Evidence of heavy nitrate contamination in valleys

Surface water in the Virtu valley in Rabat has six times the nitrate content than the safety level set by the European Union for ground water and surface waters.

This emerges from a recent study conducted by students Martha Anne Zammit and Lucia Farrugia, who tested the quality of water in nine valleys in Malta earlier this year. Hydrologist Marco Cremona assisted the students.

Since a substantial part of the surface water in valleys seeps into the water table, the high levels of nitrates found in it threatens to further undermine Malta's underground water reserve. Valleys are important water recharge points for the underlying aquifers, which are replenished with the portion of rainwater that manages to infiltrate into the bedrock and slowly seeps to the aquifer. This process may take decades.

The study showed that water samples taken from the Virtu valley exceeded 300 mg/l of nitrates, which is six times higher than limits on ground water determined by the EU Water Framework Directive.

The study attributes this high level of nitrates (and phosphates) to the proximity of farms and animal husbandry activities. In fact, valleys located further away from animal farms registered lower levels of nitrates. However, the nitrate content of the water in all valleys exceeded the safe limit, indicating that there are other influences at play, such as the application of artificial fertilisers and manures in fields.

The highest levels of nitrates were found in the Virtu valley system, which is known for the 'high density of livestock rearing and agricultural practice in the surrounding drainage area'.

On the other hand, the Luq Girgenti system registered the lowest levels of nitrates. The area is surrounded by arable land, with a lower density of pig and poultry farms. However, even in this case, nitrate concentrations only dipped below the safe nitrate level on one occasion in April.

The study concludes that high densities of animal husbandry are detrimental to water quality.

According to the EU Water Framework Directive, Malta is obliged to bring its groundwater status to 'good' by 2015, which means nitrate levels of less than 50 mg/l as nitrate. Similarly, the Nitrate Directive stipulates the need for groundwater to attain a less-than-50 mg/l nitrate level, which incidentally is also the maximum limit of nitrate in drinking water as per the Drinking Water Directive. Nitrate levels above this limit render the water unfit for drinking because it becomes carcinogenic. Although the WSC derives 40% of tapwater from groundwater, nitrate levels are kept in check by extensive blending with RO water.

In its report to the EU Commission, the Maltese authorities have stated that 90% of Malta's aquifers exceed this limit.

Hydrologist and water conservation advocate Marco Cremona expressed his concern that the studies indicate valley water is in a much worse state than groundwater.

"This means that we are currently recharging the aquifers with high-nitrate water so the situation is bound to get worse in the next decades."

Studies by the British Geological Survey have shown that surface water takes 40 years to reach the aquifer. This means that the impact of the high nitrate surface water that was tested by the students will have an impact on groundwater after 2050.

The results of the study also shows that attempts to control nitrate levels in animal farms through the obligatory use of cesspits and manure clamps have so far failed to mitigate this problem. The same applies to rules on the application of manure and soluble fertilisers on fields.

Cremona warns that these preliminary results are very worrying as they imply that facilitating the recharge of aquifers by water in valleys by means of dams will result in the worsening of an already bad situation with regard to groundwater quality.

The results also show that contrary to popular perception, urban stormwater, although contaminated with sewage, contains a small amount nitrates and could be more suitable for groundwater recharge than water in 'pristine' valleys. Moreover, there is no evidence of contaminants such as pathogens and hydrocarbons generally present in urban runoff to have reached the aquifer, despite decades of accidental and planned recharge in soakaways.

"These results indicate that it could make more sense to recharge the aquifers with low-nitrate urban runoff  rather than throwing it into the sea".

Drastic action is also required when it comes to controlling nitrate-contamination from farms and fertilisers "as the country has already lost 1.5 billion tonnes of drinking-quality groundwater, and is on course to losing it all forever.  

The results of the study were presented during the EU-funded Lake Admin Seminar on the 4 October 2012, and may be downloaded from the Malta Water Association's website: www.maltawater.org.