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1.
A confined aquifer may become unconfined near the pumping wells when the water level falls below the confining unit in the case where the pumping rate is great and the excess hydraulic head over the top of the aquifer is small. Girinskii's potential function is applied to analyze the steady ground water flow induced by pumping wells with a constant-head boundary in a mixed confined-unconfined aquifer. The solution of the single-well problem is derived, and the critical radial distance at which the flow changes from confined to unconfined condition is obtained. Using image wells and the superposition method, an analytic solution is presented to study steady ground water flow induced by a group of pumping wells in an aquifer bounded by a river with constant head. A dimensionless function is introduced to determine whether a water table condition exists or not near the pumping wells. An example with three pumping wells is used to demonstrate the patterns of potentiometric surface and development of water table around the wells.  相似文献   

2.
At an aviation gasoline spill site in Traverse City, Michigan, historical records indicate a positive correlation between significant rainfall events and increased concentrations of slightly soluble organic compounds in the monitoring wells of the site. To investigate the recharge effect on ground water quality due to infiltrating, water percolating past residual oil and into the saturated zone, an in situ infiltration experiment was performed at the site. Sampling cones were set at various depths below a circular test area, 13 feet (4 meters) in diameter. Rainfall was simulated by sprinkling the test area at a rate sufficiently low to prevent runoff. The sampling cones for soil-gas and ground water quality were installed in the unsaturated and saturated zones to observe the effects of the recharge process. At the time of the test, the water table was below the residual oil layer. The responses of the soil-gas and ground water quality were monitored during the recharge and drainage periods, which resulted from the sprinkling.
Infiltrated water was determined to have transported organic constituents of the residual oil, specifically benzene, toluene, ethylbenzene, and ortho-xylene (BTEX), into the ground water beneath the water table, elevating the aqueous concentrations of these constituents in the saturated zone. Soil-gas concentrations of the organic compounds in the unsaturated zone increased with depth and time after the commencement of infiltration. Reaeration of the unconfined aquifer via the infiltrated water was observed. It is concluded that water quality measurements are directly coupled to recharge events for the sandy type of aquifer with an overlying oil phase, which was studied in this work. Ground water sampling strategies and data analysis need to reflect the effect of recharge from precipitation on shallow, unconfined aquifers where an oil phase may be present.  相似文献   

3.
A hydrodynamic survey carried out in semiarid southwest Niger revealed an increase in the unconfined ground water reserves of approximately 10% over the last 50 years due to the clearing of native vegetation. Isotopic samplings (3H, 18O, 2H for water and 14C, 13C for the dissolved inorganic carbon) were performed on about 3500 km2 of this silty aquifer to characterize recharge. Stable isotope analyses confirmed the indirect recharge process that had already been shown by hydrodynamic surveys and suggested the tracers are exclusively of atmospheric origin. An analytical model that takes into account the long-term rise in the water table was used to interpret 3H and 14C contents in ground water. The natural, preclearing median annual renewal rate (i.e., recharge as a fraction of the saturated aquifer volume) lies between 0.04% and 0.06%. For representative characteristics of the aquifer (30 m of saturated thickness, porosity between 10% and 25%), this implies a recharge of between 1 and 5 mm/year, which is much lower than the estimates of 20 to 50 mm/year for recent years, obtained using hydrological and hydrodynamic methods and the same aquifer parameters. Our study, therefore, reveals that land clearing in semiarid Niger increased ground water recharge by about one order of magnitude.  相似文献   

