Appendix 6 - Nutrient hydrochemistry for a groundwater-dominated
catchment: the Hampshire Avon, UK
Helen P. Jarvie1, Colin Neal1, Paul J.A. Withers2, Chris Wescott3, Richard M.
Acornley3
1
Centre for Ecology and Hydrology, Wallingford,
2
ADAS, Management Group, ADAS Gleadthorpe,
3
Environment Agency, South Wessex Area, Blandford Forum
Abstract
The patterns in nitrate and phosphorus sources, loads and concentrations in a
groundwater-dominated lowland catchment, the Hampshire Avon, are examined and
water quality signatures are used to identify a typology of headwater stream types.
The major separations in water quality are linked to geology and groundwater
chemistry as modified by the impacts of point source sewage effluents. The water
quality of the major tributaries and the main stem of the River Avon are linked to the
relative contributions of these source types, the impact of further direct effluent inputs
to the main channel and in-stream processing. The tributaries and main stem of the
Avon act as net sinks for total reactive phosphorus (TRP). Low concentrations of TRP
were found in the Chalk groundwater and the groundwater system acts as an efficient
buffer, removing and retaining TRP from water draining from the catchment surface
into the aquifer. Thermodynamic analysis of CaCO3 solubility controls indicates that
this natural ‘self-cleansing mechanism’ system within the groundwater may be
directly linked to CaCO3-P co-precipitation within the aquifer matrix.
1. Introduction
This paper examines the nutrient functioning of the Hampshire Avon catchment, a
lowland and predominantly rural Chalk catchment in southern England. The Avon, in
Hampshire, has been subject to a condition known as ‘Chalk Stream Malaise’ (Defra,
2003), a growing problem in the rivers and streams of southern England, which is
threatening an aquatic environment of high conservation value (Jarvie et al, 2002b,
2004a). Chalk Stream Malaise is the term used to describe the general deterioration in
the classic Chalk stream habitat, linked to loss of key macrophytes, such as
Ranunculus spp., excessive growth of benthic and filamentous algae and increased
turbidity and siltation of gravel beds. These changes have been linked to a decline in
salmonid and coarse fish species and invertebrates (Defra, 2003). Agricultural nonpoint source pollution has been suggested as a major cause of deteriorations in the
quality of the Chalk stream environment, linked to nutrient enrichment from fertilizers
and manures and compounded by reductions in water flow and velocity linked to
abstraction and drought (Environment Agency, 2002). Concerns have also been
expressed about the capacity of Chalk streams like the Avon to assimilate diffuse
agricultural pollution as a result of the low gradient, low energy groundwater-fed
shallow stream environment, with high water residence times, promoting benthic and
epiphytic plant growth (Environment Agency, 2002, Jarvie et al, 2004a).
This paper provides the first basin-scale assessment of the spatial distribution and
temporal variability in nutrient concentrations across one of the most sensitive Chalkstream environments in the UK. It examines concentrations of nitrate and phosphorus
1
in surface water, ground water and effluent within the Avon catchment, in relation to
geology and land use. The study utilises the vast water quality datasets collected by
the Environment Agency (EA), which allows better spatial resolution of nutrient
pollution risk and temporal coverage in chemical concentrations (over a 10 year
period from 1991 – 2000), than previously available. Nutrient fluxes along the River
Avon and from the major tributaries are assessed and the relative importance of point
sources to riverine loads is estimated. This work forms the backdrop to more detailed
and focused water quality monitoring and modelling on the Avon as part of a major
research programme: Phosphorus and Sediment Yield Characterization In Catchments
(PSYCHIC), funded by the UK Department from the Environment Food and Rural
Affairs (Defra) and the EA. It also deals with the important issue of ‘self cleansing’
mechanisms for phosphorus, where inorganic P can be incorporated into calcium
carbonate lattices in Chalk streams and groundwaters (House, 1990, Neal, 2001a;
Neal et al., 2002, Jarvie et al, 2002a).
2. Study Area
The Avon, which rises on the Chalk downs north of Pewsey in southern England, has
two tributaries, the East and West Avon. The Avon catchment, down to the lowest
gauging station at Knapp Mill, has an area of 1706 km2 (Figure 1a). The upper Avon
catchment is composed of a radial pattern of major tributaries (Ebble, Nadder, Wylye,
Avon and Bourne). These tributaries converge close to the town of Salisbury, from
where the Avon flows southwards, via Ringwood, to the English Channel. The
geology of the upper Avon (to Fordingbridge) is dominated by Chalk (Figure 1b).
The Chalk is underlain by Greensand, which outcrops in the Vale of Pewsey (in the
East and West Avon catchments) and the valley of the upper Wylye, south of
Warminster. The upper Nadder valley in the extreme west of the catchment has
exposures of Purbeck and Portland Sandstones and Kimmeridge Clay. At
Fordingbridge, the River Avon flows over the sands and gravels of the Reading Beds
and London Clay and then over the acidic clays, sands, silts and gravels of the Barton,
Bracklesham, and Bagshot Beds.
The upper catchment is rural and characterized by rolling Chalk-lands, dominated by
intensive arable farming and unimproved grassland, with abandoned water meadows
within the river valleys. Large areas of the north and west of the catchment are in the
North Wessex Downs Area of Outstanding Natural Beauty (AONB) and the
Cranbourne Chase and West Wiltshire Downs AONB. The major commercial and
residential areas are located to the south, around Salisbury, Fordingbridge, Ringwood
and Christchurch. South of Salisbury, there has been extraction of sands and gravels
from the extensive, flat floodplain, which has resulted in formation of groups of large
standing water bodies. The catchment below Salisbury drains acidic sandstones,
dominated by heather moorland vegetation and part of the New Forest (Figure 1a).
