A. L. Heathwaite

Characterising phosphorus loss in surface and subsurface hydrological pathways

The magnitude and composition of the phosphorus (P) load transported in surface and subsurface hydrological pathways from a grassland catchment depends on the discharge capacity of the flow route and the frequency with which the pathway operates. Surface runoff is an important pathway for P loss, but this pathway is spatially limited and temporarily confined to high magnitude, high intensity rainfall events. High P concentrations (mean: 1.1 mg TP l(-1)) were recorded, with most P transported in the dissolved fraction. Preferential flow pathways, particularly soil macropores and field drains, are important contributors to the overall P load; most P is transported in the particulate fraction and associated with organic or colloidal P forms. High P concentrations (mean: 1.2 mg TP l(-1)) were recorded in macropore flow in the upper 0-15 cm of a grassland soil, and generally declined with increasing soil depth. On average, P concentrations in drainflow were over six times greater in stormflow compared to baseflow. Stormflow P losses in drainflow were predominantly in the particulate fraction; significant correlation (P < 0.01) was recorded with suspended sediment concentrations in drainflow. Phosphorus concentrations in groundwater were low (< 0.2 mg TP l(-1) at 150 cm), although this pathway may contribute to stream flow for the majority of the year.

Hydrological processes in abandoned and restored peatlands : an overview of management approaches

Mined peatlands do not readily recover their hydrological function, mainly because the dominant peat-forming plant genus, Sphagnum, cannot easily reestablish on the degraded surface peat found on cutover sites. Drainage and removal of the acrotelm can result in surface subsidence of up to3.7 cm y-1 m-1 of peat shortly after drainage (compression), and long-term rates up to 0.3 cm y-1 m-1 (compression and oxidation). This can decrease the hydraulic conductivity by over 75%, and decrease thewater retention capacity and specific yield. In old abandoned systems, drainage ditches may continue to facilitate a significant seasonal water loss.Colonization of abandoned sites by trees may increase the evapotranspirative losses by as much as 25%, and interception losses can be as high as 32% of rainfall. Without natural or planned occlusion of ditches, some peatlands become drier over time. Blocking ditches may largely restore water balance components, although the hydrological regime requires years to stabilise sufficiently for Sphagnum recolonization, especially where residualpeat is well decomposed, having inadequate water storage capacity. Consequently,winter precipitation (Europe) and spring snowmelt (North America) are criticalrecharge periods. Over the long term, consolidation of the peat due to drainageand methane production (where drainage systems are blocked and soils reflooded)decreases hydraulic conductivity, thereby reducing lateral seepage losses. This may actually assist in Sphagnum recolonization. A regenerated cover ofSphagnum increases soil wetness and reduces water tension (increases pore-water pressure) in the substrate, thus ameliorating its own environment. However, natural recolonization and recovery of many hydrological and ecological processes may not occur, or may require many decades. Water management and selective plant reintroduction can accelerate this. Water management options such as blocking ditches, constructing bunds, reconfiguring the surface and managing microclimate have met with varying degrees of success. No standard management prescription can be made because each site presents unique challenges.

Modelling the impact of land use change on water quality in agricultural catchments

Export coefficient modelling was used to model the impact of agriculture on nitrogen and phosphorus loading on the surface waters of two contrasting agricultural catchments. The model was originally developed for the Windrush catchment where the highly reactive Jurassic limestone aquifer underlying the catchment is well connected to the surface drainage network, allowing the system to be modelled using uniform export coefficients for each nutrient source in the catchment, regardless of proximity to the surface drainage network. In the Slapton catchment, the hydrological pathways are dominated by surface and lateral shallow subsurface flow, requiring modification of the export coefficient model to incorporate a distance–decay component in the export coefficients. The modified model was calibrated against observed total nitrogen and total phosphorus loads delivered to Slapton Ley from inflowing streams in its catchment. Sensitivity analysis was conducted to isolate the key controls on nutrient export in the modified model. The model was validated against long-term records of water quality, and was found to be accurate in its predictions and sensitive to both temporal and spatial changes in agricultural practice in the catchment. The model was then used to forecast the potential reduction in nutrient loading on Slapton Ley associated with a range of catchment management strategies. The best practicable environmental option (BPEO) was found to be spatial redistribution of high nutrient export risk sources to areas of the catchment with the greatest intrinsic nutrient retention capacity.