4.
《Journal of Hydrology》2006,316(1-4):163-183
Numerical groundwater modelling is used as the base for sound aquifer system analysis and water resources assessment. In many cases, particularly in semi-arid and arid regions, groundwater flow is intricately linked to salinity transport. A case in point is the Shashe River Valley in Botswana. A freshwater aquifer located around an ephemeral stream is depleted by the combined effect of transpiration and pumping. Quantitative system analysis reveals that the amount of water taken by transpiration is far more than the quantities pumped for water supply. Furthermore, the salinity distribution in and around Shashe River Valley as well as its temporal dynamics can be satisfactorily reproduced if the transpiration is modelled as a function of groundwater salinity. The location and dynamics of the saltwater–freshwater interface are highly sensitive to the parameterization of evaporative and transpirative salt enrichment. An existing numerical code for coupled flow/transport simulations (SEAWAT) was adapted to this situation. Model results were checked against a large set of field data including water levels, water chemistry, isotope data and ground and airborne geophysical data. The resulting groundwater model was able to reproduce the long-term development of the freshwater lens located in Shashe River Valley as well as the decline in piezometric heads observed over the last decade. Furthermore, the old age of the saline water surrounding the central freshwater lens could be explained.  相似文献   

5.
A zone of contaminated ground water has been identified in an unconfined sand aquifer adjacent to a pit into which spent pulp liquor was intermittently discharged from 1970 to 1979. A network of multilevel sampling, bundle-type piezometers was installed. Up to seven depthspecific sampling points were incorporated into each piezometer providing a cost-effective means for three-dimensional mapping of hydraulic head and water quality in the unconfined sand aquifer. Ground-water samples retrieved from this network showed an area of contamination 900 m long, 400 m wide, and more than 25 m deep. This plume is dispersed about the ground-water flow lines passing beneath the waste disposal pit, and it terminates at a vigorous ground-water discharge area located 800 m from the pit. The contaminated ground water is characterized by elevated concentrations of sodium (3,000 mg/1), chloride (590 mg/1), alkalinity (2,700 mg/1), total organic carbon (2,000 mg/1), chemical oxygen demand (10,800 mg/1), biological oxygen demand (2,000 mg/1), tannin and lignin (780 mg/1), and lower sulphate (1 mg/1) compared to background ground waters in the area. The apparent rate of sodium migration is more than 50 m/yr and is close to the average linear ground-water velocity. Removal of some organic matter by biological transformation has produced the increased alkalinity in the contaminated ground water and somewhat reduced pH. Tannin and lignin are relatively inert compared to other organic compounds found in the waste liquor. Extremely low sulphate levels occurring in the highly contaminated ground waters indicate the existence of conditions favorable for microbially-mediated sulphate reduction.  相似文献   

6.
Accurate representation of artificial recharge is requisite to calibration of a ground water model of an unconfined aquifer for a semiarid or arid site with a vadose zone that imparts significant attenuation of liquid transmission and substantial anthropogenic liquid discharges. Under such circumstances, artificial recharge occurs in response to liquid disposal to the vadose zone in areas that are small relative to the ground water model domain. Natural recharge, in contrast, is spatially variable and occurs over the entire upper boundary of a typical unconfined ground water model. An improved technique for partitioning artificial recharge from simulated total recharge for inclusion in a ground water model is presented. The improved technique is applied using data from the semiarid Hanford Site. From 1944 until the late 1980s, when Hanford's mission was the production of nuclear materials, the quantities of liquid discharged from production facilities to the ground vastly exceeded natural recharge. Nearly all hydraulic head data available for use in calibrating a ground water model at this site were collected during this period or later, when the aquifer was under the diminishing influence of the massive water disposals. The vadose zone is typically 80 to 90 m thick at the Central Plateau where most production facilities were located at this semiarid site, and its attenuation of liquid transmission to the aquifer can be significant. The new technique is shown to improve the representation of artificial recharge and thereby contribute to improvement in the calibration of a site-wide ground water model.  相似文献   