There are several major and minor sewage treatment works (STWs) across the area
and several have been modified to reduce phosphorus. The Avon and its tributaries
support a rich ecology, including nationally important examples of the Ranunculus
habitat and rare invertebrate species. Internationally-threatened species such as
Atlantic salmon, bullhead and otter are present in the Avon and its tributaries
(Environment Agency, 2002).
2
N
10 km
Figure 1a. Map of the Avon catchment showing major tributaries, settlements and
gauging stations and location within the UK.
Figure 1b. Geological map of the Avon catchment.
3
The Avon and its tributaries (not Ebble) have been designated as a candidate Special
Area of Conservation under the European Union (EU) Habitats Directive and the
Avon below Salisbury STW and the Wylye below Warminster STW are designated as
sensitive areas (eutrophic) under the EU Urban Waste Water Treatment Directive.
The rivers of the Avon catchment are largely spring-fed, which provides relatively
stable flow throughout the year, although hydrological differences are observed on
some of the tributaries, reflecting their different geologies: the Baseflow Indices (BFI)
at gauging stations in the catchment range mainly from 0.72 on the West Avon at
Upavon, which drain predominantly Upper Greensand with Chalk and Gault, to 0.92
on the Bourne at Laverstock which drains a permeable Chalk catchment (CEH, 2003).
Mean annual rainfall (1971 – 2000) ranges from 781mm for the West Avon at
Upavon to 950 mm in the western part of the catchment, the upper Wyle at Norton
Bavant. Streams draining the New Forest are hydrologically different from the springfed rivers; they are more responsive having much lower BFIs (e.g. the BFI at the
gauging station at Dockens Water is 0.34).
3. Methods
3.1 Data and analytical methodologies
Water quality data spanning 10 years (from 1991 to 2000) were supplied for the
whole of the Avon catchment from the Environment Agency of England and Wales
(EA) National Water Information Management System: 357 river monitoring sites, 33
non-river freshwater sites, 44 water-company sewage treatment works (STWs), 102
private STW, 50 trade effluent sites, 32 fish farm discharges and 220 groundwater
boreholes. Many of the river monitoring sites were sampled on a regular basis over
the 10 years, but some sites were monitored sporadically, in response to particular
incidents, such as accidental discharges or fish kills. In order to remove any bias
towards such events, data were only retrieved for sites where regular monitoring had
been undertaken over the 10-year period, i.e. where typically more than 50 samples
had been collected for a particular determinand of interest. Determinands chosen for
analysis were nitrogen and phosphorus fractions, suspended sediment, pH, alkalinity,
calcium and chloride. For phosphorus, data analysis has concentrated on the ‘Total
Reactive Phosphorus’ (TRP) fraction, which is measured most widely across the
catchments. This analysis is undertaken on unfiltered water samples by the standard
Murphy and Riley (1962) colorimetric method, using cold acidification. The TRP
analysis is not a ‘true’ measurement of orthophosphate ions (PO43-). Rather, it is a
measure of the sum of the ‘soluble reactive phosphorus’ (SRP) plus easily
hydrolysable particle-associated fractions. Alkalinity measurements are based on
acidimetric titration. There are many definitions of alkalinity, but in this case, the
methodology used, the high levels of alkalinity measured in this study mean that the
values presented represent bicarbonate alkalinity (Neal, 2001b). The alkalinity as
presented in the EA database has units of mg-CaCO3 l-1, but it is also widely
presented in units of µEq l-1. Here the latter units are used and conversion to µEq l-1
units requires values in mg-CaCO3 l-1 to be multiplied by 20.
4
Mean daily river flow data were supplied for each of the gauging stations within the
catchment by the UK National River Flow Archive at Wallingford (Figure 1a). River
water quality spot samples at gauging stations were then matched with the mean daily
flow data, to enable concentration-flow relationships to be examined and load
estimates calculated. Data manipulation and analysis was undertaken using Splus
statistical software (Insightful Corporation) and displayed within a Geographical
Information System (GIS) package (ArcView, ESRI).
3.2 Thermodynamic analyses of saturation of waters with respect to carbon dioxide
and calcite (calcium carbonate)
Dissolved inorganic carbon speciation was determined from the alkalinity and
measurements and thermodynamic information on the inter-relationships of inorganic
C species, making allowance for the dependence of equilibrium constants on
temperature and ionic strength. The degree of saturation with respect to CO2 is given
in terms of an excess partial pressure of CO2 (EpCO2). The saturation index for
calcium carbonate (calcite) solubility (SIcalcite, in logarithmic form) was determined
using data on alkalinity, Ca concentration and pH data and as with the EpCO2
assessment, allowance was made for the temperature and ionic strength. The
thermodynamic analysis was undertaken using the approach described by Neal et al.
(1998).
3.3 Load calculations
Mass loads were calculated for water quality monitoring sites that were located close
to gauging stations (Figure 1a), on an annual basis from 1993 to 2000 (inclusive).
Given the intermittent nature of the Environment Agency of England and Wales (EA)
water quality sampling (typically at monthly intervals), an interpolation technique was
employed in order to estimate annual riverine loads of nutrients. A load estimation
algorithm, based on the product of the flow-weighted mean concentration and the
mean daily flow over the period of the record was deemed most appropriate, as the
estimates produced have relatively small bias and lower variance than other
comparable estimators (Walling and Webb, 1985, Webb et al., 1997):
River load =Kr( ∑ (Ci * Qi)/ ∑Qi) * (Qr)
Where, Ci is the instantaneous concentration in the river at the time of sampling, Qi is
the instantaneous river flow at the time of sampling, Qr is the average long-term river
flow record over the period of record and Kr is a conversion factor to take account of
the units and period of record.
