Suspended sediment and particulate phosphorus in surface waters of the upper Thames Basin, UK

Journal of Hydrology (2006) 330, 142– 154

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Suspended sediment and particulate phosphorus in surface waters of the upper Thames Basin, UK Colin Neal *, Margaret Neal, Graham J.L. Leeks, Gareth Old, Linda Hill, Heather Wickham Centre for Ecology and Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, Oxon, OX10 8BB, UK Accepted 12 April 2006

KEYWORDS

Summary Suspended sediment (SS) and particulate phosphorus (PP) concentrations in surface waters of the upper Thames Basin are reported for the main stem of the River Thames, several of its tributaries, the Cherwell, Dun, Lambourn, Pang and Thame, the Kennet and Avon Canal (that drains to the Dun and Kennet) and an artificial supply reservoir (Wilton Water). For the rivers which are mainly supplied from Chalk aquifer sources, SS and PP concentrations are poorly correlated with flow and there are issues of both biological and inorganic production of SS and PP during the spring and summer months. SS and PP are better correlated with flow when the antecedent conditions are taken into account. Thus, if flows had increased the previous day, then SS and PP concentrations are augmented. Wilton Water and the Kennet and Avon Canal have, on average, higher SS and PP concentrations than the nearby Chalk fed rivers and this probably reflects increased effects of biological activity and calcite (CaCO3) precipitation under more stagnant conditions. For the rivers draining less permeable (clay dominated) catchments, then there is clearer linkage between flow and SS and PP concentrations. This feature reflects the more responsive influence of runoff from the land surface without the confounding issues of seasonally-varying groundwater discharges, intersection of groundwater levels with the ground surface and overland flow that may well characterise the permeable Chalk catchments. SS and PP are linearly correlated across the catchments. For the Chalk catchments and the associated Wilton Water and Kennet and Avon Canal, the PP:SS ratios are similar, ranging typically between 2 and 4 lg/mg. For the clay dominated catchments, the ratios are typically higher at 3–7 lg/mg. The results are considered in the light of process understanding, farming, climate change/climate-variability and the Water Framework Directive. c 2006 Elsevier B.V. All rights reserved.

Suspended sediment; Sediment; Phosphorus; Particulate; Calcite; River; Canal; Cherwell; Dun; Kennet; Lambourn; Pang; Ray; Thame; Thames; Water Framework Directive



* Corresponding author. E-mail address: [email protected] (C. Neal).



0022-1694/$ - see front matter c 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2006.04.016

Suspended sediment and particulate phosphorus in surface waters of the upper Thames Basin, UK

143

Introduction A critical concern for the ecological vitality of riverine systems in agricultural areas of the UK is the influence of suspended sediment (SS) and associated particulate phosphorus (PP). This concern is an urgent one given the remit and timeframe of the Water Framework Directive with the requirement of maintaining or improving the ecological health of European water bodies over the next decade (WFD: CEC, 2000; Dwyer et al., 2002; RPA, 2003; Defra, 2004). Within the UK, there is a strong background of studies of sediment transport to surface waters from agricultural land and within-river attenuation (Foster et al., 1995; Gurnell and Petts, 1995; Walling and Leeks, 2001; Walling et al., 2002; Walling and Leeks, 2001). In the case of phosphorus (P), the dissolved and particulate fluxes have been extensively studied (Haygarth and Jarvis, 1996; Haygarth et al., 1998; Heathwaite and Dils, 2000; Heathwaite et al., 2005; Johnes and Heathwaite, 1997; Withers et al., 2001; Wood et al., 2005). While much work has been undertaken on sediment fluxes from the land to the river, there is a clear move to examine within-river processing within the context of sediment and particulate phosphorus sources from the land and sewage effluent. There is especially need to examine the linkages between dissolved and particulate P and the influence of abiotic and biotic processes (House and Warwick, 1999; House, 2003; Jarvie et al., 2005). There are important issues of water–sediment interactions for P (Jarvie et al., 2002, 2005; Neal and Jarvie, 2005). This is because the dissolved P, particularly the inorganic form (commonly referred to as phosphate, orthophosphate and soluble reactive P) is highly bioavailable. It is important to the WFD in relation to the issue of eutrophication and inorganic P sources to the water column. In this paper, riverine SS and PP concentrations are examined for one of the key UK lowland river basins, the Thames: a region where issues of agricultural versus urban pollution is of particular importance in terms of population pressures and population intrusion into areas of ‘green belt’ and the ramifications of the Water Framework Directive with regards to eutrophication (Neal and Jarvie, 2005; Neal et al., 2006a,b). The Centre for Ecology and Hydrology (Wallingford) has undertaken extensive hydrochemical surveys of the main stem of the Thames and several of its tributaries of varying permeability. This paper provides the first integrated findings on SS and PP concentrations and variations for the Pang and Lambourn. Background material for a range of rivers in the Upper Thames Basin that drain both the Chalk and less permeable catchments are also presented. The study allows a comparison of information on agricultural systems with underlying high permeability (Chalk) and low permeability (clay) strata and a base for integrating new research within the contexts of a new lowland permeable catchment research programme, LOCAR (http:// www.nerc.ac.uk/funding/thematics/locar/) and the environmental management of agricultural systems in a strategically important river basin of southern England.

Study area The rivers studied here comprise the main stem of the Thames, four of its tributaries (the Cherwell, Kennet, Pang

Thames Basin

Cherwell

Ray Thame

Thames

Oxford

London N

Lambourn Dun Pang Kennet

Reading

10 km

Figure 1 The study area. Detailed locations of the monitoring sites for each of the rivers studied are provided elsewhere: Dun, Cherwell, Kennet, Pang/Lambourn, Thame, Thames, Neal et al., 2005, 2006a, 2000a, 2004b, 2006b, 2000b, respectively.

and Thame) and inputs to two of these tributaries, the Dun and Lambourn which join the Kennet, and the Ray which joins the Cherwell (Fig. 1). The rivers subdivide primarily into those associated with drainage from: 1. Catchments with a predominance of high permeability (Cretaceous Chalk) aquifers (Baseflow index typically greater than 0.85; the Dun, Kennet, Pang and Lambourn). 2. Catchments with a predominance of low permeability sedimentary rocks, mainly of Jurassic age (Baseflow index less than 0.55; Cherwell, Ray and Thame). 3. A mix of high and low permeability strata (the main stem of the Thames—Baseflow index 0.64). In addition to this, data are provided for Wilton Water, an artificial spring-fed lake used to supply water to the Kennet and Avon Canal, and the canal itself. The reservoir and canal waters provide important supplies and interchanges to the Dun and may well influence the Kennet as well. The salient features and key references are:

