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L'înformâtion et les sèrvices publyis pouor I'Île dé Jèrri

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Surface water monitoring

Surface and groundwater monitoring takes place to assess water pollution in our local waters. 12 sites are sampled 4 times each year.

Field analysis for the following is carried out at each site:

  • pH
  • conductivity
  • dissolved oxygen
  • temperature

Samples are taken for further chemical analysis at the laboratory of the Official Analyst to the States of Jersey.

The 12 sites monitored, are:

Millbrook

This is the most southerly site of the catchment, located to the east of the car park of the Universal Church of Our Lady.  The stream is heavily shaded, the west bank by a Cypressus hedge, the east bank primarily by willows. There are no aquatic macrophytes within the stream bed, and the substrate is dominated by silt with gravels. The underlying geology is Jersey shale formation overlain by quaternary alluvium. One pipe enters the stream from the west side.

Tesson

This site is in a semi-residential area, bounded by a road to the north and a footpath to the east.  The walls of the stream are channelised by a 0.6m high wall made of stone blocks. The stream is moderately shaded (60%) by oaks from the west bank, no macrophytes were found within the sampled stretch. The stream bed is unusual in that it contains primarily cobbles (40%) and silt (50%), with very little gravel (5%). The underlying geology is Jersey shale formation.

Pres D'Auvergne

This site is located at the point at which a steep-sided valley flattens out into a western coastal plain at Cueillette de Grantez.  The underlying geology is pleistocene / recent alluvium or wind blown sand.  Alder trees flank the southern side of the stream and a wet meadow flanks the northern side. Bankside vegetation is composed largely of: 

  • nettles (50%)
  • grass (10%)
  • marshwort (10%)
  • and other herbs

The stream substrate is composed largely of silt (50%) and gravel (30%).

Mourier

The stream lies to the west of a small triangle of grassland, bounded by 2 minor roads. To the west of the stream lies a mature oak woodland, to the east grassland. The most northerly part of the stream is shaded by oak and elder. The stream is dominated by fool's watercress (20% cover). The substrate is cobbles and gravels, with granite overlain by quaternary alluvium as the underlying geology. No pipes enter this stream.

St Catherine

This site lies immediately above Le Maseline reservoir. The stream widens allowing the flow rate to slow sufficiently for large quantities of silt to be deposited on the stream bed around the stepping stones. The site is shaded by oak and salix and the dominant macrophyte is fool's watercress. The substrate is composed mainly of silt and the underlying geology is conglomerates of the Rozel conglomerate formation.

Grouville

At this site, the stream flows north-east through grazed pasture which is bordered by a road to the south-west followed by arable fields. The stream bed is dominated by silt (90%), and the dominant macrophyte is watercress (60%) providing shade for the substrate.

Longueville

This trapezoidal shaped channelised section of Baudrette Brook flows through a cow grazed pasture. Silt covers the stream bed and it is unshaded by vegetation. There is often a dense covering of green algae in the stream bed. Upstream of the sample point there are several surface water discharges from Rue du Pres trading estate.

Grands Vaux

The stream at this site flows south through grazed and managed pasture with a wetland above and a minor road crossing the stream to the south.  The in-stream vegetation provides shade (90%) and macrophytes include hemlock water dropwort and marsh woundwort (10%). The stream is dominated by silt (50-90%) with pebbles (0-40%) and cobbles (0-10%). This site receives a groundwater input to the channel from a Jersey Water borehole to maintain flow during dry conditions.

Vallee des Vaux

This site is bordered on its west side by a steep slope of National Trust Woodland and to the east by cropped grassland and banks. The stream is shaded by:

  • oak
  • ash
  • hazel on the west bank
  • nettle (50%) on the east bank

The stream bed is composed of:

  • cobbles (50%)
  • gravel (0-50%)
  • silt (0-30%)

The stream probably receives road runoff.

Greve de Lecq

The surrounding area is woodland composed of: 

  • sycamore
  • oak
  • alder trees

The stream channel is widened at this point, resulting in shallow flow and the presence of both riffles and fast runs.  Underlying geology is granite, overlain by alluvium and head, and the stream substrate is composed of:

  • gravel (55%)
  • silt (25%)
  • boulders (15%)

Aquatic vegetation is present in several beds of fool's watercress, and scattered water-mint and bistort are also present.  Bankside vegetation is scarce, but where stands are present, nettles and bracken predominate, with other herbs growing in small quantities.

Plemont

The stream to Plemont beach in the northwest of Jersey drains a catchment of 0.38m2. Water quality in the stream is measured at the site known as PL2. The substrate is mainly gravel and the stream is often overgrown with bramble and bracken. pH levels indicate that the water is slightly acidic and nitrate concentrations have been showing a reduction in recent years.

Pont Rose

This forms part of the St Brelade catchment, which drains an area of 4.55km2. The stream enters St Aubin's bay at St Aubin's harbour. The sampling point is in a lavoir by the roadside on La Rue des Sauvalleries. The steam substrate is mainly silt and cobble and there is no aquatic vegetation. It is shaded by surrounding trees and bamboo planted in a neighbouring garden.

Hydrogeological report

 

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