Water Stewardship

Ground Water Resources of British Columbia

Chapter 5 — Ground Water Contamination: Occurrence, Sources and Transport of Contaminants in Ground Water

by

R. Allan Freeze, James Atwater, and Hugh Liebscher

5.1 INTRODUCTION

The first four chapters of this report have introduced the hydrogeological principles that control the storage and movement of ground water. Application of these principles in the development of ground water resources requires the identification of high permeability geological formations that can serve as aquifers, and an assessment of the quantity and quality of ground water that can be developed from them.

British Columbia is blessed with many major aquifers that have the potential to yield large quantities of water to serve domestic, industrial, and agricultural needs. In most cases, the natural chemical quality of ground water in the province remains very good. However, if ground water is to continue to play an important role in serving the water needs of the province, then it will have to be protected from the subsurface contamination that accompanies growth in population and increased industrial and agricultural production.

Pupp (1985) and Beak Consultants (1986) have produced a preliminary compilation of specific ground water contamination events in Canada, including those known to have occurred in British Columbia. According to their lists, the number of documented cases of ground water contamination in British Columbia is limited. In this chapter, we present an updated compilation. Even with respect to this updated list, the known sites of contamination are probably greatly outnumbered by those that have not yet come to light. Contamination incidents usually occur in areas of intensive agricultural production, and in the more highly industrialized and more densely populated regions of the province. These are the very regions where the greatest demand for increased ground water development is likely to occur. In a recent assessment of ground water contamination in Canada, Cherry (1987) provides a gloomy prognosis. He predicts that: (1) aquifer contamination that already exists will gradually spread; (2) many water supply wells that are not presently known to be contaminated will be identified as being contaminated; (3) many aquifers that are not now contaminated will become contaminated; and (4) the discharge of contaminated ground water into wetlands, streams and lakes will increase. He concludes that public concern and fear with regard to contamination will increase, fueled by the seemingly unexpected occurrences of contamination and the inability of government and industry to predict trends or to resolve the problem.

If British Columbia is to avoid being a part of the unfolding of this unpleasant ground water scenario, then it is important that the citizens and decision makers of the province understand the nature of the problem, and that they be aware of the technical and regulatory tools that are available to mitigate the occurrences and impacts. This chapter is a first attempt to provide such information. This introductory section is followed by three additional sections: Section 5.2 describes the occurrence of ground water contamination, and provides an overview of ground water contamination problems in British Columbia; Section 5.3 discusses the principles of contaminant transport; and Section 5.4 discusses available preventative measures for the protection of ground water quality.

5.2 OCCURRENCE OF GROUND WATER CONTAMINATION

5.2.1 Definition of Ground Water Contamination

Water is a solvent for many chemical constituents. As a result, most ground water contain a wide variety of dissolved inorganic chemicals (and to a much more limited extent, natural occurring organic constituents) due to the dissolution of the geological materials through which it flows. Concentrations usually lie in the parts per million (mg/L) range down to fractions of parts per billion (µg/L). Chapter 3 provided a summary of the usual ranges of concentration of the naturally occurring, dissolved, inorganic constituents of ground water. These dissolved constituents are responsible for the character of the water, be it soft, hard, tasty, or smelly. The distinct character of many of the "natural mineral waters" available for purchase is a reflection of the particular mix of dissolved constituents.

The chemical make-up of ground water is a reflection of where the water has been, and what kind of material it has flowed through or over. It depends on whether it first fell onto a farmer's field, or onto an undisturbed forest; and whether it has passed by an underground storage tank, or first served us in a household or industry before being discharged again to flow in a river or through the ground. The ground, and the water that makes up ground water, are intimately connected, and as we alter the ground water or the path that the water takes, the chemical constituent make-up of the water changes.

Ground water contamination is said to occur when the chemical constituent make-up of the ground water is altered as a result of man's activities, either directly, due to the spill or leakage of a liquid, or indirectly, through the alteration of the ground through which water passes, as is the case for agricultural fields on which fertilizers and pesticides have been applied.

In some cases, the alteration may raise the concentrations of dissolved chemical species already present. In other cases, the alteration may introduce new chemical species not formerly present in the natural background ground water chemistry.

Ground water contamination causes degradation of water quality. It becomes important when the concentrations of one or more of the constituents reach levels that render the water unsuitable for its intended use. The critical chemical species and the maximum acceptable chemical concentrations depend on the intended use. The concentrations that define suitability are often encoded in water quality standards or guidelines. Legally enforceable water quality standards exist in most jurisdictions for drinking water. Less stringent water quality guidelines often exist for water that is to be used in specific industrial or agricultural processes. The standards, guidelines and regulations pertaining to ground water quality in British Columbia are discussed in Section 5.2.4.

