摘要:

Concentrations of the Chlorofluorocarbons (CFCs) CFC‐11 and CFC‐12 were determined in groundwater from coastal plain sediments of the Delmarva Peninsula. CFC‐modeled ages were calculated independently for CFC‐11 and CFC‐12, and agreed to within 2–3 years in the majority of the waters. Recharge temperatures, determined from dissolved nitrogen and argon concentrations, varied from 9±2°C over most of the peninsula to 14±2°C at the southernmost tip of the peninsula in Virginia. The CFC‐modeled ages were examined in relation to the known hydrogeologic environment, both on regional scales and in more intensively sampled local scale networks. The CFC‐modeled recharge years and measured tritium concentrations were used to reconstruct a tritium input function that was compared to the modeled tritium plus3He distribution. Most of the present distribution of tritium in Delmarva groundwater is consistent with low dispersivities. The results of the study strongly support the use of CFCs for dating shallow,

ISSN:0043-1397    

DOI:10.1029/93WR02073

年代:1993

数据来源: WILEY

摘要:

Nuclear magnetic resonance relaxation measurements were used to identify different characteristic porosity domains in soil and aquifer materials. The porosity distribution can be inferred from these measurements by a regularization method applicable to any nuclear magnetic resonance (NMR) relaxation, or by an analytic method applicable only to multiexponential relaxations (D. Orazio et al., 1989). The porosity distribution obtained from NMR relaxation measurements strongly depends on the pore shape factor. For the Borden aquifer material, both the regularized and the analytic pore size distribution obtained from NMR relaxation measurements are consistent with those obtained by Ball et al. (1990) using Hg porosimetry and N2adsorption. For the Eustis and the Webster soils, the measured porosity domains are qualitatively consistent with those expected based on their respective composition. Our findings suggest that due to the long time required to saturate fine pores, NMR measurements of porosity distribution that are collected at short saturation times are biased toward larger pore sizes.

ISSN:0043-1397    

DOI:10.1029/93WR02302

年代:1993

数据来源: WILEY

摘要:

The water flow rate and tracer distribution has been studied in a fracture zone and averagely fractured rock in the Stripa underground research laboratory. The experimental site is located in granitic rock at a depth of 385 m below the ground surface. A 50‐m‐long drift with a diameter of 3 m was excavated. The drift was intersected by a 6‐m‐wide fracture zone. The upper part of the drift was covered by 150 plastic sheets in which water was collected. The water in the lower part of the drift was collected in sump holes. Different tracers were injected in seven locations at distances between 9.5 and 25 m from the drift. More than 50% of the water was found in one sampling area in the drift, and more than 90% in only eight sampling areas. The recovery of the tracers was also concentrated in a few sheets. Most of the flow preferentially takes place in a few paths. Two sets of tracers were used in tandem; one set of dyes which had previously been used in Stripa and one set of lanthanide‐DTPA complexes. Although shown to be nonsorbing in long‐time high‐sensitivity laboratory tests, the dyes were found to have a somewhat smaller recovery than the metal tracers. This observation was used in an attempt to estimate the flow‐wetted surface into which the dyes can diffuse and sorb. The flow porosity was estimated from the tracer residence times for all injection points. The flow in the intersected fracture zone is not like that in a porous medium; it is h

ISSN:0043-1397    

DOI:10.1029/93WR02340

年代:1993

数据来源: WILEY

摘要:

A field tracer experiment was conducted in a lateral flow field in the weathered and highly fractured upper 6 m of a 40‐m‐thick clay‐rich till plain in southwestern Ontario. In the upper 3 m where fractures are closely spaced (1 m) with concentration peaks tending to occur near visible fractures, and solute transport cannot be adequately described with a

ISSN:0043-1397    

DOI:10.1029/93WR02069

年代:1993

数据来源: WILEY

摘要:

Catchments are more spatially variable than catchment models assume them to be. Does spatial heterogeneity undermine the validity of spatially aggregated catchment geochemical models? Here we test whether catchment spatial heterogeneity invalidates a recently proposed technique that uses the bulk chemistry of runoff to predict how key water quality variables (e.g., titratable alkalinity and concentrations of base cations, hydrogen ions, and inorganic aluminum) will respond to changes in stream water in acid anion concentrations. We show that these predictions are valid for hypothetical heterogeneous catchments, created by mathematically “mixing” measured runoff chemistry data from chemically diverse streams. The chemical information in the combined runoff yields reliable acidification predictions, without any direct information concerning either the runoff chemistries of the individual source streams or the geochemical properties of their catchments. These results show that catchment acid buffering can be predicted directly from runoff chemistry in spatially heterogeneous catchments, even if the individual source regions cannot be identified and their geochemical characteristics are unkn

ISSN:0043-1397    

DOI:10.1029/93WR02202

年代:1993

数据来源: WILEY

摘要:

