Richard P. Smart

Riparian zone influence on stream water chemistry at different spatial scales: a GIS-based modelling approach, an example for the Dee, NE Scotland

A geographical information system (GIS-ARC/INFO) was used to collate existing spatial data sets on catchment characteristics to predict stream water quality using simple empirical models. The study, based on the river Dee catchment in NE Scotland, found that geological maps and associated geochemical information provided a suitable framework for predicting chemical parameters associated with acidification sensitivity (including alkalinity and base cation concentrations). In particular, it was found that in relatively undisturbed catchments, the parent material and geochemistry of the riparian zone, when combined with a simple hydrological flow path model, could be used to accurately predict stream water chemistry at a range of flows (Q95 to > Q5) and spatial scales (1-1000 km2). This probably reflects the importance of the riparian zone as an area where hydrological inputs to stream systems occur via flow paths in the soil and groundwater zones. Thus, evolution of drainage water chemistry appears to retain the geochemical characteristics of the riparian area as it enters the channel network. In more intensively managed catchments, riparian land use is a further influential factor, which can be incorporated into models to improve predictions for certain base cations. The utility in providing simple hydrochemical models, based on readily available data sets, to assist environmental managers in planning land use in catchment systems is discussed.

The influence of catchment characteristics on the seasonality of carbon and nitrogen species concentrations in upland rivers of Northern Scotland

Data from 13 catchments with no arable land in Northern Scotland were used to develop empirical linear regression models of average monthly NO3− concentrations and average summer and winter concentrations for NH4+, dissolved organic N (DON) and dissolved organic carbon (DOC) as a function of catchment characteristics. All catchments displayed a pronounced seasonal NO3− cycle. Variation in monthly mean NO3− concentration within and between catchments could be predicted from mean monthly air temperature using separate regression equations for temperatures < and ≥ 5 °C. Soil type, climate and land use influenced NH4+ concentrations. In summer, concentrations of NH4+ were largest in catchments with extensive areas of brown forest soils, which are less acidic and more base-rich than other upland soils. However, concentrations declined with increasing conifer cover and summer rainfall. In winter, however, % conifer cover had a positive effect, while higher temperature and higher humus iron podzol cover had negative influences. DON concentration decreased with increasing catchment elevation in both summer and winter. Surprisingly, concentrations of DON only displayed a positive relationship with percentage peat cover in the summer. The most important factor controlling DOC concentration was soil type, with a positive relationship being observed between DOC and peat and humus iron podzol coverage. Elevation was also important, but only in the winter when concentrations were negatively correlated with maximum catchment elevation. Overall, multivariate regression equations explained the spatial and seasonal variability in N species concentrations over a range of catchments within Northern Scotland.

Water quality in the Scottish uplands: a hydrological perspective on catchment hydrochemistry

Land above 300 m covers approximately 75% of the surface of Scotland and most of the nations major river systems have their headwaters in this upland environment. The hydrological characteristics of the uplands exert an important influence on the hydrochemistry of both headwater streams and downstream river systems. Thus, many of the spatial and temporal patterns in the chemical quality of surface waters are mediated by hydrological processes that route precipitation through upland catchments. These hydrological pathways also have an important influence on how the hydrochemistry of upland streams is responding to increasing pressures from environmental changes at the global and regional scales. At the present time, atmospheric deposition remains an issue in many parts of the Scottish uplands, where critical loads of acidity are exceeded, particularly in areas affected by increasing N deposition. Moreover, climatic change forecasts predict increasingly wetter, warmer and more seasonal conditions, which may modify the hydrochemical regimes of many river systems, particularly those with a strong snowmelt component. On a more localised scale, land management practices, including felling of commercial forests, expansion of native woodlands, agricultural decline and moorland management all have implications for the freshwater environment. Moreover, increasing public access to upland areas for a range of recreational activities have implications for water quality. Understanding the hydrology of the uplands, through integrated field and modelling studies, particularly of the hydrological pathways that regulate chemical transfers to streamwaters, will remain an important research frontier for the foreseeable future.