7.
Water from the San Joaquin Delta, having chloride concentrations up to 3590 mg/L, has intruded fresh water aquifers underlying Stockton, California. Changes in chloride concentrations at depth within these aquifers were evaluated using sequential electromagnetic (EM) induction logs collected during 2004 through 2007 at seven multiple‐well sites as deep as 268 m. Sequential EM logging is useful for identifying changes in groundwater quality through polyvinyl chloride‐cased wells in intervals not screened by wells. These unscreened intervals represent more than 90% of the aquifer at the sites studied. Sequential EM logging suggested degrading groundwater quality in numerous thin intervals, typically between 1 and 7 m in thickness, especially in the northern part of the study area. Some of these intervals were unscreened by wells, and would not have been identified by traditional groundwater sample collection. Sequential logging also identified intervals with improving water quality—possibly due to groundwater management practices that have limited pumping and promoted artificial recharge. EM resistivity was correlated with chloride concentrations in sampled wells and in water from core material. Natural gamma log data were used to account for the effect of aquifer lithology on EM resistivity. Results of this study show that a sequential EM logging is useful for identifying and monitoring the movement of high‐chloride water, having lower salinities and chloride concentrations than sea water, in aquifer intervals not screened by wells, and that increases in chloride in water from wells in the area are consistent with high‐chloride water originating from the San Joaquin Delta rather than from the underlying saline aquifer.  相似文献   

8.
Harvey FE  Sibray SS 《Ground water》2001,39(3):408-421
Across the Great Plains irrigation canals are used to transport water to cropland. Many of these canals are unlined, and leakage from them has been the focus of an ongoing legal, economic, and philosophical debate as to whether this lost water should be considered waste or be viewed as a beneficial and reasonable use since it contributes to regional ground water recharge. While historically there has been much speculation about the impact of canal leakage on local ground water, actual data are scarce. This study was launched to investigate the impact of leakage from the Interstate Canal, in the western panhandle of Nebraska, on the hydrology and water quality of the local aquifer using water chemistry and environmental isotopes. Numerous monitoring wells were installed in and around a small wetland area adjacent to the canal, and ground water levels were monitored from June 1992 until January 1995. Using the water level data, the seepage loss from the canal was estimated. In addition, the canal, the monitoring wells, and several nearby stock and irrigation wells were sampled for inorganic and environmental isotope analysis to assess water quality changes, and to determine the extent of recharge resulting from canal leakage. The results of water level monitoring within study wells indicates a rise in local ground water levels occurs seasonally as a result of leakage during periods when the canal is filled. This rise redirects local ground water flow and provides water to nearby wetland ecosystems during the summer months. Chemical and isotopic results were used to delineate canal, surface, and ground water and indicate that leaking canal water recharges both the surface alluvial aquifer and upper portions of the underlying Brule Aquifer. The results of this study indicate that lining the Interstate Canal could lower ground water levels adjacent to the canal, and could adversely impact the local aquifer.  相似文献   

9.
Allen DM 《Ground water》2004,42(1):17-31
Stable isotopes of 18O and 2H in water, and 34S and 18O in dissolved SO4, are used to verify the interpretation of the chemical evolution and proposed sources of salinity for two islands that have undergone postglacial rebound. Results for delta18O and delta34S in dissolved SO4 on the Gulf Islands, southwest British Columbia, Canada, suggest a three-component mixing between (1) atmospheric SO4 derived largely from recharge of meteoric origin, (2) modern marine SO4 associated with either modern-day salt water intrusion or Pleistocene age sea water, and (3) terrestrial SO4. The age of the marine SO4 is uncertain based on the geochemistry and SO4 isotopes alone. Two options for mixing of saline ground waters are proposed--either between current-day marine SO4 and atmospheric SO4, or between older (Pleistocene age) marine SO4 and atmospheric SO4, delta18O and delta2H compositions are relatively consistent between both islands, with a few samples showing evidence of mixing with water that is a hybrid mixture of Fraser River water and ocean water. The isotopic composition of this hybrid water is approximately delta18O = 10 per thousand. delta18O and delta2H values for many saline ground waters plot close to the global meteoric water line, which is distinctly different from the local meteoric water line. This suggests a meteoric origin for ground waters that is different from the current isotopic composition of meteoric waters. It is proposed these waters may be late Pleistocene in age and were recharged when the island was submerged below sea level and prior to rebound at the end of the last glaciation.  相似文献   

