Groundwater Problems Caused by Irrigation with Sewage Effluent.

Abstract

Increasingly, sewage effluent will be used for urban and agricultural irrigation. The main concern is the potential for infectious diseases in farm workers and city dwellers exposed to the effluent, as well as in people who consume crops irrigated with effluent, especially when those crops are eaten raw or brought raw into the kitchen. Prevention requires adequate disinfection of the effluent. The effluent also must meet normal irrigation water requirements for parameters such as salt content, sodium adsorption ratio, trace elements, and so forth. Unfortunately, little or no attention is paid to long-term effects of sewage irrigation on underlying groundwater. Since most of the water applied for irrigation in dry climates evaporates, the concentrations of non-biodegradable chemicals in the drainage or deep-percolation water going down to the groundwater can be much higher than in the effluent itself (about five times higher for an irrigation efficiency of 80 percent). These chemicals comprise not only the sa lts, nitrates, and possible pesticide residues normally expected in irrigated agriculture, but also "sewage chemicals" like synthetic organic compounds, disinfection by-products, pharmaceuticals and pharmaceutically active chemicals like endocrine disrupters, and fulvic and humic acids. These acids are known precursors of disinfection by-products that are formed when the drainage water ends up in drinking-water supplies that are then chlorinated. Thus, groundwater below sewage-irrigated areas eventually may become unfit for drinking, which raises questions of liability. More research on long-term effects of sewage irrigation on groundwater is urgently needed.

Editor's note: Through NEHA's long-standing relationship with NSF International, NEHA was granted permission by NSF International to share with the Journal's readership various papers that were presented January 12-15, 2000, at the "NSF International Small Drinking Water and Wastewater Systems International Symposium and Technology Expo" in Phoenix, Arizona. This paper, "Groundwater Problems Caused by Irrigation with Sewage Effluent," is one of them.

It is important to note that these papers were screened by an NSF International advisory committee prior to their presentation at the conference, but they have not been peer reviewed by NEHA's Journal program for technical accuracy.

Because these papers contain useful and interesting ideas and information that may be either delayed or lost if the papers were sent through the Journal's normal peer review process, NEHA has decided to publish them as presented, with only minor editorial modifications.

We hope you look forward to more of these papers in future issues of the Journal!

Introduction

Global population growth will take place mostly in developing countries and in cities, giving rise to mega-cities with mega-water needs, mega-sewage flows, and mega-problems. Populations in countries with higher living standards will grow more slowly, except in the United States, where immigration can be expected to stay high. As growing populations continue to increase demands and competition for water, management of water resources must become increasingly integrated, taking into account all aspects of the problem and using holistic approaches to achieve optimum strategies and solutions. For example, water supply management must also include demand management that determines whether more economical solutions to water shortage problems can be achieved with conservation (more efficient use), water transfers to uses with higher economic returns or other benefits, water pricing, recycling and reuse, and similar measures. Integrated water management also involves water quality management, effective pollution con trol, economics, public health, environmental and ecological aspects, sociocultural aspects, water storage (including long-term storage or "banking'), conjunctive use of surface water and groundwater, public involvement, conflict resolution, the flexibility to cope with climatic changes (McClurg, 1998) or other changes in water supply, regional rather than local approaches, weather modification, use of the virtual water concept, sustainability, and so forth. This paper focuses on water reuse through irrigation.

Treatment Requirements for Irrigation with Municipal Wastewater

If municipal wastewater is used for irrigation (agricultural irrigation or urban irrigation, including parks, road plantings, sports fields, golf courses, residential areas, etc.), the many additional chemicals found in such wastewater can pose a serious threat to groundwater quality This concern is especially acute for efficient irrigation in dry climates, where chemical concentrations in the drainage water or deep-percolation can be multiples of those in the wastewater itself (typically by factors of two to 10, depending on rainfall and irrigation efficiency). The chemicals of concern include salts, pesticide residues, nitrogen (mostly as nitrate in the drainage water), disinfection by-products, pharmaceutically active chemicals, other chemicals, and precursors of disinfection by-products, like humic substances and other dissolved organic matter, that form a new suite of disinfection by-products when the groundwater is pumped up again and chlorinated or otherwise disinfected for drinking.