Variable source and age of different forms of carbon released from natural peatland pipes

We used the carbon isotope composition (14C and δ13C) to measure the source and age of DOC, POC, dissolved CO2 and CH4 (δ13C only) released from three natural peat pipes and the downstream catchment outlet of a small peatland in northern England. Sampling under different hydrological extremes (high flows associated with storm events and low flows before or after storms) was used to explore variability in C sources as flow paths change over short periods of time. The δ13C composition of organic C differed (δ13C-DOC −28.6‰ to −27.6‰; δ13C-POC −28.1‰ to −26.1‰) from that of the dissolved gases (δ13C-CO2 −20.5‰ to +1.1‰; δ13C-CH4 −67.7‰ to −42.0‰) and showed that C leaving the catchment was a mixture of shallow/deep pipe and non-pipe sources. The isotopic composition of the dissolved gases was more variable than DOC and POC, with individual pipes either showing 13C enrichment or depletion during a storm event. The 14C age of DOC was consistently modern at all sites; POC varied from modern to 653 years BP and evasion CO2 from modern to 996 years BP. Differences in the isotopic composition of evasion CO2 at pipe outlets do not explain the variability in δ13C and 14C at the catchment outlet and suggest that overland flow is likely to be an important source of CO2. Our results also show that the sources of CO2 and CH4 are significantly more variable and dynamic than DOC and POC and that natural pipes vent old, deep peat CO2 and POC (but not DOC) to the atmosphere.

On modelling the effects of afforestation on acidification in heterogeneous catchments at different spatial and temporal scales

A modelling approach is presented for simulating and predicting future changes in streamwater Gran alkalinity throughout a large, heterogeneous river system. The methodology is based on integrating End Member Mixing Analysis (EMMA), the Model of Acidification of Groundwater in Catchments (MAGIC) and spatial data describing the catchment characteristics stored on a Geographical Information System (GIS). These are integrated within a Functional Unit Network (FUN) to predict the changes in Gran alkalinity resulting from possible future changes in atmospheric deposition and land use (low intensity afforestation) in the River Dee catchment, NE Scotland. Model results indicate that declining sulphate and constant nitrogen deposition, combined with low intensity Scots pine (Pinus sylvestris) afforestation are unlikely to contribute significantly to streamwater acidification.

Catchment characteristics controlling the mobilization and potential toxicity of aluminium fractions in the catchment of the River Dee, northeast Scotland

Elevated streamwater concentrations of aluminium have been associated with the onset of acidification, both by natural and anthropogenic means. This has important implications for river water quality. Concentrations of total, labile-inorganic and non-labile-organic fractions of aluminium were determined across the River Dee catchment, northeast Scotland. Fifty-nine subcatchments, chosen to reflect the variety of soils, parent materials and land use patterns across this major river system were sampled bi-weekly for 1 year. The distribution of aluminium was closely linked to factors of parent material and organic soil cover. Strong spatial and temporal relationships were observed between pH and all fractions of aluminium. Significant episodic peaks in aluminium occurred, these being especially pronounced when a storm event followed a period of dry weather. A weathering rate index utilizing the Na dominance of base cations was found to be a predictor of potential streamwater toxicity implied through Ca/inorganic aluminium ratios. It was demonstrated that Al was mobilized from acid headwater streams, whilst concentrations in the main stem remained much lower.

Sensitivity of blanket peat vegetation and hydrochemistry to local disturbances

At the ecosystem scale, peatlands can be extremely resilient to perturbations. Yet, they are very sensitive to local disturbances, especially mechanical perturbations (e.g. trampling). The effects of these disturbances on vegetation, and potential effects on hydrochemical conditions along the peat surface, however, are largely unknown. We used three research tracks (paths researchers use to access their study sites) differing in time of abandonment to investigate the impact of local disturbance (trampling) on the vegetation and its short-term (< or = 2 year) recovery in a flagship research blanket peatland. Additionally, we examined the effects of local disturbance on fluvial runoff events and the concentrations of dissolved organic carbon (DOC) and particulate organic carbon (POC) in runoff water. Local disturbance heavily impacted peat vegetation, resulting in large areas of scarred and churned peat. Recovery of vascular plants along abandoned tracks was slow, but a functional Sphagnum layer re-established after just one year. The absence of vegetation elicited an increase in the number of runoff events along the tracks, by which POC runoff from the tracks increased. POC concentrations were highest in the surface water from the recently abandoned track, while they were low in the runoff water from the track abandoned longest and the undisturbed control track. We attribute this to the relatively fast recovery of the Sphagnum vegetation. DOC concentrations did not differ significantly either spatially or temporally in surface runoff or soil solution waters. While at an ecosystem scale local disturbances may be negligible in terms of carbon loss, our data points to the need for further research on the potential long-term effects of local disturbance on the vegetation, and significant effects on local scale carbon fluxes. Moreover, the effects of disturbances could be long-lasting and their role on ecosystem processes should not be underestimated.