The question of what level of ground water quality contamination renders a water unsuitable for its intended use is not straightforward to answer. Certainly, if concentrations exceed standards, contamination has occurred; but then standards tend to change through time, and they often vary from one jurisdiction to another. More importantly, there are large numbers of chemical constituents for which standards have not yet been set. In such cases, the onset of concern may well be a function of the intended use for the water, and the available knowledge concerning the chemical species in question. Like the standards, both these aspects are liable to change through time and vary from one place to another. In a political sense, ground water contamination is a de facto event; if a realistic concern about ground water quality is raised, and if it is linked to man's activities, then contamination has occurred.

5.2.2 Potential for Ground Water Contamination and Loss of Water Quality

The very act of living results in changes to the world around us. The acquisition of resources to sustain us and the discharge of our wastes cause alteration to the surface environments and, therefore, result in alteration of the underlying ground water.

It is impossible to conceive of a situation in which our activities will not result in the alteration of the chemistry of ground water. That change may be the addition of one molecule of a particular material to a litre of water, a change so small that we can accept the reality of the possibility, but we cannot measure the change or the consequence of the change. On the other hand, the consequence of the activity may be the degradation of the ground water to such an extent that we can no longer use the water.

The very soils and geologic material that dictate the chemical constituent make-up of a natural ground water also affect the impact of chemicals introduced by human activities. This concept will be discussed in some detail in the section on contaminant migration, but it can be said at this point that an activity carried out in one geologic environment may result in undisputed ground water degradation, whereas the same activity carried out in another geological environment may result in negligible alteration of the ground water chemistry. With respect to the potential for ground water degradation from human activities, we then have two aspects: the activity itself, and the soil and geologic environment in which the activity takes place.

Sources of ground water contamination can be classed as: (1) point sources, or (2) non-point sources. Most so-called point sources arise from waste management facilities of one kind or another. Such facilities may be quite large, creating sources of a square mile or more; they are "point" sources only in the sense that they are concentrated and areally bounded. Among the types of facilities that have the potential to contaminate ground water are: (1) sanitary landfills for solid non-hazardous municipal waste, (2) chemical landfills for soil and liquid hazardous industrial waste, (3) tailings ponds for slurried mining waste, (4) sewage lagoons and rapid infiltration ponds for liquid municipal waste, and (5) near surface buried tanks for liquid industrial waste. In all cases, waste fluids (or leachates formed from the passage of rainwater and snowmelt through the waste) can infiltrate across the unsaturated zone and deliver contaminated water to the water table. Examples of non-point sources of ground water contamination include the areal application of herbicides, pesticides and fertilizers in agricultural areas.

In British Columbia, there are a number of activities, none of them particularly unique, that have led to ground water contamination. A suite of examples of contamination resulting from each of these activities can be found in Table 5.1. This information was compiled as of late 1987. Corrective actions subsequent to 1987 may have been taken and would not be noted. The following discussions should be read in concert with Table 5.1.

1. Agriculture

The application of fertilizers and pesticides to land to facilitate the growing of crops presents a non-point source that makes wide areas covered with these materials available for leaching. The raising of animals normally results in the generation of large quantities of waste. The spreading of these wastes on the land as fertilizers, and their storage, also constitutes a source of pathogenic material. In respect to the agricultural industry, nitrogen in the form of nitrate is the most significant contaminant, and the one that is appearing on the widest scale. Pesticides are starting to be found in British Columbia at trace levels, but as yet they have not appeared on a wide basis.

2. Sewage Disposal

The use of septic tanks and subsurface disposal fields is a common and widespread form of domestic sewage treatment and disposal in British Columbia. It is generally practiced everywhere outside the major urban centres, and even to a limited extent, in enclaves within urban centres. Phosphorous, nitrogen, and pathogenic materials are the principal constituents of concern in septic tank effluent, with nitrogen as nitrate the most significant contaminant occurring in the provincial ground waters.

Contamination of water supplies with bacteria from septic tank effluent is common, although it is generally very local in extent. Septic tank effluents are thought to be a significant source of phosphorous in ground waters entering lakes in the Okanagan Basin.

