R. C. Ferrier

Isotope hydrology of the Allt a' Mharcaidh catchment, Cairngorms, Scotland : implications for hydrological pathways and residence times

The hydrology of oxygen-18 (18O) isotopes was monitored between 1995 and 1998 in the Allt a Mharcaidh catchment in the Cairngorm Mountains, Scotland. Precipitation (mean δ18O=−7·69‰) exhibited strong seasonal variation in δ18O values over the study period, ranging from −2·47‰ in the summer to −20·93‰ in the winter months. As expected, such variation was substantially damped in stream waters, which had a mean and range of δ18O of −9·56‰ and −8·45 to −10·44‰, respectively. Despite this, oxygen-18 proved a useful tracer and streamwater δ18O variations could be explained in terms of a two-component mixing model, involving a seasonally variable δ18O signature in storm runoff, mixing with groundwater characterized by relatively stable δ18O levels. Variations in soil water δ18O implied the routing of depleted spring snowmelt and enriched summer rainfall into streamwaters, probably by near-surface hydrological pathways in peaty soils. The relatively stable isotope composition of baseflows is consistent with effective mixing processes in shallow aquifers at the catchment scale. Examination of the seasonal variation in δ18O levels in various catchment waters provided a first approximation of mean residence times in the major hydrological stores. Preliminary estimates are 0·2–0·8 years for near-surface soil water that contributes to storm runoff and 2 and >5 years for shallow and deeper groundwater, respectively. These 18O data sets provide further evidence that the influence of groundwater on the hydrology and hydrochemistry of upland catchments has been underestimated. Copyright

Hydrogeochemistry of shallow groundwater in an upland Scottish catchment

The hydrogeochemistry of shallow groundwater has been characterized in the Allt aMharcaidh catchment in the Scottish Cairngorms in order to: (i) assess the spatial and temporal variation in groundwater chemistry; (ii) identify the hydrogeochemical processes regulating its evolution; and (iii) examine the influence of groundwater on the quality and quantity of stream flow. Shallow groundwater in superficial drift deposits is circumneutral (pH∽7·1) and base cation concentrations are enriched compared with precipitation and drainage water from overlying podzolic soils. Modelling with NETPATH suggests that the dominant geochemical processes that account for this are the dissolution of plagioclase, K-feldspar and biotite. Groundwater emerging as springs from weathered granite underlying high altitude (>900 m) alpine soils shows similar characteristics, though weathering rates are lower, probably as a result of reduced residence times and lower temperatures. Chemical hydrograph separation techniques using acid neutralizing capacity (ANC) and Si as tracers show that groundwater is the dominant source of baseflow in the catchment and also buffers the chemistry of stream water at high flows: groundwater may account for as much as 50–60% of annual runoff in the catchment. Climate and land use in the Cairngorms are vulnerable to future changes, which may have major implications for hydrogeological processes in the area.

Seasonal snowpack influence on the hydrology of a sub-arctic catchment in Scotland

Snow cover, depth and water equivalent have been measured in each snow season between 1989/90 and 1993/94 in the Alit a Mharcaidh catchment in the western Cairngorms. Spatial and temporal patterns of snow accumulation vary: the peak measured water equivalent held in the catchment snowpack ranged from around 50 mm in mild winters to 200 mm in more severe ones. Snow accumulation was concentrated on north and east facing slopes in the parts of the catchment above 700 m. Point measurements of water equivalent in the areas of greatest accumulation exceeded 2000 mm as a result high densities produced by prolonged melt metamorphosis. Patterns of accumulation and melt strongly influenced the hydrology and hydrochemistry of the stream which has a distinct alpine like regime. Low base flows of high pH (>6.5) characterise cold periods of snow accumulation. Melt events are variable; slight increases in temperatures commonly melt all snow lying at low altitudes. More substantial melts involving the extensive snowpack at higher altitudes occur in the early spring as temperatures increase more significantly. Melt events strongly influence the hydrochemistry of the stream; preferential elution of Cl and SO4 were distinct characteristics of each melt season. Moreover, the most acidic (pH < 5.5) streamflows, and the highest levels of inorganic aluminium, were usually observed during snowmelt.