10.
Previous site-specific studies designed to assess the impacts of unsewered subdivisions on ground water quality have relied on upgradient monitoring wells or very limited background data to characterize conditions prior to development. In this study, an extensive monitoring program was designed to document ground water conditions prior to construction of a rural subdivision in south-central Wisconsin. Previous agricultural land use has impacted ground water quality; concentrations of chloride, nitrate-nitrogen, and atrazine ranged from below the level of detection to 296 mg/L, 36 mg/L, and 0.8 microg/L, respectively, and were highly variable from well to well and through time. Seasonal variations in recharge, surface topography, aquifer heterogeneities, surficial loading patterns, and well casing depth explain observed variations in ground water chemistry. This variability would not have been detected if background conditions were determined from only a few monitoring wells or inferred from wells located upgradient of the subdivision site. This project demonstrates the importance of characterizing both ground water quality and chemical variability prior to land-use change to detect any changes once homes are constructed.  相似文献   

11.
Harvey FE 《Ground water》2001,39(3):380-390
Stable oxygen and hydrogen isotopes have been used in ground water studies to investigate recharge, mixing, ground water/surface water interaction, advective-diffusive transport, paleohydrogeologic interactions and to estimate ground water ages. Such studies require that the isotopic composition of precipitation be known, as precipitation is a major input to ground water and surface water systems. As oxygen-18 and deuterium data for precipitation are lacking across much of the United States, there is need to establish additional local meteoric water lines as isotope input functions across the region, as well as to develop better understanding of the isotopic climate linkages that control oxygen and hydrogen isotope ratios in precipitation. In the absence of long-term monitoring stations, one possible solution to this problem is to determine the delta 18O and delta 2H values of precipitation using archive samples collected at monitoring stations managed by the National Atmospheric Deposition Program (NADP). This study describes and interprets the seasonal delta 18O and delta 2H composition of archived precipitation samples collected in eastern Nebraska near the town of Mead during the years 1992-1994. Values for delta 18O range from -23.6 to -0.7@1000. Values for delta 2H range from -172 to 0@1000. Yearly arithmetic mean delta 18O and delta 2H values for the Mead station are -8.1@1000 and -53@1000, respectively. Weighted yearly means for delta 18O and delta 2H were -7.4@1000 and -48@1000, respectively. Mead values show a strong isotopic enrichment between winter and summer precipitation, and a strong delta 18O-T correlation (r2 = 0.91) for mean monthly values of about 0.5@1000 per degree Celsius. The local meteoric water line for the Mead site is delta 2H = 7.40 delta 18O + 7.32. Deuterium excess values suggest that most of the moisture across the region is derived primarily from a Gulf of Mexico source. The results of this study demonstrate that in the absence of long-term monitoring stations such as those operated globally by the International Atomic Energy Association, NADP archive samples can be used to determine the isotopic composition of precipitation, to characterize the local meteoric water line and establish the various climatic relationships, and define the meteoric input function for use in ground water studies.  相似文献   

12.
The exact response of mangrove shorelines to anticipated sea-level rise will depend upon the balance between sedimentation and sea-level change. Within the Top End of the Northern Territory of Australia there are extensive, relatively unmodified, tide-dominated mangrove forests, where tidal processes redistribute sediment. Harbours, such as Darwin Harbour, and tidal rivers, such as the South Alligator River with its associated coastal and estuarine plains, represent opposite extremes in terms of Holocene sedimentary infill, and will respond differently to sea-level rise. In Darwin Harbour, mangrove assemblages can be recognized in geomorphologically defined habitats. Similar topography within and between creeks implies morphodynamic equilibrium with tidal processes. Tidal reworking of sediment may maintain an equilibrial profile under gradually rising sea level, with resuspension of lower intertidal and subtidal muds and their redeposition within upper intertidal mangrove habitats. In contrast, the plains along the coast and tidal rivers draining into van Diemen Gulf developed during the post-glacial marine transgression, and since sea level stabilized, around 6000 years ago, coastal plains have prograded. These broad plains are presently not extensively influenced by salt water, but are often at elevations close to, or even below, modern high-tide levels. They may, therefore, revert to saline conditions particularly rapidly if the sea rises. The pattern of change may not be directly analogous to marine incursion experienced in the early Holocene, because broad plains have been able to prograde during the last 6000 years of relatively stable sea level.  相似文献   

