Upward trend in soluble phosphorus loadings to Lough Neagh despite phosphorus reduction at sewage treatment works

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War. Res. Vol. 29, No. 4, pp. 1051-1063, 1995 Elsevier ScienceLtd. Printed in Great Britain

Pergamon

0043-1354(94)00259-2

U P W A R D T R E N D IN SOLUBLE PHOSPHORUS LOADINGS TO LOUGH NEAGH DESPITE PHOSPHORUS REDUCTION AT SEWAGE TREATMENT WORKS R. H. F O Y l*, R. V. S M I T H l, C. J O R D A N I and S. D. L E N N O X 2 ~Aquatic Sciences Research Division and 2Biometrics Division, Department of Agriculture, Newforge Lane, Belfast BT9 5PX, Northern Ireland (First receh:ed April 1994; accepted in revised form September 1994) Abstract--After allowing for changes in point source phosphorus (P) loadings from sewage treatment works (STWs) and creameries, an upward trend in the soluble reactive phosphorus (SRP) loadings in the six major rivers draining into Lough Neagh was established over the period 1974-1991. Urban SRP loadings were reduced by a P reduction programme at nine major STWs, which commenced in 1981, while the loading from other STWs was considered to have been stable as population growth was balanced by reduced P per capita loadings reflecting reductions in the P content of domestic detergents. Post 1987, creamery loadings were reduced following closures and a switch away from phosphoric acid as a cleaning agent. STWs were subjected to P reduction in four river catchments but reductions in SRP loads of these rivers were only comparable to the reduction in STWs loadings of SRP for a period of up to four years. Of the two remaining rivers, one experienced no major change in point source P loadings from 1974 to 1991, over which time a steady increase in SRP loadings occurred, while a similar SRP loading increase was also detected in the remaining river over a 12 year period when point sources were quite constant. In all river catchments, long-term changes in loadings or the short-term response to point source reductions could only be made after taking into account the variation in river SRP loads due to annual flow variations. The rates at which SRP loadings were increasing was in the range 1.30 to 1.45 kg P kin-2 yr i. Key words--phosphorus, lakes, eutrophication, agriculture, tertiary treatment, reduction, dairy industry, waste waters, sewage works

INTRODUCTION Urban population growth combined with the introduction of phosphorus (P) based detergents has increased P loadings to waterways. The control or reversal of the resulting lake eutrophication has typically been undertaken by reducing urban P loadings, through the installation of tertiary treatment at sewage treatment works (STWs) and/or limiting the P content of detergents (Sas, 1989). Effective control of eutrophication by restricting P inputs from agricultural sources has been rarely reported and neither is there a consensus as to how P loadings from agriculture are changing with time. At nine major population centres in the catchment area of Lough Neagh, a large eutrophic lake in North East Ireland, tertiary treatment of sewage began in 1981, employing alum/ferric salts to remove P (Gray, 1984). Under average flow conditions, the 1979 loading of soluble reactive phosphorus (SRP) to Lough Neagh was estimated by Foy et al. (1982) to be 256 tonnes P year -1, of which 54% originated from STWs, 6% from creameries with the remaining 40% attributed to rural background sources. The STWs within the P *Author to whom all correspondence should be addressed,

reduction programme contributed 31% of the SRP load. Phosphorus concentrations in the major rivers flowing into Lough Neagh have been measured by a consistent analytical methodology from 1974 to 1991 and river P loadings calculated, creating a data-set which can be used not only to assess the impact of urban P reduction on river P loadings but also to determine temporal trends in the rural or background P loadings. This paper examines how the SRP fiver loads have responded to P reduction from point sources. The emphasis on the SRP fraction of total phosphorus (TP) is because SRP is readily available for algal growth in contrast to the soluble unreactive and particulate phosphorus fractions (Jordan and Dinsmore, 1985). STUDY AREA The locations of the towns included in the P reduction programme are shown in Fig. 1, together with the dates of commencement of P reduction and their 1981 populations. The six major rivers delineated in Fig. 1 drain 88% of the Lough Neagh catchment area of 4453 km 2. The remaining area is referred to as direct drainage which, although only accounting for 12% of the Lough catchment area,