3. Solid Waste Disposal Sites

Where solid waste has been landfilled, water passing through the emplaced material leaches chemicals from the waste and this "leachate" can subsequently move from the landfill into surface water or ground water. The chemical constituents in the leachate reflect the kinds of materials landfilled and the age of the landfill. Every conceivable inorganic and organic material may be found in leachate, although they are not all found in all leachate. Landfills constructed in the past constitute significant point sources of contaminants since little attempt was made in earlier years to control the types of materials landfilled or the generation and movement of leachate. Landfills constructed today are not as significant a risk to ground water, primarily because of the closer scrutiny of placed materials. Fortunately, most, but not all, of the major landfills in British Columbia have been sited where they do not impact significantly on major ground water resources.

4. Mining

Widespread ground disturbances that result from mining activities can alter the flows of ground water. From a chemical alteration point of view, the most significant problems arise where acid mine drainage develops. The mining process can, in certain circumstances, result in a situation where sulphuric acid is produced as a result of chemical and biochemical action in the presence of water and oxygen. The sulphuric acid depresses the pH of the water and enhances its ability to dissolve metals. A number of these situations exist in the province. In addition, most metal mines in British Columbia have tailings ponds which create large areas of fine grained materials containing metals that are subject to leaching.

Nitrates are also available for leaching from mines. A commonly used bulk explosive consists of a mix of ammonium nitrate and diesel fuel. The nitrate source occurs as a residual on the broken rock. Increases in nitrate levels are primarily seen occurring from the mines in the south-east corner of the province.

5. Industrial Processing and Product Storage

The handling and storage of bulk chemicals is a common source of ground water contamination. Regardless of whether the chemicals are a consumptive commodity, such as gasoline, a feedstock for a particular process, or an intermediary within some process, leakage out of storage and spillage during transfer occurs. As often as not, the storage and transfer facilities are underground and losses can go undetected for extended periods. Where industrial facilities have been in place for extended periods of time, chemical loss will be extensive irrespective of how good the plant housekeeping has been. Underground storage tanks at service stations bring the sources into residential and rural settings, so the problem is not limited solely to industrial areas. Loss of petroleum derived products is the most widespread type of leakage in British Columbia. Leakage and spillage of wood preservatives, primarily in the form of chlorinated phenols, has been a common occurrence in the forest products industry.

6. Transportation

Three activities related to the transportation industry routinely result in chemical releases to the ground. The most dramatic are spills resulting from accidents on both road and rail. There have been significant increases in the types and quantities of various material being transported within the province, and as a consequence, when accidents occur, they more frequently involve commodities that can result in contamination of ground water. The highly publicized ethylene dichloride spill from a rail accident outside of Langley is perhaps the most notable contamination, particularly of rural water supplies. Secondly, transmission line and railroad right of ways are routinely sprayed with herbicides. The large areal extent of these spraying activities can potentially result in ground water contamination. The third activity is the application of de-icers at airports and on roadways. At airports, urea is used on the runways which can result in nitrate contamination. Sodium chloride or calcium chloride is normally used for road de-icing.

Of the above activities, only leachate from solid waste facilities and the bacterial content of sewage are explicitly recognized in existing regulations. Attempts are being made to deal with the acid mine drainage problem. These three are all seen as problems related to waste products and tend to fall into waste management regulations. With the exception of recent efforts to control fugitive chlorinated phenols, no such net exists for contamination that results from non-waste generating practices.

5.2.3 Risks Associated with Contaminated Ground Water

The occurrence of contaminated ground water constitutes a risk to human health. Among the many organic and inorganic chemicals that have been detected in ground water are liver and kidney toxicants, known or suspected carcinogens, and chemicals capable of damaging the reproductive and central nervous systems. Actual health damage requires that a pathway exist for the toxic chemical species from the source to the human organ. In order to make a quantitative estimate of the risk that such a pathway exists, it is necessary to understand: (1) the physical, chemical and biological properties of the contaminant species, (2) the principles of contaminant transport through the hydrogeological environment, (3) methods of estimating human exposure through ingestion, inhalation or dermal contact, and (4) the toxicological principles of dose response under acute and chronic conditions. Emphasis in this chapter is on the first two links in this chain. It is hoped that the last two links can be avoided by the setting and meeting of ground water quality standards, and by timely and effective remedial action at sites where standards have been exceeded.

Contaminated ground water can also constitute an environmental risk. If it discharges into surface bodies of water at sufficiently high concentrations, it can contribute to fish kills, and it may have a deleterious effect on animal populations that drink such water. In addition, because many plants have a limited tolerance to specific metals and organics, ground water contamination can also lead to negative impacts on plant life and crop yields.

