£A - So^-Ua weA'i /ot- 4- E n v i r o n m e n t A g e n c y Report on the By Brook low flow investigations October 2001 Volume I Report mUS En v ir o n m e n t Ag e n c y Compiled by Water Resources Team, North Wessex Area Office NATIONAL LIBRARY & INFORMATION SERVICE Environment Agency River House East Quay SOUTH WEST REGION Bridgwater Manley House. Kestrel Way. Somerset Exeter EX2 7LQ TA64YS l& V Contents Acknowledgements.................................................................................................................................5 Executive Summary................................................................................................................................6 Glossary....................................................................................................................................................8 Conversion table......................................................................................................................................9 1. Introduction.................................................................................................................................. 10 1.1 History of recent By Brook investigations...........................................................................10 1.2 Perceived problems.................................................................................................................10 1.3 Related reports and publications...........................................................................................11 2. Monitoring Network (Water Resources).................................................................................. 12 2.1 Overview.................................................................................................................................. 12 2.2 Rainfall............................................................................................................... ..................... 12 2.3 Groundwater monitoring........................................................................................................13 2.4 Streamflow measurements........................................:...........................................................14 3. The By Brook Valley: Catchment Description...................................... 1............................... 16 3.1 Topography and Outline Description................................................................................... 16 3.2 Rainfall.....................................................................................................................................16 3.3 Geology....................................................................................................................................18 3.4 Hydrogeology.........................................................................................................................19 3.4.1 Aquifer U nits................................................................................................................. 19 3.4.2 Groundwater Catchment Boundaries................................................................’.........20 3.4.3 Flow Mechanisms..........................................................................................................21 3.4.4 Recharge.........................................................................................................................21 3.4.5 Groundwater Level Monitoring...................................................................................21 3.4.6 Aquifer Properties..........................................................................................................23 3.4.7 Source Protection Zones...............................................................................................23 3.4.8 Winterboume signatures...............................................................................................23 3.5 Hydrology..................................................................................................................:............24 3.5.1 Continuous flow record at Middlehill.........................................................................24 3.5.2 Rain fall-runoff comparison.......................................................................................... 26 3.5.3 Spot flow gaugings....................................................................................................... 27 3.6 Water Quality..........................................................................................................................29 3.7 Ecology................................................................................................................................... 31 3.7.1 Ecological monitoring...................................................................................................31 3.7.2 Biological quality classification...................................................................................32 3.7.3 Detailed macro-invertebrate survey 1999.................................................................. 33 3.7.4 Macrophyte surveys......................................................................................................35 3.7.5 Water crowfoot (Ranunculus) survey W W T.............................................................39 3.7.6 Fisheries surveys...........................................................................................................40 3.7.7 Conclusions: ecology....................................................................................................41 4. Anthropogenic Influence on River Flow and Quality - Inside Catchment.........................43 4.1 Types of impacts....................................................................................................................43 4.2 Mills and ponds, hatches and sluices.................................................................................. 44 4.3 Surface water and groundwater abstractions...................................................................... 45 4.3.1 Impact of the West Kington abstraction on flows and groundwater..................... 46 4.3.2 Impact of Castle Combe Golf Course abstraction on flows.................................... 46 4.3.3 Impact of Widdenham abstraction on flows..............................................................47 4.4 Consented discharges............................................................................................................47 4.4.1 Impact of discharges from sewage treatment works on flows................................48 4.4.2 Impact of discharges from private discharge consents on flows.............................49 ENVIRONMENT AGENCY 108201 4.5 Net effect of abstractions minus discharges........................................................................50 4.6 Impact of discharges from sewage treatment w orks on water quality...........................51 4.7 Other potential sources of pollution.....................................................................................52 4.8 On-going improvements to water quality...........................................................................53 4.9 Conclusions: water quality..................................................................................................... 54 5. Impact on Stream Flow Due to Abstraction in Neighbouring Catchments...................... 55 5.1 Introduction............................................................................................................................... 55 5.2 Abstractions outside the catchment..................................................................................... 55 5.2.1 History o f Abstraction.................................................................................................... 55 5.3 Impact Assessment.................................................................................................................. 57 5.3.1 Introduction...................................................................................................................... 57 5.3.2 Public Water Supply Sources........................................................................................57 5.3.3 Stream Support Boreholes............................................................................................ 60 6. Conclusions....................................................................................................................................62 6 .1 Low Flow and Water Quality issues...................................................................................... 62 6.1.1 Abstractions within the catchment.............................................................................. 62 6.1.2 Sluices and W eirs............................................................................................................63 6.1.3 Abstraction for PWS Outside the C atchm ent.......................................................... 63 6.1.4 Abstraction for Stream Support outside the catchment..........................................63 6.1.5 Climatic Variation...........................................................................................................64 6.1.6 Septic Tanks and Diffuse Agricultural Pollution.....................................................64 6.1.7 Discharges from Sewage Treatment W orks..............................................................65 6.2 Increased Flashiness o f the By Brook...................................................................................