Wastewater provides a valuable source of water and nutrients for irrigation especially in arid and semi-arid areas. However, there are concerns about the nature of the contaminants that it contains and their fate. This becomes particularly important when the irrigated area is underlain by a useful freshwater aquifer. Without a well-managed system of surface drainage, the irrigation returns are likely to lead to a deterioration in water quality in the underlying aquifer and may ultimately restrict its usefulness.

The problem of soil salinisation is well recognised as is the slow salinisation of deeper groundwaters in irrigated areas. However, there have been relatively few studies of the impact of the organic content of the wastewater on the underlying water quality. There is a fear that potentially toxic and highly persistent organic pollutants (POPs) could be building up in the groundwater from aquifers affected by wastewater.

We have studied three areas in which wastewater is used for irrigation: the Mezquital Valley in Mexico which has received much of the wastewater from Mexico City for the last 90 years; the Turbio Valley near the industrial city of Lesn, central Mexico and Wadi Dhuleil which is downstream from the Khirbet As Samra wastewater treatment plant in Jordan. This is the third largest wastewater treatment plant in the world and receives all of the wastewater from Amman and environs. By way of contrast, a fourth area studied was the city of Hat Yai in southern Thailand where leakage from the Klong Toei canal that passes through the city has led to contamination of the shallow groundwater.

The results of this study have shown that while there is widespread evidence of contamination of the groundwaters from organic compounds, as reflected by an enhanced dissolved organic carbon concentration of a few mg/l, there was no evidence for significant concentrations of particularly toxic organic compounds. Common solvents were generally not detectable. The most frequently found organic pollutants were phthalate compounds derived from plasticizers but it was uncertain whether these resulted from contamination picked up during sampling and analysis. Some of the phthalates are suspected of being hormone-disrupting substances. Fluoranthene, one of the most common polyaromatic hydrocarbons, was detected at low concentrations in a number of the samples. Contamination also profoundly modified the basic groundwater chemistry in the affected areas both in terms of wastewater contaminants and their impact on the carbonate and redox chemistry of the aquifers. Reducing conditions could release trace elements such as nickel and arsenic as a result of the dissolution of manganese and iron oxides.

Analysis of the wastewaters by broadscan GC-MS revealed a wide variety of peaks many of which were not identified. The number of such peaks was greatly reduced in the groundwaters, but still provided a useful qualitative measure of the type and extent of groundwater contamination. Degradation, volatilization and sorption, both before infiltration and to some extent after it, were efficient in removing many of the contaminants present in the wastewater. The majority of the residual organic carbon is likely to be present in humic-like compounds. These are not of themselves toxic but can release toxic trihalomethanes after chlorination. However, there was no evidence that this was presently a problem, or likely to be a problem, at any of the public supplies sampled. Humic-like substances can also mobilize otherwise highly immobile but potentially toxic organic compounds such as PCBs. Again there was no evidence that this was currently a problem.

This study has reiterated the serious long-term consequences of salinisation. Problems of salinisation had already seriously affected groundwater in the Wadi Dhuleil region of Jordan and there is currently concern about the deeper aquifer at Lesn. It is important that these problems are anticipated by a properly-designed monitoring programme. Key indicators to monitor are: electrical conductivity (SEC), chloride and nitrate with sulphate, pH, dissolved oxygen, ammonium and dissolved organic carbon as additional useful parameters. The THM formation potential would be useful to monitor in wastewater-affected areas if the facilities are available (a gas chromatograph).

In any case, it is wise to maximise wastewater reuse efficiency, minimize direct leakage from wastewater canals and encourage as much above-ground treatment as possible.

Direct application of wastewater to agricultural land with little or no pretreatment also offers a low cost form of wastewater disposal and can protect surface drainage from potentially polluting waters. WHO has published guidelines for the safe use of wastewater in agriculture but these do not consider their impact on the underlying aquifer in any detail. Unintended groundwater recharge can also occur due to leakage of wastewater from sewers, on-site sanitation and polluted surface waters. Wastewaters are frequently heavily contaminated, often from industrial sources, and although most of the toxic heavy metals and pathogens will be effectively filtered out in the soil and the unsaturated zone, there is concern that small amounts of persistent organic chemicals might be accumulating in the groundwater. Many of the priority pollutants are trace organics. Once an aquifer is contaminated, aquifer clean-up is likely to be impractical and water treatment is expensive.

This study builds on a long programme of work carried out by BGS and DFID on the influence of wastewater on groundwater quality in various parts of the world and particularly Mexico. Four locations were selected for detailed investigations in this study:

(i) Mezquital, Mexico (in collaboration with the Comision Nacional del Agua (CNA))

Mezquital lies immediately north of the Mexico Valley and receives more than 3500 ML/day of untreated wastewater from Mexico City, the bulk of the cities' wastewater. The wastewater is used for irrigation via an extensive network of irrigation canals. It is probably the largest and longest-standing wastewater reuse project in the world. The underlying aquifer consists of a series of fissured basaltic lavas and rhyolitic tuffs along with a shallower aquifer derived from alluvial deposits. The extensive application of wastewater has totally modified groundwater recharge and groundwater flow in the valley.

(ii) Lesn, Mexico (in collaboration with Sistema de Agua Potable y Alcantrillado del Municipio de Lesn (SAPAL))

The city of Lesn in Guanajuato State, central Mexico, is a rapidly growing city with a population of 1.1 million. Demand for water, both for irrigation and public supply, is growing rapidly and this demand has been met by the construction of new surface water impoundments and wellfields often at some distance from the city. Untreated wastewater is used for irrigation. A deep regional aquifer is developed in ignimbrite and volcanic ash and is supplemented by shallower aquifers developed in Tertiary sediments composed of tuffs, sands, clays and gravels. Lesn is the centre of the Mexican leather processing and shoe manufacturing industries, and its wastewaters tend to be highly polluted especially with chromium. Irrigation with wastewater began about thirty years ago and 2000 ha are currently irrigated with a total of 80 Mm3/a of wastewater. This irrigation has affected groundwater quality in the shallow aquifer and there is concern that it will ultimately affect the deeper aquifer.