Ground water contamination represents a cost to society in that the water is no longer available for beneficial use. However, at prevailing market rates, the value of clean water is not particularly high, and it is likely that the public demand for ground water cleanup is based on a desire for risk reduction with respect to human health rather than a desire for the preservation of water resources. In fact, unless a large worth is assigned to the reduction of health risk, it is hard to develop economic justification for remedial action at many contaminated sites. Remedial costs at waste management sites can commonly reach $5-10 million, and at some are as high as $100 million or more. In cases where ground water supplies have been lost through contamination, the costs of remediation and/or alternative supplies can reach $10,000-50,000 per household.

5.2.4 Standards, Guidelines and Regulations Affecting Ground Water Quality in British Columbia

There is no federal or provincial legislation directly related to the prevention of ground water contamination. There are, however, government acts and programs at both levels that provide standards, guidelines and regulations that have an indirect bearing on ground water quality. They can be classified into three groups: (1) drinking water quality standards, (2) federal statutes, and (3) provincial statutes.

The British Columbia Drinking Water Quality Standards (B.C. Ministry of Health, 1982) grew out of a federal-provincial task force that updated the 1978 Canadian drinking water guidelines. Although they are called standards, they are actually guidelines in that they are not legally enforceable. Two types of recommended limits are specified: (1) Maximum Acceptable Concentration, and (2) Objective Concentration. Drinking water that contains substances in concentrations greater than the Maximum Acceptable Concentration is either capable of producing deleterious health effects or is aesthetically objectionable. The Objective Concentration is interpreted as the ultimate quality goal. Both types of limit are specified for: (1) Coliform Organisms, (2) Chemical Substances Related to Health, (3) Pesticides, (4) Radionuclides, (5) Physical Characteristics, and (6) Substances Related to Aesthetic and other Considerations. Table 5.2 reproduces the recommended limits for Chemical Substances Related to Health. The list emphasizes inorganics, including nitrates and several heavy metals, but it lists only three classes of organics.

There are two federal statutes that could conceivably play some role in protecting ground water quality. The Fisheries Act makes it an offence to deposit a deleterious substance in water frequented by fish. Landfill owners whose leachate enters surface streams by ground water flow paths could presumably be prosecuted under the Fisheries Act. The Canadian Environment Protection Act empowers Environment Canada and the Department of National Health and Welfare to compile of list of substances that cannot be manufactured or imported, except for designated uses. Thus far, five substances have been listed: (1) polychlorobiphenyls, (2) polybromobiphenyls, (3) polychlorinated terphenyls, (4) mirex, and (5) chloroflourocarbons. In the long run, this bill will reduce the environmental burden of these materials.

A the provincial level, British Columbia ground water quality is protected by provisions of: (1) the Environment Management Act, (2) the Environmental Management Act, (3) the Health Act, and (4) published Pollution Control Objectives.

The Environment Management Act gives the Ministry of the Environment the right to require Environmental Impact Assessments for any activity that may have adverse environmental impact. It also mandates Environmental Protection Orders that can be used to delay or stop activities which could have negative environmental consequences, and Environmental Emergency Measures that can be invoked in the case of an environmental emergency such as a major chemical spill.

The Environmental Management Act states that no waste may be introduced into the environment without a permit. Water Managers who adjudicate applications for permits under the Environmental Management Act are guided in part by Pollution Control Objectives, originally issued by the now superceded Pollution Control Board. Pollution Control Objectives have been published for: (1) Chemical and Petroleum Industries, (2) Forest Products Industry, (3) Mining, Smelting and Related Industries, (4) Food Processing, Agriculturally Orientated and Other Miscellaneous Industries, and (5) Municipal Type Waste Discharges. These publications outline minimal requirements for the siting and design of solid waste landfills, exfiltration basins, septic systems above 5,000 gal/day, and effluent disposal on land. Consideration of ground water conditions is limited to statements that facilities be 4 feet (or in some cases 5 feet) above the water table, and that siting take into account hydrogeological considerations.

Sections of the Health Act deal with small ground disposal systems (less than 5,000 gal/day) and dictate isolation distances for disposal fields with respect to placement above the water table and proximity to wells.

In 1983, the Ministry of the Environment issued updated Pollution Control Guidelines for Municipal Effluent Application to Land. In this publication, the importance of potential ground water contamination is recognized. The guidelines are divided into two sections, one for Disposal Without Significant Ground Water Recharge, and one for Disposal With Significant Ground Water Recharge. These guidelines also mandate ground water monitoring programs to determine both short- and long-term changes in ground water quality in response to waste disposal activities.

In order to determine baseline ground water quality in the province, the Ministry of Environment maintains a network of 145 observation wells (as of 1985). Systematic water quality sampling for all wells in this network was initiated in 1975. In addition, a new ground water quality monitoring program was initiated in 1983 to obtain baseline water quality in major developed aquifers.