The main quality criteria for effluent irrigation are the normal chemical requirements for crop irrigation that apply to any irrigation water (salt content, sodium adsorption ratio, sodium, chloride, trace elements, nitrogen, and bicarbonate; Ayers and Westcot, 1985; Baler and Fryer, 1973; Bouwer and Idelovitch, 1987). Most domestic effluents meet these requirements. Industrial discharges into the sewer system can cause excessively high concentrations of heavy metals and other trace elements. The main concerns in effluent irrigation are health effects in farm workers who handle the water and in people who consume the agricultural products or are exposed to the water in irrigated public areas.

Two main sets of regulations or guidelines exist: the stringent California (Title 22) requirements, which basically require tertiary treatment (coagulation, filtration, and chlorination) to produce a pathogen-free effluent, and the World Health Organization (WHO) guidelines, which require a fecal-coliform count of less than 1,000 per 100 milliliters (mL) and not more than one helminth egg per liter (Asano, 1998; Bouwer, 1993; WHO, 1989; U.s. Environmental Protection Agency 1992). Both requirements are for unrestricted irrigation, which includes irrigation of fruit and vegetables consumed raw The California rules require tertiary treatment (coagulation, filtration, and chlorination) after primary and secondary treatment (Table 1). These rules are preferred in industrialized countries with high living standards. The WHO guidelines can be achieved with lagooning (about three weeks' detention time) and, hence, are more suitable for developing countries, where expensive, high-technology treatment often is not fea sible. When the WHO guidelines are used, raw vegetables should be consumed only by local people. Export to other cities or countries also should not be allowed. Filtration of the lagoon effluent through constructed sand filters or natural sand or soil layers, as with groundwater recharge for soil-aquifer treatment, would be a big improvement (Bouwer, 1993). Effluent irrigation of fiber and seed crops or crops that are cooked before human consumption requires less treatment. Careful controls may however, be necessary to prevent use of the effluent for irrigation of fruit and vegetables that will be consumed raw As always, water management decisions must strike a balance between public health, the interests of the environment, and economics. Where the desirable balance lies depends on the local economy: the poorer the country, the less it can spend on environmental concerns and public health. Thus, the guidelines and treatment systems that are adopted for irrigation with municipal wastewater must be a compromis e between what is theoretically desirable and what is practically achievable. In quite a few cases, there is no treatment at all, and raw effluent is used for irrigation, with or without control of which crops can be grown. Needless to say, this is not a good situation--even with crop control, which often is not enforced. Ultimately, Title 22-type treatment processes should be implemented.

Effects on Groundwater

Unfortunately, the normal chemical and microorganism requirements for irrigation with municipal wastewater do not address disinfection by-products, pharmaceutically active chemicals, humic substances, and other potential contaminants. Disinfection by-products may already be present in the wastewater entering the treatment plant due to chlorination of the drinking water. By-products can also be formed by chlorination of effluent from the wastewater treatment plant, particularly with the high chlorine doses and long contact times used for Title 22-type tertiary treatment (Table 1). Disinfection with ultraviolet (UV) irradiation after granular media filtration would reduce disinfection by-product levels in the tertiary effluent. Some disinfection by-products, like trihalomethanes and haloacetic acids, have been found to be biodegradable in aquifers near aquifer storage and recovery wells (Singer et al., 1993). Still, disinfection by-products comprise a whole suite of halogenated organic compounds with yet-to-be- discovered identities, fates in the underground environment, and health effects. Trihalomethanes can include chloroform and bromodichloromethane. The latter is of concern because it may increase miscarriages in women. As with the dissolved salts, concentrations of refractory and nonvolatile disinfection by-products in the deep-percolation or drainage water would also be about five times higher than those in the effluent used for irrigation, again assuming an irrigation efficiency of 80 percent. Because of their potential toxicity and carcinogenicity, disinfection by-product levels in drinking water are continually scrutinized, and maximum contaminant levels (MCLs) may be lowered in the future. The U.S. Environmental Protection Agency (U.S. EPA) has lowered the drinking water MCL for tri-halomethanes from 100 to 80 micrograms per liter ([micro]g/L). This does not bode well for potable use of groundwater affected by deep-percolation water from effluent-irrigated areas. The challenge for the drinking-water indus try, of balancing disinfection by-product formation against microbial control (Ozekin and Westerhoff, 1998), also applies to water reuse issues. The choice is between the possibility of immediate acute illness caused by pathogens (diarrhea and worse) and much more serious diseases like cancer caused by chemicals after years of ingestion.

