15
Nitrate Vulnerable Zone (NVZ) designation, 2017 Eutrophication (lakes) Publication Date: June 2016 Page 1 of 15 NVZ Name: Chew Valley lake NVZ ID: EL109
Nitrate Vulnerable Zone (NVZ)
designation, 2017 Eutrophication (lakes)
Publication Date: June 2016
Page 1 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
Evidence of eutrophication 2017
This document provides a summary of the evidence used in proposing an area of land as one which should be, or should continue to be, designated as a Nitrate Vulnerable Zone (NVZ) for the purposes of the Nitrate Pollution Prevention Regulations 2015. A full description of the methods used in developing the NVZ proposals is set out in the detailed methodology for eutrophication-related NVZs, available via http://apps.environment-agency.gov.uk/wiyby/141443.aspx . These methods were developed under the guidance of a review group convened by the Defra for the last NVZ review (2011-2013), which included representatives from the farming and water industries as well as independent academic experts. Minor refinements to the methods have been made for the current review. NVZs are areas of land that drain to polluted waters and which contribute to the pollution of those waters. Polluted waters include those which are eutrophic or may in the near future become so if the Regulations were not to apply there. Eutrophication is defined as “the enrichment of water by nitrogen compounds, causing an accelerated growth of algae and higher forms of plant life to produce an undesirable disturbance to the balance of organisms present in the water and to the quality of the water concerned”. For both freshwaters and saline waters, a weight-of-evidence based approach to assessing the risks and impacts of eutrophication was employed. The evidence for individual water bodies was assessed against a national suite of criteria for eutrophication in the different categories/ types of water for review. The criteria are both quantitative and qualitative and reflect scientific understanding of the process and effects of eutrophication. They are broken down in the same way for each water category as follows:-
Nutrients
Plants/algae
Secondary and other effects For each designated or candidate water body which might meet the criteria for eutrophication, a datasheet such as this one was completed, bringing together information about the water body, its catchment, its uses, evidence of eutrophication and the sources of nitrogen input. This document is a record of the evidence used in the designation process, including results from national monitoring and assessment programmes, and further information supplied by Area staff. The proposals for NVZ designation are made as a result of close working between Area and national Environment Agency teams, with further quality assurance for the eutrophication designations through the use of a national expert panel. An accompanying guide to these datasheets is available, which provides an explanation of the contents, acronyms and technical terms.
Some features of the maps within this report are based on digital spatial data licensed from the Centre for Ecology and Hydrology, ©. Please note that any maps shown here have not used detailed field boundaries and therefore represent the indicative 'soft' boundary only. The definitive NVZ area can be seen on the “What’s in Your Backyard” (WIYBY) website ((http://apps.environment-agency.gov.uk/wiyby/141443.aspx).
Page 2 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
43096WB ID: 2013 NVZ status: Designate
Section 1. Lake and catchment characteristics
Wessex
356913
464.7
Yes
High alkalinity,shallow,large,lowland
EA Area
Lake grid co-ordinates (Easting/Northing)
Lake waterbody area (ha)
Is the lake heavily modified according to WFD?
WFD Lake Typology
Lake attributes
159798/
Lake Alkalinity (ueq/L) 3341
Mean lake depth (m) 4.3
Does stratification occur? Well mixedYesIs this waterbody a reservoir?
Natural or artificial lake:
n/k --- Chew Valley Lake is a reservoir/impounded lake. It receives water through natural hydrological routes (surrounding streams) some water is pumped into the lake therefore I am not sure whether this would be classed as a Natural or Artificial Lake (see below) . Bristol Water abstracts water from Chew Valley Lake for potable supply. Raw water from Chew Valley Lake supplies Stowey Water Treatment Works directly, and Barrow WTW indirectly via the 3 Barrow tanks. Chew Valley Lake is fed by the River Chew, Hollow Brook, Herons Green and other small streams within the topographical catchment. Chew Valley Lake also receives water pumped from the Chew Magna / Winford Brook catchment. (Bristol Water, SGZ action plan 2013).