The permeable catchments: Pang and Kennet The Pang drains to the Thames at Pangbourne (catchment area 171 km2). Six sites have been sampled at various times since 1997 together with a spring discharge, the Blue Pool, that provides significant flow during baseflow to the lower Pang (Neal et al., 2004a,b). The Kennetdrains to the Thames at Reading (catchment area 1200 km2). The upper half of the Kennet has been sampled at 10 sites since 1997 (Neal et al., 2000a). Two tributaries to the Kennet have also been studied. Firstly, there is the Dun which drains to the Kennet near Hungerford (catchment area 110 km2). Here, one reservoir (Wilton Water), six Kennet and Avon Canal and six river sites (including two small streams—Froxfield and the Shalbourne) have been sampled since 2000 (Neal et al.,

144 2005). Secondly, there is the Lambourn, which drains to its confluence with the Kennet at the town of Newbury (catchment area 250 km2). Here, three sites have been sampled since 2002 (Neal et al., 2004a,b).

The low permeability catchments: Cherwell and Thame The Cherwell drains to the Thames at Oxford (catchment area 943 km2) and there are 3 monitoring points plus one on its main tributary, the Ray. The uppermost site on the Cherwell is in the market town of Banbury, about 1km upstream of Banbury sewage treatment works (STW). The intermediate site on the Cherwell is about 5 km downstream of Banbury STW and the lowest site is just upstream of the junction with its main tributary, the Ray, near the village of Islip. The Ray (catchment area 284 km2) drains about 30% of the Cherwell catchment and has been sampled in the village of Islip. These four sites have been sampled since 2000 (Neal et al., 2006a). The Thame drains to the Thames near the village of Dorchester, upstream of both the town of Wallingford and the Thames monitoring site. In the upper part of the Thame is the main town of the area, Aylesbury. Three sites have been sampled on the Thame. The uppermost site is near to Aylesbury and about 1.2km upstream of Aylesbury STW. The intermediate site is located about 4 km downstream of Aylesbury STW, near the village of Cuddington, and the furthest downstream site is about half way along the length of the Thame near the village of Wheatley (Neal et al., 2006b). Monitoring began in 1998 for the Thame at Wheatley and in 2002 for the other two sites. The catchment area of Thame at Wheatley is 800 km2.

Mixed permeability catchment: the main stem of the upper Thames The main stem of the upper Thames has been sampled, beginning in 1997, at one site near Howberry Park, about 2km upstream of Wallingford (Neal et al., 2000b): catchment area to this point is 3500 km2. In terms of land use, all the catchments are primarily a mix of arable and grassland types, typically covering over 80% of the area, the remaining mainly being woodland covered together with some urban areas. For most of the permeable catchments, the mix of arable to grassland is about 2:1, while for the low permeable catchments it is around 1:1. Monitoring across the study sites has taken place over a variety of sampling periods (see individual references for details of the monitoring periods). The periods of monitoring varied from 1 to 7 years according to the differing research needs and funding availability, and it has largely been on a weekly basis. In addition, monitoring ceased at most sites between the 28th February 2001 to the 13th June 2001 because of lack of access due to an outbreak of Foot and Mouth Disease. The SS concentrations were determined gravimetrically using GFC filters, the samples being dried at 110 C. Three phosphorus fractions were determined on the water samples collected. Firstly, there was a measure of the total phosphorus (TP: dissolved plus particulate) for unfiltered samples. Secondly, the total dissolved phosphorus (TDP) was measured on filtered samples: the TDP represents

C. Neal et al. phosphate plus dissolved organic bound P and polyphosphates. For both these determinations, the method of Eisenreich et al. (1975) was used, the only difference being an unfiltered and a filtered (0.45lm membranes), respectively, was used. Thirdly, dissolved phosphate (termed soluble reactive P) was measured using an automated version of the method of Murphy and Riley (1962), as modified by Neal et al. (2000d). Particulate phosphorus was calculated as the difference between PP and DHP. Details of the full sampling periods, hydrological and water quality regimes are provided in the papers cited above for each catchment. Water samples were returned to the CEH Wallingford laboratory for determination of SS and PP concentrations. Information on daily flow is used in this paper to investigate the influence of flow on SS and PP concentration change. Flow data for gauging sites across the region was taken from the Water Archive of CEH Wallingford (http://www.ceh. ac.uk/data/NWA.htm:, Marsh and Lees, 2003). Within the analysis, focus is placed on examining SS and PP concentration variations and the average concentrations across the region on both standard and a flow-weighted mean basis, rather than as flux where the total flow measurement is critical. The flow weighted average was calculated as the product of the concentration times the average daily flow on the day of sampling divided by the sum of the daily flows for the days of sampling. There was only data for one flow gauging station each for the Pang (Tidmarsh). Flow weighted averages were calculated for each site along the Pang using this flow information together with the concentrations at each site. The same procedure was used for the Lambourn with one gauging station at Shaw. It was taken that such normalisation is reasonable given the similar geologies for each catchment and an expected similar flow response. It is not possible here to provide what could be identified within the analysis as a highly accurate assessment of the average and flow-weighted average SS (and PP) concentrations due to the limited amount of data available: c.f. the detailed assessment of flux uncertainties with regards to environmental water quality data provided by Littlewood (1992), Littlewood et al. (1998).

Results General findings Sediment transport in British rivers is generally very responsive to high flows, which occur over a small proportion of the time. For example, on the Swale, Wass and Leeks (1999) found that 90% of the total SS flux occurred within 11% of the time. Systematic weekly sampling of instantaneous SS concentration in rivers through the year can miss the highest flow events which display the highest sediment concentrations. The data from the Thames and its tributaries show a large range in the concentrations of SS (0.1– 440 mg/l) and PP (0.0–1501 lg/l). The streams draining the high permeability catchments typically have SS and PP concentrations lower than those for the low permeability catchments. For example, the SS and PP concentrations average 8.7 mg/l SS and 25.0 lg/l PP, for the permeable catchments, while the corresponding values for the low permeable catchments are 22.7 mg/l SS and 100.8 lg/l PP, i.e.