(iii) Hat Yai, Thailand (in collaboration with the Department of Mineral Resources (DMR) and Prince of Songkla University)

Hat Yai is the third city in Thailand with a rapidly growing population (140,000 in 1989 and estimated to reach 330,000-420,000 by the year 2000). It is situated in the southern part of Thailand about 50 miles from Peninsular Malaysia. The main industries include rubber, food and timber. The city is typical of many in SE Asia being located on a low-lying coastal plain and is underlain by a thick sequence of alluvial sediments. It is largely dependent on groundwater for water supply mainly from an unconfined to semi-confined sand and gravel aquifer at depths of 20-40 m beneath the city. The city is unsewered and the disposal of urban drainage is either directly into surface water courses or into soakaways. A significant part of the recharge to the underlying aquifers in the city centre is from a series of highly polluted canals and there is concern that this may ultimately affect the drinking water supply for the city.

(iv) Wadi Dhuleil and Amman Jordan (in collaboration with Water Authority of Jordan (WAJ))

Wastewater in Jordan is currently treated at some fourteen municipal wastewater treatment plants (WTP's) of which by far the largest is the Khirbet As Samra (KS) plant some 40 km north east of Amman currently treats approximately 80% of Jordan's wastewater and is the third largest wastewater treatment plant in the world. This now receives the wastewater from the cities of Amman, Ruseifa and Zarqa. The treatment works became progressively more overloaded but was substantially upgraded in 1997. The effluent from the KS plant flows along Wadi Dhuleil, then the Zarqa River where it forms the major flow of the river. Water is drawn off along its length for small-scale irrigation. Ultimately it reaches the King Talal reservoir some 42 km west of the KS plant. In the Wadi Dhuleil area, the main changes in groundwater quality in the underlying limestone aquifer are attributed to irrigation return flows and salinization as a result of overpumping. The groundwater salinity has approximately doubled since 1985. As the salinity has increased, the range of tolerant crops that can be grown has become more and more restricted. In some areas, the water is now so saline that it can only be used for irrigating olives.

Are organics a problem in wastewater-affected areas and are they likely to be so in the future?

The short answer is >probably no= to both questions. It is difficult to be more positive since it is practically impossible to prove complete absence of toxic organic compounds in water especially when they are potentially toxic at such low concentrations.

Although we occasionally found identifiable organic contaminants in the groundwaters from the wastewater-affected areas, these included local contaminants such as those derived from diesel fuel and so had not necessarily been derived from the wastewater. We did not find detectable concentrations of any serious contaminants on a regular basis. This probably arises from a combination of factors: low source term because some of the areas studied were not strongly industrialised (but some were) and loss through volatilization (solvents), strong adsorption to organic-rich sludges and soil (POPs) and degradation during transport and treatment.

There did appear to be a consistently enhanced concentration of DOC in the affected groundwaters confirming the >carbon shadow= observed earlier. This is not of itself a problem but could lead to two problems indirectly: increased production of THMs on chlorination and solubilization and transport of otherwise highly insoluble and toxic organic pollutants such as various POPs.

The timescale of any changes will depend on the aquifer characteristics but is likely to be slow - measured in decades. The greatest rate of change is likely to occur in aquifers with low specific yields and where turnover is relatively rapid and the rate of internal cycling is high. It is clear from both the situation downstream of the Khirbet As Samra wastewater treatment plant in Jordan and in the irrigated area outside the Lesn industrial area, Mexico that significant deterioration to groundwater quality can occur in a matter of one or two decades.

Therefore while there are residual uncertainties with unknown risks concerning the role of organics, it is likely that the greatest threat from the leakage of wastewater to underlying aquifers lies in the buildup of salinity and nitrate.

In the long term, the use of high quality water for disposing of waste products should be reduced as much as possible: 'minimize, concentrate and reuse' should be the guiding principles rather than 'dilute and disperse'.

There needs to be a groundwater management plan in place wherever wastewater is used for irrigation or where there is significant leakage from sewers etc to underlying aquifers.

A regular and sustained monitoring program for key pollution indicators is recommended. These are: SEC, chloride, nitrate plus possibly alkalinity, sulphate, ammonium, pH and dissolved oxygen. Long term continuity is the most important factor as changes tend to be slow.

Dissolved organic carbon and/or trihalomethane forming potential would be the most useful additional parameters to monitor the general level of organic contamination, possibly on a less frequent basis.

BGS, CNA, SAPAL, WAJ, DMR and PSU (1998) Protecting groundwater beneath wastewater recharge sites. Volume 1: Final Report. BGS Technical Report WC/98/39.

BGS, CNA, SAPAL, WAJ, DMR and PSU (1998) Protecting groundwater beneath wastewater recharge sites. Volume 2: Analytical Details, Results and Appendices. BGS Technical Report WC/98/39.

BGS, CNA, SAPAL, WAJ, DMR and PSU (1998) Protecting groundwater beneath wastewater recharge sites. Volume 3: Broadscan GC-MS Spectra. BGS Technical Report WC/98/39.

Continuing to seek new opportunities in the area.

Tel: +44 1491 692293
Fax: +44 1491 692345
Email: dgk@bgs.ac.uk