Influence of snow on the hydrology and hydrochemistry of the Allt a' Mharcaidh, Cairngorm mountains, Scotland

Abstract Systematic studies of winter snow pack accumulation and melt have been carried out in the Allt a Mharcaidh catchment in the western Cairngorms, Scotland since 1989/90. These reveal marked variations in the annual snow pack accumulation which, in terms of peak volume (in water equivalents), ranges from 180 mm in severe ones. This volume, and the rate at which it melts, has a clear impact on the annual flow regime of the Allt a Mharcaidh: major melt produces high peak flows, whilst the absence of a substantial snow pack in mild winters can result in low flows. The dynamics of snow pack chemistry have also been examined. In the severe winter of 1993/94 the results suggest enhanced acidic enrichment at greater altitude (>900 m). In contrast, the smallest concentrations of acid anions (NO 3 , SO 4 , and Cl) were recorded during the mild winter of 1991/92. Limited snowfall at this time resulted in minimal accumulation of pollutants through dry deposition. Snowmelt events cause dramatic changes in the chemistry of the Allt a Mharcaidh. High flows are accompanied by major acidic episodes produced by preferential elution of pollutants from snow which may have severe consequences for aquatic life. The most direct link between the quality of snow and runoff is the release of SO 4 and Cl. Spatial variability in the extent and magnitude of melting produced a damped response in the stream signal over a short temporal scale. Ions derived from marine sources were enhanced by a factor of 2 in the snowpack compared to the damped concentration in the meltwater. The Cairngorm mountains is a high amenity area renowned for its outstanding natural beauty. It is widely recognised that this fragile environment is of national importance, which is further justification for continued research interest in this area.

Development and application of a distributed catchment-scale hydrological model for the River Ythan, NE Scotland

Mathematical models are being used to develop a decision support system for integrated management of the Ythan catchment in NE Scotland. One component of this has involved the development of a distributed catchment-scale hydrological model. The model is based on subsurface flow routing and calculates the contribution to stream flow from each 50 m×50 m cell in the 548 km2 catchment. It uses two topographic parameters, slope and distance to stream following the main line of flow, and five physical parameters. The topographic analysis and distributed flow accumulation are performed by linking the single cell model with a geographic information system. Preliminary results from a three-year simulation of daily flows indicate that the model successfully predicts the main characteristics of the catchment flow.

Assessment of wet deposition mechanisms in an upland Scottish catchment.

A network of collectors were installed at various altitudes and degrees of exposure in the Allt a Mharcaidh catchment, northeast Scotland, in an attempt to obtain an accurate assessment of wet deposition loading. Results indicate that the quantity and quality of bulk deposition is constant over the whole catchment. “Enhancement deposition” as measured by a filter-gauge interception collector indicated that there was greatest deposition at altitude. The concentrations of all elements, except for hydrogen, were greater than that of catchment bulk deposition at the higher altitudes; at lower altitude enrichment was only appreciable for sodium and chloride. Input/output chloride budgets were used to assess catchment evapotranspiration rates and the relative proportions of enhancement deposition within different altitudinal ranges. The calculation gives a catchment evapotranspiration of 18.5% and a chloride enhancement deposition 2.5 times greater at higher altitudes than at lower altitudes. Rainfall chemistry in this high-level Cairngorm catchment appears independent of the positioning of the rainfall collectors. Different altitudes within the catchment receive an additional loading due to enhancement deposition, dependent upon the frequency of cloud/mist cover. This additional loading must be included in the assessment of total catchment loadings and in the calculation of evapotranspiration.

Interaction of nitrogen deposition and land use on soil and water quality in Scotland: issues of spatial variability and scale.