Suspended sediment and particulate phosphorus in surface waters of the upper Thames Basin, UK a factor of three to four times difference. Suspended sediment and PP concentrations are characterised by approximately log normal distributions and standard deviations are around the same magnitude as the mean (SS standard deviations average 1.3 times the mean with a range 0.24– 2.40 times the mean: the corresponding values for PP are 1.08, 0.39 and 2.06). On a flow-weighted basis, the range and sequence of range in concentration across the low permeability sites are similar to that for the un-weighted averTable 1

145

ages. However, flow-weighted average concentrations are typically higher than the unweighted averages as would be anticipated given the higher energy riverine environment during spates: 11.5 mg/l SS and 30.1 lg/l PP for the permeable catchments and 44.5 mg/l SS and 128.1 lg/l PP for the low permeability catchments. Note however that the link between concentration and flow is complex, especially for the permeable catchment case as discussed later in this paper. Summary statistics are provided in Tables 1 and 2,

Suspended sediment and particulate phosphorus statistics for the upper Thames Suspended sediment

Particulate phosphorus

Avg (mg/l)

Avg fwt (mg/l)

Min (mg/l)

Max (mg/l)

Avg (lg-P/l)

Avg fwt (lg-P/l)

3.0 7.0 5.2 1.6 3.7 6.6 10.6

3.3 6.9 6.6 1.9 4.5 9.7 17.0

0.3 0.3 0.1 0.0 0.5 0.1 0.6

10 152 14 6 23 0 440

11.9 21.7 27.2 8.8 11.6 21.5 25.4

12.5 20.3 24.4 7.1 13.0 25.6 35.6

0.0 0.0 0.0 0.0 0.0 0.0 0.0

4.1 3.7 4.3

4.3 4.3 4.5

0.5 0.1 0.5

16 20 12

13.3 14.4 14.5

13.8 14.5 14.3

Kennet Clatford Glebe House Mildenhall Stichcombe Axford Ramsbury Knighton Hungerford Kintbury Woolhampton

11.8 9.8 9.7 10.6 10.2 11.5 11.1 9.5 12.0 12.0

16.9 12.3 13.3 15.6 14.9 15.3 14.8 14.2 18.0 11.8

0.3 0.3 0.5 0.3 0.3 0.6 0.9 0.3 0.4 5.2

298 89 106 103 102 151 235 141 75 39

25.6 29.0 31.2 28.0 27.8 29.4 26.7 23.4 32.1 38.2

Dun Fore Bridge Great Bedwyn Dun Cottage Hungerford Froxfield Shalbourne

17.6 14.8 12.3 8.4 5.4 17.0

24.2 21.1 17.3 11.4 5.3 20.6

1.9 0.4 0.7 0.4 0.2 4.8

222 132 134 63 35 79

Thame Aylesbury Cuddington Wheatley

22.8 18.2 19.9

43.4 30.7 32.1

0.7 3.7 5.4

Cherwell Banbury Kings Sutton Islip Ray Islip

28.0 28.5 24.1 19.8

62.9 64.2 48.2 38.5

Thames Howberry Park

14.8

26.4

Pang Frilsham Bucklebury Upst. of Blue Pool Blue Pool Downst. of Blue Pool Bradfield Tidmarsh Lambourn E. Shefford Boxford Shaw

Min (lg-P/l)

PP in SS Max (lg-P/l)

Avg (mg-P/g-SS)

Avg fwt (mg-P/g-SS)

52 274 112 49 75 0 477

3.98 3.10 5.22 5.64 3.18 3.26 2.39

3.83 2.95 3.70 3.76 2.90 2.63 2.10

1.0 0.0 1.0

44 111 39

3.26 3.89 3.41

3.18 3.38 3.18

33.5 33.1 37.5 37.9 38.0 36.9 34.1 31.4 43.6 36.3

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.0

23 101 11 28 73 18 0 227 146 86

2.16 2.94 3.22 2.65 2.72 2.55 2.41 2.46 2.67 3.18

1.99 2.70 2.81 2.44 2.55 2.41 2.30 2.21 2.42 3.09

45.0 37.2 33.9 24.1 14.4 41.1

58.2 49.1 45.6 31.7 14.7 48.4

1.0 0.0 6.0 0.0 1.0 9.0

496 228 364 277 67 191

2.56 2.51 2.77 2.87 2.66 2.41

2.41 2.33 2.63 2.78 2.77 2.34

380 98 167

107.0 112.4 136.8

147.2 119.5 131.7

21.0 14.0 0.0

1012 1068 1501

4.69 6.19 6.87

3.39 3.89 4.10

0.9 1.9 4.1 3.5

285 387 208 99

89.0 87.3 65.9 89.0

155.7 151.4 103.8 83.0

4.0 0.0 5.0 0.0

618 667 249 670

3.18 3.06 2.73 4.49

2.47 2.36 2.15 2.15

1.3

130

61.1

182.8

0.0

676

4.13

6.93

The italics denote (a) Blue Pool is a spring and (b) Froxfield and Shalbourne are small streams entering the Dun. Fwt = flow-weighted.

