Ground Water Vulnerability Assessment Predicting Relative Contamination Potential Under Conditions of Uncertainty (1993) / Chapter Skim
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3 Approaches to Vulnerability Assessments
3 Approaches to Vulnerability Assessments
Pages 42-103
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From page 42... ...
The potential for contaminants to leach to ground water depends on many factors, including the composition of the soils and geologic materials in the unsaturated zone, the depth to the water table, the recharge rate, and environmental influences on the potential for biodegradation. The composi tion of the unsaturated zone can greatly influence transformations and reactions.
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From page 43... ...
Conversely, longer flow paths from land surface to the water table can lessen the potential for contamination by chemicals that sorb or degrade along the Towpath. The recharge rate is important because it affects the extent and rate of transport of contaminants through the unsaturated zone.
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From page 44... ...
REVIEW OF CURRENT APPROACHES Combinations of some or all of the factors noted above are included in the various approaches used to assess ground water vulnerability. These approaches range in complexity from a subjective evaluation of available map data to the application of complex contaminant transport models.
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From page 45... ...
Each class is broken down further into specific types of approaches, such as aquifer sensitivity assessment methods which consider only hydrogeologic factors; hybrid methods, which consider hydrogeologic and pesticide factors; and ground water vulnerability assessment methods, which consider hydrogeologic, pesticide, and agronomic factors. Statistical tools are also noted for their usefulness in validating methods or providing hydrogeologic setting information.
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From page 46... ...
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From page 48... ...
. Variables used in the overlay and index methods typically include approximate depth to the water table, ground water recharge rate, and soil and aquifer material properties.
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The extent to which flow systems of different spatial scales can be defined as parts of regional assessments of ground water vulnerability is subject to significant limitations. Nonetheless, the identification of recharge and discharge zones may be one of the more important elements of a vulnerability assessment.
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Overlay methods are commonly used for vulnerability assessments at
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From page 52... ...
1984~. Several types of indices have been developed for ground water vulnerability assessments.
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From page 53... ...
Such complex models have not been used to evaluate ground water vulnerability on a regional scale; therefore, this discussion will focus on simpler process-based models of one-dimensional transport through the vadose zone. Table 3.5 indicates the various process representations used in several simulation models that have been used to predict pesticide behavior in the unsaturated zone.
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From page 54... ...
Process Water Flow Simulation Model CMLS (Ver.
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From page 55... ...
Erosion based on the Universal Soil Loss Equation. Convective transport of solute based on water flow between soil increments.
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+ * Volatile losses + - Runoff - + + 1A plus sign indicates that the output parameter values are provided, and a minus sign indicates that the parameter values are not provided.
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From page 57... ...
, the associated variances, and in some cases probability distributions. In practice, the lack of realistic probabilistic information from deterministic models can be overcome by employing Monte Carlo simulation techniques, which require assumptions about the probability density functions representing the spatial or temporal variability of the input parameters in the simulation models.
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From page 58... ...
These findings suggest that models based on simplified process representation may be more useful for certain types of vulnerability assessments. The convective-dispersive solute transport approach employed in LEACHM and other models predicts the asymptotic behavior and is least likely to be valid when used for shallow depths.
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From page 59... ...
The foregoing discussion suggests that a principal limitation of simulation models currently used to make vulnerability assessments may be their failure to account for flow and transport processes at spatial scales either smaller or larger than those for which the models were developed.
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From page 60... ...
Since ground water vulnerability is a probabilistic notion, statistical methods should have more application in vulnerability assessments than they have had to date. Statistical methods can more easily deal with differences in scale than other methods that are based on the description of physical relationships.
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From page 61... ...
that ground waters taken from opposing sides of the Columbia River were distinctly different. Such cases bring into question the logic of using surface water drainage basins as regions for conducting vulnerability assessments.
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From page 62... ...
not defined by any single constituent." Thus multivariate statistical techniques may be well suited to analysis of water quality data and other regional data, which can include soils and geologic information, vegetal coverage and land management practices. Vulnerability assessments that use overlay/indexing techniques are an eyeballed form of multivariate discriminant analyses that lack probability estimates.
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From page 63... ...
In general, the more complex approaches require greater expertise for interpretation and are best used in a team effort, where the team is composed of a statistician and other experts familiar with available data and their sources. UNCERTAINTY IN VULNERABILITY ASSESSMENT METHODS "To be absolutely certain about something, one must know every thing or nothing about it." Olin Miller Uncertainties inherent in all approaches to ground water vulnerability assessments may be derived from: (1)
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From page 64... ...
Because each step in a vulnerability assessment requires some degree of interpretation, Table 3.7 lists interpretation as one source of error in each class.
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From page 65... ...
Errors Due to Natural Spatial and Temporal Variability Many, if not most, of the data used in vulnerability assessments display significant spatial and temporal variability. Thus, large sampling errors can occur because different estimates of attributes or model parameters will be obtained from different samples at different locations or time periods.