13.
Field-based experiments were designed to investigate the release of naturally occurring, low to moderate (< 50 microg/L) arsenic concentrations to well water in a confined sandstone aquifer in northeastern Wisconsin. Geologic, geochemical, and hydrogeologic data collected from a 115 m2 site demonstrate that arsenic concentrations in ground water are heterogeneous at the scale of the field site, and that the distribution of arsenic in ground water correlates to solid-phase arsenic in aquifer materials. Arsenic concentrations in a test well varied from 1.8 to 22 microg/L during experiments conducted under no, low, and high pumping rates. The quality of ground water consumed from wells under typical domestic water use patterns differs from that of ground water in the aquifer because of reactions that occur within the well. Redox conditions in the well can change rapidly in response to ground water withdrawals. The well borehole is an environment conducive to microbiological growth, and biogeochemical reactions also affect borehole chemistry. While oxidation of sulfide minerals appears to release arsenic to ground water in zones within the aquifer, reduction of arsenic-bearing iron (hydr)oxides is a likely mechanism of arsenic release to water having a long residence time in the well borehole.  相似文献   

14.
Impact of Sea-Level Rise on Sea Water Intrusion in Coastal Aquifers   总被引：17，自引：0，他引：17  
Despite its purported importance, previous studies of the influence of sea-level rise on coastal aquifers have focused on specific sites, and a generalized systematic analysis of the general case of the sea water intrusion response to sea-level rise has not been reported. In this study, a simple conceptual framework is used to provide a first-order assessment of sea water intrusion changes in coastal unconfined aquifers in response to sea-level rise. Two conceptual models are tested: (1) flux-controlled systems, in which ground water discharge to the sea is persistent despite changes in sea level, and (2) head-controlled systems, whereby ground water abstractions or surface features maintain the head condition in the aquifer despite sea-level changes. The conceptualization assumes steady-state conditions, a sharp interface sea water-fresh water transition zone, homogeneous and isotropic aquifer properties, and constant recharge. In the case of constant flux conditions, the upper limit for sea water intrusion due to sea-level rise (up to 1.5 m is tested) is no greater than 50 m for typical values of recharge, hydraulic conductivity, and aquifer depth. This is in striking contrast to the constant head cases, in which the magnitude of salt water toe migration is on the order of hundreds of meters to several kilometers for the same sea-level rise. This study has highlighted the importance of inland boundary conditions on the sea-level rise impact. It identifies combinations of hydrogeologic parameters that control whether large or small salt water toe migration will occur for any given change in a hydrogeologic variable.  相似文献   

15.
Ground water recharge and flow characterization using multiple isotopes   总被引：2，自引：0，他引：2  
Stable isotopes of delta(18)O, delta(2)H, and (13)C, radiogenic isotopes of (14)C and (3)H, and ground water chemical compositions were used to distinguish ground water, recharge areas, and possible recharge processes in an arid zone, fault-bounded alluvial aquifer. Recharge mainly occurs through exposed stream channel beds as opposed to subsurface inflow along mountain fronts. This recharge distribution pattern may also occur in other fault-bounded aquifers, with important implications for conceptualization of ground water flow systems, development of ground water models, and ground water resource management. Ground water along the mountain front near the basin margins contains low delta(18)O, (14)C (percent modern carbon [pmC]), and (3)H (tritium units [TU]), suggesting older recharge. In addition, water levels lie at greater depths, and basin-bounding faults that locally act as a flow barrier may further reduce subsurface inflow into the aquifer along the mountain front. Chemical differences in ground water composition, attributed to varying aquifer mineralogy and recharge processes, further discriminate the basin-margin and the basin-center water. Direct recharge through the indurated sandstones and mudstones in the basin center is minimal. Modern recharge in the aquifer is mainly through the broad, exposed stream channel beds containing coarse sand and gravel where ground water contains higher delta(18)O, (14)C (pmC), and (3)H (TU). Spatial differences in delta(18)O, (14)C (pmC), and (3)H (TU) and occurrences of extensive mudstones in the basin center suggest sluggish ground water movement, including local compartmentalization of the flow system.  相似文献   