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Fig. 1. Lough Neagh catchment showing major rivers and towns ( I ) within P reduction programme. Values refer to date of commencement of P reduction and 1981 urban populations. Sewage from Muckamore diverted to Antrim 199l.

contained 26% of the total urban population of 222.4k in 1981 and 36% of the urban population covered by the P reduction programme; namely the STWs serving Antrim, Craigavon and Lurgan. Two of the six rivers, Ballinderry and Six Mile Water, were not directly affected by the P reduction programme as none of the nine population centres were in these catchments. However, the Six Mile Water did undergo changes in P loading from STWs as, prior to 1976, Antrim STW discharged directly into it. In 1991, sewage from Muckamore STW, which formerly discharged into the Six Mile Water, was diverted to Antrim STW. Over the period of study, land use throughout the Lough catchment remained quite constant with 85-87% of the area devoted to grass and rough grazing and only 7 - 9 % under arable cultivation. Further details of the Lough catchment land use can be found in Smith (1977) and Foy e t al. (1982).

METHODS River loadings

Annual loadings of SRP were calculated for each of the six rivers shown in Fig. 1 using log load vs log flow relationships which were based upon weekly samples taken at locations close to the river entry points to Lough Neagh (Smith, 1977; Smith and Stewart 1977). The errors in estimating annual SRP loadings associated with the log load vs log flow method and the weekly sampling have previously been assessed by comparing estimated loadings with loadings based on continuous monitoring of SRP concentrations and river flows (Stevens and Smith, 1978; Stevens and Stewart, 1981). The comparisons showed that estimated loadings were within 10% of measured SRP loadings. SRP was determined in 0.45 #m filtered samples by the method of Murphy and Riley (1962). As the six rivers sampled drain only 88% of the Lough Neagh catchment, an estimate of the SRP loading to the Lough from the entire catchment was made by uprating the combined SRP loadings of the six major rivers to take into account not only the extra area involved but also the higher urban population density of the direct drainage area.

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Lough Neagh phosphorus reduction P removal at S T W s

Loadings of TP from STWs were calculated from daily time based composite TP concentrations analysed by the operator of the STWs. Flow data were available for STWs at Antrim, Ballymena and Magherafelt, but only from the remaining STWs after 1989. W h e n flows were not available, the reduction in P loading from a STW was estimated by reference to the mean TP concentration measured in 1979/80 at the STW prior to P reduction; so that after P reduction, if TP concentration had decreased by 50% from 1979 values, TP loading was assumed to have also decreased by 50%. The 1979/80 reference TP loadings were determined by Gray (1984). Annual reductions in P loadings from STWs were only available as TP loadings which were converted to reductions in SRP loadings using a conversion factor of 0.728 x TP reduction. This relationship was derived from operational details of STWs serving Antrim, Magherafelt and Ballymena before and after P reduction given by Storey (1990) and takes into account the proportionally greater reduction of the SRP loading compared to TP loading (89 vs 81%) as a result of P reduction. Prior to P reduction, the SRP fraction formed 64.8% of TP in treated effluent, but after P reduction, SRP accounted for 38.5% of TP. In 1974 and 1975, when Antrim STW discharged directly into the Six Mile Water, the SRP loading from Antrim STW was calculated from the product of the town population and the P per capitas (see below) for each year, giving values of 7,0 tonnes P in 1975 and 6.0 tonnes P in 1974. In the same river catchment, the 1991 diversion of M u c k a m o r e STW to Antrim STW was estimated to have reduced the Six Mile Water SRP load by 1.6 tonnes P year-~. Creameries