River mass loads are expressed as means and ranges in annual loads. As mass loads
increase with catchment size according to increases in river flows, mass loads per
hectare and flow-weighted mean concentrations were also calculated for each of the
gauging stations to examine the relative contributions of different parts of the
catchment irrespective of catchment area and river flow.
Estimates were also made of the nutrient loads from sewage and industrial effluents
discharging to each of the subcatchments. For each gauging station, an effluent
loading was calculated from all of the EA-monitored effluent inputs discharging
5
upstream of the gauging station. In the absence of a detailed flow record for effluent
discharges, effluent loads were calculated as the product of the dry weather
consented-discharges and mean of the upper 75% of recorded effluent concentrations
(Jarvie et al., 2003):
Effluent load = K(DWC * DWF)
DWC is an estimate of the average dry weather effluent concentration, based
on the mean of the upper 75% of effluent concentrations, while DWF is the EA
effluent consented dry weather flow and K is a conversion factor to take account of
the units and the period of the record.
4. Results
4.1 General chemistry of river waters, groundwaters and effluents in the Avon
catchment
The Avon river and ground waters are calcium bearing, with high alkalinity (Table
1a) and they are characteristic of Chalk areas (Neal et al, 2000a,b, 2002).
Groundwater alkalinity concentrations show only a small degree of fluctuation (less
than a factor of 3), whereas there is much higher variability in river water alkalinity in
terms of spatial distribution, with much lower alkalinity values in streams draining the
Avon catchment to the south of Salisbury (Figure 2a). Nitrate concentrations, like
alkalinity (and Ca), show little differentiation between groundwater and river water,
but there are considerably lower concentrations of NO3 (and Ca) in the Avon rivers to
the south of Salisbury (Figure 2b). The groundwaters of the Avon catchment show
lower pH values than the river waters and these differences reflect variations in
EpCO2. Average EpCO2 in the groundwaters are 39 times atmospheric pressure,
compared with only 6 times atmospheric pressure in the river and this reflects carbon
dioxide degassing from the water column as the groundwater emerges from springs
and passes through the river and carbon dioxide uptake by photosynthesizing plants
within the river. As the EpCO2 decreases on passage from groundwater to river water,
the concentration of carbonic acid also reduces, producing higher pH in the river.
Total reactive phosphorus shows much higher concentrations in river water, where
mean concentrations are 0.23 mg-P l-1 compared with 0.05 mg-P l-1 in groundwater.
60% of groundwater samples have TRP concentrations of <0.05 mg-P l-1; 87% have
TRP concentrations <0.1 mg-P l-1. Only 8% of groundwater samples have TRP
concentrations >0.2 mg-P l-1. These atypical groundwaters were for the upper Nadder
catchment, in the upper Greensand, in contrast to the other boreholes which abstract
from the Chalk aquifer.
The saturation index for calcite is usually higher in river water compared to
groundwater (Figure 3). The rivers mainly have a mean SIcalcite of around 1 (i.e. about
10 times saturation), while the corresponding values for groundwater average about 0
(i.e. about calcite saturation). The groundwaters do not exceed on average 5 times
saturation with respect to CaCO3, with some groundwaters show up to 5 times
undersaturation with respect to CaCO3. The lower SIcalcite values for groundwater
indicate that either CaCO3 precipitation is actively occurring within the groundwater
or calcite is dissolving (when SIcalcite < 0). Oversaturation in the river water indicates
that CaCO3 precipitation may be kinetically inhibited. CaCO3 precipitation in
6
Table 1a. Averages and ranges in chemical concentrations of major determinands in river water and groundwater samples from the Avon
catchment (1991- 2000). These are based on individual samples for each site, not mean values for each site.
pH
Ca
(mgl-1)
Alkalinity
(μE l-1)
EpCO2
SICaCO3
(x atm
press)
TRP
NO3
(mg-Pl-1)
(mg-N l-1)
Cl
(mgl-1)
NH3
(mgl-1)
SS
(mgl-1)
Groundwater
Mean
Median
Max
Min
No samples
7.3
7.3
7.6
6.7
274
107
106
214
4.4
274
3938
3918
5328
2295
274
39
34
169
9.8
274
0.18
0.19
0.70
-0.91
274
0.05
0.02
0.97
<0.02
9665
5.33
5.70
38.9
0.001
10673
20
17
156
4
9812
0.02
0.01
3.52
0.01
10709
NA
NA
NA
NA
NA
Rivers
Mean
Median
Max
Min
No samples
8.0
8.1
8.8
4.5
4680
105
111
174
3.4
4680
3920
4320
5880
200
4680
6.2
5.4
119
0.4
4680
0.76
1.06
1.73
-4.28
4680
0.23
0.18
9.5
<0.02
16652
5.49
5.56
28.1
0.07
14510
23
21
258
0
12061
0.02
0.05
20.5
0.002
16615
13
8
2070
1.8
15314
TRP = Total Reactive Phosphorus
SICaCO3= saturation index for CaCO3
EpCO2 = excess partial pressure of CO2
SS = suspended sediment (no data for groundwater; NA)
7
Table 1b. Averages and ranges in chemical concentrations of major determinands in effluent samples from the Avon catchment (1991 – 2000).
These are based on individual samples for each site, not mean values for each site.