Geochemical reactions and hydrologic regime both affect runoff chemistry in many catchments. Each factor can obscure the effects of the other. This complicates efforts to interpret and predict catchments' geochemical and hydrological behavior. Here we present simple empirical techniques for separating effects of discharge fluctuations and effects of changes in acid anion concentrations in catchment monitoring data. We illustrate these techniques with data from central Ontario. Statistically subtracting acid anion effects reveals that important water quality variables (e.g., sum of base cations, acid neutralizing capacity, H+, Ali) are simple functions of the logarithm of catchment discharge. Likewise, correcting for discharge effects reveals that these water quality variables are roughly linear functions of runoff acid anion concentrations. The linear functions linking acid anion concentrations to water quality can be reliably predicted from bulk catchment runoff chemistry. Therefore acid anions' effects on water quality can be reliably predicted, even if they are obscured by discharge fluctuations in catchment monitoring data.

ISSN:0043-1397    

DOI:10.1029/93WR02203

年代:1993

数据来源: WILEY

摘要:

This research combines mass transport theory with stochastic methods to derive a stochastic, variable flow, surface water quality model for water quality policy analysis. Steady‐state flow assumptions and a series of differential equations reduce the information and number of equations required to completely specify the model. The newly developed hydrology model is applied to river salinity in the Colorado Basin to evaluate the effectiveness of alternative salt load reduction strategies in water quality management. Model results are used to illustrate the influence of salt load reduction, stochastic river flows, and water conservation on river water qualit

ISSN:0043-1397    

DOI:10.1029/93WR02464

年代:1993

数据来源: WILEY

摘要:

Methods for identifying the size, or scale, of hillslopes and the extent of channel networks from digital elevation models (DEMs) are examined critically. We show that a constant critical support area, the method most commonly used at present for channel network extraction from DEMs, is more appropriate for depicting the hillslope/valley transition than for identifying channel heads. Analysis of high‐resolution DEMs confirms that a constant contributing area per unit contour length defines the extent of divergent topography, or the hillslope scale, although there is considerable variance about the average value. In even moderately steep topography, however, a DEM resolution finer than the typical 30 m by 30 m grid size is required to accurately resolve the hillslope/valley transition. For many soil‐mantled landscapes, a slope‐dependent critical support area is both theoretically and empirically more appropriate for defining the extent of channel networks. Implementing this method for overland flow erosion requires knowledge of an appropriate proportionality constant for the drainage area‐slope threshold controlling channel initiation. Several methods for estimating this constant from DEM data are examined, but acquisition of even limited field data is recommended. Finally, the hypothesis is proposed that an inflection in the drainage area‐slope relation for mountain drainage basins reflects a transition from steep debris flow‐dominated channels to lower‐gradient allu

ISSN:0043-1397    

DOI:10.1029/93WR02463

年代:1993

数据来源: WILEY

摘要:

Detailed laser‐Doppler velocity and Reynolds stress measurements over fixed two‐dimensional bed forms are used to investigate the coupling between the mean flow and turbulence and to examine effects that play a role in producing the bed form instability and finite amplitude stability. The coupling between the mean flow and the turbulence is explored in both a spatially averaged sense, by determining the structure of spatially averaged velocity and Reynolds stress profiles, and a local sense, through computation of eddy viscosities and length scales. The measurements show that there is significant interaction between the internal boundary layer and the overlying wake turbulence produced by separation at the bed form crest. The interaction produces relatively low correlation coefficients in the internal boundary layer, which suggests that using local bottom stress to predict bed load flux may not only be erroneous, it may also disregard the essence of the bed form instability mechanism. The measurements also indicate that topographically induced acceleration over the bed form stoss slope has a more significant effect in damping the turbulence over bed forms than was previously supposed, which is hypothesized to play a role in the stabilization of fully developed bed fo

ISSN:0043-1397    

DOI:10.1029/93WR01932

年代:1993

数据来源: WILEY

摘要:

A stochastic approach is developed to estimate the probability of wet days, and the mean and standard deviation of local daily nonzero precipitation reflecting global climate change scenarios. The approach is based on the analysis of daily atmospheric circulation patterns (CPs) and the linkage between types of CPs and daily precipitation. Three CP data sets are used for the 500‐hPa pressure field: 40‐year historical, 10‐year 1×CO2, and 10‐year 2×CO2scenarios obtained from the atmospheric general circulation model of the Canadian Climate Centre. CP types obtained by clustering techniques and their frequency distribution are similar for the three data sets. The linkage between CP types and precipitation considers an additional variable, the spatial average pressure height within each CP type. The difference in pressure heights among the three CP data sets makes it possible to estimate the effect of global change on local precipitation statistics. Under the dry continental climate of eastern Nebraska the effect of 2×CO2scenario on local precipitation regime is spatially variable and significant: the number of wet days slightly decreases but both the mean and variance of daily precipitation increase resulting in a more variable precipitat

ISSN:0043-1397    

DOI:10.1029/93WR01711

年代:1993

数据来源: WILEY