5.3 PRINCIPLES OF CONTAMINANT TRANSPORT

Most ground water contamination arises from sources at the ground surface. Landfill leachates, spills, and non-point agricultural contaminants are all introduced at or near the surface, and before they enter the ground water system, there is an initial percolation phase in which the contaminants move downward through the unsaturated soil moisture zone to the water table. The unsaturated zone plays a significant role in determining what percentage of the mass of the contaminants migrates from the ground surface to the underlying ground water. Some materials, such as some fertilizers and pesticides, can initially find their way into the ground in a dry state, or at a moisture content or total volume, that allows for considerable retention in the unsaturated zone. Subsequent movement of the constituents requires dissolution in the soil moisture and downward percolation under the influence of infiltration due to precipitation or applied irrigation water. Geochemical conditions and transport mechanisms in the unsaturated zone are quite complex and they will not be discussed in detail here. Our emphasis will be on migration of contaminants in ground water, after they have reached the water table by percolation from above.

5.3.1 Properties of Contaminants

From the point of view of understanding contaminant migration in ground water, it is necessary to differentiate between three types of contaminants: (1) soluble contaminants that dissolve in ground water, (2) insoluble contaminants that are lighter than water, and (3) insoluble contaminants that are heavier than water.

Most inorganic and many organic chemical constituents fall into the first category. They dissolve in ground water and form elongated plumes of contamination that arise at a point source and stretch out in the direction of the ground water flow paths. The formation and behaviour of contaminant plumes are discussed in the next section.

Many petroleum products, and especially gasoline and its derivatives, fall into the second category. They do not dissolve readily in ground water but rather occur as a separate phase, and because this phase is lighter than water, it floats. Contaminants of this type occur as pools and thin films floating on the water table surface.

Many chlorinated solvents, most pesticides, and many other types of liquid organic contaminants, fall into the third category. They occur as a separate phase that is heavier than water. Such contaminants sink through course grained sand and gravel aquifers and pool on the top of fine grained clay and silt aquitards.

The transport of insoluble contaminants, both lighter than water and heavier than water, is treated in Section 5.3.3.

Table 5.3 lists a variety of common organic contaminants together with their solubility and specific gravity. Those that have a solubility greater than 1000mg/l are highly soluble; those that have a solubility less than 100mg/l are practically insoluble. For the insoluble contaminants, those with a specific gravity less than 1.0 are "floaters"; those with a specific gravity greater than 1.0 are "sinkers".

5.3.2 Soluble Contaminants; the Formation and Behaviour of Contaminant Plumes

When a soluble contaminant species is introduced into a ground water flow system, there will be an increase in concentration of that species relative to its background concentration. This zone of increased concentration constitutes a contaminant plume. For a point source, this zone will initially be quite small. In the case of agricultural sources or septic tank fields, the many overlapping point sources may coalesce to form a very large plume. If the source provides a continuous supply of contaminants, the plume will grow in size as a function of time, and the plume front will migrate through the ground water flow system. The rate of migration of the contaminant plume is controlled by three transport mechanisms: (1) advection, (2) dispersion, and (3) retardation.

Advection is the primary transport mechanism in high permeability aquifers. The term refers to the carrying along of dissolved contamination by flowing ground water. The rate of advective contaminant migration is equal to the rate of ground water flow. As shown in Figure 5.1a, the result of advective transport from a point source is a "plug-flow" situation in which the contaminant plume has sharp boundaries, with concentrations equal to the source concentration Co inside the plume and concentrations equal to background (taken as zero on Figure 5.1a) outside the plume. If a monitoring well were installed so that samples could be taken at point X* at regular intervals through time, the concentration, C(t), measured there could be used to develop a breakthrough curve like that shown in Figure 5.1c. For advective transport, the breakthrough curve is a step function, with C=0 up until t=t* and C=Co, thereafter. It is common to plot breakthrough curves in terms of relative concentration, C/Co, so that C/Co=0 until t=t* and C/Co=1, thereafter.

In reality, dissolved contaminant plumes tend to spread laterally and longitudinally. This spreading process is called dispersion. Comparison of Figure 5.1b with Figure 5.1a shows that the effect of dispersion is to create a large plume of lower concentration. The breakthrough curve at X* now shows an earlier arrival of the first contamination, but the concentration at X* at time t=t* is now only half what is was under advection alone. For the range of leachate concentrations commonly encountered, and for most common contaminants of interest, the relevant water quality standards usually translate into a relative concentration of C/Co=0.1 or less. Under these circumstances, the time of travel for a critical concentration to reach some critical distance from a specific source will be less in a dispersed plume than in an advective plume.