146 Table 2

C. Neal et al. Suspended sediment and particulate phosphorus concentrations in the Dun, Kennet and Kennet and Avon Canal Suspended sediment Avg (mg/l)

Avg fwt (mg/l)

Avg (lg-P/l)

Avg fwt (lg-P/l)

Min (lg-P/l)

22.7

27.5

1.2

222

77.4

83.4

Canal 27.7 35.9 25.9 33.0 34.3 23.6

27.1 30.1 32.8 24.6 38.0 20.2

2.4 2.2 3.4 3.0 1.9 3.8

133 160 372 97 436 82

63.7 79.5 63.2 72.3 82.1 72.5

Averaged data Kennet and Avon Canal Canal average 29.0

28.6

23.6

35.9

River Dun and Kennet Dun 13.3 Kennet 10.8

18.5 14.7

8.4 9.5

Pang and Lambourn Pang 6.5 Lambourn 4.0

8.7 4.4

Thame and Cherwell Thame 20.3 Cherwell 25.1 Thames Thames

Wilton Water Wilton Water Kennet and Avon Crofton Fore Bridge Great Bedwyn Dun Cottage Hungerford Kintbury

14.8

Min (mg/l)

Particulate phosphorus Max (mg/l)

PP in SS Max (lg-P/l)

Avg (mg-P/g-SS)

lg/mg fwt (mg-P/g-SS)

3.0

544

3.42

3.04

64.6 70.9 74.9 57.1 76.9 61.4

14.0 7.0 5.0 7.0 7.0 15.0

268 440 625 188 425 174

2.30 2.21 2.44 2.19 2.40 3.07

2.38 2.36 2.29 2.33 2.03 3.03

73.0

69.9

63.2

82.1

2.58

2.49

17.6 12.0

35.1 29.1

46.1 36.2

24.1 23.4

45.0 38.2

2.68 2.69

2.54 2.49

3.0 3.7

10.6 4.3

21.5 14.1

23.7 14.2

11.6 13.3

27.2 14.5

3.59 3.52

3.04 3.25

35.4 53.5

18.2 19.8

22.8 28.5

118.7 82.8

132.8 123.5

107.0 65.9

136.8 89.0

5.92 3.37

3.79 2.28

26.4

14.8

14.8

61.1

182.8

61.1

61.1

3.67

7.04

Fwt = flow-weighted.

while Figs. 2 and 3 provide illustrations of the linkages between SS concentration and flow as well as PP concentrations with SS concentrations. In order to test the statistical significance of differences between the average concentrations, two-tailed student t-tests were undertaken on the log-transformed values— logged values being used to give an approximately normal distribution. The results show in most cases that the average concentrations for SS across the sites are significantly different statistically (p < 0.001) when N > 50. Table 3 provides an example of the analysis for the Pang. A similar test was undertaken for five subgroups: two for Chalk dominated systems of contrasting SS and PP concentrations (Pang/ Lambourn, N = 870, and Kennet/Dun, N = 2102), standing waters (K&A Canal/Wilton Water, N = 430), the low permeability catchments (Thame/Cherwell. N = 531) and the mixed permeability catchment of the main stem of the Thames (N = 272) and in all cases p < 0.001. The permeable catchments For the rivers supplied from the Chalk, SS and PP concentrations vary between sites in both their averages and ranges. In general, the lowest SS and PP concentrations are for the Blue Pool and Froxfield stream, both of which are essentially local spring discharges with sluggish flows to stagnant conditions, where the contributions to the rivers from within

catchment sources of sediment are expected to be minimal. The sediment associated with the Blue Pool probably comes primarily from precipitation of calcium carbonate (calcite) as the waters degas CO2 from the groundwater and the surface waters become oversaturated with respect to calcite (Neal et al., 2000c). For the Froxfield stream, there may be a similar chemical precipitation source of sediment, but there may also be detritus from the extensive weed growth in this stream and from phytoplankton. For the other Chalk supplied tributaries, the Pang and Lambourn have the lowest average and flow-weighted average SS and PP concentrations compared to the Dun and Kennet where concentrations are typically twice as high. The range in average concentration varies by a factor of about 2–3 across the sites. The average concentrations for the Pang and Lambourn are 5.2 mg/l (range 3.0–10.6 mg/l) for SS and 17.8 lg/l (range 11.6–27.2 lg/l) for PP. The corresponding values for the Dun and Kennet are 12.1 mg/l (range 8.4–17.6 mg/l) SS and 32.1 lg/l (range 24.1–45.0 lg/l) PP. This difference in average concentration between the Pang/Lambourn and Dun/Kennet is statistically significant (p < 0.001 for the logged data). The greatest range in SS and PP concentrations for the Chalk fed catchments is for the Pang. This feature is probably associated with differing situations. Firstly, the lowest concentrations are associated with the uppermost monitoring point at Frilsham, where the Pang is

Suspended sediment and particulate phosphorus in surface waters of the upper Thames Basin, UK

Flow vs SS

SS vs PP

Kennet at Clatford

Kennet at Clatford

400

500 400 PP (μg/l)

SS (mg/l)

300 200 100

300 200 100

0

0 0

5 10 15 Flow (comecs)

20

0

Lambourn at E. Shefford 40 PP (μg/l)

SS (mg/l)

300

Lambourn at E. Shefford

15 10 5

30 20 10

0

0 0

1

2 3 4 Flow (cumecs)

5

6

0

Pang at Tidmarsh

5

10 15 SS (mg/l)

20

Pang at Tidmarsh 250

120

200 PP (μg/l)

90 SS (mg/l)

100 200 SS (mg/l)

50

20

60 30

150 100 50

0

0 0

Figure 2

147

2

4 6 Flow (cumecs)

8

0

30

60 90 SS (mg/l)

120

SS versus flow and PP versus SS for representative permeable (Chalk) catchments.

ephemeral and mainly supplied from local spring inputs. Secondly, the highest concentrations occur for the lowest monitoring point at Tidmarsh, which is partially supplied from low permeability Eocene clays. Wilton Water and the Kennet and Avon Canal have relatively high mean concentrations for SS (29 mg/l, range 23.6–35.9 mg/l) and PP (73.0 lg/l, range 63.2–82.1 lg/l) for the Chalk catchments: the difference is statistically significant (p < 0.001 for logged values). Whilst one might argue that the high SS and PP concentrations in the Kennet and Avon Canal might be associated with low storage volumes and high boat traffic, this does not tie in with the high concentrations in Wilton Water, where there is no boat traffic. Further, while one might assume that the higher SS and PP concentrations in the Dun and Kennet might be associated with an interchange of water between the Kennet and Avon Canal and the river, this is not the case. Thus, while upstream of Hungerford, the course of the river Kennet deviates away from the Kennet and Avon Canal and the Dun remains close to the Kennet and Avon Canal, the SS and PP concentrations remain about the same. The differences in SS and PP concentration are probably a result of three main factors. Firstly, a high biological activity within Wilton Water, other artificial ponds and connecting waterways, and the Kennet and Avon

