Appendix 6 - Nutrient Hydrochemistry for a Groundwater-Dominated Catchment: the Hampshire

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 1Centre for Ecology and Hydrology, Wallingford, 2ADAS, Management Group, ADAS Gleadthorpe, 3Environment 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 non- point 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 Chalk- stream 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).

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 spring- fed 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 3- analysis is not a ‘true’ measurement of orthophosphate ions (PO4 ). 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 -1 presented in the EA database has units of mg-CaCO3 l , 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 -1 requires values in mg-CaCO3 l to be multiplied by 20.

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

4 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 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

5 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 Alkalinity EpCO2 SICaCO3 TRP NO3 Cl NH3 SS (mgl-1) (μE l-1) (x atm (mg-Pl-1) (mg-N l-1) (mgl-1) (mgl-1) (mgl-1) press) Groundwater Mean 7.3 107 3938 39 0.18 0.05 5.33 20 0.02 NA Median 7.3 106 3918 34 0.19 0.02 5.70 17 0.01 NA Max 7.6 214 5328 169 0.70 0.97 38.9 156 3.52 NA Min 6.7 4.4 2295 9.8 -0.91 <0.02 0.001 4 0.01 NA No samples 274 274 274 274 274 9665 10673 9812 10709 NA

Rivers Mean 8.0 105 3920 6.2 0.76 0.23 5.49 23 0.02 13 Median 8.1 111 4320 5.4 1.06 0.18 5.56 21 0.05 8 Max 8.8 174 5880 119 1.73 9.5 28.1 258 20.5 2070 Min 4.5 3.4 200 0.4 -4.28 <0.02 0.07 0 0.002 1.8 No samples 4680 4680 4680 4680 4680 16652 14510 12061 16615 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.

TRP NO3 Cl NH3 SS (mg-Pl-1) (mg-N l-1) (mgl-1) (mgl-1) (mgl-1) Sewage Mean 6.1 17.2 76 2.0 15 Effluent Median 5.8 15.5 68 1.2 13 (Water Max 64 67 432 49 2850 Company Min 0.06 0.2 19 0.02 2 STW) No samples 3659 3501 3376 4685 4509

Sewage Mean 8.1 13.9 74 6.5 43 Effluent Median 7 11.2 57 2.1 23 (Private STW) Max 148 63.4 1173 55.9 1376 Min <0.02 0.001 10 0.02 2 No samples 296 784 774 383 376

Trade Effluent Mean 4.0 8.3 153 13.2 24.3 Median 0.5 4.7 65 1.64 9.4 Max 22.5 79.6 2845 279 354 Min <0.02 0 6.9 0.003 2 No samples 544 435 544 634 610

Fish Farm Mean 0.2 5.57 20 0.24 8.3 effluent Median 0.16 5.56 19 0.18 5.5 Max 6.6 129 1070 135 224 Min <0.02 0.09 9.0 0.01 1.2 No samples 2502 2264 2237 3252 3576

TRP = Total Reactive Phosphorus SS = Suspended Sediment

8 Figure 2a Figure 2b

Figure 2c

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

9 water TRP concentrations show a well-defined reduction in concentration with flow, indicating the influence of point-source dilution (Figure 4a).

1.5 1 0.5

0 CaCO3 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 -0.5 I

S -1 -1.5 -2 groundwater river water -2.5 -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 EAST MILLS EAST AVON AT UPAVON RIVER AVON AT AMESBURY 5 . 0 5 . 5 . 0 4 0 . 0 3 3 . . 3 . 0 0 0 2 . 0 ) 1 1 . . 1 1 0 . 0 - l 0

- 10 20 30 40 50 60 1 2 3 4 0 5 10 15 20 g m

( WEST AVON AT UPAVON RIVER AVON AT KNAPP MILL RIVER WYLYE AT NORTON BAVANT

n 8 . 5 4 o 0 . . i 1 0 t a 6 . r 3 0 . t 0 . 0 n 1 4 e . 2 0 . c 0 5 n . 0 2 o . 1 0 . c 0 P R

T 0 1 2 3 4 5 10 20 30 40 50 60 1 2 3 4 5 6 RIVER BOURNE AT LAVERSTOCK RIVER NADDER AT WILTON RIVER WYLYE AT S. NEWTON 4 . 0 5 4 . 2 . 0 0 3 . 3 0 . 0 5 1 . 2 2 . 0 . 0 0 5 1 1 . . 0 . 0 0 0

0 2 4 6 5 10 15 0 5 10 15 20 25 Flow (m3s-1) Figure 4a

11 RIVER AVON AT EAST MILLS EAST AVON AT UPAVON RIVER AVON AT AMESBURY 9 9 8 8 6 7 7 6 5 6 ) 1 5 - l 5

4 4 N 4 - g

m 10 20 30 40 50 60 1 2 3 4 0 5 10 15 20 (

n WEST AVON AT UPAVON RIVER AVON AT KNAPP MILL RIVER WYLYE AT NORTON BAVANT o i t 4 a 1 r 7 t 2 1 6 n 0 e 1 c 6 8 n 4 o 6 c 5

4 2 e t 2 a 4 r t i

N 0 1 2 3 4 5 10 20 30 40 50 60 1 2 3 4 5 6 BOURNE AT LAVERSTOCK RIVER NADDER AT WILTON RIVER WYLYE AT S. NEWTON 0 . 9 8 7 8 7 0 . 6 7 6 6 0 . 5 5 5 4 0 4 . 4 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 TRP NO3 Cl pH SS N:P SRP:Cl (mg-P l-1) (mg-N l-1) (mg l-1) (mg l-1) Type I: Permeable headwaters with no Upper Wylye (u/s of Warminster) 0.085 5.69 15.1 8.09 10 74 0.006 point major STW influence Allen River 0.036 6.40 16.2 7.81 7 216 0.002 Sweatsford Water 0.037 9.95 18.0 7.80 8 268 0.002