NO3
(mg-N l-1)
17.2
15.5
67
0.2
3501
Cl
(mgl-1)
NH3
(mgl-1)
SS
(mgl-1)
76
68
432
19
3376
2.0
1.2
49
0.02
4685
15
13
2850
2
4509
Sewage
Effluent
(Water
Company
STW)
Mean
Median
Max
Min
No samples
TRP
(mg-Pl-1)
6.1
5.8
64
0.06
3659
Sewage
Effluent
(Private STW)
Mean
Median
Max
Min
No samples
8.1
7
148
<0.02
296
13.9
11.2
63.4
0.001
784
74
57
1173
10
774
6.5
2.1
55.9
0.02
383
43
23
1376
2
376
Trade Effluent
Mean
Median
Max
Min
No samples
4.0
0.5
22.5
<0.02
544
8.3
4.7
79.6
0
435
153
65
2845
6.9
544
13.2
1.64
279
0.003
634
24.3
9.4
354
2
610
Fish Farm
effluent
Mean
Median
Max
Min
No samples
0.2
0.16
6.6
<0.02
2502
5.57
5.56
129
0.09
2264
20
19
1070
9.0
2237
0.24
0.18
135
0.01
3252
8.3
5.5
224
1.2
3576
TRP = Total Reactive Phosphorus
SS = Suspended Sediment
8
Figure 2b
Figure 2a
Figure 2c
N
10 km
Figure 2. Spatial distribution of mean concencentrations of (a) Alkalinity, (b) Nitrate
and (c) Total Reactive Phosphorus.
groundwater is probably able to occur as a result of the greater availability of
nucleation sites within the aquifer (Neal et al., 2002). In contrast, low availability of
nucleation sites within the river water may contribute to inhibition of CaCO3
precipitation. The much lower concentrations of TRP in the Chalk groundwater may
reflect uptake of TRP by co-precipitation of CaCO3 and TRP in the soil and/or
groundwater (Neal et al, 2000, 2002). In contrast, the higher TRP concentrations in
the upper Nadder boreholes may reflect groundwater transport that is predominantly
fissure flow. For this situation, uptake by the carbonate matrix is likely to be limited
by lower water residence times and flow which bypasses the aquifer matrix.
A small cluster of river sites have much lower mean SIcalcite values (c. 20 times
undersaturation) and low mean TRP concentrations (<0.05mg-P l-1; Fig. 3) and these
are the streams draining the New Forest. The low SIcalcite reflects local geology and
runoff from base-poor sandstones in contrast to the calcareous Chalk and Greensand
that dominates across the rest of the catchment. The low TRP, NO3 and Cl
concentrations in the New Forest streams (Figure 2a,b,c) also reflect low population
density and thus low sewage effluent inputs as well as low intensity agricultural
inputs. For the main calcareous Avon catchment, upstream of Fordingbridge and the
influence of the New Forest , the wider range in TRP concentrations in river water
compared with groundwater, reflects variable dilution of effluent inputs. Indeed, river
water TRP concentrations show a well-defined reduction in concentration with flow,
indicating the influence of point-source dilution (Figure 4a).
9
1.5
1
CaCO3
0.5
SI
0
0
-0.5
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
-1
-1.5
-2
groundwater
-2.5
river water
-3
TRP (mg-P/l)
Figure 3. Plot of mean Saturation Index for CaCO3 (SIcalcite) plotted against mean
Total Reactive Phosphorus (TRP) concentration for groundwater and river water
monitoring sites in the Avon catchment.
Nitrate concentrations in the Avon rivers show increasing concentrations with flow
(Figure 4b), indicating the importance of diffuse-source of NO3 to the rivers.
Effluent discharges in the Avon catchment are divided into water company sewage
works, private sewage works, trade effluents and fish farms. Concentrations of TRP,
NO3, Cl and NH3 are considerably higher in the effluent samples compared with river
water, with the exception of TRP, NO3 and Cl in fish farm effluent, which are close to
the river-water concentration values (Tables 1a and b). Highest concentrations of
TRP are found in private sewage works, with a mean concentration of 8 mg-P l-1,
compared with 6mg-P l-1 in water company STW effluent, 0.2 mg-P l-1 in fish farm
effluent and river water. Nitrate concentrations are highest (mean 17 mg-N l-1) in
water company STW effluent.
4.2 Use of river water chemistry to identify a typology of stream source types
Closer investigation of the spatial distribution of water quality signatures across the
Avon catchment has revealed a separation of headwater stream into a series of type
localities, linked to geology and source inputs (Table 2, Figures 2 and 5 ). Given the
elevated concentrations of Cl in sewage and the conservative transport of Cl in river
water (it is chemically unreactive and therefore does readily undergo sorption to
sediments and biological uptake), Cl is employed here as a tracer of sewage effluent
10
RIVER AVON AT AMESBURY
0.5
EAST AVON AT UPAVON
0.1
0.1
0.3
0.3
0.5
0.1 0.2 0.3 0.4 0.5
20
30
40
50
60
1
3
4
5
10
15
20
RIVER WYLYE AT NORTON BAVANT
0.1
0.5
0.2
1.0
0.3
0.6
0.4
0.2
0
RIVER AVON AT KNAPP MILL
0.4
0.8
WEST AVON AT UPAVON
2
1.5
10
1
2
3
4
5
10
30
40
50
60
2
4
6
2
3
4
5
6
RIVER WYLYE AT S. NEWTON
0.3
0.2
0.15
0.1
0.05
0
1
RIVER NADDER AT WILTON
0.25
RIVER BOURNE AT LAVERSTOCK
20
0.4
0
0.1 0.2 0.3 0.4
TRP concentration (mg-P l-1)
RIVER AVON AT EAST MILLS
5
10
15
0
5
10
15
20
25
Flow (m3s-1)
Figure 4a
11
EAST AVON AT UPAVON
RIVER AVON AT AMESBURY
6
5
5
4
4
4
10
20
30
40
50
60
1
WEST AVON AT UPAVON
2
3
4
0
RIVER AVON AT KNAPP MILL
5
10
15
20
RIVER WYLYE AT NORTON BAVANT
4
2
5
4
6
6
7
2 4 6 8 10 12 14
1
2
3
4
5
10
30
40
50
60
7
8
1
RIVER NADDER AT WILTON
8
9
BOURNE AT LAVERSTOCK
20
2
3
4
5
6
RIVER WYLYE AT S. NEWTON
7.0
0
4
4.0
4
5
5
5.0
6
6
7
6.0
Nitrate concentration (mg-N l -1)
5
6
7
7
6
8
8
9
9
RIVER AVON AT EAST MILLS
0
2
4
6
5
10
15
0
5
10
15
20
25
Flow (m3s-1)
Figure 4b
Figure 4. Relationships between (a) Total Reactive Phosphorus (TRP) and flow and (b) Nitrate and flow for gauged rivers in the Avon
catchment.