Canal is likely to lead to high concentrations of algae and calcite precipitates which will add to the SS and PP concentration. Secondly, the maintenance of the riverbanks during the spring and summer periods, including reed cutting, leads to sediment disturbance (this occurs for the Kennet as well), as do other activities linked to river and canal maintenance such as dredging. Thirdly, the high agricultural activity near to Wilton Water and the Kennet and Avon Canal and Wilton Water leads to high sediment discharges to the surface waters (Neal et al., 2005). Wilton Water and the Kennet and Avon Canal will have lower dilution potential than the nearby river. For Wilton Water, chemical precipitates are observed within the shallower areas at the points where submerged springs emerge. N.B. the high SS and PP concentrations in Wilton Water and the Kennet and Avon Canal contrasts with the low concentrations in the Blue Pool. This difference probably reflects the very different conditions in the waterbody. For the Blue Pool, there is a small and shallow (<1 m depth) overtopping system with limited water storage and low water residence time, resulting in low dissolved P concentrations and low biological activity. Correspondingly, Wilton Water is a more substantial reservoir system (up to several metres depth) with a much longer residence times and a complex ecosystem, with SS and PP

148

C. Neal et al.

Flow vs SS

SS vs PP

Thames at Howberry Park

Thames at Howbury Park

150 PP (µg/l)

SS (mg/l)

600 100 50

400 200

0

0 0

50 100 150 Flow (cumecs)

200

0

Thame at Wheatley 1500 PP (µg/l)

150 SS (mg/l)

150

Thame at Wheatley

200

100 50

1000 500 0

0 0

10 20 Flow (cumecs)

30

0

PP (µg/l)

280 240 200 160 120 80 40 0 0

5 10 15 Flow (cumecs)

60 120 SS (mg/l)

180

Cherwell at Banbury

Cherwell at Banbury

SS (mg/l)

50 100 SS (mg/l)

20

700 600 500 400 300 200 100 0 0

100 200 SS (mg/l)

300

Figure 3 Plots of SS versus flow and PP versus SS for mixed permeability (main stem of the Thames) and low permeability catchment runoff (Thame and Cherwell).

concentrations typically an order of magnitude higher than the Blue Pool. In terms of the relationship between SS and PP, the concentrations are largely linearly correlated. The PP:SS ratio is typically around 2.7 lg/mg for the Kennet system including Wilton water and the Kennet and Avon Canal. The ratios are moderately higher (around 3.6 lg/ mg) for the Pang and Lambourn, although values overlap between the averages across the tributaries and the range of values along each tributary.

Cherwell (153 lg/l) compared with the Thame (133 lg/l) by about 15%. The lowest average and flow-weighted average PP concentrations are for the lower Cherwell and the Ray. The SS and PP concentrations are linearly correlated. The phosphorus content of the SS is typically around 5.9 and 3.4 mg/g for the Thame and Cherwell, respectively. There is thus a clear separation in the range of values between the sites on the two rivers: the SS is enriched in P for the Thame compared with the Cherwell.

The low permeability catchments For the low permeability catchments, there are some statistically significant differences in SS and PP concentrations, and the upper part of the Cherwell has the highest SS values. Thus, the Cherwell at Banbury and Kings Sutton has average flow-weighted SS concentrations (28 and 63 mg/l, respectively) twice to three times higher than the lower Cherwell, the Ray, and the Thame. For PP, there are some marked differences between the Cherwell and the Thame and between average and flow-weighted averages. The Thame has average PP concentrations (119 lg/l) that are higher than the upper Cherwell (88 lg/l) by about a third, while the flow-weighted averages are higher for the upper

Mixed permeability catchments In most British rivers there is a general trend towards increases in SS concentrations in a downstream direction. The SS concentration for the main stem of the Thames is intermediate between those for the permeable and low permeability tributary catchments. Since the Thames is relatively slow flowing, some depositional loss of the higher SS inputs, from the low permeability parts of the catchment, might well be expected. However, the average PP concentrations for the main stem of the Thames (61.1 lg/l) are lower than the tributaries of the low permeability catchments (100.8 lg/l) and higher than the tributaries of the permeable catchments (30.1 lg/l). Correspondingly, the flow-weighted

Suspended sediment and particulate phosphorus in surface waters of the upper Thames Basin, UK Table 3

149

Two tailed t-test statistics for suspended sediment and particulate phosphorus in the Pang Avg

Suspended sediment Frilsham Bucklebury Upstream of Blue Pool Blue Pool Downstream of Blue Pool Bradfield Tidmarsh Particulate phosphorus Frilsham Bucklebury Upstream of Blue Pool Blue Pool Downstream of Blue Pool Bradfield Tidmarsh

Geom. mean

N

Frilsham

Bucklebury

Upst. Blue Pool

Blue Pool

Downst. Blue Pool

Bradfield

p

p

p

p

p

p

0.451 0.000

0.000

0.006 0.000

0.634

3.0 7.0 5.2 1.6 3.7

2.6 4.1 3.0 1.0 2.9

85 85 31 31 85

0.000 0.483 0.000 0.264

0.259 0.000 0.006

0.001 0.828

0.000

6.6 10.6

4.7 6.6

34 358

0.000 0.000

0.471 0.000

0.667 0.003

0.000 0.000

11.9 21.7 27.2 8.8 11.6

9.2 14.7 18.8 4.9 8.7

85 85 31 31 85

0.000 0.000 0.002 0.655

0.215 0.000 0.006

0.000 0.000

0.004

21.5 25.4

14.8 16.0

34 358

0.011 0.000

0.975 0.444

0.303 0.371

0.000 0.00

average PP concentration for the main stem of the Thames is higher than that of the tributaries (by 40% or more). The SS and PP concentrations are positively correlated and the average PP content of the SS is similar to that for the Pang and Lambourn and Cherwell (around 3.6 mg-P/g-SS). However, on a flow-weighted basis the average PP content of the SS for the main stem of the Thames is 7.0 mg-P/g-SS, which is about twice as high as that for any of its tributaries. Clearly, for some situations the sediment within the main stem of the Thames is P enriched. This is perhaps to be expected as the main stem of the Thames is a conduit for many of the sewage out-falls of towns by the river upstream (e.g. Oxford), as well as from the upstream tributary sources with effluent discharges from towns adjacent to them (n.b. during the study period), there have been significant reductions in dissolved P from STWs on the Thame upstream of Wheatley, the Cherwell upstream of Kings Sutton and main stem of the Thames upstream of Howberry Park. Preliminary examination of the data collected as part of this study indicates little effect as any significant recovery will only occur with the depletion of the large SS and PP stores currently within the river and the interchange of P between the sediment and the water column that will take decades to achieve.