16.
Ground-water levels in the Upper Floridan aquifer beneath the southeastern coast of South Carolina have undergone pumpage-induced declines approaching 20 ft below sea level at the southern end of Hilton Head Island. This scenario suggests the potential exists for the inducement of recharge to the Upper Floridan aquifer across the island, which could affect the quality of water being pumped by wells. However, low radiocarbon concentrations in ground-water samples (0.5 to 1.4 ± 0.1 PMC) indicate that most of the water is relict ground water reflecting prepumpage ground-water flow conditions in the Upper Floridan aquifer. The isotopic data indicate long residence times and water-chemistry evolution more characteristic of ground-water recharge occurring farther inland prior to the commencement of pumpage in the late 1800s. Radiocarbon concentrations (as Percent Modern Carbon) and stable carbon isotope ratios (as δ13C in dissolved inorganic carbon) determined during this study and reported in other studies on and around Hilton Head Island varied in a systematic manner. Heavier δ13C values (–2.8 to –1.6 per mil) in ground water beneath southern Hilton Head Island reflect ground-water discharge from prepumpage flowpaths originating over 100 miles away, hence a depletion in radiocarbon concentration with corrected ground-water ages no younger than 16,000 yrs BP. In contrast, lighter δ13C values (–13.9 to –8.67 per mil) beneath the northern part of the island indicate recent recharge as a result of water-level declines, and recharge in areas off the island that have not changed as a result of pumpage (evidenced by enrichment in radiocarbon with corrected ground-water ages no older than 4,000 yrs BP). This suggests that the δ13C composition of ground water in the Upper Floridan aquifer is a useful indicator of mixing between ground waters from different sources, and can be used to delineate recharge-discharge patterns. This approach may be applicable to other aquifers of highly evolved ground-water chemistry in regional carbonate aquifer systems that may be receiving recent recharge. Moreover, this approach could prove useful in delineating the contribution of recent water being captured by pumped wells as part of wellhead protection programs designed to assess aquifer vulnerability from surficial contaminant sources.  相似文献   

17.
When fugitive methane migrates upward along boreholes of oil and gas wells, it may migrate into shallow ground water or pass through overlying soil to the atmosphere. Prior to this study, there was little information on the fate of fugitive methane that migrates into ground water. In a field study near Lloydminster, Alberta, Canada, we found hydrogeochemical evidence that fugitive methane from an oil well migrated into a shallow aquifer but has been attenuated by dissimilatory bacterial sulfate reduction at low temperature ( approximately 5 degrees C) under anaerobic conditions. Evidence includes spatial and temporal trends in concentrations of methane and sulfate in ground water and associated trends in concentrations of bicarbonate and sulfide. Within 10 m of the oil well, sulfate concentrations were low, and sulfate was enriched in both 34S and 18O. Sulfate concentrations had a strong positive correlation with delta13C values of bicarbonate, and sulfide was depleted in 34S compared to sulfate. These data indicate that bacterial sulfate reduction occurred near the production well. Near the oil well, elevated concentrations of bicarbonate were observed, and the bicarbonate was depleted in 13C. Modeling indicates that the main source of this excess 13C-depleted bicarbonate is oxidized methane. In concert with the sulfate concentration and isotope data, these results support an interpretation that in situ bacterial oxidation of methane has occurred, linked to bacterial sulfate reduction. Bacterial sulfate reduction may play a major role in bioattenuation of fugitive natural gas in ground water in western Canada.  相似文献   