12
Table 2. Typology of headwater stream chemistry (based on average concentrations) and average chemical concentrations at sites on the major
tributaries and the main River Avon.
River Types
Example sampling sites
NO3
(mg-N l-1)
5.69
6.40
9.95
Cl
(mg l-1)
15.1
16.2
18.0
pH
Upper Wylye (u/s of Warminster)
Allen River
Sweatsford Water
TRP
(mg-P l-1)
0.085
0.036
0.037
N:P
SRP:Cl
8.09
7.81
7.80
SS
(mg l-1)
10
7
8
Type I: Permeable headwaters with no
point major STW influence
74
216
268
0.006
0.002
0.002
Type II: Permeable headwaters; sewage
impacted
Upper West Avon (Echilhampton Brook)
Upper Wylye (d/s of Warminster)
0.337
0.712
5.99
6.55
42.6
33.0
7.96
8.0
14
13
31
13
0.011
0.022
Type III: Impermeable (clay) headwaters;
sewage impacted
Sem
East Knoyle Stream
Hays Stream
0.212
1.704
0.314
3.22
10.5
4.45
27.3
38.9
38.1
7.8
7.79
7.67
23
25
23
18
8
22.6
0.008
0.039
0.008
Type IV: Moorland catchments draining
acidic sandstones of the New Forest
Plateau
Linford Brook
Dockens Water
Huckles Brook
0.023
0.024
0.039
0.55
0.33
0.91
27.8
19.6
21.0
7.27
7.09
7.11
6
7
9
21
14
16
0.001
0.002
0.002
Major tributaries sampled close to
confluence with River Avon
Wylye
Nadder
E Avon
W Avon
Bourne
Ebble
0.193
0.161
0.343
0.305
0.172
0.052
5.38
5.51
7.0
5.65
6.48
6.65
20.7
20.0
24.5
31.9
16.9
15.6
8.19
8.13
8.04
8.09
8.08
7.90
9
22
13
12
10
6
33
48
21
24
50
120
0.009
0.007
0.014
0.01
0.01
0.004
Main stem of the river Avon , flowing from
North to South
Netheravon
Amesbury
Stratford Sub Castle
u/s Salisbury STW
d/s Salisbury STW
Longford
0.249
0.257
0.223
0.174
0.591
0.232
5.89
5.50
5.45
5.65
5.85
5.39
23.7
20.3
19.2
18.9
23.5
20.1
8.05
8.03
8.16
8.02
7.93
8.07
11
9
12
16
16
11
27
25
29
39
16
32
0.012
0.013
0.011
0.009
0.023
0.011
13
Echilhampton Brook
W
.A
n
v
R. A
R. Wy
River Sem
N
lye
r
R. Nadde
le
R. Ebb
Sweatsford
Water
Allen River
R. Avon
Hays
Stream
R.
on
Upper Wylye u/s
of Warminster
East Knoyle
Stream
E. von
A
vo
Bo
urn
e
Upper Wylye d/s
of Warminster
Netheravon
Amesbury
Stratford Sub
Castle
Upstream of Salisbury
STW
Downstream of Salisbury
STW
Longford
Huckles Brook
Dockens Water
10 km
Linford Brook
Figure 5
Figure 5. Map of the Avon catchment showing headwater streams (used in Table 2 to
illustrate typological classification of stream source types), and sampling sites on the
major tributaries and on main River Avon.
and ratio of TRP to Cl (TRP:Cl) used to assess changes in TRP concentrations in
relation to a sewage signal. Average TRP:Cl ratios in effluent are 0.08, compared
with 0.0025 in groundwater. Four types of typology have been distinguished. These
are 


Type I: Permeable (Chalk/Greensand) headwater catchments with no major point
source inputs. These streams include the Upper Wylye (upstream of Warminster),
the Allan River and Sweatsford Water and they are characterized by low TRP (<
0.1 mg-P l-1) and low Cl (<20 mg l-1). These sites have high N:P ratios (>70) and
low TRP:Cl ratios (<0.007).
Type II: Permeable (Chalk/Greensand) headwater catchments which are sewage
impacted. These streams include Echilhampton Water in the Upper West Avon
catchment and the Upper Wylye, downstream of Warminster STW and are
characterized by much higher TRP and Cl concentrations (>0.3 mg-P l-1 and >25
mg-Cl l-1, respectively). These sites have much lower N:P ratios (<40) and higher
TRP:Cl ratios (>0.01), compared with the Type I permeable catchments which
have major no point source influence.
Type III: Impermeable (Clay) catchments which are sewage impacted. These
streams include the River Sem, East Knoyle Stream, Hays Stream in the Upper
Nadder valley, draining the Kimmeridge Clay. These streams also have high TRP
and Cl concentrations (>0.3 mg-P l-1 and >25 mg-Cl l-1), with TRP:Cl ratios
14

>0.008. However, suspended sediment concentrations are higher in the clay
catchments (>25mg-SS l-1) as a result of greater near-surface runoff and catchment
erosion.
