are mainly due to SO4 dissolution, dedolomitisation and SO4 reduction, coupled with microbial degradation of lignite.  相似文献   

15.
Water resources are a key factor, particularly for the planning of the sustainable regional development of agriculture, as well as for socio-economic development in general. A hydrochemical investigation was conducted in the Friuli Venezia Giulia aquifer systems to identify groundwater evolution, recharge and extent of pollution. Temperature, pH, electric conductivity, total dissolved solids, alkalinity, total hardness, SAR, Ca2+, Na+, K+, Mg2+, Cl−, SO4 2−, NO3 −, HCO3 − , water quality and type, saturation indexes and the environmental stable isotope δ18O were determined in 149 sampling stations. The pattern of geochemical and oxygen stable isotope variations suggests that the sub-surface groundwater (from phreatic and shallow confined aquifers) is being recharged by modern precipitations and local river infiltrations. Four hydrogeological provinces have been recognised and mapped in the Friuli Venezia Giulia Plain having similar geochemical signatures. These provinces have different degrees of vulnerability to contamination. The deep confined groundwater samples are significantly less impacted by surface activities; and it appears that these important water resources have very low recharge rates and would, therefore, be severely impacted by overabstraction.  相似文献   

16.
Continental carbonates of Quaternary age in southern Italy commonly exhibit the facies of calcareous tufa, often reported as related to shallow aquifers fed by meteoric waters and to organic processes. A close spatial relationship exists between the mappable tufa deposits and major Quaternary extensional faults. With respect to the Ca‐Mg‐HCO3 composition of limestone aquifers’ springs, tufa‐depositing springs exhibit higher salinity and alkalinity, are slightly warmer, have lower pH and are enriched in SO4 and CO2. Their δ13C values are systematically positive and compatible with a deep‐seated carbon source. A clear input of soil‐derived organic carbon is indicated only for small, non‐mappable tufas deposited by perched springs. The dataset indicates that the large tufa deposits owe their origin to a supplementary source of CO2 advected by degassing through active faults, as a necessary prerequisite for inducing a rise of total dissolved salts and alkalinity. Meteoric waters that have come from a shallow aquifer are able to precipitate only limited amount of carbonates.  相似文献   

17.
We made direct measurements of the partial pressure of CO2 (PCO 2) in the tidal-freshwater portion of the Hudson River Estuary over a 3.5-yr period. At all times the Hudson was supersaturated in CO2 with respect to the atmosphere. PCO 2 in surface water averaged 1125±403 (SD) μatm while the atmosphere averaged 416±68 μatm. Weekly samples at a single, mid-river station showed a pronounced and reproducible seasonal cycle with highest values (～2000 μatm) in mid-to-late summer, and lowest values (～500 μatm) generally in late winter. Samples taken along the length of the 190-km section of river showed a general decline in CO2 from north to south. This decline was most pronounced in summer and very slight in spring. Diel and vertical variation were small relative to the standing stock of CO2. Over six diel cycles, all taken during the algal growing season, the mean range was 300±114 μatm. CO2 tended to increase slightly with depth, but the gradient was small, about 0.5 μmol m?1, or an increase of 190 μatm from top to within 1 m of the bottom. For a large subset of the samples (n=452) we also calculated CO2 from measurements of pH and total DIC. Calculated and measured values of CO2 were in reasonably good agreement and a regression of calculated versus measured values had a slope of 0.85±0.04 and an r2 of 0.60. Combining our measurements with recent experimental studies of gas exchange in the Hudson, we estimate that the Hudson releases CO2 at a rate of 70–162 g C m?2 yr?1 from the river to the atmosphere.  相似文献   

18.
《Applied Geochemistry》1993,8(3):207-221
The gases dissolved in Lake Nyos, Cameroon, were quantified recently (December 1989 and September 1990) by two independent techniques: in-situ measurements using a newly designed probe and laboratory analyses of samples collected in pre-evacuated stainless steel cylinders. The highest concentrations of CO2 and CH4 were 0.30 mol/kg and 1.7 mmol/kg, respectively, measured in cylinders collected 1 m above lake bottom. Probe measurements of in-situ gas pressure at three different stations showed that horizontal variations in total dissolved gas were negligible. Total dissolved-gas pressure near the lake bottom is 1.06 MPa (10.5 atm), 50% as high as the hydrostatic pressure of 2.1 MPa (21 atm). Comparing the CO2 profile constructed from the 1990 data to one obtained in May 1987 shows that CO2 concentrations have increased at depths to below 150 m. Based on these profiles, the average rate of CO2 input to bottom waters was 2.6 × 108 mol/a. Increased deep-water temperatures require an average heat flow of 0.32 MW into the hypolimnion over the same time period. The transport rates of CO2, heat, and major ions into the hypolimnion suggest that a low-temperature reservoir of free CO2 exists a short distance below lake bottom and that convective cycling of lake water through the sediments is involved in transporting the CO2 into the lake from the underlying diatreme. Increased CH4 concentrations at all depths below the oxycline and a high14C content (41% modern) in the CH4 4 m above lake bottom show that much of the CH4 is biologically produced within the lake. The CH4 production rate may vary with time, but if the CO2 recharge rate remains constant, CO2 saturation of the entire hypolimnion below 50 m depth would require ∼140a, given present-day concentrations.  相似文献   