In 1979, SRP loading from creameries was estimated from a multiple regression analysis to have been 16 tonnes P year-i (Foy et al., 1982). A 1986 questionnaire survey found that the consumption of P in cleaning agents in creameries was 13.6 tonnes P year -~, of which 57.5% was phosphoric acid and 12.9% phosphoric acid derivatives. Due to creamery closure and a switch from phosphoric acid as a cleaning agent, the P loading from creameries decreased in two river catchments. In the Blackwater catchment, the annual reductions were 0.8, 8.9, 8.9 and 9.3 tonnes P yr -~ from 1988 to 1991 respectively and, in the Six Mile Water, 1.2 tonnes P yr -~ in 1988 and 1.6 tonnes P yr -~ from 1989 to 1991. As these reductions were primarily due to the lower usage of phosphoric acid and make no allowance for reductions in P

discharged from waste milk or milk by-products, they were assumed to represent a reduction in SRP loading. Other phosphorus sources

The SRP loadings from towns not included in the P reduction programme were calculated as the product of their urban populations and annual SRP per capita values given in Table I. The per capita values are based on three components: a constant dietary TP contribution of 0.44 kg P person -~ yr-L; a variable fabric detergent P component based upon the annual U K average value sourced from the Soap and Detergent Industry Association and Patrick (1983) and a variable P component from dishwasher cleaning agents. The calculation of the latter is based on extrapolation of the following data: dishwasher product consumption rate in 1989 of 0.99 k g y r i (Clarke et al., 1992); a product P content of 7% which is based on product labels listing phosphate contents in the range < 5 30%; households using dishwashers increasing from 3% in 1983 (D.M.R., 1991) to 9% in 1988 and 14% in 1992 (Anon, 1992). To convert the combined TP per capitas to per capitas for SRP, it was assumed that 10% of TP entering a STW was retained within the works and removed as sludge and that SRP formed 64,8% of the remaining TP in the emuent (Storey, 1990). The SRP per capitas in Table I were also used to assess the potential changes in loadings from the rural population. U r b a n and rural populations were estimated on an annual basis from the census results of 1971, 1981 and 1991 and, for inter-census years, changes in the electoral rolls which are updated annually. SRP loadings from increased commercial rainbow trout production were calculated from Department of Agriculture Northern Ireland (DANI) production statistics and a TP loss rate of 10 kg P tonne fish t, of which 50% was assumed to be SRP. Annual rates of fertiliser P usage for Northern Ireland between 1974 and 1991 were also obtained from D A N I internal statistics while those from 1942 to 1952 can be found in M c C o n a g h y and McAllister (1955). Slatistics

The sign + denotes 95% confidence limits of the mean of a variable estimate. RESULTS

T h e i m p a c t o f P r e d u c t i o n at S T W s c a n be c o n s i d e r e d best by c o m p a r i n g river m a s s l o a d s w i t h t h e q u a n t i t y o f P r e m o v e d at S T W s b u t first t h e i m p a c t

Table 1. Changes in detergent P, dishwasher P and urban SRP per capita 1974-1991 Annual P per capita consumption of Year

Detergent P (kgP person lyr i)

1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

0.449 0.475 0.577 0.591 0.504 0.573 0.540 0.646 0.569 0.602 0.577 0.624 0.624 0.639 0.551 0.533 0.500 0.398

Dishwasher P (kgP person lyr t)

Net annual SRP per capita* (kgPperson lyr ~)

0.004 0.007 0.015 0.022 0.026 0.037 0.047 0.062 0.073 0.073 0.084 0.099

0.518 0.532 0.591 0.601 0.549 0.590 0.572 0.637 0.598 0.619 0.606 0.640 0.648 0.663 0.619 0.608 0.597 0.544

*Annual SRP per capita = sum of detergent, dishwasher and dietary (0.438 kg P person ~yr ~) less 10% retention of P within STWs and 64.8% of TP as SRP.

