D. Fortalecimiento Institucional e Infraestructura
V. PROYECTOS ESTRATEGICOS DEL FMA
1. Alli Allpa: Programa para la promoción de cadenas productivas
MICHAELD. TROJAN Minnesota Pollution Control Agency
St. Paul, Minnesota
For this discussion, groundwater sensitivity is defined as the likelihood for contaminants to reach a specified position in the groundwater system after introduction at some location above the specified position (1,2). Sensitivity is a widely applied concept that has several uses. Sensitivity analyses often lead to the development of maps showing the relative sensitivity of groundwater across a geographic area. These maps are used as a screening tool for land-use decisions and have increasingly been used in source water protection. Sensitivity analyses are also used in program development, such as identifying areas where groundwater monitoring will occur. The concept of groundwater sensitivity also has educational value because hydrogeologic concepts are easily expressed and the audience can visualize relationships between land use and aquifer protection.
WHAT MAKES AN AQUIFER SENSITIVE TO CONTAMINATION?
Groundwater sensitivity applies to a variety of situations; each requires a different level of analysis. Sensitivity, vulnerability, and susceptibility, which are often defined differently, are used interchangeably in this discussion. The simplest scenario involves transport of a conservative contaminant in water from the land surface to groundwa- ter. This scenario can be modified to consider land use, account for attenuation or retardation of contaminants, consider a point below the top of the saturated zone, or consider risk to human or ecological receptors at some point within an aquifer.
Under the scenario where a conservative contaminant is transported from the land surface to groundwater, factors that affect sensitivity include the permeability of geologic materials, the thickness of the unsaturated zone, and recharge. Highly permeable geologic materials
transmit water quickly and thus increase sensitivity (see HYDRAULICCONDUCTIVITY/TRANSMISSIBILITY). In heterogeneous deposits, the geologic material that has the lowest permeability typically has the greatest effect on the rate of water movement, provided the geologic material is sufficiently thick, laterally continuous, and does not have extensive macroporosity. As the thickness of the unsaturated zone increases, sensitivity decreases because water can be stored and has a farther distance to travel. Sensitivity increases as the quantity of recharge increases because contaminants are transported by recharge water. Recharge is often computed from a water budget by subtracting evaporation from precipitation and assuming no runoff or soil storage.
Even in sensitive hydrogeologic settings, groundwater will not become contaminated unless there is a source of contamination. We can generalize about how groundwater quality will be affected by land use (see LANDUSEIMPACTS ON GROUNDWATER QUALITY). Agriculture can contribute significant quantities of nitrates and pesticides to an aquifer, nonsewered residential areas can contribute large quantities of nitrates and chlorides (3), and industrial and older sewered residential areas can contribute large quantities of volatile organic chemicals (VOCs), chlorides, and some metals such as manganese. Within each of these land uses, specific management practices further affect contaminant volumes. Several studies show direct, although not necessarily linear relationships, between the quantity of chemical applied at or near the land surface and concentrations in sensitive aquifers. Thus, for example, groundwater is more sensitive in areas that use greater fertilizer application rates.
Nearly all contaminants attenuate to some extent within the unsaturated zone. The extent and rate of attenuation depend on the properties of the contaminant and on the chemical, physical, and biological properties of the geologic materials (Table 1). These properties are too numerous to discuss individually, but adsorption and degradation are the two primary attenuation processes that affect contaminants. Contaminants that are not readily adsorbed percolate to groundwater and result in increased sensitivity. Except in highly weathered, acidic soils, these are chemicals that have a negative charge, such as chloride, nitrate, and acid herbicides (2,4-D, Dicamba). Contaminants that are quickly degraded biologically or chemically represent a lower risk to groundwater than more persistent contaminants. In addition to the properties of a chemical, degradation is affected by factors such as temperature, the presence of oxygen, and pH. For example, most halogenated contaminants are persistent except under reducing conditions, nitrate is persistent in the presence of oxygen, and benzene is persistent in the absence of oxygen.