CONTRIBUTION OF NITROGEN SPECIES AND PHOSPHORUS FRACTIONS TO STREAM WATER QUALITY IN AGRICULTURAL CATCHMENTS

The contribution from agricultural catchments to stream nitrogen and phosphorus concentrations was assessed by evaluation of the chemical composition of these nutrients in agricultural runoff for both surface and subsurface flow pathways. A range of land uses (grazed and ungrazed grassland, cereals, roots) in intensive agricultural systems was studied at scales from hillslope plots (0.5 m2) to large catchment (>300 km2). By fractionating the total nutrient load it was possible to establish that most of the phosphorus was transported in the unreactive (particulate and organic) fraction via surface runoff. This was true regardless of the scale of measurement. The form of the nitrogen load varied with land use and grazing intensity. High loads of dissolved inorganic nitrogen (with >90% transported as NH4-N) were recorded in surface runoff from heavily grazed land. In subsurface flow from small (2 km2) subcatchments and in larger (>300 km2) catchments, organic nitrogen was found to be an important secondary constituent of the total nitrogen load, comprising 40% of the total annual load.

A procedure for the simultaneous determination of total nitrogen and total phosphorus in freshwater samples using persulphate microwave digestion

Abstract In the absence of a suitable method for routine analysis of large numbers of natural river water samples for organic nitrogen and phosphorus fractions, a new simultaneous digestion technique was developed, based on a standard persulphate digestion procedure. This allows rapid analysis of river, lake and groundwater samples from a range of environments for total nitrogen and phosphorus. The method was evaluated using a range of organic nitrogen and phosphorus structures tested at low, mid and high range concentrations from 2 to 50 mg l −1 nitrogen and 0.2 to 10 mg l −1 phosphorus. Mean recoveries for nitrogen ranged from 94.5% (2 mg l −1 ) to 92.7% (50 mg l −1 ) and for phosphorus were 98.2% (0.2 mg l −1 ) to 100.2% (10 mg l −1 ). The method is precise in its ability to reproduce results from replicate digestions, and robust in its ability to handle a variety of natural water samples in the pH range 5–8.

Making process-based knowledge useable at the operational level: a framework for modelling diffuse pollution from agricultural land

In much of Western Europe, Northeast US and parts of Australia, increased inputs of nitrogen and phosphorus to land in the form of fertilisers, manures and biosolids means that agricultural runoff now comprises a greater share of these nutrients in rivers and lakes and associated water quality problems. While numerous site-specific field studies have quantified the potential export of nutrients in agricultural runoff, it is clear that to meet the requirements of end-users, the research effort needs to shift towards developing generic models of diffuse source pollutant export from land that are based on expert knowledge but remains simple to use and easy to apply. Data-hungry process-based models, while elegant and all-encompassing, may not be suitable for the simple decision support frameworks required by end-users such as government agencies, water utilities and farmers.

Understanding nutrient biogeochemistry in agricultural catchments: the challenge of appropriate monitoring frequencies.

We evaluate different frequencies of riverine nutrient concentration measurement to interpret diffuse pollution in agricultural catchments. We focus on three nutrient fractions, nitrate-nitrogen (NO3-N), total reactive phosphorus (TRP) and total phosphorus (TP) observed using conventional remote laboratory-based, low-frequency sampling and automated, in situ high-frequency monitoring. We demonstrate the value of low-frequency routine nutrient monitoring in providing long-term data on changes in surface water and groundwater nutrient concentrations. By contrast, automated high-frequency nutrient observations provide insight into the fine temporal structure of nutrient dynamics in response to a full spectrum of flow dynamics. We found good agreement between concurrent in situ and laboratory-based determinations for nitrate-nitrogen (Pearsons R = 0.93, p < 0.01). For phosphorus fractions: TP (R = 0.84, p < 0.01) and TRP (R = 0.79, p < 0.01) the relationships were poorer due to the underestimation of P fractions observed in situ and storage-related changes of grab samples. A detailed comparison between concurrent nutrient data obtained by the hourly in situ automated monitoring and weekly-to-fortnightly grab sampling reveals a significant information loss at the extreme range of nutrient concentration for low-frequency sampling.

