Discrete wetland groundwater discharges revealed with a three-dimensional temperature model and botanical indicators (Boxford, UK)

Abstract

Wetlands provide unique goods and services, as habitats of high biodiversity. Hydrology is the principal control on wetland functioning, hence understanding the water source is fundamental. However, groundwater inflows may be discrete and easily missed. Research techniques are required with low cost and minimal impact in sensitive settings. In this study, the effectiveness of using a three-dimensional (3D) temperature model and botanical indicators to characterise groundwater discharge is explored at the CEH (Centre for Ecology and Hydrology) River Lambourn Observatory, Boxford, UK. This comprises a 10-ha lowland riparian wetland, designated for its scientific interest and conservation value. Temperature data were collected in winter at multiple depths down to 0.9 m over approximately 3.6 ha and transformed into a 3D model via ordinary kriging. Anomalous warm zones indicated distinct areas of groundwater upwelling which were concurrent with relic channel structures. Lateral heat propagation from the channels was minimal and restricted to within 5-10 m. Vertical temperature sections within the channels suggest varying degrees of groundwater discharge 2 along their length. Hydrochemical analysis showed warmer peat waters were akin to deeper aquifer waters, confirming the temperature anomalies as areas of groundwater discharge.Subsequently, a targeted vegetation survey identified Carex paniculata as an indicator of groundwater discharge. The upwelling groundwater contains high concentrations of nitrate which is considered to support the spatially restricted growth of Carex paniculata against a background of poor fen communities located in reducing higher-phosphate waters.

Long-term Holocene groundwater fluctuations in a chalk catchment: evidence from Rock-Eval pyrolysis of riparian peats

“…This Boxford site is a special area of conservation because of the habitat it provides for Desmoulins whorl snail ( Vertigo moulinsiana ) and is also as a site of special scientific interest because of its wetland habitats (Old et al , ). It has been the subject of much recent work aimed at understanding the functioning of the wetland (House et al , 2015a, b) which has included the installation of boreholes (Allen et al , ; Newell et al , ) and geophysical surveys, including electrical resistivity tomography (Chambers et al , ) and ground penetrating radar (Musgrave and Binley, ). Historic maps dating back to the 1880s show a network of predominantly linear conduits, sluices and aqueducts characteristic of a managed water meadow system (Newell et al , ).…”

“…Groundwater level and river behaviour are thus intimately linked, and the headwaters of chalk streams can move many kilometres up and down the catchment in response to changing groundwater level (Grapes et al , ). The development of densely vegetated riparian wetlands in chalkland settings is similarly linked to groundwater input (Grapes et al , ; House et al , ), and it is therefore possible that any accumulated peat profiles may contain some record of past groundwater levels in the catchment. An understanding of the timing and magnitude of groundwater fluctuations over the past millennia is difficult to quantify but is important not only in reconstructing palaeoclimate, palaeohydrogeology and palaeohydrology, but because it may help constrain predictions on how climate change will impact Chalk aquifers in the future (Jackson et al , ; Jimenez‐Martinez et al , ).…”

“…In this paper, we consider the organic geochemistry of the Boxford peats over the past 4000 years and discuss whether this provides any useful information on environmental change within the catchment. The paper contributes to the growing body of recent work in the Lambourn catchment, focussing particularly at the Boxford wetland site, aimed at understanding the hydrological and hydrogeological functioning of the chalkland catchment (Gooddy et al , ; Grapes et al , ; Griffiths et al , ; Mullinger et al , ; Allen et al , ; Musgrave and Binley, ; Chambers et al , ; House et al , ; Newell et al , ).…”

Modelling groundwater/surface water interaction in a managed riparian chalk valley wetland

“…Surface flooding is a combination of seepage from upwelling GW and overbank flow routed from the channels by the relic drainage network. The simulated areas of GW mounding in the north meadow and associated flooding support earlier findings in the field (House et al ., ). To the east, steeper head gradients correspond with the mouth of a dry valley.…”

“…The degree to which water sources interact will affect plant species distribution through the available nutrient budget. Previous hydrochemical analysis has shown that chalk GW upwelling into the peat contains high concentrations of NO 3 and SO 4 and low P concentrations (House et al ., ). Elsewhere, the peat contains reducing waters low in NO 3 and SO 4 , yet high in P. These different chemical environments were found to promote distinct plant species.…”