We are often interested in the sensitivity at some point below the top of the saturated zone, such as at a well. In addition to having a greater distance to travel, a contaminant may be affected by geochemical changes within the saturated zone. Trojan et al. (4), for example, showed that in the glacial aquifers of Minnesota, nitrate was quickly attenuated in the upper few meters of groundwater, even though these aquifers were mapped
SENSITIVITY OF GROUNDWATER TO CONTAMINATION 57 Table 1. Characteristics of Different Classes of Contaminants. This Information is Generalized and Varies within Each Contaminant Class and Between Different Geologic Deposits and Hydrologic Environments
Contaminant Class Examples Adsorption Persistence Toxicity
Halogenated organics with one or more benzene rings
PCBs, dioxins, many pesticides, chlorobenzenes
High, increases with greater extent of halogenation
High, increases with greater extent of halogenation
High, increases with greater extent of halogenation Polyhalogenated
aliphatics
Industrial solvents (TCE, PCE), some pesticides
Low, increases with greater extent of halogenation
Moderate, increases with greater extent of halogenation
Moderate, increases with greater extent of halogenation Nonhalogenated
polynuclear aromatics
Pyrene, benzo(a)pyrene Moderate but increases rapidly with increasing molecular weight
Moderate but increases rapidly with increasing molecular weight Moderate but increases rapidly with increasing molecular weight Other nonhalogenated aromatics
Benzene, toluene Low Low to high, depending
on geochemical environment
Low except for some chemicals, such as benzene
Nonhalogenated aliphatics
Oil, alkanes, Moderate to high Low to high, depending
on geochemical environment
Low
Metals Lead, copper Moderate to high High Moderate to high
Nonmetals Arsenic, boron Moderate but varies
widely
High Moderate to high
Anions Chloride, nitrate Low High Low
Radionuclides Cesium-137, radon-222 High High High
as sensitive to contamination. In this case, denitrification was the most likely cause for disappearance of the nitrate. In other scenarios, contaminants may be adsorbed within an aquifer.
Few sensitivity methods consider the risk to receptors. Risk analysis does not consider whether a chemical will reach groundwater but whether it will pose a risk to receptors. Risk is a function of dose and toxicity. Thus, consumption of 1,1,2-trichloroethene (TCE) at a concentration of 5µg/L poses a greater risk than consumption at 1µg/L; consumption of TCE at 5 µg/L [maximum contaminant level (MCL)= 5 µg/L] poses a greater risk than consumption of xylene at 5000µg/L (MCL= 10,000 µg/L). Because we have to consider specific exposure points (e.g., a well), the quantity of contaminant being transported, and the contaminant toxicity, estimates of sensitivity based on risk can be very complicated.
Table 2 provides a summary of factors that affect groundwater sensitivity to contamination. Figure 1 pro- vides a schematic showing how different factors affect sensitivity. Figure 2 shows how sensitivity varies when different receptor points or contaminants are considered.
METHODS FOR ASSESSING SENSITIVITY
The Commission on Geosciences, Environment, and Resources (2) provides an excellent discussion of methods for assessing sensitivity. The most commonly employed methods are overlay and index methods. These involve combining various physical properties of the hydrogeologic system. Each property is assigned a score or other sensitivity value based on perceived sensitivity. Overlay and index methods use many of the factors in Table 2. DRASTIC (5) is a widely employed index and overlay
method that uses seven factors in the sensitivity assessment. Overlay and index methods can be relatively simple and may require small amounts of information. Sensitivity analyses using these methods often result in plan view maps depicting relative groundwater sensitivity, usually through a color-coded scheme. These maps are useful interpretive and screening tools. The scale of these maps is usually not appropriate for site-specific decisions, although they have been used for this purpose.
Analytic or numeric methods predict the time it takes contaminants to reach groundwater. These methods typically consist of mathematical models. Attenuation of chemicals is often considered. Examples include PRZM (6), GLEAMS (7), and LEACHM (8). An advantage of analytic and numeric methods is that they quantify the processes that affect the movement of water and contaminant. The accuracy of these methods depends on the quality of data used in the model.
Statistical methods use statistical techniques, such as regression analysis, to predict the likelihood of con- tamination. These methods require data on contaminant concentrations and may require additional information to derive sensitivity estimates. For example, concentrations of nitrate may be correlated with depth to water and sand content in the vadose zone. Because they use actual data, statistical methods can provide accurate estimates of sensitivity, information on variability in a sensitivity analysis, and estimates of certainty. They are, however, data intensive.