Type of artificial lake:
n/a
Lake perimeter (% artificial)
<20% (impounded reservoir, some margin is natural.)
Significant changes in lake level due to seasonal drawdown:
Y --- The drawdown on this lake is significant enough that it may impact the macrophyte community.Pumped storage or other reservoir:
Pumped storage --- Water pumped from the Chew Magna / Winford Brook catchment. Bristol Water operate a pumped storage scheme, pumping water from Winford Brook that is stored in Chew Magna compensation reservoir to Chew Valley Lake. The modelled inflows have been used against an abstraction rule to derive the quantity that can be pumped to Chew Valley Lake while maintaining adequate stored water for compensation requirements.
Information on abstraction (if available)
Water pumped from the Chew Magna / Winford Brook catchmentLicence No 17/53/014/S/016Sources of Supply and points of abstraction- Chew Valley Lake - ST 570 615- Chew Magna Reservoir - ST 565 631- Chew Stoke Stream - ST 552 615There is another licence no 17/53/014/S/014 known as the line of works which can put water into Chew Valley Lake (7 points on this licence).
Page 3 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
Lake catchment attributes5911Lake catchment area (ha)
If pumped, pumped catchment area (ha)
Primarily surface water --- Chew Valley Lake is fed by the River Chew, Hollow Brook, Herons Green and other small streams within the topographical catchment
Hydrological character
Comments on accuracy of lake catchment:
Is the map representative of the natural catchment?
n/a --- Chew Valley Lake is fed by the River Chew, Hollow Brook, Herons Green and other small streams within the topographical catchment. Chew Valley Lake also receives water pumped from the Chew Magna / Winford Brook catchment.
Is the map representative of the artficial catchment?
n/a
Please note that the map above has not used the detailed field boundaries and is the indicative 'soft' boundary.
The definitive NVZ area can be seen on the Environment Agency website (www.environment-agency.gov.uk)
Page 4 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
Section II - Waterbody uses
Controlled water (Section 104 of Water Resources Act):
YesPublic Water Supply:
YesDrinking Water Protected Area:
YesUWWTD designation
NoUsed for hydropower or flow regulation:
Flow regaultion --- Some of the water from the lake is used to maintain the flow in the River Chew.
Accessiblity to public:
Easily accessible, many visits --- Bath, Bristol and Weston Supermare are in the local area. A bus service runs to site as well as being connected by a cycleway. It has two large landscaped picnic areas. Facilities include a tea shop, with indoor and outdoor seating areas with a view of the lake, and information centre. There are also a souvenir shop and small art galleryRecreational fishing:
Moderate benefit --- Some restricted use for recreational activities is permitted by the owner, Bristol Water, including dinghy sailing and fishing, primarily for trout. In late summer large shoals of roach (Rutilus rutilus) and perch (Perca fluviatilis) fry congregate around the margins and weed beds, which are predated by the brown trout (Salmo trutta morpha fario) and rainbow trout (Oncorhynchus mykiss). Chew Valley also holds a large population of pike (Esox lucius), and fishing for these is permitted at certain times of the yearContact watersports
Signifcant benefit --- Chew Valley Lake Sailing Club has a large sailing area, approximately 1.9 miles (3 km) long, for dinghy sailing. The club can cater for fleets of over one hundred boats and it hosts large national eventsNature of watersports (if applicable):
--- Other public benefit visits:
Signifcant benefit- Two nature trails. The Grebe Trail[4] is a hard-surfaced, all-weather path suitable for pedestrians, pushchairs and wheelchairs and covers a circuit 0.75 miles (1.2 km) long, starting and finishing at the wooded picnic area. The Bittern Trail[5] is reached from the Grebe Trail by the footbridge over Hollow Brook. The grassy path is often waterlogged in winter, but there is a boardwalk over a short section. This trail runs along the east shore, visits an open bird hide and returns to the footbridge, making a 1-mile (1.5-kilometre) circuit. Bristol Water imposes conditions on visitors, particularly related to the areas where dogs are allowed. During 2005–2006 Bristol Water started restoring two artificial islands. These are intended to provide safe nesting and roosting sites for a range of wildfowl. Permits to enter the reservoir enclosure and to use the access roads, paths and bird hides are available