Many ground water contamination incidents resulting from point sources occur in high permeability surficial sand and gravel aquifers underlain at shallow depth by clay silt aquitards. In these cases, flow paths are near horizontal, advective velocities are large, and lateral dispersion tends to be very small. The result for a source of small to moderate size is a long, skinny plume with relatively sharp boundaries. Longitudinal dispersion creates concentration gradients parallel to the flow path, but observed travel times are nevertheless close to those calculated assuming advective flow alone.

Advective flow velocities are calculated from Darcy's law:

v = Ki/n

where

v = advective velocity [L/T]
K = hydraulic conductivity of aquifer [L/T]
n = porosity of aquifer [decimal fraction]
i = hydraulic gradient [decimal fraction]

In most hydrogeologic settings, hydraulic gradients in aquifers lie in the relatively narrow range, 0.01 to 0.001. Aquifer porosities in unconsolidated deposits also lie in a narrow range around 0.3. Advective velocities are therefore highly dependent on the value of the hydraulic conductivity of the geologic materials, which can vary over many orders of magnitude (Freeze and Cherry, 1979). In sand and gravel aquifers, advective plume migration rates of 10 to 100 m/y are not uncommon. In less permeable silts and clays, migration rates may be less than a few cm/y. In fractured rocks porosities tend to be much smaller, and advective migration rates can become quite large, even if the hydraulic conductivity is low.

Dispersion is apparently caused by a combination of two phenomena, both of which are related to the presence in most aquifers of lenses and layers of lower permeability, fine grained material. The first, which affects lateral dispersion, is the divergence of flowlines around such lenses, leading to a natural spreading of dissolved constituents. The second, which affects longitudinal dispersion, is the capture of some of the dissolved constituents through molecular diffusion into and out of the fine-grained layers.

Retardation is a catch-all term that describes the reduction in migration rates due to a wide variety of chemical reactions between the contaminated water, the native water, and the geological materials. These include adsorption desorption reactions, solution-precipitation reactions, oxidation-reduction reactions, and biochemical transformations. The effect of retardation on a breakthrough curve is shown in Figure 5.1e. For a retardation factor of three, migration rates are reduced threefold and travel times are increased threefold.

One of the most common causes of retardation is adsorption of contaminant species from the pore water onto the grains of the porous medium. This process is most effective in fine-grained geological materials with high organic content. Retardation factors are contaminant dependent. They are highly correlated with the octonal water partition coefficient, which is a chemical property easily measured in the laboratory. Table 5.3 includes the octonal water partition coefficients for some organic contaminants. Those species with value near 1.0 are only slightly retarded in the presence of organic water; those species with values above 3.0 are significantly retarded.

A contaminant spill is a point source that differs from that produced at a waste management facility in that it is not continuous. A spill introduces a slug of contaminant into the ground water flow system. In the ideal case, it will take on an elliptical shape that is elongated in the direction of flow. The centre of mass will migrate at the advective flow rate, or at a reduced rate under the influence of retardation. With time, the size of the ellipse will increase and the concentrations will decrease, under the influence of dispersion.

It is possible to provide a more quantitative description of contaminant transport in ground water than has been presented here. Such a description would be in the form of a set of equations that describe these phenomena. In modern hydrogeological practice, these equations are the foundation for computer models that are used to predict the migration of contaminant plumes in ground water flow systems. Such models have proven useful in the analysis of point source ground water contamination events and in the design of remedial action at contaminated sites.

5.3.3 Insoluble Contaminants

Oil spills and leakage from gasoline tanks result in the downward seepage of petroleum products across the unsaturated zone to the water table (Figure 5.2a). Because they are lighter than water and relatively insoluble, the oil or gas tends to accumulate on the water table and spread laterally along it. However, because many hydrocarbon fractions are not totally insoluble, the petroleum accumulations can act as a secondary source for a dissolved plume of hydrocarbon contaminants. Concentrations tend to be low, but even very low concentrations produce an unpleasant taste and odor.

Figure 5.2 Movement of insoluble contaminants in ground water

The same situation arises with respect to heavier than water contaminants that form pools at the base of aquifers on the top of underlying fine-grained aquitards (Figure 5.2b). The slight solubility of these contaminants is sufficient to produce plumes of dissolved constituents emanating from these subsurface sources. For many of the most common organics, even the very low concentrations that arise are bound to exceed the extremely low water quality standards that are being proposed for many organic contaminants.