The relationship between suspended sediment concentration and flow There is a stronger linkage between SS concentration and flow for the less permeable catchments than the more permeable ones as shown both graphically (Figs. 2 and 3) and statistically, but in all cases there is a statistically significant relationship (p < 0.01). Traditionally, the suspended sediment concentrations are linked to flow by a power relationship of the type SS = a * Flowb where ‘a’ and ‘b’ are constants. There is a need to examine if just linear or

logarithmic relations apply between SS and flow. Using the data for the subset of sites with the longest data records yields regression lines for SS (mg/l) and average daily flow (cumecs) as well as the log to the base 10 equivalents are: Kennet at Clatford SS

(mg/l) = 2.63 ± 1.06 * flow (cumecs) + 2.5 ± 47.2 r2 = 0.075, N = 303 log(SS (mg/l)) = 0.257 ± 0.147 * log(flow (cumecs)) + 0.665 ± 0.866 r2 = 0.035, N = 303 Kennet at Mildenhall SS (mg/l) = 1.70 ± 0.60 * flow (cumecs) + 4.0 ± 27.0 r2 = 0.094, N = 310 log(SS (mg/l)) = 0.354 ± 0.119 * log(flow (cumecs)) + 0.633 ± 0.575 r2 = 0.102, N = 310 Pang at Tidmarsh SS (mg/l) = 8.2 ± 1.5 * flow (cumecs) + 2.2 ± 19.6 r2 = 0.252, N = 357 log(SS (mg/l)) = 0.772 ± 0.095 * log(flow (cumecs)) + 0.947 ± 0.510 r2 = 0.414, N = 357 Thames at Howberry Park SS (mg/l) = 0.307 ± 0.036 * flow (cumecs) + 4.4 ± 21.4 r2 = 0.516, N = 271 log(SS (mg/l)) = 0.504 ± 0.065 * log(flow (cumecs)) + 0.369 ± 0.478 r2 = 0.503, N = 271 Thame at Wheatley SS (mg/l) = 1.63 ± 0.45 * flow (cumecs) + 11 ± 36 r2 = 0.255, N = 150 log(SS (mg/l)) = 0.384 ± 0.085 * log(flow (cumecs)) + 1.01 ± 0.43 r2 = 0.330, N = 150 The ± represents twice the standard error. The sites are listed in order of decreasing permeability. The Kennet sites are essentially Chalk drained, while the Pang drains the Chalk but has a component of runoff from Eocene clays in

150

C. Neal et al.

the area. The Thames is a mix of permeable and low permeable areas, while the Thame is mainly low permeability areas. The regression shows that in most cases it makes little difference if logged or linear regression is used as the correlation coefficients are approximately equal. The possible exception to this is the Pang, where a more marked and statistically significant non-linear relationship occurs, presumably due to the influence of the low permeability parts of the catchment (the Bourne) at higher flow rates.

Area weighted suspended sediment and particulate phosphorus fluxes Although the total fluxes cannot be estimated easily, because of an insufficient number of flow gauging stations, the flux per unit area can be approximately estimated since the runoff per unit area is much less variable. Here the flux per unit area is calculated as the product of the long-term average runoff in mm/yr and the flow-weighted SS and PP concentrations. Across the sites, the long term annual runoff in mm/yr increases moderately downstream, and the lowest values are for the Pang and the highest for the Kennet (Table 4). The values are, in downstream direction and from low to high values, • Pang: 72 mm/yr at Frilsham, 114 mm/yr at Pangbourne. • Cherwell: 114 mm/yr at Banbury, 220 mm/yr at Enslow Mill. • Lambourn: 151 mm/yr at East Shefford, 231 mm/yr at Shaw. • Thame: 195 mm/yr at Cuddington, 231 mm/yr at Wheatley. • Dun: 229 mm/yr at Hungerford. • Kennet: 190 mm/yr at Mildenhall, 267 mm/yr at Knighton and 294 mm/yr at Theale. • Thames at Days Lock: 258 mm/yr. In terms of flux per unit area, clear patterns emerge and the SS and PP fluxes are not constant across the catchments or between the rivers:

Table 4 Estimates of the suspended sediment and particulate phosphorus fluxes for the upper Thames and its tributaries Suspended sediment (kg/ha/yr)

Particulate phosphorus (g/ha/yr)