LIMITATIONS OF THE SENSITIVITY CONCEPT
In 1993, the Commission on Geosciences, Environment, and Resources prepared a report on ground water vulner- ability assessments (2). The Commission, ‘‘in struggling with the manifold technical and practical difficulties
58 SENSITIVITY OF GROUNDWATER TO CONTAMINATION
Table 2. Summary of Factors that Affect Groundwater Sensitivity
Property Effect on Sensitivity Low Sensitivity High Sensitivity
Hydrologic Factors Aquifer material Rate at which water
moves within an aquifer
Shale, most hard rocks, clay, silt
Limestone, sandstone, sand, gravel
Depth to bedrock Distance that water must travel to reach groundwater Large distances (>50 feet) Short distances (<10 feet) Depth to water Distance that water must
travel to reach groundwater Large distances (>50 feet) Short distances (<10 feet)
Recharge Amount of water
reaching groundwater
Small quantities (<1 inch)
Large quantities (>10 inches) Soil material Rate at which water
moves to or within groundwater
Clay, silt, organic soils Sand, gravel
Thickness of confining layers
Rate at which water moves to or within groundwater Large thickness (>20 feet) Small thickness (<5 feet)
Topography Amount of water
reaching groundwater
Upland areas where water runs off
Lowland areas where water accumulates
Type of bedrock Rate at which water moves to or within groundwater
Unfractured shale and hard rocks
Limestone, sandstone, fractured rock
Other Factors
Soil/aquifer material Attenuation of chemical High organic or clay content
Low organic or clay content Contaminant Persistence, mobility,
toxicity
Rapidly degraded, low mobility, low toxicity
Persistent, highly mobile, highly toxic
Land use Amount and type of
chemical released
Low chemical inputs (forest, grassland)
Large chemical input (row crop agriculture, nonsewered communities)
affecting the performance of vulnerability assessments today, nearly concluded that their limitations are so great as to be of no use in management decision-making.’’ Though the concept of sensitivity is applicable to land- use decisions, program development, or education, there are two important concerns over their use.
First, the analyses and resulting products, usually maps, are only as good as the data that go into them. Sensitivity analyses generally do not result in collection of new data and rely instead on existing information. Geologic maps, soil maps, and climate data are often available, but values for depth to water and hydraulic conductivity may be difficult to find. Data become more limiting when factors such as specific contaminants, land use, and risk are considered.
Second, maps or other products developed from sensitivity analyses can be misused. Many people who make land-use decisions rely on these maps. They often assume that sensitive areas will become contaminated and less sensitive areas will not, and they are often not aware of the way the maps were developed. Examples of misuse include
• using a 1:100,000 sensitivity map to site a landfill; • assuming that all areas mapped as sensitive have
equal sensitivity;
• assuming that the same data were available throughout a mapped area; and
• using a sensitivity analysis based on a conserva- tive tracer to predict sensitivity to contamination with pesticides.
Researchers have increasingly modified existing meth- ods to provide better tools to local users of sensitivity maps. Rupert (9) improved the effectiveness of a modified DRASTIC ground water vulnerability map by calibrating the point rating schemes to actual groundwater qual- ity data by using nonparametric statistical techniques and a geographic information system. Snyder et al. (10) used a particle tracking model in conjunction with DRAS- TIC to evaluate groundwater vulnerability. Erwin and Tesoriero (11) related the occurrence of elevated nitrate concentrations in samples from public supply wells to natural factors to assess aquifer susceptibility using logis- tic regression. Holtschlag and Luukkonen (12) used a vulnerability model that incorporated several contami- nant properties to predict atrazine leaching. Although researchers have generally focused on improving the accuracy of sensitivity estimates, the problems dis- cussed before can also be overcome by improving data accessibility, providing information about the reliability
Limestone aquifer has low sensitivity because overlying layer is clay
Unconfined sand aquifer aquifer has high sensitivity
Limestone aquifer moderately sensitive because overlying clay layer
is thin
Limestone aquifer highly sensitive because overlying layer is coarse sand
Limestone aquifer has low sensitivity because overlying layer is shale
Water table Sand
Shale Clay
Area where sand aquifer has highest sensitivity because of shallow water
table and more recharge
Limestone
Figure 1. Schematic representation of geologic factors that affect groundwater sensitivity.