The high nutrient and organic-carbon levels in effluent also can be expected to enhance plant growth and bioactivity in the soil. Decaying roots and other plant parts and biomass can then form humic and fulvic acids as stable end products. These are non-biodegradable, and they are known disinfection by-product precursors in water that is to be chlorinated. Thus, when groundwater from below effluent-irrigated areas is pumped and chlorinated for potable use, a new suite of disinfection byproducts can be formed.

Concern also is rising about pharmaceutically active chemicals that have entered the sewers with domestic, industrial, pharmaceutical, and hospital waste discharges (Zullei-Seibert, 1998). Pharmaceutically active chemicals seem to survive wastewater treatment and may not adsorb well to soil particles, so they could be rather refractory in the underground environment. While these chemicals may not be directly toxic, they can produce adverse health effects by affecting the immune and hormone systems of animals and humans (i.e., they can act as endocrine disrupters) (Goodbred et al., 1997). At least 45 chemicals have been identified as potential endocrine disrupting contaminants, including industrial contaminants like dioxins and PCBs, insecticides like carbaryl and DDT, and herbicides like 2,4-D and atrazine (Colborn et al., 1993). More research is needed on the occurrence and fate of pharmaceutically active chemicals in the underground environment and about synergistic effects when a whole spectrum of pharmac eutically active chemicals and other contaminants is present.

In addition to all these organic chemicals, drainage water from effluent-irrigated fields often contains excessive levels of salts and nitrates, as well as pesticide residues from the agricultural practices (Bouwer, 1990).

To estimate the rate at which chemicals in drainage water move downward from root zones, the average flux is divided by the volumetric water content. If, for example, 100 centimeters (cm) of irrigation water is applied per year and the consumptive use of water (evaporation plus transpiration) is 80 cm per year, the downward flux is 100 - 80 = 20 cm per year, assuming insignificant rainfall. If the volumetric water content in the unsaturated zone is 15 percent, the downward pore velocity of the water molecules then is 20/0.15 = 133 cm per year. This would also be the downward velocity of nonreactive, or conservative, dissolved chemicals. Adsorption of chemicals to soil particles reduces the rate of chemical movement, while preferential flow increases it (Bouwer, 1991). When the drainage water joins the groundwater and there is no other recharge or discharge from the aquifer, the rise in groundwater level is estimated as the downward flux divided by the saturation deficit or fillable porosity of the unsaturate d zone (difference between total porosity and volumetric water content). Assuming for the previous example that the total porosity is 32 percent, the fillable porosity would then be 32 - 15 = 17 percent, so that a downward flux of 20 cm per year would cause a rise in groundwater level of 20/0.17 = 118 cm per year. This is in line with observed groundwater level rises in irrigated areas. If irrigation is seasonal, similar calculations can be made for shorter time periods. Where rainfall is significant, it also should be taken into account.

After joining the aquifer and causing groundwater levels to rise, the drainage water tends to remain in the top portion of the aquifer according to the vertical-stacking principle. Thus, shallow wells that derive their water from the top of the aquifer will first show evidence of groundwater contamination. Deeper wells that pump groundwater from deeper portions of the aquifer or from underlying confined aquifers will show contamination later.

Summary and Conclusions

Water reuse for irrigation makes a lot of sense and looks very good on the surface. A more integrated and in-depth look at the practice, however, reveals some serious potential problems with groundwater quality and groundwater levels. Unfortunately, in almost all effluent irrigation projects, existing or planned, long-term effects on groundwater are not considered. While farmers are not held liable for "normal" agricultural contamination of groundwater (salts, nitrate, and pesticides), municipal contamination by effluent irrigation with its additional chemicals may be different. Municipalities have deeper pockets than farmers, and since effluent irrigation will tend to be concentrated in or near cities, where there can also be wells for municipal and domestic water supply, adverse effects from contaminated groundwater may be more serious and more visible than in rural areas. Municipalities may then be held just as liable for contaminating groundwater as industries, leaking underground storage tanks, and other point-source polluters.