Avg

Avg

Min

Max

25 29 80 106

14 22 63 73

41 33 116 133

Min

Max

Pang Lambourn Kennet Dun

9.1 9.0 32.5 42.4

3.8 6.8 23.4 26.1

19.4 10.4 48.1 55.4

Thames

68.1

na

na

472

na

na

Thame Cherwell

72.9 109.3

59.9 84.7

84.6 141.2

275 251

233 183

304 333

• The lowest flux per unit area is for the Pang and Lambourn, both in terms of SS (average 9 kg/ha/yr) and PP (29 g/ha/yr for PP). • The Kennet and Dun have higher SS and PP flux per unit areas (average 37 kg/ha/yr SS and 92 g/ha/yr for PP) than their Chalk counterparts (the Pang and Lambourn) by about a factor of four. • The Kennet and Dun have much more managed catchments, in terms of flow management and sluices, and a higher input of SS and PP laden waters from reservoir, pond and canal, sources high in SS and PP compared to the Pang and Lambourn. This higher degree of management is reflected in the flux differences in the same way as that described earlier in relation to concentration differences. • The Thames has an intermediate flux per unit area of SS between the permeable and low permeability catchments (68 kg/ha/yr), but it has the highest flux for PP across the catchments (472 g/ha/yr). For SS, the flux difference is to be expected because of the relative inputs for permeable and low permeable catchments. Correspondingly, the high PP concentrations reflect augmentation of P from sewage sources, as discussed above in terms of concentration. • The low permeability catchments of the Thame and Cherwell have the highest flux per unit area of SS (average 91 kg/ha/yr) and the second highest flux per unit area for PP (average 63 g/ha/yr). • Across the catchments, both the SS and PP flux per unit area varies by a factor of about two. In the case of the Pang, the flux per unit area increases downstream as the area increases, while for the Dun the reverse is observed with flux per unit area decreasing downstream and for the rest of the catchments (Thame, Cherwell, Lambourn and Kennet) there is no clear pattern of behaviour. Clearly, there seem to be significant and spatially variable within-catchment and within-river attenuation processes and activities for both SS and PP, but the details of the drivers and responses are not clear. Presently, in situ sensors are being used to monitor turbidity within the LOCAR programme for the Chalk catchments at a 15 min time interval, and there are associated data from the Environment Agency for both permeable and low permeability catchments. These data need to be exploited in the future. For the LOCAR studies of the Pang and Lambourn, it remains premature to undertake a detailed analysis. This is because the instrumentation of the Pang and Lambourn has proved extremely challenging. Thus, the characteristically clear, nutrient rich and low energy environments of these rivers mean that biological productivity is extremely high. Turbidity sensor lenses are frequently obscured by either biofilm growth or macrophyte debris. The collection of high quality data relies on a particularly intensive field maintenance programme, with unusually high frequencies of sensor cleaning and the application of detailed data processing and quality assurance procedures. Initially, the sensors were cleaned automatically every 6 h. However, this rate of cleaning proved, insufficient and hourly automated cleaning was introduced late in the programme to significantly reduce fouling of the sensor. The analysis of the more reliable

Suspended sediment and particulate phosphorus in surface waters of the upper Thames Basin, UK in situ turbidity data has now started. Here it is worth showing some of the findings for a storm period in November 2002 (Fig. 4) that illustrates a complex relationship between flow and SS as well as the need for the collection of high resolution SS data. The data show a variety of behaviour of the relationship between SS and flow and it simply cannot be viewed in terms of high scatter (Fig. 5). Rather, there is variability in response that changes between linear, curvilinear and hysteretic. Thus, short lived flow events (down to sub daily) occur that (a) are only adequately sampled by high resolution monitoring and (b) show flow-SS patterns that vary over time in relation not only to the wetting and drying up of the catchment, but also with depletion of the available sediment stock for mobilisation through an event. Where there are hysteretic patterns they exhibit a ‘clockwise spiralling’ (c.f. arrows in Fig. 4), reflecting initial high releases of SS on the rising limb of an event with a subsequent reduction in SS concentration as flow remains high and then starts to decline. This is most clearly observed for the storm of the 14th November 2002 (Fig. 5) where SS concentrations sharply increase from <50 mg/l to around 300 mg/l and then decline to around 200 mg/l while flow remains high and then subsequently returns to concentrations of <50 mg/l as flows subsequently decline. This feature fits very well with the regression analysis presented earlier in the paper, where a strong linkage exists between SS concenFlow Flow (cumecs)

2.5 2 1.5 1 0.5 10

11

12

13 14 15 November 2002

16

17

18

16

17

18

SS

SS (mg/l)

300 200 100 0 10

11

12

13 14 15 November 2002

Flow vs SS

SS (mg/l)

300 200 100 0 0.5

1

1.5 Flow (cumecs)

2

Figure 4 SS concentration versus relationships based on 15 min data for the Pang at Tidmarsh for individual days from 12th to 17th November 2002.

151

tration and the differential of flow from the previous day with regards to the rising but not the falling limb.

Discussion The analysis of the data presented here leads to the identification of some important differences in SS and PP concentrations and fluxes per unit area across the region, and indicates some avenues for further research. It broadly shows that: • The surface runoff from the high permeability catchments has generally lower concentration and flux per unit area of SS and PP than the less permeable counterparts for the upper Thames. • There are clear differences in concentrations and fluxes across each of the catchments. • There are marked contrasts between the Chalk drained Pang/Lambourn and the Kennet/Dun catchments that cannot easily be explained on the basis of uncertainties in measurement and agricultural usage. • SS and PP transport cannot simply be considered in terms of a physical process for the Chalk areas as there is strong evidence for chemical precipitation (calcite) and diatom/phytoplankton development (Neal et al., 2000a,b, c,d, 2005). In the broad context of UK river sediment fluxes, the average suspended load estimates for the study catchments (Table 4) are within the range reported for UK rivers (e.g. Walling and Webb, 1987; Wass and Leeks, 1999; Foster and Lees, 1999) and tend to be the low end of the range (from less than 1 kg/ha/yr up to nearly 5000 kg/ha/yr) with a typical value of approximately 500 kg/ha/yr, which is also low by global standards (Walling and Webb, 1987). The position with regard to UK river sediment fluxes may reflect lower delivery or supply to the river from catchment surfaces in lower gradient lowland catchment systems, and/or methodological differences in the estimation methodology and dataset, which places less weight upon high flows than is the case in the experimental catchment studies reported in the literature. However, it must be borne in mind that there will be in-river sediment movement (together with PP) together with in-river water quality processing which are not fully quantified by down-stream catchment yield estimates. The dynamics of SS (and PP since the two are linearly correlated) differ between permeable and impermeable catchments, with the permeable catchments exhibiting a more complex response linked to antecedent conditions. This is to be expected for a number of reasons. Fluctuating water tables and periods of overland flow linked to groundwater levels will in part determine SS and PP concentration variations in the rivers of the permeable catchments. The extent of SS and PP transport will be partly determined by the extent of wetting up of the catchment and the degree of overland flow where the groundwater reaches the catchment surface. Also, for extreme storm events there may be infiltration excess. New models are currently being produced to describe the wetting up and recession (e.g. within the LOCAR

C. Neal et al. 100 90 10/11/2002 80 70 60 50 40 30 20 10 0 0.6 0.7 0.8 0.9 Flow (cumecs)

350 300 SS (mg/l)

SS (mg/l)

152

0 1

1.1

1

1.4 1.6 1.8 Flow (cumecs)

2

2.2

1.75

1.8

15/11/2002 150 SS (mg/l)

SS (mg/l)

1.2

200

11/11/2002

40

100 50

20 0.7

0.8 0.9 Flow (cumecs)

0 1.5

1

1.55

1.6 1.65 1.7 Flow (cumecs)

100

150

12/11/2002

80

100

SS (mg/l)