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From page 66... ...
Data Processing Errors Data processing errors include inaccuracies in the computational scheme used to obtain numerical values from a model or other quantitative measures of ground water vulnerability. These errors can include numerical dispersion errors, discretization errors, round-off errors, precision errors, possible solution convergence, and uniqueness errors.
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From page 67... ...
This inability to distinguish differences between adjacent cells with differing vulnerability scores increases with increasing magnitude of the relevant model and data uncertainties. Few published vulnerability assessments account for uncertainties from either model or data errors.
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From page 68... ...
Of these techniques, only FOUAs statistical sampling methods, and stochastic modeling techniques have been applied to vulnerability assessments. A description of the FOUA technique is presented in Box 3.1 and an example of the use of Monte Carlo methods in Box 3.2.
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From page 69... ...
The objective is to estimate the uncertainty, such as variance, in the derived parameter given the error in the independent variables. Here, we will briefly examine the basis for FOUA approximation method and its application in estimating the uncertainty in numerical measures of ground water vulnerability calculated by simple methods.
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From page 70... ...
The simulation models discussed in this chapter are primarily deterministic models that attempt to simulate the soil system without considering the inherent uncertainty in both the soil processes and system characteristics. Several attempts have been made to integrate the application of Monte Carlo techniques with simulation models for predicting pesticide leaching and migration processes within a framework that allows consideration of uncertainty in both process and soil and ground water system characterization.
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From page 71... ...
Thus, the Monte Carlo simulation analysis is more efficient. The output frequency distributions are then used as a basis for evaluating the probability that the maximum pesticide leaching depth is sufficient to result in ground water contamination.
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From page 72... ...
The output probability distributions produced by the integration of simulation modeling and Monte Carlo techniques allow consideration of uncertainties in soils properties, pesticide characteristics, meteorology, and hydrogeologic conditions of pesticide migration to and within ground water systems. Th is type of information wil I help managers make the types of decisions needed to protect vulnerable grou nd water regions with scarce resou rces.
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From page 73... ...
157- 45' 0. FIGURE 3.3 Map for Island of Oahu, Hawaii depicting relative vulnerability to ground water contamination with the pesticide ethylenedibromide (EDB J
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From page 74... ...
Thus, indices, such as AF, derived from simplified models may be adequate for vulnerability assessments because model errors are likely to be acceptable, in this case for relative rankings of contamination potentials. Data errors, and their effects on uncertainty in vulnerability assessments, have been examined in some detail for the AF and RF indices.
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From page 75... ...
Estimates of uncertainty were determined by first-order uncertainty analysis of the pesticide parameter KoC and soil parameters (e g, Pb, foe, and ~FC) - Note the large change in the RF map rating classes, largely resulting from uncertainty in foc and KoC The large change in vulnerability ratings shown in Figure 3.4b, caused simply by accounting for a single standard deviation due to data errors, suggests the need for considerable caution in making regulatory decisions that distinguish between vulnerabilities in adjacent cells.
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From page 76... ...
The practical significance of uncertainty analyses in providing reliable and cost-effective vulnerability assessments for resource managers and regulatory officials can be summarized as follows (Heuvelink et al.
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From page 77... ...
Thus, in many cases uncertainty analyses cannot be performed or themselves contain uncertainty. Despite these limitations, uncertainty analyses should be done to the extent practicable when conducting vulnerability assessments and can be useful in determining the impact of different levels of uncertainty on the results of the vulnerability assessment.
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From page 78... ...
Second, it is likely that the available data will be much more detailed at the field scale than at the regional scale. For comprehensive evaluation of a regional vulnerability assessment model, the application at the field-plot scale should be based on the same type and detail of data as exists at the regional scale to the greatest extent possible.
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From page 79... ...
evaluation should be an established protocol for environmental quality models in all media, recognizing that the particular implementation of this may differ for surface water, air and ground water quality models" (EPA 1989) (Note: Parenthetical text added by authors)
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From page 80... ...
Ultimately, the purposes of model testing and evaluation are to identify the level of confidence in the form and structure of the model and to provide statements on the appropriate use of the model and its outputs. Regional-Scale Testing and Evaluation Ideally, regional vulnerability assessments could be tested against field observations of vulnerability to lead to improved methods and a better understanding of the factors affecting aquifer vulnerability to contamination.
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From page 81... ...
, who compared the frequency of detections of DBCP to a vulnerability index applied to part of the San Joaquin Valley in California (see Box 3.4~. The use of ground water quality data to examine differences among vulnerability classes should be done with considerable caution for a number of reasons.
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From page 82... ...
The utility of the LPI index for making regional-scale vulnerability assessments was evaluated by Meeks and Dean (1990) for a 381 square mile (975 km2)
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From page 83... ...
