During recent years much of the emphasis in groundw ater investigations in industrialized countries has shifted from problems of groundw ater supply to considerations of groundw ater quality. As a result of our consumptive w ay of life，the groundw ater environment is being assaulted w ith an ever-increasing number of soluble chemicals. Current data indicate that in the United States there are at least 17 million w aste disposal facilities emplacing more than 6. 5 billion cubic meters of liquid into the ground each year. As time goes on，the vast subsurface reservoir of fresh w ater，w hich a few decades ago w as relatively unblemished by man's activities，is gradually becoming degraded.
The problem of w ater quality degradation of rivers and lakes has been evident for a long time. In general，solutions to this problem have been found in the implementation of effective legislation for discontinuing contaminant emissions. Already in some parts of the w orld， effective emission abatement measures have led to great improvements in surface-w ater quality. Unfortunately，problems of groundw ater quality degradation are in many w ays more difficult to overcome. Because of the heterogeneities inherent in subsurface systems，zones of degraded groundw ater can be very difficult to detect. The U. S. Environmental Protection Agency in 1977 reported that almost every know n instance of aquifer contamination w as discovered only after a w ater-supply w ell w as affected. Often by the time subsurface pollution is conclusively identified，it is too late to apply remedial measures that w ould be of much benefit. From a w ater quality view point，degradation of groundw ater often requires long periods of time before the true extent of the problem is readily detectable. Long periods of groundw ater flow are often required for pollutants to be flushed from contaminated aquifers. Groundw ater pollution often results in aquifers or parts of aquifers being damaged beyond repair.
Whereas the problem of achieving acceptable quality of surface w aters focuses mainly on decreasing the know n emissions of pollutants to these systems，the problem facing scientists and engineers involved in the protection of groundw ater resources is to identify the areas and mechanisms by w hich pollutants can enter groundw ater flow systems and to develop reliable predictions of the transport of contaminants w ithin the flow systems. This is necessary as a basis for minimizing the impact of existing or proposed industrial，agricultural，or municipal activities on groundw ater quality.
The purpose of this section is to provide some insight into the physical and chemical factors that influence the subsurface migration of dissolved contaminants. To this end the behavior of nonreactive solutes and of solutes that undergo reactions during subsurface migration w ill be considered. Follow ing this，more specific contamination problems related to activities such as agriculture，mining， nuclear pow er development， and disposal of refuse， sew age， and industrial w astes w ill be briefly review ed.
Throughout this section all solutes introduced into the hydrologic environment as a result of man's activities are referred to as contaminants，regardless of whether or not the concentrations reach levels that cause significant degradation of water quality. The term pollution is reserved for situations where contaminant concentrations attain levels that are considered to be objectionable.
The emphasis in this section is on the occurrence and processes that control the migration of dissolved contaminants in groundw ater. Groundw ater can also be contaminated by oily substances that exist in a liquid state in contact w ith w ater in a manner that does not lead to mixing of the oils in a dissolved form. The oily liquid is said to be immiscible in the w ater. The physical processes that control the movement of immiscible fluids in subsurface systems w as described by Bear in 1972 and w as introduced in a later section.
Before proceeding w ith discussions of the principles of contaminant behavior in groundw ater flow systems and of sources of groundw ater contamination， w e w ill briefly examine some of the more important w ater quality standards. These standards serve as a basis for appraisal of the results of chemical analyses of w ater in terms of suitability of the w ater for various intended uses. The most important of these standards are those established for drinking w ater ( Table 13. 1) . The recommended limits for concentrations of inorganic constituents in drinking w ater have existed for many years. Limits for organic constituents such as pesticide residues are a recent addition. There is considerable controversy w ith regard to the specific organic constituents that should be included in drinking w ater standards and the concentration limits that should be established for them.
Table 13. 1 Drinking water standards
continued
Sources: U. S. Environmental Protection Agency， 1975 and Word Health Organization ( WHO ) ， European Standards，1970.
* Recommended concentration limits for these constituents are mainly to provide acceptable esthetic and taste characteristics.
Limit for NO－3expressed as N is 10 mg / L according to U. S. and Canadian standards; according to WHO European standards，it is 11. 3 mg / L as N and 50 mg / L as NO－3.
Maximum permissible concentration are set according to health criteria.