Conservation value of lake:
National --- It is a national centre for birdwatching, with over 260 species recorded, including some unusual sightings. The lake has indigenous and migrant water birds throughout the year, and two nature trails have been created. The flora and fauna provide habitats for some less common plants and insects. Providing further protection, much of the area around the lake also falls within the Mendip Hills Area of Outstanding Natural Beauty (AONB).Habitats Directive site:
Water Supply:
Recreational use:
Conservation status:
Page 5 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
SPA --- The lake is an important site for wildlife and has been dedicated as a Site of Special Scientific Interest (SSSI) and a Special Protection Area (SPA)SPA or SAC for aquatic interest features
---
SSSI or local conservation designation:
SSSI
Description of Aquatic interest features:
--- The site is an internationally important staging post for migratory birds, especially waders, terns, warblers and hirundines. The grasslands and reedbeds are a critical autumn feeding ground for Reed Warblers Acrocephalus scirpaceus and Sedge Warblers A. schoenobaenus, and the lake is one of the most important inland waters in Britain for wintering wildfowl.
Page 6 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
1.2
42
Low
Not confident
2011 - 2014
75th percentile annual TON (mg/l)
Total number of TON samples
Confidence of annual 75th %ile TON exceeding 1 mg/l
Confidence of annual 75th %ile TON exceeding 2 mg/l
Date range of TON samples
Total oxidised nitrogen (TON) data
TON monitoring data
0
0.5
1
1.5
2
2.5
24/08/2011
24/10/2011
24/12/2011
24/02/2012
24/04/2012
24/06/2012
24/08/2012
24/10/2012
24/12/2012
24/02/2013
24/04/2013
24/06/2013
24/08/2013
24/10/2013
24/12/2013
24/02/2014
24/04/2014
24/06/2014
24/08/2014
24/10/2014
sample date
TO
N m
g/l
A4070453
0.4
20
Mean summer TON (mg/l)
Total number of summer TON samples
Section III - Causes - Nutrients
Page 7 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
1.4
41
High
Not confident
2011 - 2014
Mean annual TN (mg/l)
Total number of TN samples
Confidence of annual mean TN exceeding 1 mg/l
Confidence of annual mean TN exceeding 2 mg/l
Date range of TN samples
Total nitrogen (TN) data
TN monitoring data
0
0.5
1
1.5
2
2.5
3
24/08/2011
24/10/2011
24/12/2011
24/02/2012
24/04/2012
24/06/2012
24/08/2012
24/10/2012
24/12/2012
24/02/2013
24/04/2013
24/06/2013
24/08/2013
24/10/2013
24/12/2013
24/02/2014
24/04/2014
24/06/2014
24/08/2014
24/10/2014
sample date
TN
mg
/l
A4070453
Does any other(e.g. EA non-WFD, or third party) monitoring data for the lake
provide improved evidence of significantly elevated nutrient nitrogen?
--- The total oxidized nitrogen concentrations for Chew Valley Lake (Bristol Water’s data) follow a seasonal pattern with increased levels from autumn through to spring. From the above data it would seem that the very high levels of phosphorus and elevated levels of nitrogen are major factors causing the formation of algal blooms in Chew Valley Lake. (Bristol Water SGZ action plan 2013).
Page 8 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
143
Poor
100%
2011 - 2014
Annual geometric mean TP (ug/l)
WFD face value TP class
Confidence of moderate or worse TP status
Date range of TP samples
Total phosphorus (TP) data
TP monitoring data
0
100
200
300
400
500
600
24/08/2011
24/10/2011
24/12/2011
24/02/2012
24/04/2012
24/06/2012
24/08/2012
24/10/2012
24/12/2012
24/02/2013
24/04/2013
24/06/2013
24/08/2013
24/10/2013
24/12/2013
24/02/2014
24/04/2014
24/06/2014
24/08/2014
24/10/2014
sample date
TP
ug
/l
A4070453
Does any other(e.g. EA non-WFD, or third party) monitoring data for the lake
provide improved evidence of significantly elevated nutrient phosphorus?
