Ni^A-Angiian 14-5 ^2 (aO'-V C o N ^ r ^ ^ c r r Investigation of Salinity in the River Thurne Catchment of North-East Norfolk October 1993 PMVIROKfolMT AGENCY n @A & 5 environm ent a g e n c y n a t i o n a l U B M W k ^FORMATION SERV1C h e a d OFFICE Rio House. Waterside Drive. Aztec W est, Almondsbury. A Bristol BS32 4UD I P Holman, University of East Anglia K M Hiscock, University of East Anglia Anglian Region Operational Investigation OI 535/1/A NRA National Rivers Authority Investigation of Salinity in the River Thurne Catchment of North-East Norfolk October 1993 *1 P Holman, University of East Anglia K M Hiscock, University of East Anglia • I P Holman, now Soil Survey & Land Research Centre, Cranfield University, York Anglian Region Operational Investigation OI 535/1/A National Rivers Authority Anglian Region Kingfisher House “ ” Goldhay Way Orton Goldhay Peterborough PE2 5ZR Tel: 0733 371811 Fax: 0733 231840 © National Rivers Authority 1993 All rights reserved. No part of this document may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior permission of the National Rivers Authority. Dissemination Status: Internal: Released to Regions External: Public Domain Research Contractor: This document was produced under Anglian Region OI Contract 535 by: School of Environmental Sciences University of East Anglia Norwich Norfolk NR4 7TJ Tel: 0603 56161 Fax: 0603 507719 NRA Project Leader: The NRA’s Project Leader for Anglian Region OI Contract 535: David Burgess - RHO, Kingfisher House, Peterborough Additional Conies: Further copies of this document can be obtained from R&D Co-ordinator, Anglian Region. CONTENTS Page LIST OF TABLES iii LIST OF FIGURES EXECUTIVE SUMMARY iv 1. INTRODUCTION 1 1.1 Background 1 1.2 Aims and Objectives 2 1.3 Research area 2 2. METHODS 4 2.1 Hydrogeology ' 4 2.2 Saline intrusion investigation 4 2.3 Catchment water balance 4 3. RESULTS 5 3.1 Hydrogeology 5 3.1.1 The geometry and structure of the Crag aquifer S 3.1.2 Aquifer parameters 8 3.1.3 The elevation and seasonal fluctuation of the Crag groundwater table 8 3.1.4 Perched aquifers 8 3.1.3 Recharge mechanisms 9 Rainfall 9 Surface water recharge 9 3.1.6 Groundwater discharge zones 10 Drained marshland 10 Happisburgh cliffs 11 3.2 Location of the saline intrusion 12 3.2.1 Surface water salinity survey 12 3.2.2 Groundwater salinity investigation 12 Electro-magnetic survey 12 Electrical resistivity survey 13 3.3 The influence of land drainage 14 3.3.1 Dyke water level survey 14 3.3.2 Composition of the drainage water 15 Saline water 16 3.4 Catchment water balance 18 3.4.1 Water balance results 18 3.4.2 Errors and innaccurades in the water balance 20 _ 3.4.3 Available groundwater resource 20 ~ 3.4.4 The effect of raised dyke water-levels on drainage pump discharge 20 4. DISCUSSION 22 4.1 Mechanisms governing the location of the saline intrusion 22 4.2 Influence of the internal structure of the Crag aquifer on saline intrusion 24 4.3 Stability of the saline intrusion front 26 4.4 Contribution of drainage pump discharges to the catchment water . balance 27 4.5 Effect of raising dyke water levels 27 CONCLUSIONS RECOMMENDATIONS ACKNOWLEDGEMENTS REFERENCES Page LIST OF TABLES 3.1 The range of groundwater levels near HickliAg Broad 9 3.2 Mean values and range of values of dyke water levels recorded at the pump inlets during the period April 1991 to April 1993 15 3.3a Chlorinity data for the Group 1 land drainage pumps of the Tburne catchment 16 3.3b Chlorinity data for Group 2 land drainage pumps 16 3.4 Summary of the results of the catchment water balance 18 3.5 Results of the catchment water balance 19 3.6 Available groundwater resource and the reduction in drainage discharge if abstracted 20 3.7 The effect of raising the dyke water levels of the Brograve and West Somerton levels 21 4.1 Groundwater head imposed by inland drainage pumps relative to that of the West Somerton New pump and the resulting predicted depth of the saline intrusion from a consideration of hydrostatic equilibrium 26 4.2 Groundwater head imposed by inland drainage pumps relative to that of the Brograve pump and the resulting predicted depth of the saline intrusion 26 4.3 Nominally available groundwater surplus in the Thume catchment 27 LIST OF FIGURES LI Map of the study area showing the River Thurae catchment 3 3.1 Groundwater and surface water elevations on 1st March, 1993 and the revised groundwater catchment boundary 5 3.2 Contour map of the base of the Crag 6 3.3a A section through the Crag aquifer showing the proposed structure 7 3.3b Contour