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Fig. 2. Seasonality of SRP concentrations in (a) River Main and (b) Ballinderry Rivers 1974-1991. Horizontal bars denote 1974-91 mean monthly concentrations. Vertical bars denote deviations from long term mean over period 1974-1991. on river SRP concentrations will be considered briefly by comparing two rivers: River Main which experienced P reduction at the two Ballymena STWs from 1981 and the Ballinderry River, which has not experienced any P reduction (Fig. 2). The annual cycle of mean (1974-1991) monthly SRP concentrations in each river is presented by the horizontal bars, while the time series of deviations from each monthly mean are shown by the vertical bars. Summer SRP concen-

trations of each river are shown to have been substantially higher than winter concentrations and this annual cycle is the reverse of the annual flow cycle. In the River Main, the impact of the P reduction on SRP concentrations is evident as a series of negative deviations of each mean over the final 60% of each time series (Fig. 2a). These negative deviations are greater in the summer months than in the winter when the deviations are not only smaller but less

Table 2. Total phosphorus loading in 1979/80 and TP removed at STWs within P reduction programme River

Main

Upper Bann

Moyola

Blackwater

Direct drainage

1979/80 TP Loading (tonnes P yr t) year

21.4

26.2

5.0

32.8

38.8

1981 t982 1983 1984 1985 1986 1987 1988 1989 1990 1991

6.9 10.9 8,4 15.9 14.2 16.0 12,5 10.3 13,2 11.2 16,2

0,0 12,5 20.8 17.7 16.7 19.0 22.1 18,1 20,4 14,4 17.3

TP Removed (tonnes P yr ~) 0.0 1.8 4.1 3.3 3,6 3,5 3.9 3.9 4,1 2,2 1.8

0.0 9.0 12.9 5.4 9.5 15,4 15.4 17,6 17.7 22. I 24.8

5.5 13.3 17.5 19.2 23.9 24.1 29.3 29.6 23.9 11.5 18,7

Lough Neagh phosphorus reduction consistent towards the end of the time series. For the Ballinderry River, there is clear evidence that SRP concentrations have increased over the years, particularly in the winter (October-March) as the time series for these months show a clear transition from negative deviations in the first half of each series followed by positive deviations in the second half (Fig. 2b). The operational results of the P reduction programme are summarised in Table 2 with variations in the efficiency of the programme reflecting a variety of causes including problems with the ferric-aluminium liquor initially employed, overloading of STWs and plant failure. Between 1983 and 1991, TP loadings from STWs were reduced by an average of 73.1 tonnes P year ~; equivalent to 58.8% of the reference loadings from STWs prior to P reduction (Table 2). Within the major river catchments, the 1983-1991 average TP reduction was 50.2 tonnes P year-~, equivalent to a SRP reduction of 36.9 tonnes P year -~. This reduction was substantially less than the range of 147 tonnes P year -~ between the maximum and minimum annual river loadings of SRP observed prior to P reduction (Table 3), Over the period 1974-1980, SRP loads were dependent on flow, with the minimum load in 1975 occurring when flows were least with the converse being true in 1979. To allow for the dependency of SRP loadings on flow, the impact of P reduction on river SRP loadings was assessed by comparing river SRP loads in the years following P reduction with SRP loadings predicted from regressions of annual SRP load v s i/annual flow. These regressions covered the years prior to P reduction and were employed as a standard against which subsequent river loadings could be

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compared. All the SRP v s l/flow regressions were significant at the p < 0.001 level for the rivers within the P reduction programme (Table 4). The assessment procedure is shown in detail for the River Main where, prior to P reduction, variation in SRP loads closely corresponded to variation in flows, as demonstrated by the close fit of loads from 1974 to 1980 to the curve of load v s 1/flow regression (Fig. 3). Following P reduction in 1981, all but two of the annual SRP loads were located below the load v s l/flow regression curve indicating that the reductions in river loadings which occurred following P reduction were not consistent (Fig. 3). When residuals from the load v s 1/flow regression curve (observed-predicted values) in Fig. 3 are plotted v s year it is evident that they were negative to a degree comparable with the reduction in P loadings from the Ballymena STWs only between 1981 and 1985 (Fig. 4). This indicates a good agreement between the reduction in river SRP loads and the quantity of SRP removed at the STWs over these years. However, it is also evident that subsequently residual values increased despite continuing reduction of P from STWs (Fig. 4). By 1989-91, residuals were close to zero, indicating that, for a given flow, river SRP loadings were little different from those prior to 1981 despite an average reduction in SRP load from STWs of 9.9 tonnes P year ~. When plotted v s time, the residuals from the three remaining pre-P reduction annual SRP load vs 1/annual flow regression curves each demonstrate a sequence similar to that observed for the River Main (Fig. 5). Following P reduction, the residuals were initially lower by an amount which corresponded to

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Flow (106 m 3 yr -1) Fig. 3. River Main annual SRP loads v s flows: Pre P reduction ( i ) 1974-1980 and post P reduction (0) 1981-1991. Line: fit of 1974--1980 regression of SRP load vs l/flow.