--- Bristol Water’s monitoring data for orthophosphate and total phosphorus (both as μg P/l) in Chew Valley Lake. The levels of phosphorus in the lake show a seasonal pattern, peaking in late summer. Bristol Water started monitoring for Total phosphorus in 2013. From the above data it would seem that the very high levels of phosphorus and elevated levels of nitrogen are major factors causing the formation of algal blooms in Chew Valley Lake. (Bristol Water SGZ action plan 2013).
Page 9 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
134823
120914
1362
150093
From all agricultural sources
From agricultural sources minus atmospheric deposition
From urban sources
From all sources
From all sources (minus atmospheric deposition)
Nitrogen loading estimates based on catchment map area
4.6
0
5.3
Leached N
(kgN/yr)Conc. (mg/l)
4.3
136184 4.8
Nutrient sources
Significance of loading from agricultural sources to the catchment of the lake
Principal source --- Largely agricultural up stream, few other sources except line of works and Chew head springs which have had agricultural pollution in the past. 'The greatest input of nitrogen to Chew Valley lake, by sector comes from agriculture (95%)' (Bristol Water SGZ action Plan 2013).
Significance of loading from human habitation to the catchment of the lake
Minor source --- Upstream villages are mainly sewered but there may be some septic tanks and package plants. Chew stoke STW discharges in the vicinity of the lake. . The following is from Bristol Water SGZ action plan 2013: agriculture (95%) agriculture (63%), and mainly from livestock Valley Lake, by sector, comes from STWs (i.e. East Harptree STW) (16%)Significance of any other sources of nutrient loading to the lake or its catchment
--- Bristol Water have fish rearing cages in reservoir
Local assessment 2015
65Ranking based on nitrogen loading from agricultural sources
NEAP - N assessment 2014
Page 10 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
Section IV - Response - Plants/Algae
14
Moderate
91%
10
Annual mean Chlorophyll (ug/l)
WFD face value Chlorophyll class
Confidence of moderate or worse status
Chlorophyll Good/Moderate boundary value
total number of Chlorophyll samples
Chlorophyll data
Chlorophyll monitoring data
0
50
100
150
200
250
300
350
400
450
500
24/08/2011
24/10/2011
24/12/2011
24/02/2012
24/04/2012
24/06/2012
24/08/2012
24/10/2012
24/12/2012
24/02/2013
24/04/2013
24/06/2013
24/08/2013
24/10/2013
24/12/2013
24/02/2014
24/04/2014
24/06/2014
24/08/2014
24/10/2014
sample date
Ch
loro
ph
yll u
g/l
A4070453
40
2011-2014Date range of Chlorophyll samples
Page 11 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
0.24
Moderate
100%
2011 - 2013
EQR
WFD face value class
Confidence of moderate or worse
status
Date range of samples
Total number of samples/surveys
Phytoplankton
(Pluto EQR)
Macrophytes
(EQR)
Diatoms
(EQR)
0.5
Moderate
87%
2010 - 2013
0.3
Poor
100%
2011 - 2013
2 6
Palaeolimnology
No dataChange as Square Chord Distance
Change in Diatom community No data
1Number of years when algal blooms were observed based on
reactive monitoring 2010-2015:
Does any other (e.g. EA non - WFD, or third party monitoring data for the lake
provide improved evidence of eutrophication? (local judgement)
Strengthens evidence --- Bristol Water’s raw water monitoring data for Chew Valley Lake show that algal blooms are putting Chew Valley Lake at risk of failing the objective of Article 7.3 of the Water Framework Directive. The frequency and scale of blue-green algal blooms have increased since 2006. No blooms >20,000 cells/ml were recorded in 2012 or 2013. Six incidences of blooms >20,000 cells/ml were recorded in 2014. Bristol Water’s monitoring data also indicate that Total algal cell counts have increased since 2006; this is due to diatom blooms as well as blue-green algae blooms.
