Lisa Jackson says she takes no risks with the water for her 9-month-old baby son William - she boils it.
But that boiling, combined with the chemicals in the water, may be placing him at increased risk.
Mrs Jackson, her husband, Simon, and their children William, and Charlotte, 3, live at Wheatstone, a cropping area south of Tinwald in mid-Canterbury.
Their home is on the edge of an area that Canterbury's regional council has warned residents has raised nitrate levels.
That area extends in a thin line almost to the coast from south of Tinwald. It runs through farming areas such as Huntingdon and down to Flemington and Wheatstone, and the council claims farmers' "general land-use practices" are the likely cause of the nitrate contamination.
Irrigation, especially on porous soils, can greatly increase the amount of nitrate leaching into groundwater, according to the Ministry for the Environment.
Spraying effluent on to pasture increases the risks further, especially when irrigation is poorly managed.
In extreme cases, nitrates consumed by bottle-fed babies during their first six months may cause what is known as "blue baby syndrome".
Bacteria in newborn babies' digestive tracts convert nitrate to nitrite, which reacts with the haemoglobin carrying oxygen in their blood, to form methaemoglobin, which cannot carry oxygen.
Simple household treatments such as boiling, filtration, disinfection, and water softening do not remove the nitrate, and boiling actually increases the concentration in the remaining water.
Medical Officer of Health Dr Daniel Williams said that drinking water with nitrate concentrations above guidelines could pose health risks to three groups - bottle-fed infants under 6 months, pregnant women, and some adults with rare metabolic disorders.
The Jacksons are about to test their water to ensure it is safe. After a shallow house well ran dry, they have been drawing it from an irrigation well.
The house water passes through a filter, but concern over the groundwater is growing. Mr Jackson said he was not aware of any problems with the water, but would get it tested to make sure.
The regional council's regulation director Mike Freeman said 300 residents in the area had been informed of the results of an investigation into nitrate levels.
An initial investigation in April identified an area south of Tinwald where concentrations in shallow groundwater exceeded levels set by the Ministry of Health.
The ministry has set the maximum safe level for nitrate nitrogen in domestic water supplies at 11.3g/m3 (equivalent to 11.3mg/litre).
Latest sampling had confirmed the width of the contaminated area, although the length has extended towards the coast.
Residents had been told that anyone in the at-risk group should seek alternative water supplies or treat water.
Mr Freeman said the nitrate concerns seemed to be confined to wells less than 30m deep.
The regional council would continue to monitor levels and its proposed natural resources regional plan would help to introduce practices that reduce the impact of land use on groundwater.
The latest problem follows three recent investigations into nitrate contamination in the Ashburton-Rakaia plains.
The first area to be investigated was northeast Ashburton. Monitoring showed an overall decrease in concentrations and levels in the centre were close to guideline levels.
There have also been investigations of wells in the Seafield area and of groundwater quality in the Chertsey-Dorie area.
Of the 155 wells sampled in the first three investigations, samples from 39 wells had nitrate levels above guidelines.
* Irrigation can increase nitrate leaching to groundwater and spraying effluent on to pasture increases the risks further.
* In extreme cases, nitrates consumed by bottle-fed babies in their first six months may cause what is known as "blue baby syndrome".
* Simple household treatments such as boiling, filtration, disinfection, and water softening do not remove nitrate from water. Boiling increases the nitrate concentration of the remaining water.