1974 1975 1976 1977 t978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991

Area (km 2) Year

472.2 343.1 464.6 518.9 596.0 606.0 554.0 679.1 588.7 357.9 520.1 586.9 596.0 480.1 650. I 391.2 629.8 536.6

Flow (106m 3)

37.6 29.9 36.9 44.3 50.9 50.0 47.8 44.4 36.0 23.0 31.4 34.2 38.5 38.3 53.0 29.9 57,2 40.2

SRP Load (tonnes P)

Main 709.2

380.3 203,3 313.0 270.4 342.3 535.5 335.9 377.5 355.6 204.8 285.4 305.4 306.8 205.2 309.7 169.3 299.1 265.9

Flow (106m 3) 44.8 25, I 34.2 31.5 45.6 60.6 40.3 52.6 31.0 18.8 22.6 37.0 31,5 26.6 38,4 21,4 35,5 27.2

SRP Load (tonnes P)

Upper Bann 660,6

242.8 165.5 241.3 246.8 274.8 279.5 255.7 342.1 343.1 255.8 256.2 305,6 233.8 262.0 341.4 238.0 324.6 274.5

Flow (106m 3) 12.3 8.2 12.6 12.8 16.0 18.3 18.5 22.3 18.2 14.1 13.6 17,7 1 1.8 16.4 19.2 13.3 19.7 17.0

SRP Load (tonnes P)

Moyola 301.4

840.2 482.8 717.5 800.9 875.4 918.0 895.7 939.1 942,2 764.7 859.7 844,0 964.4 775.9 1140.4 743.7 875.9 681.5

Flow (106m3) 61,2 35.8 63.0 69,7 95.4 82.2 95.1 102.3 79.6 74,1 73,1 82.5 78.5 64.4 94.0 53.9 62.1 47.6

SRP Load (tonnes P)

Blackwater 1480,3

290.7 178.8 271.8 337.3 323.1 361.2 320.8 331.7 322.3 273.2 291.7 342.9 359.8 295,3 368.3 227.8 312.3 239.6

17.1 13.1 21.7 22.6 23.7 25.7 26.6 27.0 25.5 25.3 25.1 30.7 32.5 32.5 40.7 24.6 35.3 24.6

SRP Load (tonnes P)

Ballinderry 430.1

Flow (106m3)

Table 3. Catchment areas and annual SRP loads and flows of major rivers 1974-1991

183.3 123.1 192.5 174.7 218.3 188.6 192,4 256.2 225.3 154.5 201.3 214.3 242.0 206.4 257.5 197.6 224.9 182.0

Flow (106m 3)

21.8 16,3 18.8 16,3 19.5 18.7 21, I 26.5 23.4 19.6 21.5 24.0 26.2 27,3 31.4 26.3 28,9 23.4

SRP Load (tonnes P)

Six Mile Water 301.4.

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Table 4. Regression analysis of annual SRP load (tonnes P) v s l/annual flow ( × 109 m 3 yr i ) for years prior to introduction of P reduction at STWs River Period Observations R2 In~rcept (Standard error) Slope (Standard error) Student's t statistic

Main 1974-80 7 0.895 77.37 (2.79) - 17.14 (2.62) -6.54***

Upper Bann

Moyola

Blackwater

1974-81 8 0.850 78.69 (4.85) - 11.86 (2.04) - 5.82**

1974-81 8 0.758 32.75 (2.39) -4.35 (I.00) -4.34**

1974-81 8 0.722 149.34 (12.76) - 57.19 (14.48) - 3.95**

Significance levels: **0.01 > p > 0.001; ***0.001 > p.