19.
The storage of low level radioactive waste in trenches overlying an unconfined groundwater flow system in sands has generated a contaminant plume (with chemical characteristics of dilute sanitary landfill leachate) containing 14C both as dissolved inorganic and organic C. In the groundwater, dissolved organic compounds account, on average, for 22% of the total C and 10% of the 14C. Approximately 300 m from the waste management site, the groundwater discharges to the surface in a wetland containing up to 3 m of peat and an extensive tree cover. Drainage from the wetland passes through a gauged stream. Radiocarbon input to the groundwater discharge area in 1991 was determined to be between 3.3 and 4.2 GBq, based on data from a line of sampling wells along the groundwater input boundary of the wetland, with control provided by water and tritium balance data. During the 1991 study year, only 1.5–2% of both the inorganic and organic 14C inputs left the wetland in surface water drainage. Vegetation growth in the wetland during the study year contained 8–10% of the released radiocarbon. If the rate of 14C accumulation in the peat has been constant, 7–9% of the annual radiocarbon input has been retained in the organic soil. Much of this soil accumulation can be attributed to litter from standing vegetation, making distribution coefficients an inappropriate model for 14C partitioning between groundwater and soil. The plant/soil 14C concentration ratio was 24 to 33, but application of a concentration ratio to describe the transfer of radiocarbon to plants is also believed to be inappropriate. This study indicates that over 80% of the groundwater radiocarbon is rapidly lost to the atmosphere when the groundwater comes to surface, and we infer that most of the 14C accumulation in vegetation occurs by CO2 transfer from the air to the plant.  相似文献   

20.
We estimated CO2 and CH4 emissions from mangrove-associated waters of the Andaman Islands by sampling hourly over 24 h in two tidal mangrove creeks (Wright Myo; Kalighat) and during transects in contiguous shallow inshore waters, immediately following the northeast monsoons (dry season) and during the peak of the southwest monsoons (wet season) of 2005 and 2006. Tidal height correlated positively with dissolved O2 and negatively with pCO2, CH4, total alkalinity (TAlk) and dissolved inorganic carbon (DIC), and pCO2 and CH4 were always highly supersaturated (330–1,627 % CO2; 339–26,930 % CH4). These data are consistent with a tidal pumping response to hydrostatic pressure change. There were no seasonal trends in dissolved CH4 but pCO2 was around twice as high during the 2005 wet season than at other times, in both the tidal surveys and the inshore transects. Fourfold higher turbidity during the wet season is consistent with elevated net benthic and/or water column heterotrophy via enhanced organic matter inputs from adjacent mangrove forest and/or the flushing of CO2-enriched soil waters, which may explain these CO2 data. TAlk/DIC relationships in the tidally pumped waters were most consistent with a diagenetic origin of CO2 primarily via sulphate reduction, with additional inputs via aerobic respiration. A decrease with salinity for pCO2, CH4, TAlk and DIC during the inshore transects reflected offshore transport of tidally pumped waters. Estimated mean tidal creek emissions were ～23–173 mmol m?2 day?1 CO2 and ～0.11–0.47 mmol m?2 day?1 CH4. The CO2 emissions are typical of mangrove-associated waters globally, while the CH4 emissions fall at the low end of the published range. Scaling to the creek open water area (2,700 km2) gave total annual creek water emissions ～3.6–9.2?×?1010 mol CO2 and 3.7–34?×?107 mol CH4. We estimated emissions from contiguous inshore waters at ～1.5?×?1011 mol CO2?year?1 and 2.6?×?108 mol CH4?year?1, giving total emissions of ～1.9?×?1011 mol CO2?year?1 and ～3.0?×?108 mol CH4?year?1 from a total area of mangrove-influenced water of ～3?×?104 km2. Evaluating such emissions in a range of mangrove environments is important to resolving the greenhouse gas balance of mangrove ecosystems globally. Future such studies should be integral to wider quantitative process studies of the mangrove carbon balance.  相似文献   

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