Effects of abstraction on salinity regime – Ant Broads and Marshes Summary • This paper looks at a 17 page report produced by Mr Linford-Wood of the Environment Agency, which considers the potential relationship between water abstraction and increased salinity. • The Ant Valley fens are highly vulnerable to saline incursion; salinity having been demonstrated by various researchers to have a negative impact upon the distribution of rare plant species, SAC Annex 1 fen communities and uncommon species of invertebrate. • There are some indications of an ongoing upward trend in salinity in the Ant catchment with this trend having been identified by the early 1990s. • Groundwater and surface water abstraction has a direct effect on river flows. • The EA consider that the effect of real fully licenced abstraction has been to reduce freshwater river flows in the Ant system by approximately 5%, however this value is disputed. • Other hydrologists suggest that the in-combination effects of abstraction are considerably in excess of the suggested 5% of river flow quoted by the EA. • In some of the Ant floodplain fens, the reduction in diffuse flow through the model cells under ‘average’ conditions is above 10% and in some cases up to 50%. At low flows, the figure is in excess of 50% in parts of Catfield Fen, Broad Fen, Reedham Marshes and Snipe Marsh. • Flows in the River Bure upstream of the confluence with the Ant are reduced by as much as 50% at Q95; and between -10 and -20% at Q10 conditions. It is clear that the very considerable reduction in modelled flows in the Bure under all flow conditions will be significantly increasing the likelihood of saline incursion into the Ant. • Mr Linford-Wood’s analysis suggests that there is “no evident salinity impacts from seawater incursion” further upstream than Barton Broad. The Environment Agency’s own monitoring data shows that high chloride levels resulting from saline incursion can be seen as far upstream as Wayford Bridge, which is well above the upper extent of the Ant Broads and Marshes SSSI and c4km upstream of Barton Broad. • In the mid Ant valley the mean water electrical conductivity levels are already well above the tolerance of species in the ‘Freshwater’ category as developed by Panter et al (2011). • Data from a range of sources show the extreme sensitivity of a number of plant species and communities to salinity. Recent work indicates that the upper limit of the freshwater category proposed by Panter et al of 300mg/l chloride may be set too high for freshwater fen plant species and should be ‘fine-tuned by the introduction of a ‘very fresh’ category with an upper limit of 100mg/l chloride as proposed by Dutch researchers. • Mean chloride levels in the River Ant above and below Barton Broad are already well in excess of 100mg/l, and thus fen plant species and communities in the mid and lower valley should be regarded as already being at a ‘tipping point’. • In this context, the maintenance of strong freshwater flow downstream and flushing through the fens on either side of the river is critically important in preventing river water from penetrating far into these fens. • The net change in flushing resulting from reduced flows as a consequence of abstraction, whether an ‘average’ of 5% as hypothesised by Mr Linford-Wood or the much higher figure suggested by Dr Bradley could prove to be critically important. Background and context A 17 page report prepared by Simon Linford-Wood of the Environment Agency which considers the potential relationship between water abstraction and increased salinity was circulated to the group on the 18th September1. Mr Linford-Wood’s report summarises the main contributing factors to the salinity regime of Ant Broads and Marshes (ABM) and assesses the potential impacts of abstraction. Mr Linford-Wood is a hydrologist and groundwater specialist and consequently his paper is focussed upon the salinity regime and not the implications of salinity for freshwater organisms, and fenland plants and invertebrates. By way of background, it may be helpful to consider the points made in a document by Drs Bradley and Parmenter in November 2017 “The hydroecology of the Ant Valley, Norfolk, and the environmental implications of abstraction”, the main conclusions of which were: • It is widely acknowledged that the fen system of the Ant Valley is perhaps the finest in Western Europe and this is recognised in its European and international designations as a SAC and Ramsar site. • The habitat types for which the SAC is designated are dependent upon irrigation of the fen by calcareous water, either directly from groundwater, or indirectly from groundwater via the river network. • The Broadland fen habitats support the great majority of the species for which this area is important, and the most valuable area in this respect is the Ant valley fen