Type IV: Moorland catchments draining acidic sandstones of the New Forest
These streams include Linford Brook, Dockens Water, Huckles Brook and are
characterized by low TRP (<0.04 mg-P l-1) and NO3 (<1 mg-N l-1) concentrations
and low pH (<7.3).
The water quality of the major tributaries and the main stem of the River Avon are
linked to the relative contributions of these source types, the impact of further direct
effluent inputs to the main channel and in-stream processing. With the exception of
the New Forest streams, NO3 concentrations remain relatively constant across the
Avon rivers, with TRP in the tributaries and main stem showing reductions in
concentration linked to dilution downstream of STWs and at large catchment scales.
If the reductions in TRP concentrations were simply attributable to hydrological
dilution, then ratios of TRP:Cl would remain constant as TRP concentration declined.
In contrast, TRP:Cl ratios decline at larger catchment scales and on passage
downstream from a STW. For example, on the main River Avon TRP:Cl declines
from 0.012 at Nether Avon to 0.009 just upstream of Salisbury STW. The decline in
TRP:Cl indicates that TRP is being removed from the water column, which probably
results from in-stream uptake of TRP by river sediments and/or aquatic plants.
4.3 Quantifying TRP and Nitrate loads in the Avon
Total Reactive Phosphorus
Mean annual flow-weighted TRP concentrations are lowest on the Nadder and Bourne
(0.13 and 0.14 mg-P l-1, respectively) and highest on the upper Wylye at Norton
Bavant downstream of Warminster STW (0.41 mg-P l-1) and East Avon (0.33 mg-P l1
) (Table 3). The full variability in annual TRP flow-weighted means across the Avon
gauging sites is 81% (from 0.1 to 0.54 mg-P l-1). Mean annual TRP loads per hectare
show similar pattern to the flow-weighted mean concentrations, with lowest mean
annual loads of 0.15 kg-P ha-1 a-1 on the Bourne and highest annual mean annual loads
on the upper Wylye at Norton Bavant (1.4 kg-P ha-1 a-1) and on the East Avon (2.5
kg-P ha-1 a-1). Mean annual river mass loads are driven by the combined effects of
concentration and river flow, so the highest mean annual loads are found at the
catchment outlet (Knapp Mill; 121 t-P a-1) and are lowest in the Bourne (3.8 t-P a-1).
Effluent loads ranged from 1 t-P a-1 discharging to the East Avon and 145 t-P a-1
discharging to the catchment outlet at Knapp Mill. The highest load contributions
came from Warminster STW (14 t-P a-1), Christchurch STW (45 t-P a-1) and Salisbury
STW (47 t-P a-1). Downstream of Warminster STW on the upper River Wylye at
Norton Bavant, effluent loads accounted for 92% of the mean annual river load. In
contrast, in the main River Avon the downstream of Salisbury, the cumulative input
from effluent exceeds the in-stream mean annual TRP load. This indicates that instream processes provided significant removal of TRP from the water column, as
indicated by the downstream reductions in TRP:Cl ratios described above.
15
Table 3. Means and ranges in annual flow-weighted mean concentrations and annual loads (1993-2000); mean annual loads per ha and effluent
loads to the Avon rivers.
Mean
annual
flowweighted
mean
(mg l-1)
NADDER AT WILTON
Mean
annual
load
(t a-1)
Minimum
annual
load (t a-1)
Maximum
annual load
(t a-1)
Catchment
Area (ha)
Areaweighted
mean
annual
load
(kg ha-1 a-1)
Effluent
load (t a-1)
Effluent
load as %
of river
load
0.10
0.28
0.21
0.54
13.5
15.4
9.3
13.0
18.3
17.9
22100
11200
0.61
1.38
2.9
14.2
22
92
0.15
0.33
0.11
0.29
0.22
0.42
20.6
9.1
15.1
6.8
27.4
13.5
44500
3600
0.46
2.53
16.4
3.4
80
38
0.26
0.14
0.21
0.11
0.32
0.21
6.3
3.8
2.9
2.4
11.0
6.7
7600
26400
0.83
0.15
1.0
6.5
16
169
AVON AT EAST MILLS
0.21
0.22
0.18
0.18
0.32
0.32
25.3
107
15.9
92.6
38.7
125
32400
147800
0.78
0.72
16.3
145
64
136
AVON AT KNAPP MILL
0.19
0.14
0.28
121
97.3
138
170600
0.71
145
120
NADDER AT WILTON
5.76
7.16
5.12
6.51
6.06
7.70
649
233
355
144
995
343
22100
11200
29.4
20.8
10.9
42.2
2
18
5.88
6.94
5.55
5.29
6.30
8.79
837
201
367
118
1322
290
44500
3600
18.8
55.9
47.7
7.6
6
4
5.31
5.94
4.37
5.62
5.65
6.20
173
202
47
77
274
359
7600
26400
22.8
7.67
3.9
21.4
2
11
7.03
6.12
6.64
5.10
8.03
6.55
735
2903
285
1530
1130
3565
32400
147800
22.7
19.6
42.4
392
6
14
5.78
5.41
6.09
3503
1719
4761
170600
20.5
392
11
WYLYE AT S. NEWTON
EAST AVON AT UPAVON
WEST AVON AT UPAVON
BOURNE AT LAVERSTOCK
AVON AT AMESBURY
WYLYE AT NORTON BAVANT
WYLYE AT S. NEWTON
NITRATE
(AS N)
Maximum
annual flowweighted
mean
(mg l-1)
0.13
0.41
WYLYE AT NORTON BAVANT
TOTAL
REACTIVE
PHOSPHORUS
(AS P)
Minimum
annual
Flowweighted
mean
(mg l-1)
EAST AVON AT UPAVON
WEST AVON AT UPAVON
BOURNE AT LAVERSTOCK
AVON AT AMESBURY
AVON AT EAST MILLS
GSTN 43021: AVON AT KNAPP
MILL
16
Nitrate
Mean annual flow-weighted NO3 concentrations range from 5.31 mg-N l-1 on the
West Avon to 7.16 mg-N l-1 on the Upper Wylye at Norton Bavant. The full
variability in annual NO3 flow-weighted means across the Avon gauging sites is 50%
(from 4.37 to 8.79 mg-N l-1), which is considerably lower than the variability in TRP
flow-weighted means, indicating more consistent inputs of NO3 to the Avon rivers.