SS (mg/l)

150 50

60

0 0.6

200 100

100 80

14/11/2002

250

50

16/11/2002

60 40 20

0 0.6

0.8

1 1.2 Flow (cumecs)

0 1.4

1.4

100

80

60 40 20 0 1.1

1.6 1.7 Flow (cumecs)

1.8

100

13/11/2002 SS (mg/l)

SS (mg/l)

80

1.5

17/11/2002

60 40 20

1.2

1.3 1.4 Flow (cumecs)

1.5

0 1.2

1.25

1.3 1.35 Flow (cumecs)

1.4

Figure 5 Suspended sediment concentration relationships with flow based on 15 min data for the Pang at Tidmarsh from 12th to 17th November 2002.

programme). They may well provide new insights into how sediment are transported in permeable catchments and the sediment data itself may well provide a guide as to how the hydrology might function with regards to overland flow. Even taking into account the antecedent conditions, simple linear regression of the compiled data is not sufficient to describe SS and PP concentration variations in terms of flow, as the relatively low r2 values from this study indicate. However, crucially, it must be borne in mind that there are management activities and inorganic and biological processes that may well come into play that strongly influence the SS and PP concentrations. These processes are simply not factored into most of the current modelling activities and process based studies in a general enough way at present. For the low permeability catchments, there will be a dominance of rapid runoff from overland flow and rapid water transit routes such as mole drainage. Thus, the linkage between SS and PP concentrations with flow seems less complex than that for the Chalk. Nonetheless, the SS

and PP concentration relationship with flow is far from perfect in relation to data scatter from linearity and even in this case there may well be an underlying complexity linked to issues such as fractal processing, which may reflect complex within-catchment routing and highly variable residence times. Such fractal processing may well also be a feature of the Chalk catchments as well with its dual and multiple porosity functioning. This feature is now being shown by spectral analysis of continuous conductivity data for many of the catchments studied here, that reveals a 1/f structure. It is readily acknowledged here that much more intensive and extensive measurements are required to estimate precisely and accurately SS (and PP) fluxes (Littlewood, 1992; Littlewood et al., 1998; Webb et al., 1997). However, such intensive monitoring is of particular importance for examining the dynamics of SS and PP concentration change and this is crucial for model calibration and testing and as a benchmark for spectral analysis to test for 1/f fractal scaling.

Suspended sediment and particulate phosphorus in surface waters of the upper Thames Basin, UK The variations in SS and PP described in this study are of importance in relation to water quality standards and the Water Framework Directive. Indeed, the study of the Thames, one of the main rivers within England, provides an image of a matrix of permeable and impermeable sub catchments, water usage (e.g. reservoir, ponds and canals) and management. The prime issue within the WFD is the maintenance of waters of good ecological status and high amenity value. Addressing the challenge of the WFD requires a new outlook from the classical approach of minimising fluxes of sediment and contaminated sediment to ecologically impacted or sensitive surface water systems. Indeed, there is a strategic management need for a critical pathway and ‘issues’ analysis for these ecologically impacted and sensitive systems. In particular, the WFD puts an emphasis on biological impacts. This directly links to the lower flow periods when dilution potential is at its lowest (i.e. water–sediment interactions are at their highest), biology is at its most active and amenity value is at its highest (Jarvie et al., 2006; Neal and Jarvie, 2005). In the case of the Chalk areas at least, there may well be important sources of SS and PP linked to river, pond and canal management such as reed bed clearance and dredging, as well as from chemical and biological processes. This feature leads to the complex issues of inorganic and biological feedback mechanisms that can affect aquatic ecosystem functioning but which is hard to pin down. Indeed, the biologically active backwaters and areas of flow separation with slower moving and warmer conditions during the major growth period may well act as ‘seed sources’ for biological response within the river. Nutrient cycling and the interactions between the SS/PP and the water column will also affect the feedback loops. However, the nature and quantification of such processes remain inconclusively assessed. Nonetheless, such an assessment may well prove of fundamental importance for the Chalk areas with regard to the WFD, and the social and economic maintenance of our agricultural heritage, its resource, and its sustainability.

Wider comment A number of points regarding relevance to future environmental research and lessons from the framing of past research programmes’ management can be drawn. This leads to the question of how best to take the research forward in order to address the needs for further research, environmental management and WFD. Here a ‘broad-brush’ approach has been taken rather than focussing in on extremely detailed studies. Such ‘broad-brush’ approaches are often taken as relatively simplistic, but they have much to commend them. The approach is close to the traditional approach of the EA and by the Scottish Environmental Protection Agency (SEPA). It is hoped that in the years to come there will be a synergy of approaches and joint endeavours—much of which have been started with studies such as the Land Ocean Interaction Study (LOIS; Leeks and Jarvie, 1998) and many of the component studies developed here for the upper Thames. There has been a relatively long tradition in the UK environmental sciences, outside that of the agricultural community, for studying catchment systems that seem relatively

153

unperturbed with no confounding influences of communities such as towns and cities. Some exceptions to this rule include large-scale process studies, such as the LOIS which have examined major river basins with a matrix of rural, agricultural and urban settings and a mixture of permeable and less permeable catchments (Leeks and Jarvie, 1999). Another exception has been the Urban Regeneration and the Environment Programme (URGENT: http://urgent.nerc.ac.uk) with its emphasis on urban environments. But even here, there is a lack of emphasis on the relationship between urban and rural communities. LOCAR itself has had a rural focus. However, many environmental issues in lowland catchments confront both urban and rural dwellers and habitats. Hence communities and environments cannot be conveniently divided. Such separations do not do justice to the needs of interactive rural and urban communities, dealing with sewage sources and over abstraction of water, dealing with the issue of housing and ‘the invasion of the green belt’, the needs of the WFD, and the requirements of the taxpayer. For the upper Thames, there is a complex set of issues linked to a matrix of permeable and less permeable catchments, catchments being influenced by varying community levels and infrastructure (e.g. sewage sources, watercourse management, amenity usage, townships, demographic redistribution, etc). Such issues are multidimensional in both scope and responses need. In addition to human pressures and responses there are also issues of climate instability and climate change that will affect all aspects of surface water quality and water resources management for the foreseeable future. It is within this wider context that the new research must focus.

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