A hydrochemical assessment of groundwater-surface water interaction in the Woodham Burn, a Magnesian Limestone catchment in County Durham

Palumbo-Roe, B.; Brauns, B.; Banks, V.J.. 2020 A hydrochemical assessment of groundwater-surface water interaction in the Woodham Burn, a Magnesian Limestone catchment in County Durham. Nottingham, UK, British Geological Survey, 118pp. (OR/20/059) (Unpublished)

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Abstract/Summary

The interaction between groundwater and surface water, in particular in the hyporheic zone, is recognised to influence chemical fluxes between river and groundwater and to transform reactive chemistries such as nutrients or legacy contaminants. Characterising this connectivity in the Skerne catchment in Co. Durham has been recognised to be important by the Environment Agency (EA) in order to protect the underlying Magnesian Limestone aquifer and dependent features. Of particular concern is the presence of an eastward moving, sulphate-rich, mine water plume related to the recovery of groundwater levels in the underlying Coal Measures strata and mine workings. Building on a previous investigation across the entire Skerne catchment, this work, in collaboration with the EA, aimed to understand the existence of hydraulic connection between groundwater and surface water and the hyporheic zone characteristics in a 500 m stretch of the Woodham Burn, a tributary of the Skerne. We have employed multiple methods, both at the reachscale and smaller sediment-scale, for identifying source areas of sulphate to the stream, characterising the subsurface flow and estimating the controls on sulphate fluxes and potential natural attenuation. The Woodham Burn was monitored during three sampling events in: April 2018, August 2018, and February 2019. The stream water chemistry survey has confirmed sulphate concentrations in excess of the drinking water standard of 250 mg/l, with a median of 510 mg/l in the surface water, range 235- 790 mg/l. Stream flow measurements complementary to the water chemistry analysis were not possible and therefore loads (flow multiplied by concentration) of sulphate could not be calculated. Given the lack of tributaries, the changes in surface water chemistry were, nevertheless, useful to attribute the observed changes to groundwater losses or gains, where groundwater and surface water concentrations are significantly different. The spatial survey of downstream changes in stream water chemistry has delineated a sulphate-rich recharge zone within the study reach, which is very localised, with sulphate concentrations up to 800 mg/l and electrical conductivity of 2000 µS/cm. The source appears to be groundwater discharging directly into the stream channel and in the form of a seep on the western bank. The contribution of this source to the stream was quantified as up to 50 percent increase in dissolved sulphate in surface water. An additional area potentially recharging sulphate to the stream, more diffuse in nature, was identified through analysis of the water-soluble fraction of soil samples collected by augers and additional bank seepage measurements and it corresponds to the northern banks upstream of the first monitoring point. Temperature vertical profiling of the riverbed along the 500 m study reach together with a corresponding survey of specific electrical conductivity (SEC) variation in the surface water were used to further detect areas of potential flow of surface water to groundwater or flow of groundwater to surface water, and to inform the site selection for the monitoring of subsurface flow at smaller scale with piezometers and minipiezometers. At four locations, piezometers were installed with data loggers to provide continuous observations of hydraulic heads, temperature, and (at two points) SEC: two paired piezometers of shallow (0.4 m) and deeper (1.4 m) depth at three of the locations and only a single shallow piezometer for one site. Due to the loss of the surface water logger after a storm event, precise water level fluctuations in Woodham Burn were unknown, and recorded fluctuations in the subsurface were more difficult to correlate. At the same locations, plus an additional one, a network of multilevel minipiezometers (two to three per site) were driven into the hyporheic zone to a fixed depth of 0.9 m below the riverbed and used to draw pore water from 10, 20, 50 and 90 cm depth. The evidence from vertical gradients of conservative elements, chloride and lithium, measured in each multilevel minipiezometer, and evidence from the diurnal temperature variations and hydraulic head from logged data, converged to indicate an increase of hyporheic exchange flow (HEF) moving downstream in the burn, corresponding to the transition in the superficial deposits from alluvium to lacustrine deposits, while the most upstream sites showed the near absence of HEF and at least one clearly gaining reach in correspondence of the sulphate-rich instream discharge. ix Evaluation of natural attenuation in the hyporheic zone was carried out via comparison of conservative and non-conservative solute gradients. In most of the sites where there was sufficient HEF, both nitrate and sulphate showed various extents of non-conservative behaviour compared to chloride in the subsurface flow. In particular, the most significant losses of nitrate were observed in piezometers at the most downstream section of the reach (-48% to -98%). Sulphate losses were generally lower than those for nitrate and varied greatly (9% to 100%), often larger at depth. Although nitrate and sulphate losses were observed during surface water downwelling in the studied hyporheic zone, a correspondent decrease in stream water concentrations was not evident. It is recommended to test the significance of hyporheic natural attenuation to improve the stream water quality at catchment scale, by carrying out surface water flow measurements combined with water quality analysis, which enable calculation of mass gains and losses to identify the net flux integrated over the entire stream. The analysis of the deep hyporheic zone chemistry, the least affected by shallow hyporheic exchange of downwelling surface water, gave insights into hydrochemical differences along the reach and indicates potentially distinct groundwater sources. These differences appear to be related to geology: the most upstream monitoring locations sited on the alluvium have greater similarities to the Magnesian Limestone aquifer, as inferred by cluster analysis with additional EA groundwater monitoring boreholes. As previously identified points of limited HEF and groundwater dominated hyporheic water, these locations plausibly represent a groundwater recharge zone. On the other hand, the most downstream points located on the lacustrine deposits show a different hyporheic zone composition, distinctly closer to hyporheic waters previously sampled from Rushyford Beck, also on lacustrine deposits. Beside these hydrochemical differences, a greater one is represented by the discrete spring (Bubbly Spring) discharging through the stream bed and western stream banks. Its chemistry has strong similarities to other seeps in the burn and also to Stony Hall C borehole water, which is sourced from the Coal Measures. The spring composition (Mg-SO4 water type) was very stable throughout the monitoring period and distinctively enriched in SO4 (median 811 mg/l) together with Sr (median 984 μg/l) , Li (median 162 μg/l), Rb (median 7.19 μg/l) and U (median 4.04 μg/l) compared to all the other waters in Woodham Burn, while it was lower in Si, Ba, Mn and Fe. Two, both plausible, reaction paths can explain the spring composition: one is gypsum dissolution and dedolomitisation, the other one is acid neutralisation of coal mine water through the dissolution of dolomite. To explain the physical processes underpinning the emergence of this groundwater enriched in sulphate and the origin of this sulphate further investigation is needed. In particular residence time studies and isotope analysis of water and dissolved sulphate are recommended. To gain a broader perspective on groundwater discharge areas a spatial hydrochemical survey of springs and seeps in the catchment should also be undertaken.

Item Type:	Publication - Report
Funders/Sponsors:	British Geological Survey
Additional Information. Not used in RCUK Gateway to Research.:	This item has been internally reviewed, but not externally peer-reviewed.
Additional Keywords:	GroundwaterBGS, Groundwater
Date made live:	17 Dec 2020 11:07 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/529185

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