Mean annual nitrate loads per hectare range from 7.7 kg-N ha-1 a-1 (the Bourne) to 56
kg-N ha-1 a-1 (East Avon). Mean annual loads range from 173 t-N a-1 for the West
Avon to 3503 t-N a-1 at Knapp Mill. Effluent inputs account for a very small
percentage of riverine nitrate loads compared with TRP loads, with effluents
accounting for 2% of the river NO3 loads for the Nadder and West Avon and 18% of
river NO3 loads for the upper Wylye at Norton Bavant.
5. Discussion
Four catchment typologies have been identified in relation to nutrient water quality
for the Avon. The major separations in water quality are linked to geology and to the
relative impacts of point source sewage effluents. River flow in the Avon is
groundwater-dominated and groundwater chemistry therefore has a major influence
on the Avon water quality, particularly in relation to NO3 and Ca concentrations: i.e.
groundwater is the major source of NO3 and Ca in the Avon rivers. However, TRP in
groundwater is generally low, with median TRP concentrations of 0.02 mg-P l-1 and
over 60% of groundwater samples exhibiting TRP concentrations less than 0.05 mg-P
l-1. Elevated concentrations of >0.1 mg-P l-1 are restricted to a few boreholes in the
upper Nadder catchment that extract groundwater from the upper Greensand. The
Chalk boreholes have consistently low TRP concentrations, typical 0.02 to 0.03 mg-P
l-1, despite intensive agriculture with high rates of P fertilizer applications as well as
septic tanks and soakaways discharging to the groundwater. This suggests that the
soil and groundwater system act as a major buffer for phosphorus, removing TRP
from water draining through the soil profile and into the groundwater. SIcalcite values
are close to equilibrium in the groundwater. This indicates that precipitation of CaCO3
is occurring within the groundwater. Further, the link between low TRP
concentrations and SIcalcite close to equilibrium indicates that co-precipitation of
CaCO3 and P may explain TRP retention and buffering within the groundwater.
Higher TRP concentrations in the upper Nadder boreholes may be linked to lower
water residence times resulting from preferential fissure flow and bypassing of matrix
P-uptake buffer mechanism. NO3 is much more soluble than TRP and does not
undergo co-precipitation interactions with CaCO3. Given the high inputs of NO3 from
agriculture and the low chemical reactivity, elevated concentrations of NO3 in
groundwater, compared with TRP, would be expected.
Within the rivers, the major source of TRP is sewage effluent, as demonstrated by (i)
dilution of TRP concentrations with flow, linked to point source dilution and (ii)
effluent accounts for the vast majority of the TRP load generally within the Avon and
its tributaries. In contrast, NO3 shows increasing concentrations with flow. This may
result from one or a combination of two processes: (1) mobilization of near-surface
soil waters under higher flow conditions and a strong ‘diffuse source signal’ and (2)
biological uptake of NO3 during the spring and summer low-flow periods (Neal et al.,
2004a).
17
The critical times of ecological risk within these Chalk streams are at baseflow during
the spring and summer periods when rivers are subject to eutrophication and problems
of excessive growth of epiphytic algae smothering the Ranunculus macrophyte
vegetation occur (Jarvie et al., 2004a). This is the time when plant uptake is at its
highest. Here, we show that at these ecologically sensitive periods the overwhelming
source of TRP to the rivers (acknowledged to be the limiting nutrient for aquatic plant
growth), is sewage effluent and that uptake of P by the plants does not overcome the
lack of point source dilution in this case – in other cases biological uptake can be very
high indeed under the low flow conditions (Neal et al., 2005).
The rivers of the Avon catchment also act as net sinks for TRP, as demonstrated by
the reductions in TRP:Cl ratios downstream of STWs and also observations that more
TRP enters the rivers from STWs than is transported out of the catchment on an
annual basis. This may well be a common occurrence for lowland Chalk aquifer
dominated systems (Neal et al., 2004a,b; 2005b). Total Reactive Phosphorus may be
removed from river water by uptake into aquatic plants and sorption to river
sediments which are retained on the river bed. Recent work by Jarvie et al (2005) has
indicated that bed sediments act as a net sink for SRP in most of the rivers sampled
within the Avon, particularly those that are sewage-impacted. In these rivers, the SRP
concentration of the river water exceeds the equilibrium P concentration (EPC0) of the
bed sediments, resulting in potential for net uptake of SRP by the river sediments.
The results by Jarvie et al (2005) also suggested that, in subcatchments which were
not impacted by STWs (and where SRP concentrations were typically <0.05 mg-P l-1),
SRP<EPC0, resulting in potential for net release of SRP from the bed sediments to the
water column. The implication of this may be that reductions in SRP concentrations
in river waters (by removing or reducing point-source inputs), may result in bed
sediments switching from sinks to sources of SRP/TRP. However, further detailed
monitoring is required to confirm whether this effect might occur within the rivers.
6. Conclusions
This study shows that there are four water quality typologies for the Chalk
groundwater-dominated Avon subject to ecological deterioration linked to
eutrophication and ‘Chalk Stream Malaise’. These typologies link to differences in
aquifer characteristics, geology and to point source inputs of nutrients. The
hydrochemical behaviour of N and P are different within the Avon.
