Understanding the hydrochemical evolution of a coastal dune system in SW England using a multiple tracer technique

Abstract

An improved knowledge of the hydrology of coastal dune systems is desirable for 1 successful management of their diverse ecology under a changing climate. As a near-pristine 2 coastal dune spit system, Braunton Burrows (SW England) is an ideal location for the study 3 of the natural processes governing recharge to the dune groundwater system and the 4 evolution of its water quality. Whereas previous investigations have tended to focus on inter-5 dune slacks, this study has also given attention to infiltration through the high dunes. Cores 6 were taken through dunes and the resulting sand samples processed to provide information on 7 grain size distribution and porewater chemistry. Groundwater samples were obtained from 8 beneath dunes and slacks. A variety of geochemical techniques were applied including 9 hydrochemistry, stable isotopes and residence time indicators. The unsaturated zone profiles 10 indicate the existence of piston flow recharge with an infiltration rate of 0.75-1 m/yr, 11 although faster rates probably also occur locally. Groundwater beneath the high dunes gave 12 ages in the range 13-16 yr, compared to the dune slack groundwater ages of 5-7 yr, and an 13 age of 22 yr for groundwater from the underlying mudstone aquifer. The chemistry of waters 14 in both unsaturated and saturated zones is dominated by Ca and HCO 3 , supplemented by 15 variable amounts of other ions derived from marine aerosols and limited reaction with sand 16 grains and their coatings. The main chemical evolution of the porewaters occurs rapidly 17 through the mobilisation of surface salt crusts and dissolution of shell carbonate. This 18 situation changes little in the underlying groundwater, though an evolution towards reducing 19 conditions increases the concentrations of redox-sensitive species such as Fe and Mn. The 20 rapid chemical evolution of the infiltrating water means that its composition will respond 21 quickly to changes in the supply of shell material and/or marine salts, which are possible 22 consequences of climate change. However, the residence time measurements suggest the 23 dune aquifer has a relatively long turnover time which will to some extent buffer such 24 changes. The results of the present study should be transferable to natural dune systems in 25 similar coastal situations. 26

Modelling small groundwater systems: experiences from the Braunton Burrows and Ainsdale coastal dune systems, UK

“…z) is relatively well-constrained by the data from the geophysical survey, there remains large uncertainty relating to the distribution of hydraulic conductivity (K) within the dune system. The inability of the model to reproduce the observed water level response in some parts of the model may indicate that the distribution of hydraulic conductivities across the model domain is not as uniform as Burden (1998) and Allen et al (2014) suggest. Recent drilling in the central part of the dune system has revealed the presence of coarse sands and gravels at depths within the dunes (D. White, 2015, personal communication), hence challenging the general assumption that sand-blown dune systems consist of uniform, well-sorted granular sediments throughout.…”

“…Whilst the system itself is of relatively modest size (8 km 2 ) and has been subject to a variety of surveys and studies (e.g. Mcfarlene 1955; Kidson & Carr 1960;Kidson et al 1989;Burden 1998;May 2001;Stratford et al 2013;Allen et al 2014), quite a high degree of uncertainty was discovered during model development relating to the definition of the model boundary conditions (e.g., drainage conditions in the northeast of the model domain) and the transmissivity distribution within the model domain.…”

“…recharge, aquifer depth, permeability). Previous studies suggest that there is little spatial variation in aquifer depth, particle size or hydraulic conductivity across the dune system (Burden, 1998;Allen et al, 2014). Hence, a uniform distribution of hydraulic conductivities (10 m d…”