The Mixed Pond is one of a series of ponds in the south-west corner of Hampstead Heath
formed by the damming of the Hampstead Brook. Access to the 4,270 square metre swimming
area is via a concrete diving platform. There is a charge for swimming. There is no
beach but there is a grassy sunbathing area next to the diving platform.
The Environment Agency monitoring found occasional problems with faecal pollution
in this bathing water, so in 2009 the Environment Agency introduced a DNA tracing
technique that helps us identify whether the source was human or animal. This enables
us to target further investigations and to identify appropriate courses of corrective
action. Following reduced water quality results in 2009 and 2010 this technique was
used, however findings were inconclusive.
The misconnection of foul drainage to surface water may potentially affect the water
quality of the Mixed pond. If the Environment Agency investigations indicate that
reduced water quality is caused by misconnected foul drainage the Environment Agency
will work with the local authorities and Thames Water to investigate and rectify the
problem.
Modern sewerage systems have two separate systems, one takes foul sewage to sewage
treatment, the other takes rainwater runoff through surface water drains to rivers,
lakes and the sea. Misconnections occur when waste water pipes are plumbed into surface
water drains instead of the foul water sewerage system. This can give rise to pollution
when the waste water is discharged directly to the environment through the surface
water drain. For example, a washing machine or toilet may be incorrectly plumbed so
that it discharges to the surface drain rather than the foul sewage drain.
For the four year (2020-2023) assessment period where data is available, phytoplankton
(microscopic algae) was not assessed as being sufficient to be objectionable, but
was observed as being present on 30% of visits. Phytoplankton (microscopic algae)
increase in number at certain times of the year. This process is known as a phytoplankton
bloom. Blooms of phytoplankton can result in the water appearing discoloured or a
foam forming on the water. If you observe a suspected bloom you are advised to not
enter the water and to contact your local authority health contact for help and advice.
There are no active pollution risk forecasts made at this bathing water. However any
bathing water has the potential to be affected by a pollution incident and if this
occurs a pollution risk warning with associated advice against bathing will be issued
on this website.
Environment Agency samplers make observations of litter present on the beach at every
visit, this includes assessments of sewage debris, litter and tar. At Hampstead Heath
(Mixed Pond) for the four year (2020-2023) assessment period where data is available,
sewage debris was not noted at this site. Litter was not assessed as being sufficient
to be objectionable, but was observed as being present on 29% of visits. Tarry residue
was not noted at this site.
The Hampstead Brook, which feeds the Mixed Pond, is fed by several springs which rise
less than one kilometre away in the western corner of Hampstead Heath. The catchment
is predominantly parkland consisting of woods and grassland, but also includes a small
part of the suburban area known as ‘Vale of Health'.
Seaweed (macroalgae) and phytoplankton (microscopic algae) are a natural part of the
marine and freshwater environment. Below we note whether these have been recorded
in quantities sufficient to be a nuisance.
The majority of sewers in England are “combined sewers” and carry both sewage and
surface water from roofs and drains. A storm overflow operates during heavy rainfall
when the sewerage system becomes overwhelmed by the amount of surface water. The overflow
prevents sewage from backing up pipes and flooding properties and gardens. An emergency
overflow will only operate infrequently, for example due to pump failure or blockage
in the sewerage system.
Heavy rain falling on pavements and roads often flows into surface water drains or
highway drains, ending up in local rivers and ultimately the sea. The quality of bathing
water may be adversely affected as a result of such events.
It is the Environment Agency role to drive improvement of water quality at bathing
waters that are at risk of failing higher standards. It is natural for water to run
off the land to the sea. Water quality at a bathing water is dependent upon the type
and area of land (the catchment) draining to the water and the activities undertaken
in that catchment.