For N (mainly present as NO3), the groundwater is enriched due to the high fertilizer
inputs coupled with its high solubility and the aquifer being ineffective at removing it
from the groundwater. Due to this, the aquifer provides an important contribution of
nitrate loading in rivers. As groundwater residence times are high (years to decades
and longer) there can be considerable delay between times of major fertilizer
application to the catchment and runoff in the river (Burt et al., 1993). Under high
flow conditions NO3 concentrations increase and this is relates to the input of NO3
from diffuse surface/near-surface agricultural sources and from NO3 removal by
biological processes under baseflow conditions.
For P, the hydrochemical processes determining TRP concentrations in the ground
and river waters are much more complex than for N. Low concentrations of TRP were
18
found in the Chalk groundwater, demonstrating that groundwater from this source is
not a major source of phosphorus to the Avon. There are high P inputs from
fertilizers and manures, as a result of intensive arable cultivation, as well as septic
tanks and soakaways. Thus, the groundwater system is currently acting as an efficient
buffer, removing TRP from water draining from the catchment surface into the
aquifer. Thermodynamic analysis indicates that CaCO3-P co-precipitation may be
occurring within the aquifer. This observation is consistent with data for the upper
Thames valley (Neal et al., 2002) and it is probably a general feature of the Chalk.
Groundwaters from the upper Nadder catchment, sampled from non-Chalk aquifers
(upper Greensand), exhibited much higher TRP concentrations than the Chalk
groundwaters. This probably results from the lack of calcite nucleation sites (which
are abundant in the Chalk), but fracture flow by-passing of the aquifer matrix may
also be a factor. As with N, there are issues of long water residence times within the
aquifer and long delays before fertilizer inputs are translated to river outputs.
However, the ability for the catchment to remove phosphorus may well be
longstanding owing to the high levels of calcite within the aquifer both in terms of
gravimetric content and number of nucleation centres for precipitation (Neal et al.,
2002). Within the rivers, there is no clear evidence for CaCO3-P co-precipitation as a
mechanism for removal of TRP within the water column. This is consistent with
earlier studies for UK rivers (House and Denison, 1997; Neal, 2001a; Neal et al.,
2002): it probably reflects (a) the lack of sufficient calcite nucleation centres within
the river water column and (b) high concentrations of TRP and other components such
as dissolved organic carbon within river waters which inhibit calcite nucleation and
precipitation (House, 1987, 1990; Neal, 2001a). Nonetheless, there may be some
calcite precipitation with associated loss of P from the water at algal biofilm surfaces
(Hartley et al., 1997; Jarvie et al, 2002a).
Sewage effluent is the major source of TRP to the Avon and its tributaries and its
influence on TRP concentrations are highest under baseflow conditions, when there is
minimal input from near-surface runoff. At the times of baseflow, which occur mainly
during the times of highest biological activity, there is the most risk of eutrophication
and nuisance algal growth. Phosphorus is the key limiting nutrient for freshwater
aquatic plant growth in the Avon and therefore a first step in controlling
eutrophication is to target sources of phosphorus at these times.
With regards to particulate P, diffuse-source controls to reduce erosion losses may
have an important role in reducing sediment and sediment-associated P inputs to
rivers. However, these diffuse sources contribute to P loads under high-flow
conditions, particularly during the winter, at ecologically insensitive periods.
Although some of the diffuse-source sediment will be retained within the river
channels, recent work on the P-sorption capacity of surface bed sediments (Jarvie et
al., 2004b), showed that most of the bed sediments sampled in the Avon catchment
had potential for net dissolved P uptake, particularly at sites impacted by STW
effluent discharges. At sites which are upstream of the influence of point-source
discharges, the low TRP concentrations from groundwater may mean that bed
sediments could be sources rather than sinks of TRP. However, internal loadings of P
are marginal in comparison with sewage effluent sources at the catchment scale.
19
7. Wider Comment
The relative impacts of point and diffuse sources of P on the aquatic health of lowland
UK Rivers is of key importance to environmental management decision-making with
regards to P removal from STWs, agricultural sustainability and amenity value.
Within the research, there remains the need to provide a detailed assessment of P
sources, flux transfers and within-catchment and within-river sources and sinks that
can be incorporated within environmental models for research and environmental
management purposes. Here, a missing component is an assessment of flux transfers
to the river and biological impacts within the river of STW effluents and septic tank
sources which discharge to the unsaturated zone near to the river. New work is
required to characterize the P load contributions from bed sediments at sites upstream
of point source influences and to examine whether river bed sediments switch from
sinks to sources of dissolved P if SRP concentration were reduced significantly by
widespread point-source controls. And, finally, there is a need to link changes in
pollutant loadings to biological response and this relates to processes such as feedback
mechanisms and aquatic ecosystem functioning. All of these issues are central to the
goals of the Water Framework Directive in terms of
(1) Establishing Reference Sites for defining Good Ecological Status in lowland
Chalk catchments.
(2) Assessing the implications of bed sediment controls on baseflow P concentrations
at times of ecological risk, in relation to diffuse and point source inputs.
(3) Determining the relative importance of point and diffuse sources to declines in
the health of river ecology.
(4) Establishing achievable and ecologically-relevant target P concentrations in river
water.
8. Acknowledgements
This work was carried out with funding from the UK Department for Environment,
Food and Rural Affairs (Defra), the Environment Agency and English Nature (project
PE0202: Development of a Risk Assessment and Decision-Making Tool to control
Diffuse Loads of Phosphorus and Particulates from Agricultural Land). The views
expressed within this paper are those of the authors and are not necessarily those of
Defra, the Environment Agency or English Nature. The authors also wish to thank
Rachel Anning and Emily Orr at the Environment Agency’s National Data Centre for
supplying the water quality data used in this study.
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