map of the upper clay layer within the Crag aquifer 7 3.4 A geo-electric section of the area between Hickling Broad and Horsey showing the brackish groundwater originating in the Broad 10 3.5 Groundwater and surface water levels in the Brograve Level during the period from April 1991 to April 1992 11 3.6 Groundwater and surface water ieveis in the Horsey Level during the period from April 1991 to April 1992 12 3.7 Contour map of ground conductivity at an exploration depth of 7.5 m from measurements taken with an EM-34 13 3.8 Contour map of the depth to the saltwater table, based upon data from electrical resistivity soundings 14 3.9 Dyke water levels in die Brograve and Eastfield drainage Levels on the 8th March, 1993 15 3.10a The chlorinity of water samples collected at a Group 1 pump during the period April 1991 to April 1993 17 3.10b The chlorinity of water samples collected at a Group 1 pump against discharge 17 3.11a The volume of seawater pumped each week by a Group 1 pump during the period April 1991 to April 1993 17 3.11b Tbe volume of seawater pumped by a Group 1 pump against total discharge 17 3.12 The chlorinity of water samples collected at a Group 2 pump during the period April 1991 to April 1993 18 41 Tbe relationship between saline inflow into the dyke systems and die directions of the hydraulic gradients resulting from the dyke water levels 23 4.2 The proposed mechanism of fixed equipotential heads imposed by the main dykes of the Brograve and Eastfield land drainage pumps 24 4.3 Conceptual model of tbe mechanisms con trolling saline intrusion in the Tbume catchment 25 iii EXECUTIVE SUMMARY This R&D Note summarises the results of a three-year PhD investigation, funded by the Natural Environment Research Council and the National Rivers Authority, into the hydrogeology of the Norwich Crag aquifer in the River Thume catchment in north-east Norfolk. This low-lying coastal aquifer is subject to saline intrusion as a result of land drainage that has been carried out in the marshland during the past two centuries. Changes in the economics of arable farming and the perception that changes in the land drainage regimes will produce beneficial effects on the water quality of the River Thume have necessitated the need for an increased understanding of the hydipgeology of this complex, highly managed aquifer. Section 1 of this Note introduces the catchment and the background to the issues involved in the future management of the catchment. Section 2 outlines the methodology used in this inter-disciplinary study. Hydrogeological, hydrological, hydrochemical and geophysical techniques have been extensively used. Section 3 summarizes the results of the three main avenues of research. The hydrogeology of the Crag aquifer is described; the location and the mechanisms controlling the saline intrusion are presented; and the results of a catchment water balance are given. The water balance has been used to assess the available groundwater resource for the catchment and to predict the likely effects of changes in the land drainage regimes on the quantity and quality of water discharged into the River Thume. Finally, the principal results have been brought together in Section 4 to demonstrate the links between aquifer structure, saline intrusion and land drainage, and to discuss the possible outcome of some future management options. 1. INTRODUCTION 1.1 Background Most of tbe Norfolk Broads and tbe dykes that drain the associated marshland contain freshwater. Some of them, in the lower readies of die Broadland rivers, are brackish as a result of tidal influences. However, those in the Tburae catchment, even though they lie upstream of the saline effects of all but the highest tides, are also brackish. This unusual saline nature of the Thume river system in north-east Norfolk has been reported since the turn of tbe century, when the source of tbe salinity was thought to be "probably due to salt springs’1 within Hickling and Horsey Mere (Gurney 1904). Following the work of Pallis (1911), it is generally recognised that the source of the salinity is by direct underground communication between the sea and the waters of the Thume district The two main potential conduits are die underlying Norwich Crag aquifer, which outcrops beneath the sea, or the peat within the marshes. Goldsworthy (1972) concluded that the peat was the main transmission zone, there being recurrent exposures of peat between the tidemarks on the foreshore between Horsey and tbe northern end of the Winterton Ness (Lambert et al 1960).