Over large areas of the Scottish uplands anthropogenic sulfur (S) deposition is declining in response to stringent national and European controls on S emissions. At the same time, however, the relative contribution of nitrogenous (N) compounds to the total anthropogenic deposition loading has increased. To investigate the significance of N deposition on the potential acidification of surface waters, national, regional, and catchment databases were developed to assess the relationships between N deposition, soil C/N ratios, land use and surface water NO3 concentrations. National classification schemes for land use and soils were used as only limited empirical data are available at such large spatial scales. Data were screened to eliminate areas where N inputs are dominated by non-atmospheric sources. From these screened datasets, it was apparent that areas with the highest risk of N leaching were situated predominantly in the upland areas of south-west and west Scotland (areas with low soil C/N ratios). At the regional scale, surface-water NO3 concentration in afforested catchments was negatively correlated with soil C/N ratios below 20. This relationship was not evident in moorland catchments, where NO3 leaching was strongly related to N deposition and the loch/catchment ratio, rather than the soil C/N ratio. Temporal trends of regional water quality highlighted as increasing loch NO3 concentrations between 1988 and 1996-1997, presumably reflecting an increase in N deposition, enhanced leaching losses from the terrestrial component of the catchment, or altered in-lake processes. The hydrochemical records for two catchments in NE Scotland (Lochnagar and Allt a Mharcaidh) highlight the importance of within catchment process in controlling the nitrogen response observed in surface waters. The potential mechanisms through which vegetation and soils may modify incoming deposition are discussed.

Hydrological and hydrochemical fluxes through vegetation and soil in the Allt a' Mharcaidh, western Cairngorms, Scotland: Their effect on streamwater quality

Abstract A detailed investigation of the hydrochemical alteration of input water by vegetation and soils was undertaken in an upland catchment in the Cairngorm Mountain region of Scotland. The composition of catchment outflow water reflects the hydrological routing of water through different soil horizons and the importance of long residence time water. There is uptake of nitrogen and neutralization of incoming anthropogenic acidity by the vegetation, and sulphate adsorption in the mineral soils. Streamwater quality is dominated by the contribution of long residence time water, especially during base flow. Sulphate retention and cation release are the major neutralization mechanisms buffering outflow chemistry at this site.

Sulphate dynamics of podzols from paired impacted and pristine catchments.

Abstract The similarity in the physical and chemical parameters of the soil profiles located at Loch Chon (central Scotland) and Hoylandet (mid Norway) enabled the comparison of soil sulphate adsorption and accumulation under two different levels of sulphate deposition. The Loch Chon site, due to its proximity to the industrialized central belt in Scotland, has received a history of anthropogenic sulphur loadings, while the Hoylandet site is situated in an area of Norway subjected to precipitation generated from unpolluted air masses originating over the North Atlantic ocean. Both soils show accumulation of water-soluble sulphate in the organic horizons (45 mg S kg −1 at Chon and 65 mg S kg −1 at Hoylandet), with low levels (0–5 mg S kg −1 ) being found in the mineral horizons. The iron podzol from the pristine (unpolluted) site at Hoylandet had levels of adsorbed sulphate in the range of 0–17 mg S kg −1 , which may have arisen through microbial mineralization of translocated organic matter and dead root tissue. In contrast, the humus iron podzol from Loch Chon, which had similar levels of mineralizable sulphate, showed a build-up of high levels of adsorbed sulphate throughout the profile (15–90 mg S kg −1 ). This additional sulphate may reflect a history of high anthropogenic sulphate deposition in this area, as laboratory based adsorption isotherm studies indicated that both soils were capable of adsorbing sulphate. Regression analysis indicated that adsorbed sulphate was correlated (5% level) with dithionite-soluble aluminium in the Loch Chon soil, but with the similar iron fraction in the Hoylandet soil.

Changes in soil and stream hydrochemistry during periods of spring snowmelt at a pristine site in mid-Norway

A detailed study of hydrochemical changes during spring snowmelt was undertaken at a pristine site in mid-Norway. The composition of catchment outflow water reflects hydrological pathways through different soil horizons and the contribution of pre-event or long-residence time water. There was evidence of secondary snowpack elution and ionic enrichment of water collected below the organic horizons. The observed time lag in commencement of BC soil horizon flow indicated that during the initial stages of the melt, water did not penetrate the deeper mineral horizons, or that initially the soil was not at field capacity. Increased soil and surface thawing is proposed as an important mechanism affecting the routing and subsequent chemical alteration of soil water leachate. The relative importance of elution, pathways, and dilution processes within the study catchment is discussed and a scenario for the effects of winter anthropogenic inputs at this pristine site is presented.