Type II: Permeable headwaters; sewage Upper West Avon (Echilhampton Brook) 0.337 5.99 42.6 7.96 14 31 0.011 impacted Upper Wylye (d/s of Warminster) 0.712 6.55 33.0 8.0 13 13 0.022

Type III: Impermeable (clay) headwaters; Sem 0.212 3.22 27.3 7.8 23 18 0.008 sewage impacted East Knoyle Stream 1.704 10.5 38.9 7.79 25 8 0.039 Hays Stream 0.314 4.45 38.1 7.67 23 22.6 0.008

Type IV: Moorland catchments draining Linford Brook 0.023 0.55 27.8 7.27 6 21 0.001 acidic sandstones of the New Forest Dockens Water 0.024 0.33 19.6 7.09 7 14 0.002 Plateau Huckles Brook 0.039 0.91 21.0 7.11 9 16 0.002

Major tributaries sampled close to Wylye 0.193 5.38 20.7 8.19 9 33 0.009 confluence with River Avon Nadder 0.161 5.51 20.0 8.13 22 48 0.007 E Avon 0.343 7.0 24.5 8.04 13 21 0.014 W Avon 0.305 5.65 31.9 8.09 12 24 0.01 Bourne 0.172 6.48 16.9 8.08 10 50 0.01 Ebble 0.052 6.65 15.6 7.90 6 120 0.004

Main stem of the river Avon , flowing from Netheravon 0.249 5.89 23.7 8.05 11 27 0.012 North to South Amesbury 0.257 5.50 20.3 8.03 9 25 0.013 Stratford Sub Castle 0.223 5.45 19.2 8.16 12 29 0.011 u/s Salisbury STW 0.174 5.65 18.9 8.02 16 39 0.009 d/s Salisbury STW 0.591 5.85 23.5 7.93 16 16 0.023 Longford 0.232 5.39 20.1 8.07 11 32 0.011

13 Echilhampton Brook

Upper Wylye d/s of Warminster Netheravon Upper Wylye u/s of Warminster Amesbury

Stratford Sub East Knoyle CastleUpstream of Salisbury Stream STW R Downstream of Salisbury .

Hays A STW

v Longford

Stream o

Sweatsford n River Sem Water Huckles Brook Allen River N 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 -1 -1 characterized by low TRP (<0.04 mg-P l ) and NO3 (<1 mg-N l ) 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 l- 1) (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 in- stream 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.

16 Nitrate -1 Mean annual flow-weighted NO3 concentrations range from 5.31 mg-N l on the

Mean Minim Maximu Mean Minimu Maximu Catchme Area- Effluen Effluen annual um m annual m m nt Area weighte t load t load flow- annual annual load annual annual (ha) d mean (t a-1) as % of weight Flow- flow- (t a-1) load (t a- load (t a- annual river ed weighte weighted 1) 1) load load mean d mean (kg ha-1 (mg l-1) mean (mg l-1) a-1) (mg l-1)

NADDER AT WILTON 0.13 0.10 0.21 13.5 9.3 18.3 22100 0.61 2.9 22 WYLYE AT NORTON 0.41 0.28 0.54 15.4 13.0 17.9 11200 1.38 14.2 92 BAVANT WYLYE AT S. NEWTON TOTAL 0.15 0.11 0.22 20.6 15.1 27.4 44500 0.46 16.4 80 REACTIVE EAST AVON AT 0.33 0.29 0.42 9.1 6.8 13.5 3600 2.53 3.4 38 PHOSPHOR UPAVON US (AS P) WEST AVON AT 0.26 0.21 0.32 6.3 2.9 11.0 7600 0.83 1.0 16 UPAVON BOURNE AT 0.14 0.11 0.21 3.8 2.4 6.7 26400 0.15 6.5 169 LAVERSTOCK AVON AT AMESBURY 0.21 0.18 0.32 25.3 15.9 38.7 32400 0.78 16.3 64 AVON AT EAST MILLS 0.22 0.18 0.32 107 92.6 125 147800 0.72 145 136 AVON AT KNAPP MILL 0.19 0.14 0.28 121 97.3 138 170600 0.71 145 120

NADDER AT WILTON 5.76 5.12 6.06 649 355 995 22100 29.4 10.9 2 WYLYE AT NORTON 7.16 6.51 7.70 233 144 343 11200 20.8 42.2 18 BAVANT WYLYE AT S. NEWTON 5.88 5.55 6.30 837 367 1322 44500 18.8 47.7 6 NITRATE EAST AVON AT 6.94 5.29 8.79 201 118 290 3600 55.9 7.6 4 (AS N) UPAVON WEST AVON AT 5.31 4.37 5.65 173 47 274 7600 22.8 3.9 2 UPAVON BOURNE AT 5.94 5.62 6.20 202 77 359 26400 7.67 21.4 11 LAVERSTOCK AVON AT AMESBURY 7.03 6.64 8.03 735 285 1130 32400 22.7 42.4 6 AVON AT EAST MILLS 6.12 5.10 6.55 2903 1530 3565 147800 19.6 392 14 GSTN 43021: AVON AT 5.78 5.41 6.09 3503 1719 4761 170600 20.5 392 11 KNAPP MILL 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.

17 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).

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

18 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

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 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

19 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.

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.

20 (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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