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The nitrates level should not cause too much of a problem, theoretical risks are somewhat academic if WHO guidelines are only breached by say 10%.. What would concern me would be if nitrate levels suddenly increased actually as a result of much higher borehole extraction rates. MONITOR levels daily as a matter of routine, particularly as spectrophotometers offer a simple technology option. Briefly, Nitrate and nitrite (NO3 and NO2 respectively) are, like ammonia, generally expressed as mg/l N. Nitrite is an intermediate oxidation state of nitrogen in the biochemical oxidation of ammonia to nitrate. Surface waters do not normally carry high levels of nitrite unless badly polluted (<0.1mg/lN).
High levels of NO2 and NH4 generally indicates gross sewage pollution in a river. Nitrates in groundwater can be reduced to nitrites. A prime source of nitrate contamination of surface waters is run-off containing fertiliser chemicals, particularly with the onset of winter rains.
Waters containing high levels of nitrates are potentially very harmful to young children and are increasingly being linked with enteric cancers; principally because of the formation of nitrosamines. In young children bacteria in the digestive tract can reduce nitrates to nitrites which are then absorbed into the blood stream across the gut wall and convert the oxygen-carrying haemoglobin into methhaemoglobin.
Infants have a high fluid intake relative to body weight and the proportionate reduction in the oxygen-carrying capacity of the blood has been known to make infants go blue, hence the conditions name "blue baby syndrome". In parts of the UK it was not uncommon for Water Plcs to issue bottled water for young children.
There are no straightforward methods for reducing nitrate levels during water treatment. Demineralisation and biological denitrification methods are both not suited for large scale operations.