Consider the common solvent, trichloroethene (TCE). The proposed maximum contaminant limit in the U.S. Safe Drinking Water Act is 0.005 mg/l. A plume 1,000 m long, 100 m wide, and 10 m deep at a concentration 10 times the standard contains only 50 kg of TCE. Serious ground water contamination over relatively large volumes of aquifer can arise from very small spills or leaching rates.

5.4 PREVENTATIVE MEASURES FOR THE PROTECTION OF GROUND WATER QUALITY

If the quality of the ground waters of the province are to be protected in the long-term, then some fundamental decisions must be made with respect to land usage in those areas where there are high permeability near surface aquifers. At this time in this province, the solid waste disposal facilities which are receiving a fair amount of environmental attention probably have less impact on the ground water resource than do the widely accepted practices of septic tank effluent disposal, traditional agricultural application of fertilizers, herbicides and pesticides, and the transportation and storage of commodities. Notwithstanding the above, there are accepted steps than can and should be taken in the short term to protect wells, to site disposal facilities, and to remediate areas that are presently contaminated.

5.4.1 Good Well Design

The British Columbia Ministry of Environment (1982) has published a set of Guidelines for Water Well Construction in British Columbia. From the point of view of protection of a well against the entry of contaminants, the essence of good well design involves three aspects: (1) an effective seal, (2) well disinfection, and (3) sampling and analysis of well water.

The guidelines recommend that all wells should be effectively sealed with grout to a depth of 15 feet if possible, such that the wellhead construction ensures that no contaminant can enter the well or seep down around the annulus that may exist between the drillhole and the well casing.

They also recommend that all wells be disinfected by the drilling contractor after completion. Before using the well water for consumption, the well should be pumped until the smell of chlorine from the chlorine solution used in the disinfecting process has dissipated.

Lastly, the well water should be sampled and analysed for chemical and bacteriological quality. Results should be compared against the British Columbia Water Quality Standards.

5.4.2 Siting and Design of Waste Disposal Activities

The prevention of ground water contamination from waste management facilities must be achieved through some combination of siting and engineering design.

In years past very little of either was done. Landfills were often sited in abandoned gravel pits; tailings ponds were sited purely on the basis of convenience; few precautions were taken to cope with spills at wood preservative dipping tanks. As a result, contaminated leachates were a common occurrence, and they often led to the formation of contaminant plumes in high permeability surficial aquifers.

There are an emerging set of design features that offer hope for improvement. Modern landfill design features basal liners, surficial caps and leachate collection systems. The basal liner can be a natural clay material or a synthetic polyethylene membrane. It is not uncommon in the United States for regulatory agencies to require more than one liner at the base of a hazardous waste landfill.

There is concern, however, that modern design technology does not really prevent ground water contamination, but merely postpones it. There is little solid information yet on the expected life of synthetic liners, for example, but estimates run in the range of 10 to 50 years. It is known that liners can be breached by pinhole flaws, improperly constructed seams, animal burrows, and breakdown by acidic or corrosive leachates. The next few decades will provide needed information on whether improved engineering provides a long-term solution to waste containment at land-based facilities.

Given these doubts about the effectiveness of improved design, the issue of siting takes on greater importance. It is clear from Section 5.3 that the rate of migration of contaminants in ground water is controlled by the hydraulic conductivity of the geological materials at the site. If land-based waste management facilities could be sited on thick, unfractured clay or silty clay deposits, then contaminant migration rates would not exceed a few centimeters per year. Good siting is the single most important step that could be taken to reduce ground water contamination from waste management facility. Unfortunately, siting issues tend to lie in the political arena rather than the technical arena, and there seems little likelihood that this situation will change.

If good siting cannot be guaranteed and good design is still unsure, the need for effective monitoring networks and good leachate collection systems becomes paramount. Land-based waste disposal facilities seldom contain waste fluids completely; their design must be based on a philosophy of leachate management.

5.4.3 Remedial Action at Contaminated Sites

Once a contaminant plume has migrated any distance into an aquifer, corrective action is far from simple. Remediation is expensive and technically difficult, and complete success is unlikely. There are no remedial case histories yet on record where the ground water quality in an aquifer has been returned to pre-contamination levels. Nevertheless, there are techniques available to arrest the further migration of contaminated water into uncontaminated regions, and to reduce the levels of contamination in areas already affected. Most techniques fall into one of two categories: (1) source control, or (2) plume control.