A B
C D
E
Groundwater flow directions
Figure 2. Schematic illustrating how a variety of factors affect groundwater sensitivity. Considering transport of a conservative chemical, the groundwa- ter is considered sensitive at each of five well locations (A, B, C, D, E). At location A, the aquifer is most sensi- tive because the well is screened at the water table. The chemicals of concern are nitrate and pesticides from agricul- ture. At location B, the well may be less sensitive than at A because con- taminants can be degraded or adsorbed within the aquifer. The chemicals of concern are nitrate and pesticides from agriculture. At location C, the well may be less sensitive than at B if the con- taminant is degraded or adsorbed, but if the aquifer is fractured, sensitivity may be greater than at B. The chemicals of concern are nitrate and pesticides from agriculture. At location D, the well may be less sensitive than at all other well locations because the overlying land use does not contribute contaminants. Con- taminants introduced at other locations may be degraded or adsorbed before reaching the well. At location E, sen- sitivity is similar to that at B but the contaminants of concern are VOCs and trace elements from commercial, indus- trial, and residential land use.
60 WATER CONTAMINATION BY LOW LEVEL ORGANIC WASTE COMPOUNDS IN THE HYDROLOGIC SYSTEM of data, providing estimates of variability or uncertainty,
and training users.
SUMMARY
Understanding groundwater sensitivity and identifying areas where groundwater is sensitive to contamination is a potentially useful tool for managers, planners, and educators. Sensitivity analyses often lead to the production of maps showing relative sensitivity across a geographic area. Despite a potential for misuse, researchers have increasingly focused on improving the accuracy of these maps, including calibration, for local applications. BIBLIOGRAPHY
1. United States Environmental Protection Agency. (1993). A Review of Methods for Assessing Aquifer Sensitivity and Ground Water Vulnerability to Pesticide Contamination. US EPA, Washington, DC.
2. Commission on Geosciences, Environment and Resources. (1993). Ground Water Vulnerability Assessment: Predicting Relative Contamination Potential Under Conditions of Uncertainty. National Academy Press, Washington, DC. 3. Trojan, M.D., Maloney, J.S., Stockinger, J.M., Eid, E.P., and
Lahtinen, M.J. (2003). Effects of land use on ground water quality in the anoka sand plain aquifer of minnesota. Ground Water. 41(4): 482–492.
4. Trojan, M.D., Campion, M.E., Maloney, J.S., Stockinger, J.M., and Eid, E.P. (2002). Estimating aquifer sensitivity to nitrate contamination using geochemical information. Ground Water Monitoring and Remediation 22(4): 100–108. 5. Aller, L., Bennett, T., Lehr, J.H., Petty, R.J., and Hackett, G.
(1987). Drastic: A Standardized System for Evaluating Ground Water Pollution Potential Using Hydrogeologic Settings. EPA-600/2-87-035. US EPA, Ada, OK.
6. Carsel, R.F., Smith, C.N., Mulkey, L.A., Dean, J.D., and Jowise, P. (1984). User’s Manual for the Pesticide Root Zone Model (PRZM): Release 1. US EPA, Environmental Research Laboratory, Athens, GA.
7. Knisel, W.G., Leonard, R.A., and Davis, F.M. (1989). GLEAMS User Manual. Southeast Watershed Research Laboratory. Tifton, GA.
8. Wagenet, R.J. and Hutson, J.L. (1987). LEACHM: A Finite- Difference Model for Simulating Water, Salt, and Pesticide Movement in the Plant Root Zone, Continuum 2. New York State Resources Council, Cornell University, Ithaca, NY. 9. Rupert, M.G. (2001). Calibration of the drastic ground
water vulnerability mapping method. Ground Water 39(4): 625–630.
10. Snyder, D.T., Wilkinson, J.M., and Orzol, L.L. (1997). Use of A Ground-Water Flow Model With Particle Tracking to Evaluate Ground-Water Vulnerability, Clark County, Washington. USGS Water-Supply Paper 2488.
11. Erwin, M.L. and Tesoriero, A.J. (1997). Predicting Ground- Water Vulnerability to Nitrate in the Puget Sound Basin. USGS Fact Sheet FS-061-97.
12. Holtschlag, D.J. and Luukkonen, C.L. (1998). Assessment of Ground-Water Vulnerability to Atrazine Leaching in Kent County, Michigan: Review, Comparison to Results of Other Studies, and Verification. USGS Water-Resources Investigations Report 98–4006.