§ Limit depends on average air temperature of the region; fluoride is toxic at about 5 － 10 mg / L if water is consumed over a long period of time.
** 1pCi = 3. 7 ×10－ 2Bq.
In Table 13. 1 the major constituents for w hich recommended permissible limits are listed are total dissolved solids ( TDS) ，sulfate，and chloride. Consumption by humans of w aters w ith concentrations somew hat above these limits is generally not harmful. In many regions groundw ater used for drinking-w ater supply exceeds the limits of one or more of these parameters. Several hundred milligrams per liter of chloride must be present in order for saltiness to be detected by taste.
Hardness of w ater is defined as its content of metallic ions w hich react w ith sodium soaps to produce solid soaps or scummy residue and w hich react w ith negative ions，w hen the w ater is evaporated in boilers，to produce solid boiler scale. Hardness is normally expressed as the total concentration of Ca2 +and Mg2 +as milligrams per liter equivalent CaCO3. It can be determined by substituting the concentration of Ca2 +and Mg2 +，expressed in milligrams per liter，in the expression
total hardness = 2. 5 ［Ca2 +］+ 4. 1［Mg2 +］
Each concentration is multiplied by the ratio of the formula w eight of CaCO3to the atomic w eight of the ion; hence the factors 2. 5 and 4. 1 are included in the hardness relation. Water w ith hardness values greater than 150 mg / L is designated as being very hard. Soft w ater has values less than 60 mg / L. Water softening is common practice in many communities w here the w ater supply has a hardness greater than about 80 － 100 mg / L. Water used for boiler feed w ill cause excessive scale formation ( carbonate-mineral precipitation) if the hardness is above about 60 － 80 mg / L.
Of the recommended limits specified for minor and trace inorganic constituents in drinking w ater，many have been established for reasons other than direct hazard to human health. For example iron and manganese are both essential to the human body. Their intake through drinking w ater is normally an insignificant part of the body requirement. The recommended limits placed on these metals in the standards are for the purpose of avoiding，in household w ater use，problems associated w ith precipitates and stains that form because oxides of these metals are relatively insoluble. The recommended limit for zinc is set at 5 mg / L to avoid taste produced by zinc at higher concentrations. Concentrations as high as 40 mg / L can be tolerated w ith no apparent deteriment to general health. Zinc concentrations as low as 0. 02 mg / L are， how ever，toxic to fish. Zinc contamination can be regarded as severe pollution in ecological systems w here fish are of primary interest but may be only of minor significance if human consumption is the primary use of the w ater.
The most common identifiable contaminant in groundwater is nitrate ( NO－3) . The recommended limit for nitrate in drinking water is 45 mg / L expressed as NO－3or 10 mg / L expressed as N. In Europe the limit recommended by the World Health Organization is 50 mg / L as NO－3and 11. 3 mg / L as N. Excessive concentrations of NO－3have potential to harm infant human beings and livestock if consumed on a regular basis. Adults can tolerate much higher concentrations. The extent to which NO－3in water is viewed as a serious pollutant therefore depends on the water use.
The constituents for w hich maximum permissible concentration limits have been set in drinking w ater standards ( Table 13. 1 ) are all considered to have significant potential for harm to human health at concentrations above the specified limits. The specified limits are not to be exceeded in public w ater supplies. If the limits for one or more of the constituents are exceeded， the w ater is considered to be unfit for human consumption. The limits indicated in Table 13. 1 are representative of the current standards in the United States and Canada. The limits are continually being appraised and modifications occur from time to time. As more is learned about the role of trace constituents in human health，the list of constituents for w hich maximum permissible limits exist may expand，particularly in the case of organic substances.
In many regions the most important uses of groundw ater are for agriculture. In these situations it is appropriate to appraise the quality of groundw ater relative to criteria or guidelines established for livestock or irrigation. Recommended concentration limits for these uses are listed in Table 13. 2. The list of constituents and the concentration limits are not as stringent as for drinking w ater. These w ater quality criteria do serve to indicate， how ever，that concentration increases in a variety of constituents due to man's activities can cause serious degradation of groundw ater quality even if the w ater is not used for human consumption.
Table 13. 2 Recommended concentration limits for water used for livestock and irrigation crop production
( Source: Freeze et al. ，1979)
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