Source control attempts to stop or reduce the influx of contaminants into the ground water system at their source. For small sources, it can take the form of excavation and incineration. In cases where excavation is not economically feasible, containment is often attempted by the construction of vertical slurry-walls of low-permeability materials. Soil bentonite or cement bentonite mixtures are fed into deep narrow trenches around the source. The method is most applicable where a relatively shallow surficial aquifer is bounded at depth by a low permeability aquitard.

Plume control is carried out with a network of pumping wells. The wells serve two purposes: first, they remove contaminated water from the plume, and second, they alter the natural flow system by drawing the ground water flow lines into the wells, thus preventing downstream migration of the plume. The contaminated water produced by the pumping wells is treated on surface, usually by carbon adsorption or air stripping towers. In some cases, plume control systems involve injection wells as well as pumping wells. Often, the injection wells can be located in such a way that they provide a "waterdrive" to push contaminated water more efficiently toward the pumping wells.

5.4.4 The Need for Improved Regulation

Existing federal and provincial statutes, and the administrative structures that support them, do not provide an integrated and comprehensive program to protect ground water quality in Canada. Weaknesses in the regulatory process have been identified at the federal level by Cherry (1987), and at the provincial level by the Association of Professional Engineers of B.C. (1985). Among the specific identified needs are: (1) improvements in the drinking water standards, (2) improved handling of "special" wastes, (3) tighter monitoring requirements, (4) identification and remediation of contaminated sites, and (5) increases in funding for surveillance an enforcement.

Current drinking water standards do not reflect recent advances in knowledge on health risks associated with synthetic organic chemicals. The three organic groups listed on Table 5.2 represent a small subset of the organic pollutants currently under scrutiny. There is a need to update Canadian standards. However, it should be recognized that a tightening of Drinking Water Standards does not in itself protect ground water quality. It will protect the consumer and it may very well sensitize decision-makers to the cost of ground water contamination. The implementation of tighter standards normally results in the need to find new supplies or in increasing the level of treatment on the existing supply.

Provincial Pollution Control and Waste Management Objectives once allowed the disposal of hazardous and toxic wastes in landfills under certain circumstances. It is now recognized that separate waste facilities for these "special" wastes are needed and the overall requirements for management of such wastes is embodied in the new regulations. A provincial commission is investigating potential sites and disposal methods. It is worth noting, however, that the current Pollution Control Objectives for the Forest Products Industry do not take special note of wood preservative spills, and those for the Mining Industry to not address leakage from tailings ponds.

Protection of ground water quality in the vicinity of waste management sites requires careful monitoring. There is a need for tighter ground water monitoring requirements in revised Pollution Control Guidelines.

It is likely that there are cases of contaminated ground water in British Columbia that have not yet come to light. A concerted effort is needed to identify such sites and assess the health risks associated with them. There may be special problems associated with non-point contamination from agricultural pesticides and fertilizers.

The setting of regulations, and the issuing of permits that include specific conditions and restrictions, have little value if the regulatory agencies do not have adequate funds to check for compliance, and enforce regulations where compliance has not been achieved. Surveillance and enforcement have historically been the weak links in the application of Canadian environmental policy.

Ground water is a resource of the Crown in British Columbia. However, no rights to its usage, nor charge for its use exist, and as such, the Province does not at this time have a vested interest in protecting the resource for its own sake. In the long term, there is a need for an Act that protects the resource.

The existing legislation embodied in the Waste Management Act and the Health Act can be strengthened to minimize releases of wastes, but it would not be reasonable to use these acts to control agricultural practices, land-use issues, and commodity storage practices. Legislative action on these fronts is necessary if there is to be long term maintenance of the quality of ground water resources in British Columbia.

Information Sources for Table 5.1

Information is known to exist on each contamination incident, however all the information may not be readily available due to litigation or specific confidentiality agreements. Published material is subscripted with a "(p)".

1. Association of Professional Engineers (p)
2. Atwater (p)
3. Atwater Personal Communication
4. Beak (p)
5. B.C. Ministry of Environment and Parks (now Ministry of Environment) Survey
6. B.C. Ministry of Environment and Parks (now Ministry of Environment) — Ground Water Section
7. Burnett (p)
8. Consultants Reports to Clients
9. Dakin (p)
10. Environmental Protection — Pacific Region
11. IWD Data
12. Jasper (p)
13. NHRI Files
14. Okanagan Basin Study (p)
15. Pacific Hydrology Consultants
16. Reports to Environment Canada — Pacific Region
17. Reports to Waste Management Branch — Surrey
18. Soper (p)
19. Stroscher, Mark — Personal Communication
20. Vancouver Sun Newspaper (1986)

The authors wish to acknowledge the efforts and contributions of the numerous individuals who assisted in the acquisition of information for Table 5.1.

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