TABLE 3.9 Percentage of Wells Tested with Detectable DBCP Categorized by Leaching Potential Index (Adapted from Meeks and Dean 1990. Evaluating ground-water vulnerability to pesticides.
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From page 84... ...
In addition to analytical errors, the accuracy of the water quality data is constrained by how ground water samples are taken (Nelson and Dowdy 1990~. The methods for obtaining representative ground water samples are relatively controversial, and errors can occur when: (1)
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From page 85... ...
Thus when water levels are low, only a small water column will be in the screened portion of the monitoring well. When water levels are high, the water table will still be in the screen, although there will be a larger column of water in the well (Driscoll 19861.
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From page 86... ...
Given all the uncertainties of sampling, standard monitoring wells may in fact provide only relative qualitative information on the concentrations of many contaminants. COMPUTING ENVIRONMENTS FOR VULNERABILITY ASSESSMENTS Regardless of the approach chosen, a large quantity of data (attribute and geographic)
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From page 87... ...
Building and Making Databases Available for Assessment Approaches Ground water vulnerability assessments require the input of data into an assessment approach where they are manipulated or analyzed to produce a measure of vulnerability. The large volumes of data often needed have led to a growing demand for computerized data sets and the development of computerized databases.
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From page 88... ...
More useful may be a definition of the system GIS is the computer hardware, specialized spatial database software, database management system, spatial and attribute databases, and applications software that is interfaced or integrated with the GIS software, the data, and the people necessary to operate the system. This definition is illustrated in Figure 3.7.
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From page 89... ...
APPROACHES TO VULNERABILITY ASSESSMENTS it. · Workstation and peripherals Spatial database management tools Nonspatial database tools Soils · Field boundaries Topography Orthophotography · Roads Streams · Hydrogeology · Weather stations · Management practices Soil characterization · Pesticide properties Temperature & precipitation Soil and water conservation planning Ecosystem management Land use planning Water quality modeling Cultural resource management Expert knowledge · Interpretations Decision making Understanding the system 11 - ~ Rev ~.~/ /)
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From page 90... ...
County soils information is included for almost 8,O00 soil series with information on soil layering, soil texture class, percent sand, percent clay, bulk density, percent organic matter, available water, hydrologic soil group, and potential crops. From these basic data extracted from NSSAD/SIRS, DBAPE provides procedures for calculating model parameters for wilting point, water content at field capacity (0.1 and 0.33 bar tension)
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From page 91... ...
As discussed in the section on Uncertainty in Vulnerability Assessment Methods, such errors may substantially increase the uncertainty of the vulnerability assess ment. Geographical Display of Assessment Results In most ground water vulnerability assessments, the results are portrayed on a map of the study area, typically in the form of polygons shaded or colored to depict the levels of vulnerability for all locations on the map.
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From page 92... ...
The uncertainty associated with the color intensities should be included on the map legend. Analytical Functions GIS technology can support ground water vulnerability assessments in the analysis and modeling of spatial and physical relationships of critical environmental elements.
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From page 93... ...
In these examples, the computations are applied across the map, with the equations applied independently for each point on the map. Although the concept of adding two maps or multiplying several maps may seem unusual, this is a routine capability of GIS technology and is duplicated in most non-GIS based approaches that attempt to deal with the spatial distribution of simple models like USLE.
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From page 94... ...
Furthermore, limited field experimentation with pesticide simulation models suggests that models based on simplified process representation may be more useful for many vulnerability assessments than more complicated models. Most approaches for ground water vulnerability assessment assume undisturbed surficial deposits with spatially uniform percolation.
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From page 95... ...
Overlay and index methods are based on assumptions that a few major factors largely control ground water vulnerability and that these factors are known and can be weighted (explicitly in index methods or implicitly in overlay methods)
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From page 96... ...
This conclusion may be summarized as the Third Law of Ground Water Vulnerability: The obvious may be obscured and the subtle indistinguishable. Uncertainty in vulnerability assessments needs to be better recognized and revealed in the outputs.
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From page 97... ...
Several approaches for vulnerability assessments are available, and each has its own strengths, and limitations. All approaches combine uncertainty and should explicitly capture or reflect that uncertainty.
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From page 98... ...
National Water Summary 1986- Hydrologic Events and Ground-Water Quality. Water-Supply Paper 2325.
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From page 99... ...
1990. Effects of land use and ground-water flow on shallow ground-water quality, Delmarva Peninsula, Delaware, Maryland, and Virginia.
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From page 100... ...
1992. Agricultural Chemical Use and Ground Water Quality: Where Are the Potential Problems?
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From page 101... ...
1990. Evaluating ground-water vulnerability to pesticides.
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From page 102... ...
1991. Ground Water Vulnerability Assessment: Snake River Plain, Southern Idaho.
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From page 103... ...
1987. A new model for the evaluation of groundwater vulnerability to non-point contamination by pesticides.
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