system. • The Ant Valley fens are vulnerable to saline incursion; salinity having been demonstrated to have a negative impact upon the distribution of both rare plant species and uncommon species of invertebrate. • Sea level is projected to rise by ~24cm by 2050, and the Broadland Fens, including those of the Ant Valley, are likely to come under increased pressure both from storm surge events and an overall increase in salinity. • There are some indications of an ongoing upward trend in salinity in the Ant catchment, either due to increased incursion of saline water, or reduced down-stream flow. • Groundwater and surface water abstraction has a direct effect on river flows: for example the combined total of groundwater and surface water abstractions represents approximately half of the flow of the River Ant at Honing Lock. The effect of abstraction on river flows Mr Linford-Wood has concluded that, relative to estimated naturalised (i.e. no abstraction or discharge) flow conditions, the net result of historic and real fully licenced abstraction has been to reduce freshwater river flows in the Ant system by approximately 2% and 5% respectively and considers that, as a simple rule of thumb it might be expected for flushing periods to be extended by a similar extent. The Environment Agency’s comparison of modelled flow differences out of Barton Broad under the different abstraction scenarios suggest indicative ‘turnover’ times (the ratio of outflows to volume of water stored) are increased by 2-5% and some specific time estimates are provided for high storage and low storage conditions. These figures of 2% and 5% appear, although this is not expressly stated in Mr Linford-Wood’s paper, to be derived from a paper by Mr Gavin Sharpin, of the Environment Agency, entitled ‘The 1 Linford-Wood, S. 2018 Effects of abstraction on salinity regime – Ant Broads and Marshes. September 2018 Impact of artificial influences on surface water levels in Ant Broads and Marshes SSSI’ dated September 20182, which was also circulated on the 18th September. We are aware that there is some considerable dispute amongst other hydrologists as to whether these figures are actually correct; for example Dr Chris Bradley3 observes that these figures are “difficult to reconcile with the licensed abstraction volumes”, and that “there appears to be a discrepancy between modelled freshwater flows and observed freshwater flows, which suggests the figure of 2-5% should be regarded with extreme caution”. In the context of the monitored freshwater flows the inference being that “the in-combination effects of abstraction … will be considerably in excess of the suggested 2%-5% of riverflow quoted in the Sept. 2018 papers”. Prof. Gilvear of the University of Plymouth has also criticised the findings of the paper by Mr Sharpin and concludes that “the hydrological significance, and associated hydro-chemical effects, of reduced flows on the River Ant due to water abstractions has still not been firmly established …. it is poor practice to quote precise values for the impact on water levels when it is acknowledged within the report that there are some very large uncertainties and likely that there are significant errors associated with the analysis ….”. This is of particular concern, when the figures produced by Mr Sharpin are then used by other authors as a representation of the degree of change. Prof. Gilvear notes “I am also concerned that any studies which go on to develop on and extrapolate from the figures presented in the Sharpin report would be subject to similar uncertainty.”4 We also note that whilst Mr Linford-Wood reports that a “….net result of historic and real fully licenced abstraction has been to reduce freshwater river flows in the Ant system by approximately 2% and 5% respectively”, Mr Sharpin refers to a 5% reduction in the variation of the observed Barton Broad water level record under the fully licensed scenario and 3% under the historic scenario but fails to state (or estimate) the consequent reduction in river flows. Irrespective of the true value of the in-combination effect of abstraction upon flows in the main river, the impact of reduced freshwater flow in one part of the Ant system may well be higher than in another and so a figure of 5%, if correct, should be taken as the average change for the main river and not the entire floodplain system. For example, we know from recent modelling work undertaken by the Environment Agency (Appendix 2) that the fully licensed impact of combined surface and groundwater abstraction is to reduce freshwater flows through some of the fen sites (notably Catfield Fen, Mrs Myhills Marsh, Broad Fen, Snipe Marsh and the western part of Reedham Marshes) by over 50% under ‘dry’ Q95 conditions. Flows through Catfield Common and Hickling Broad under the same scenario are reduced by between 30 and 50%.