Much more research needs to be done on the effects of effluent irrigation on groundwater. Both laboratory and field studies are needed to improve understanding of how long-term effluent irrigation will affect underlying groundwater and the quality of water from wells in the area--and to help determine what should be done to avoid undesirable developments. Meanwhile, groundwater impact analyses should be performed for all significant effluent irrigation projects to make the best possible predictions about how the projects will affect groundwater quality and water table elevations, how the situations can best be handled, and what kinds of damage and liability can be expected. The groundwater impact analysis must, of course, address long-term aspects, so that the AMTO (after-my-term-of-office) principle is applied rather than the NIMTO (not-in-my-term-of-office) attitude.

(Adapted with permission from NSF Proceedings of the Small Drinking Water and Wastewater Systems International Symposium and Technology Expo, January 12-15, 2000, Phoenix, Arizona.)

Corresponding Author: Herman Bouwer, U.S. Department of Agriculture--ARS, U.S. Water Conservation Laboratory, 4331 E. Broadway Rd., Phoenix, AZ 85040.

REFERENCES [*]

Takashi Asano, (ed.), Wastewater Reclamation and Reuse, Technomic Publishing Co. Inc., Lancaster PA. pp. 1528, 1998.

R.S. Ayers, and D.W. Westcot, "Water quality for agriculture," FAO irrigation and Drainage Paper 29, Food and Agriculture Organization of the United Nations, Rome, p. 174, 1985.

D.C. Baier, and W.B. Fryer, "Undesirable plant responses with sewage irrigation," J. Irr. and Drain. Div., Proc. Am. Soc. Civil Engrs., Vol. 99-IR2:133-141, 1973.

H. Bouwer, "Agricultural chemicals and groundwater quality," J. Soil and Water Conservation. Vol. 45:184-189, 1990.

H. Bouwer, "Simple derivation of the retardation equation and application to preferential flow and macrodispersion," Ground Water, Vol. 29, No. 1, pp. 41-46, 1991.

H. Bouwer, "From sewage farm to zero discharge," European Water Pollution Control, Vol. 3, No. 1, pp. 9-16, 1993.

H. Bouwer, and E. Idelovitch, "Quality requirements for irrigation with sewage effluent," J. Irrig. and Drain. Div. ASCE, Vol. 113, No. 4, pp. 516-535, 1987.

T. Colborn, ES. Vom Saal, and A.M. Soto, "Developmental effects of endocrine disrupting chemicals in wildlife and humans," Env. Health Perspectives, Vol. 101 pp. 378-384, 1993.

Goodbred, S.L., R.J. Gilliom, T.S. Gross, N.P. Denslow, W.L. Bryant, and T.R. Schoeb, "Reconnaissance of 17[beta]-estradiol, 11-ketotestosterone, vitellogenin, and gonad histopathology in common carp of United States streams: potential for contaminant-induced endocrine disruption," U.S. Geol. Survey Open-File Report 96-627, Sacramento, California, 1997.

S. McClurg, "Climate change and water: what might the future hold?" Western Water, May/June, pp. 4-13, 1998.

K. Ozekin, and Paul Westerhoff, "Bromate formation under cryptosporidium inactivation conditions," Water Quality International, May/June, pp. 16-17, 1998.

P.C. Singer, R.D.G. Pyne, M. AVS, C.T. Miller, and C. Mojonnier "Examining the impact of aquifer storage and recovery on DBPs," J. Am. Water Works Assoc., Nov. Vol. 1993, pp. 85-94, 1993.

U. S. Environmental Protection Agency, "Guidelines for water reuse," EPA Manual 625/R-92/004, U. S. Env. Prot. Ag., Washington, DC, 1992.

World Health Organization, "Health guidelines for the use of wastewater in agriculture and aquaculture," Tech. Bull. Ser. 77, WHO, Geneva, Switzerland, 1989.

N. Zullei-Seibert, "Your daily drugs in drinking water? State of the art for artificial groundwater recharge," In Third Internat, Symp. on Artificial Recharge of Groundwater ed. J.M. Peters, A.A. Balkema, Rotterdam, The Netherlands; pp. 405-407, 1998.

(*.) Editor's note: Because this paper was originally published in the NSF Proceedings of the Small Drinking Water and Wastewater Systems International Symposium and Technology Expo, the references do not conform to normal Journal of Environmental Health style.