Tracing the paths of pollution
The aquifers of Costa Rica's Central Valley supply fresh water to over one million people, more than 25% of the national population. But these underground reservoirs, lying in complex systems of volcanic rock are, like many groundwater reserves across the globe, in danger, and recent research has suggested that rapid, poorly controlled urban growth and intensive coffee production are, in the Central Valley, the main culprits. The research, conducted by four Costa Rican institutions and co-ordinated by the Environmental Hydrology Laboratory, at the Universidad Nacional, stemmed from fears expressed in 1997 that wells and springs drawing from the valley's aquifers were becoming contaminated by nitrates and possibly other pollutants. Excess nitrogen, in the form of nitrate, is one of the most common, and serious pollutants of water. Using nuclear and conventional tools, hydrologists have analysed the composition of water in the aquifers, and their findings suggest that there are indeed strong links between agriculture, urbanisation and groundwater contamination in the valley.
Isotope fingerprints
Much of the evidence has been provided by isotope analysis. This technique, which is being strongly promoted and supported by the International Atomic Energy Agency (IAEA), enables scientists to examine the concentration of differently weighted atoms (isotopes) in a sample of water. This then gives an unmistakable 'fingerprint' profile of a sample, and allows them to trace water taken from an aquifer back to the area where it fell as rain. In combination with conventional methods, isotope analysis can be a very quick and cost effective way of building a map of water movements within a hydrological system. The methods can also be used to identify the sources of contaminants, such as the 15 N and 18 O isotopes found in nitrates, and to find out where they entered the system. If, for example, farmers are using irrigation in areas that are also recharge zones for aquifers, the chances of groundwater pollution may be high. The technique has enabled scientists to trace oxides present in acid rain to specific industrial emissions, and isotopic composition can also indicate the extent to which pollutants are being naturally broken down by bacteria.
The Costa Rican teams have used the methods to find out how nitrates had been moving through the volcanic soils and rock, and whether groundwater reservoirs have indeed become contaminated. Their results have confirmed the initial fears. In some springs and wells, nitrate levels have already reached the maximum limit recommended by the World Health Organisation, and when traced, these nitrates were found to come partly from fertilizers used in coffee plantations. Further studies of nitrogen absorption by coffee plants supported their findings. Field trials in some Central Valley plantations showed that the coffee plants were only absorbing between thirty and forty percent of the nitrogen provided in fertilizers, and that this figure fell to six percent towards the end of the growing season. Nitrogen that is not taken up by the growing crop is a waste of farm inputs as well as a pollution risk.
Supporting legislation
Such information needs now to be used to justify better regulation by Costa Rica's water management authorities. Clearly the intensive production methods that coffee farmers in the valley have been using since the 1960s need some control. These methods, which involve regular, high dosage application of fertilizer and pesticides, have given the farmers one of the highest production rates per hectare in the world. However, as the field based trials indicated, over-dosing is a common problem, and sensible limits need to be imposed. Such limits should properly be applied to pesticides as well as fertilizers. There is preliminary evidence that other contaminants are finding their way into aquifers, in addition to nitrates. Residues of carbaryl pesticide have appeared in some of the tested samples. The research findings also support the need for greater regulation and management of urban waste. Some nitrates were traced back to newer urban areas where sewerage facilities have failed to keep pace with rapid growth. In these areas human wastes tend to be disposed of via septic tank systems, but because of the valley's porous soils, such systems may be inappropriate.
In a recent development, Costa Rica's Asamblea Legislativa has begun discussion of a major change in water legislation. The researchers have been pleased that their findings are being taken into close consideration for many of the new articles.
