Solute transport dynamics in small, shallow groundwater-dominated agricultural catchments: insights from a high-frequency, multisolute 10 yr-long monitoring study

Abstract

Abstract. High-frequency, long-term and multisolute measurements are required to assess the impact of human pressures on water quality due to (i) the high temporal and spatial variability of climate and human activity and (ii) the fact that chemical solutes combine short-and long-term dynamics. Such data series are scarce. This study, based on an original and unpublished time series from the Kervidy-Naizin headwater catchment (Brittany, France), aims to determine solute transfer processes and dynamics that characterise this strongly human-impacted catchment.The Kervidy-Naizin catchment is a temperate, intensive agricultural catchment, hydrologically controlled by shallow groundwater. Over 10 yr, five solutes (nitrate, sulphate, chloride, and dissolved organic and inorganic carbon) were monitored daily at the catchment outlet and roughly every four months in the shallow groundwater.The concentrations of all five solutes showed seasonal variations but the patterns of the variations differed from one solute to another. Nitrate and chloride exhibit rather smooth variations. In contrast, sulphate as well as organic and inorganic carbon is dominated by flood flushes. The observed nitrate and chloride patterns are typical of an intensive agricultural catchment hydrologically controlled by shallow groundwater. Nitrate and chloride originating mainly from organic fertilisers accumulated over several years in the shallow groundwater. They are seasonally exported when upland groundwater connects with the stream during the wet season. Conversely, sulphate as well as organic and inorganic carbon patterns are not specific to agricultural catchments. These solutes do not come from fertilisers and do not accumulate in soil or shallow groundwater; instead, they are biogeochemically produced in the catchment. The results allowed development of a generic classification system based on the specific temporal patterns and source locations of each solute. It also considers the stocking period and the dominant process that limits transport to the stream, i.e. the connectivity of the stocking compartment. This mechanistic classification can be applied to any chemical solute to help assess its origin, storage or production location and transfer mechanism in similar catchments.

Hydrological controls on DOC : nitrate resource stoichiometry in a lowland, agricultural catchment, southern UK

“…DOC is also transport rather than supply limited at this site, showing a slight increase in concentration with increasing discharge, and a more pronounced increase in spring which is not associated with a rise in discharge. In fact all three sites -on Chalk, Greensand, and clay -have elevated DOC concentrations in spring, which could arise from production, leaching and export of DOC from catchment soils as soil temperatures rise (Aubert et al, 2013), and/or in-stream production.…”

“…In addition, there is an increased understanding that establishing the factors that control water-borne carbon fluxes is key to predicting the likely implications of climate change for patterns and magnitude of organic carbon transport through freshwaters (Aitkenhead and McDowell, 2000). Although dissolved organic carbon (DOC) plays a crucial role in stream ecology (influencing processes such as nutrient uptake and the balance between heterotrophy and autotrophy) our understanding of terrestrial-to-aquatic transfers, aquatic processing of DOC, and its character in lowland, agricultural streams is incomplete (Aubert et al, 2013;Stanley et al, 2012;Yates et al, 2016), as much of the effort in this area has been focused o forested catchments, boreal peatlands, and/or upland landscapes with significant wetland cover (Frost et al, 2006;Ågren et al, 2007).…”

“…Therefore, integrated annual measurements risk masking important seasonal patterns in terrestrial-to-aquatic transfers and export of DOC and nitrate, arising from variations in hydrological pathways throughout the year, such as the interplay between groundwater and shallower lateral flows due to wetting-up of upper soil horizons in response to autumn rain (Prior and Johnes, 2002;Sandford et al, 2013;Outram et al, 2014;Yates and Johnes, 2013). Such intra-annual variations in solute chemistry have been termed the "hydrochemical signature" of the catchment (Aubert et al, 2013). This hydrochemical signature is espe-cially important to consider across an agricultural landscape characterized by a wide range of baseflow index (BFI).…”

New molecular evidence for surface and sub-surface soil erosion controls on the composition of stream DOM during storm events

“…Previous studies conducted on this site provide evidence for the structuration of the hydrological year into three different periods: period A, where the water table reaches the surface in down-slope wetlands but stays below the surface in the slope; period B, where rainfall intensification is responsible for the rise of water table in the slope which creates a hydrological connection between the slope soils, riparian soils and stream (Fig. 2); and period C, which is characterized by the return of the water table to deep soil horizons, resulting in the progressive drying of wetland soils (Aubert et al, 2013;Humbert et al, 2015;Lambert et al, 2013;Molenat et al, 2008). With the water table rise during period B, the water flow path geometry in riparian wetlands changes from a vertical to horizontal circulation.…”

Valuing knowledge on temporal dynamics from long-term monitored basins for neighbouring sites

“…The project is focused on multi-scale temporal variation of water and solute transfers under the effects of agriculture and climate. The knowledge produced has improved our understanding of hydrological processes (Aubert et al 2013), but such stations can also be directly useful to comprehend the water security issues of surrounding basins, thus tackling the challenges of both Panta Rhei and PUB (Hrachowitz et al 2013, Montanari et al 2013. …”