18.
The D/H and18O/16O ratios of fumarole condensates from White Island, an andesite volcano in the Bay of Plenty, have been measured during the period 1965 to 1969 to determine the origin of the water and the changes which occur as the volcanic activity changes. The D/H and18O/16O ratios of all of the samples were correlated with a slope of ?2. The δ18O values were proportional to the logarithm of the chloride concentration but with distinctly different relationships between the period from 1965 to 1967, and the period of tephra eruptions in 1968. In the latter period the chloride contents were close to that of sea water while in the former quieter period the contents were lower. The δD values follow a similar pattern but with a poorer correlation, indicating that a more variable process is controlling the deuterium results. Possible hydrothermal models are: mixing of near surface water and magmatic water; progressive leaching of chloride from underground rocks and exchange of isotopes (Craig, 1966); and lastly, equilibrium evaporation at about 255°C of either sea water or local surface water. The first of these models is considered most unlikely because of the incompatible logarithmic relationship between the chloride content and the oxygen isotope results. The second model has some merit, but the available evidence favours the last of the three models in which the 1965–67 samples derive from surface water or vapour from a boiling chloride water aquifer of sea water origin and the 1968 samples derive from the boiling chloride water, because of disturbance of the system during the tephra eruptions.  相似文献   

19.
Riparian cottonwood forests in dry regions of western North America do not typically receive sufficient growing season precipitation to completely support their relatively high transpiration requirements. Water used in transpiration by riparian ecosystems must include alluvial groundwater or water stored in the potentially large reservoir of the unsaturated soil zone. We used the stable oxygen and hydrogen isotope composition of stem xylem water to evaluate water sources used by the dominant riparian cottonwood (Populus spp.) trees and shrubs (Shepherdia argentea and Symphoricarpos occidentalis) in Lethbridge, Alberta, during 3 years of contrasting environmental conditions. Cottonwoods did not exclusively take up alluvial groundwater but made extensive use of water sourced from the unsaturated soil zone. The oxygen and hydrogen isotope compositions of cottonwood stem water did not strongly overlap with those of alluvial groundwater, which were closely associated with the local meteoric water line. Instead, cottonwood stem water δ18O and δ2H values were located below the local meteoric water line, forming a line with a low slope that was indicative of water exposed to evaporative enrichment of heavy isotopes. In addition, cottonwood xylem water isotope compositions had negative values of deuterium excess (d‐excess) and line‐conditioned (deuterium) excess (lc‐excess), both of which provided evidence that water taken up by the cottonwoods had been exposed to fractionation during evaporation. The shrub species had lower values of d‐excess and lc‐excess than had the cottonwood trees due to shallower rooting depths, and the d‐excess values declined during the growing season, as shallow soil water that was taken up by the plants was exposed to increasing, cumulative evaporative enrichment. The apparent differences in functional rooting pattern between cottonwoods and the shrub species, strongly influenced the ratio of net photosynthesis to stomatal conductance (intrinsic water‐use efficiency), as shown by variation among species in the δ13C values of leaf tissue.  相似文献   

20.
Data from a large-scale canal-drawdown test were used to estimate the specific yield (sy) of the Biscayne Aquifer, an unconfined limestone aquifer in southeast Florida. The drawdown test involved dropping the water level in a canal by about 30 cm and monitoring the response of hydraulic head in the surrounding aquifer. Specific yield was determined by analyzing data from the unsteady portion of the drawdown test using an analytical stream-aquifer interaction model (Zlotnik and Huang 1999). Specific yield values computed from drawdown at individual piezometers ranged from 0.050 to 0.57, most likely indicating heterogeneity of specific yield within the aquifer (small-scale variation in hydraulic conductivity may also have contributed to the differences in sy among piezometers). A value of 0.15 (our best estimate) was computed based on all drawdown data from all piezometers. We incorporated our best estimate of specific yield into a large-scale two-dimensional numerical MODFLOW-based ground water flow model and made predictions of head during a 183-day period at four wells located 337 to 2546 m from the canal. We found good agreement between observed and predicted heads, indicating our estimate of specific yield is representative of the large portion of the Biscayne Aquifer studied here. This work represents a practical and novel approach to the determination of a key hydrogeological parameter (the storage parameter needed for simulation and calculation of transient unconfined ground water flow), at a large spatial scale (a common scale for water resource modeling), for a highly transmissive limestone aquifer (in which execution of a traditional pump test would be impractical and would likely yield ambiguous results). Accurate estimates of specific yield and other hydrogeological parameters are critical for management of water supply, Everglades environmental restoration, flood control, and other issues related to the ground water hydrology of the Biscayne Aquifer.  相似文献   

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