The Role of Natural Soil Pipes in Water and Carbon Transfer in and from Peatlands

Natural piping has been reported in peatlands around the world. This chapter reviews the role of natural pipes in peatland hydrology and carbon fluxes. There is a growing body of evidence to suggest that pipes are important hydrological agents in peatlands typically delivering over 10% of streamflow. Deep and shallow pipes respond rapidly to rainfall inputs demonstrating strong connectivity with the peat surface. While ephemeral pipes respond quickly to rainfall events, they also appear to obtain water from deep peat layers and underlying mineral strata. This mix of different sources of water results in highly variable concentrations of dissolved organic carbon and dissolved carbon dioxide and methane within pipe water. Early results from an intensive monitoring study in a blanket peatland in northern England suggest that pipe flows may account for around half of the dissolved organic carbon that is delivered to the stream network. Episodic pulses of particulate organic carbon from pipe outlets are common during storm events. Work that is underway to understand more about the sources of carbon being released from pipe networks is outlined, and several areas of further research are highlighted including examination of the role of natural pipes in bog pool hydrology and carbon cycling.

Calibration of the sodium base cation dominance index of weathering for the River Dee catchment in north-east Scotland

Previously the dominance of base cations by Na+ in river water in upland catchments with low weathering rates and influenced by marine-derived aerosols has been suggested as a quantitative index of weathering rate upstream of the sampling point. Using data for 59 sites from a study of the River Dee catchment in NE Scotland, the index has been fully calibrated against catchment weathering rates and net alkalinity production, derived through input output budget methods, for both upland and agricultural catchments and over a wide range of parent materials. It is shown that the relationship between Na+ dominance and weathering rate is logarithmic, rather than linear as initially suggested. The excellent correlations highlight the potential use of this Na+ dominance index for the direct quantification of catchment susceptibility to acidification at fine spatial resolution, using a few simple and inexpensive measurements. Stronger correlations were observed between the % Na+ dominance and net annual flux of alkalinity than between % Na+ dominance and weathering rate derived from summation of base cation fluxes. This demonstrates the importance of mechanisms controlling the transport of base cations out of catchments, namely in association with organic matter and with anthropogenically derived SO42−. These processes are shown to reduce the residual alkalinity derived through weathering. The partial neutralization of organic acidity by internally generated alkalinity has implications in the context of using the mass balance approach for setting critical loads for catchments.

Tolerance of calluna vulgaris and peatland plant communities to sulphuric acid deposition

Critical loads offer a unique way of evaluating impacts of acid deposition by quantifying environmental sensitivity. The critical loads of acidity for UK peat soils have been based upon an arbitrary reduction in pH of 0.2 units. This chemical shift needs to be better related to adverse effects on sensitive biological receptors. It is known that effective precipitation pH equates closely to soil solution pH, and the latter is directly linkable to biotic effects of pH change. On continuation of a long-term experiment assessing impacts of simulated acid rain on peat microcosms in a realistic outdoor environment, Calluna vulgaris continued to flourish at acid deposition loads well above the existing critical load. Calluna plants were harvested and analysed, and acid deposition treatments to the microcosms continued to allow natural vegetation to regenerate. A diverse mixture of moorland plants and bryophytes established at acidity treatments well above the existing critical load, and only a very high acid load resulted in no natural regeneration. A critical effective rain pH value of 3.6 is suggested as a basis for setting critical loads. At this pH, Calluna grows well, and a healthy diverse vegetation community re-establishes when harvested. It is suggested that the peat critical load should be set at the acid load that, at any specific site, would result in a mean effective precipitation pH of 3.6.