Chemical fractionation of lake sediments to determine the effects of land-use change on nutrient loading

Lake studies allow contemporary sediment and nutrient dynamics to be placed in a historical context in order that trends and rates of change in catchment inputs may be calculated. Here, a synthesis of the temporal information contained in catchment and lake sediment records is attempted. A chemical fractionation technique is used to isolate the different sediment sources contained in the lake core, and 210Pb dates provide an accurate record of changes in lake sediment sources over the past 100 years. The extent to which land-use records, collated from agricultural census returns, and process-based studies of sediment and nutrient export from different catchment land uses can be used to explain the trends observed in the lake sediments is examined. Sediment influx to the study lake has increased from less than 2 mm year−1 prior to the Second World War to over 10 mm year−1 at present. The source of the sediment is largely unaltered and unweathered allochthonous material eroded from the catchment. Land-use records suggest that the intensification of agriculture, characterized by a shift towards arable land immediately postwar, followed by an increase in the area of temporary grass in the 1960s, may be the cause of accelerated catchment erosion; both land-use changes would have increased the area of ploughed land in the catchment. An increase in the number of cattle and sheep in the catchment from around 2000 and 6000, respectively, in the 1940s, to a peak of nearly 7000 cattle and over 15 000 sheep in the 1980s, provides a further source of sediment and nutrients. Livestock are grazed on permanent grassland which is commonly located on steep hillslopes and in riparian zones where saturation-excess surface runoff may be an important hydrological pathway. Rainfall simulation experiments show that surface runoff from heavily grazed grassland has a high suspended sediment, ammonium-nitrogen and particulate phosphorus load. The combined effect of the long-term increase in the organic loading from livestock and the inorganic N and P load from fertilizers, may be the source of nutrient enrichment in the lake.

Development of an iron oxide-impregnated paper strip technique for the determination of bioavailable phosphorus in runoff

Bioavailable phosphorus (P) requires separate analysis because it is independent of physical and chemical P fractions present in runoff. Iron oxide-impregnated paper strips (Pi-strips) provide a simple technique for determining bioavailable P in aqueous samples. In this study, some controversial strip preparation procedures were examined and subsequently modified, and the efficiency of the technique was compared to the conventional physico-chemical fractionation approach. Pi-strips prepared using Whatman 50 filter paper rapidly drawn through iron chloride solution then immersed in ammonium hydroxide solution produced the most accurate and precise results. A strong asymptotic relationship between P uptake and the duration of shaking, coupled with rapid intra-sample transformations between P fractions, indicated that shaking for 8 h was sufficient for complete P uptake from natural water samples and standard solutions (0–250 μg P l−1). A single Pi-strip was used because multiple strips increased analytical error. The contribution to Pi-strips from adhering sediment particles was negligible for samples with low concentrations of solids but turbid samples (>0.75 mg l−1 suspended solids) were noticeably influenced by P derived from particulate material. Fractionation of agricultural runoff demonstrated a strong relationship between the dissolved inorganic (MRP and Pi-strip fraction, but only 85% of the total dissolved fraction (TDP) was retained by the Pi-strip, suggesting not all soluble P was available for biological uptake.

The interplay between transport and reaction rates as controls on nitrate attenuation in permeable, streambed sediments

Anthropogenic nitrogen fixation and subsequent use of this nitrogen as fertilizer has greatly disturbed the global nitrogen cycle. Rivers are recognized hotspots of nitrogen removal in the landscape as interaction between surface water and sediments creates heterogeneous redox environments conducive for nitrogen transformations. Our understanding of riverbed nitrogen dynamics to date comes mainly from shallow sediments or hyporheic exchange flow pathways with comparatively little attention paid to groundwater-fed, gaining reaches. We have used 15N techniques to quantify in situ rates of nitrate removal to 1m depth within a groundwater-fed riverbed where subsurface hydrology ranged from strong upwelling to predominantly horizontal water fluxes. We combine these rates with detailed hydrologic measurements to investigate the interplay between biogeochemical activity and water transport in controlling nitrogen attenuation along upwelling flow pathways. Nitrate attenuation occurred via denitrification rather than dissimilatory nitrate reduction to ammonium or anammox (range = 12 to >17000 nmol 15N L-1 h-1). Overall, nitrate removal within the upwelling groundwater was controlled by water flux rather than reaction rate (i.e. Damkohler numbers 80% of nitrate removal occurs within sediments not exposed to hyporheic exchange flows under baseflow conditions, illustrating the importance of deep sediments as nitrate sinks in upwelling systems.