To which biological element(s) does it relate?
More than one (describe) ---
Evidence that designated aquatic interest features associated with the lake show
evidence of eutrophic disturbance? (local judgement)
Not affected or likely to be- In favourable conditionStrength of evidence (local judgement)
n/a --- Local judgement on the evidence of eutrophic disturbance
Y, definitely --- Severe algal blooms in Chew Lake in 1968 and the consequent filtration difficulties led to the establishment of a collaborative research programme between the University of Bristol and the Bristol Waterworks Company, to define the present biological status of the lakes, and to predict their possible future development. Physical and chemical aspects of Chew Valley and Blagdon Lakes, two eutrophic reservoirs in North Somerset, England R. S. WILSON, M. A. SLEIGH, T. R. A. MAXWELL In FBA JOURNAL. Bristol WATER publishes “warnings” on its web site. 2014 weight of evidence shows this water-body is very certain eutrophic. Bristol Water evidence for DrWPA also supports very certain eutrophication.
30
Other responses
Page 12 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
Review of evidence and recommendations
Existing designation - check needed
Current assessment of weight of evidence supporting
designation in 2017
Comments and decisions
WFD Weight of evidence for eutrophication:
WFD overall ecological status : Poor
Confidence in WFD status : Very Certain
Existing designation. N is still elevated >1mg/l and there is evidence of eutrophic impact. Recommend continued designation.
Agreed with provisional decision to continue designation
Recommended action :
First national panel
Comments from second national panel :
Comments from first panel:
Second national panel
Local summary and recommendation:
Evidence base is strong for Chew Valley Reservoir and recent Bristol Water data continues to support this view.
Certainty of eutrophication problem based on core WFD tools: Very certain eutrophication problem
Certainty of eutrophication problem based on overall weight of
evidence:
Very certain eutrophication problem
Recommendation: Continued designation
Page 13 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
Final summary (2017)
2008 Decision: Precautionary designation
2013 Decision: Designate
Lake Description:
Chew Valley lakes is part of a pumped storage reservoir system. Bristol Water abstracts water from Chew Valley Lake for potable supply. However, it functions as a natural lake.
Why the lake should be designated as a Polluted Water (eutrophic):
Chew Valley Lake is an existing eutrophic NVZ designation. Nitrogen concentrations are greater than 1mg/l, with no evidence of any overall decline. There are a number of discharge consents in the catchment but the source of nitrogen is chiefly agricultural and there are frequent algal blooms. There is evidence of impact on the ecology of the reservoir.
Phosphorus:
WFD status classification for total phosphorus is Poor. TP concentrations peak in summer, which may indicate release of phosphorus from sediment.
Nitrogen:
The 75th percentiale TON and mean annual TN have stayed at similar concentrations since the previous review, TON at 1.3mg/l (compared with 1.2mg/l previously) and TN at 1.5 (compared to 1.4mg/l). TON decreases through the summer growing period indicating that nitrogen limitation is occurring.
Supplementary evidence:
Bristol Water's raw water monitoring data shows that there were 6 algal blooms in 2014 and their Safeguard Zone Action Plan (2013) suggests that the elevated nitrogen, together with high levels of phosphorus, are major factors in the formation of algal blooms.
Ecological response:
Diatoms are at Poor status, and chlorophyll, overall phytoplankton and macrophytes are all at Moderate WFD status. Although the Environment Agency have only recorded 1 algal bloom in the designation period the water company have recorded many more. There is strong evidence of nitrogen limitation occurring in the summer months, when TON declines rapidly, which suggests further reduction in nitrogen load would have an impact on phytoplankton growth.
2017 Recommendation: Continued designation
Page 14 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
Page 15 of 15
NVZ Name: Chew Valley lake
NVZ ID: EL109
