Watercress is a popular food and is traditionally served cooked although it is increasingly
consumed raw either in salads or as a garnish. Watercress is harvested from waterways both
for personal use and for commercial sale in New Zealand. Watercress grows in an aquatic
environment and is subject to potential chemical and microbiological contamination from the
water/sediment it grows in.
The consumption of watercress infected with enteric pathogens could potentially cause
serious gastrointestinal illness (e.g. Campylobacteriosis) and people gathering watercress
could also be at risk of infection from contact with contaminated surface waters. Excessive
heavy metal contamination of watercress may also cause adverse health effects. The aims of
this study were therefore to:
• investigate potential microbiological and heavy metal contamination of watercress and
growing waters from eleven streams in the Wellington and Wairarapa regions; and
• assess public health risks regarding harvesting and consumption of watercress from these
streams.
The stream sites selected covered a range of urban, semi-urban and rural catchments and a
range of water quality and sediment characteristics. The sites selected were representative of
catchment types found elsewhere in New Zealand.
All of the sites showed variable but significant levels of E. coli in both the watercress and
water samples and therefore the potential for enteric waterborne pathogens to be present.
Campylobacter was detected in the growing waters at all sites (80% of the samples) and in
11% of the watercress samples. The water results also showed levels of E. coli well above
recommended freshwater recreational contact safety guidelines at most sites. In rural areas
there is the potential for fascioliasis in persons consuming infected raw watercress.
Except for zinc on one occasion, heavy metal levels in watercress did not exceed the NZ Food
Regulations (1984) at any of the sites. Watercress at urban sites had higher levels of specific
heavy metals, relative to other sites. In streams subject to industrial discharges or natural
processes e.g. geothermal activity, watercress may potentially bio-accumulate heavy metals to
levels in excess of food regulations.
Based on the findings of this study:
• Watercress harvested from any uncontrolled surface water source in New Zealand should
not be consumed unless the watercress is thoroughly cooked in boiling water.
• Watercress should not be eaten raw unless it can be demonstrated that the growing waters
are strictly controlled and adequately monitored.
• People gathering watercress may be at risk of waterborne illnesses through contact with
contaminated water.
• Watercress grown in sediments/water subject to significant heavy metal contamination
may bio-accumulate heavy metals to levels in excess of health guidelines. Watercress (Nasturtium sp., Figure 1) is an aquatic perennial herb that grows wild along the
margins of slow moving rivers, streams, ditches and drains. It is common in New Zealand
especially in the North Island (Roy et al., 1998). Watercress is a popular food and is
traditionally served cooked although it is increasingly being consumed uncooked in salads or
as a garnish, and is being sold as a salad vegetable in restaurants/cafes. Watercress is
harvested both for personal use and for commercial sale in New Zealand. As watercress grows in an aquatic environment (sometimes completely submerged), it is
subject to potential chemical and microbiological contamination from the water/sediment in
which it grows. Sites favourable for watercress usually have comparatively small volumetric
flows, which do not allow for much dilution of the pollutants they receive and are often either
in urban or intensive agricultural areas.
Many streams in New Zealand are subject to faecal contamination from urban and rural runoff
(diffuse and point sources), which can introduce enteric pathogens e.g. Camplyobacter, into
the receiving waters (Ball and Till, 1998). The consumption of watercress infected with
enteric pathogens could potentially cause cases of gastrointestinal illness and therefore could
be a potential risk factor contributing to New Zealand’s high incidence of certain enteric
diseases e.g. Campylobacteriosis (Ministry of Health, 1998). Persons gathering watercress
could also be at risk of infection from contact with contaminated surface waters.
Despite the fact that salad vegetables are generally regarded as low risk foods they can
harbour a range of pathogens. Since these foods are not necessarily cooked before
1
consumption they can pose a health risk to the consumer (Graham, 1999). Watercress may
pose a significantly greater risk than other common salad vegetables (e.g. lettuce) as it grows
in waterways potentially subject to waterborne contaminants. Other common salad
vegetables, would not be exposed to the majority of waterborne pathogens due to their land
based or controlled hydroponic growing environments.
Although watercress has not formally been linked to any enteric disease outbreak in New
Zealand, many overseas outbreaks of human gastro-enteritis have been linked to the
consumption of contaminated fresh vegetables (Beuchat, 1996), and watercress is included in
a list of salad vegetables which have caused extensive outbreaks of salmonellosis (ICMSF,
1998).
In terms of heavy metal contamination, a number of studies undertaken on Central North
Island Rivers, e.g. the Waikato River, have revealed levels of arsenic in watercress above the
World Health Organisation limit for food stuffs. This is attributed to the geothermal activity
in the area and the impact of geothermal power stations (Robinson et al., 1995; Deely, 1998).
Overseas studies have also demonstrated watercress’s ability to uptake certain heavy metals
from contaminated water/sediments e.g. (Wong, 1985). To get a good representation of different growing sites it was decided to sample watercress
and growing waters from a number of surface water sites in the Greater Wellington Region.
To assess heavy metal contamination, the concentrations of the heavy metals referred to
above would be measured. These heavy metals are commonly monitored as part of resource
consent conditions for wastewater discharges in the Wellington Region. Studies in the lower
reaches of Waiwhetu Stream have also confirmed that the sediments are strongly enriched
with lead and zinc, and moderately enriched with cadmium, chromium, nickel, copper, and
arsenic (Aitken, 1998). To assess potential microbiological contamination, suitable faecal
indicator organisms (e.g. E. coli) and a common waterborne pathogen (Campylobacter) would
be tested.
briefly write the main points from the above information
- Watercress is a popular food in New Zealand and is traditionally served cooked, but is increasingly consumed raw in salads or as a garnish.
- Watercress is harvested from waterways, which exposes it to potential chemical and microbiological contamination.
- Microbiological and heavy metal contamination of watercress and growing waters from eleven streams in the Wellington and Wairarapa regions was investigated.
- All sites showed variable but significant levels of E. coli in both watercress and water samples, indicating potential for enteric waterborne pathogens.
- Campylobacter was detected in the growing waters at all sites, with potential risks of gastrointestinal illness for consumers.
- Excessive heavy metal contamination did not exceed regulations in watercress, but urban sites did have higher levels of heavy metals.
- Based on the findings, watercress harvested from uncontrolled surface water sources should be cooked before consumption.
- Watercress should not be eaten raw unless growing waters are strictly controlled and monitored.
- People gathering watercress may be at risk of waterborne illnesses.
- Watercress grown in water subject to heavy metal contamination may bio-accumulate heavy metals to levels exceeding health guidelines.