the reduction in SRP from point sources but, subsequently, the values of the residuals increased. In the Blackwater River, this upward trend in the residuals was least evident, partly due to the steady increase between 1982 and 1990 in the reduction of P from STWs and creameries, rather than the reduction occurring in a single step as was the case in the other rivers (Fig. 5c and Table 2). The above results suggest that background (i.e. non-point source) SRP loadings were increasing in each of the four rivers over time. This increase was quantified by calculating the differences between the residuals from the SRP load v s flow regressions and the reductions in SRP from point sources loads. A value close to zero of this variable indicates no change in river SRP loadings relative to the annual SRP load v s flow relationship prior to P reduction, with a strongly positive value indicating a marked increase in river loadings relative to loadings measured prior to 1981 or 1982. To allow for differences in catchment size between rivers, the data was

expressed on an areal basis and plotted v s time (Fig. 6). The regression of this variable with time gave a r 2 value of 0.18 which was significant at the 0.001 < p <0.01 level (equation 1). The exclusion from this regression of all Upper Bann results, which were exceptionally high in 1983, 1985 and 1989, increased the r 2 value to 0.41, significant at p < 0.001 (equation 2). The slopes of equations (1) and (2) provide a measure of the underlying increase in P loading in units of kg P km -2 year ~. y=-l.62+l.38x

n = 4 1 , r2=0.18

equation 1

y=-3.46+l.23x

n = 3 1 , r2=0.41

equation2

where y = [(SRP

removed - SRP load flow /catchment area] (kg P km 2)

x = time from the introduction of P reduction (year). The two remaining rivers, Ballinderry (1974-1991) and Six Mile Water (1976-1987), provided the

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Fig. 4. R i v e r M a i n a n n u a l S R P l o a d i n g s ( O ) , flows ( 0 ) a n d residuals f r o ) o f S R P vs l / f l o w r e g r e s s i o n e q u a t i o n f r o m 1974 to 1991. B a r s d e n o t e r e d u c t i o n o f S R P f r o m S T W s .

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10 0 -10 -20 [ ] Creamery-P -30 1974 1976 1978 1980 1982 1984 1986 1988 1990 1975 1977 1979 1981 1983 19851987 1989 1991 Year Fig. 5. Time plots of residual values ( 1 ) for M o y o l a (a), U p p e r Bann (b) and B l a c k w a t e r (c) rivers. Bars d e n o t e reduction o f S R P loadings from STWs and creamery P.

Table 5. Summary results of linear and multiple regression analyses of Ballinderry and Six Mile Water SRP loads vs (i) J/fiow (109 m 3) and (ii) I/flow and time (1974 = year 1) River

Ballinderry (i)

Period Observations R2 Intercept (Standard error) Slope: I/flow (Standard error) Student's t statistic Slope: Time (Standard error) Student's t statistic

1974--91 18 0.398 45.64 5.12 -5.65 1.74 -3.25**

(ii) 1974-91 18 0.834 35.77 2.77 -5.04 0.95 -5.32*** 0.796 0.127 6.28***

Significance levels: **0.01 > p > 0.001; ***0.001 > p.

Six Mile Water (i) 1976-87 12 0.498 39.54 2.63 -3.56 1.13 -3.15"*

(ii) 1976-87 12 0.878 29.55 1.37 2.70 0.61 4.43** 0.628 0.119 5.30***

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Time (years) Fig. 6. Time plot of changes in areal SRP loadings. Main (11), Moyola (0), Blackwater (O) and Upper Bann (A). Year 1 = denotes start of P reduction in each river. o p p o r t u n i t y to examine changes in river S R P loads over extended periods which were i n d e p e n d e n t of m a j o r changes in u r b a n P inputs. In addition to subjecting the S R P loads from these rivers to the regression o f S R P v s l/flow, multiple regression analyses of S R P load v s 1/flow a n d a yearly time variable (1974 = 1) were u n d e r t a k e n (Table 5). F o r each river, time was positively correlated with river S R P loads at the p < 0.001 level o f significance a n d was associated with 44 a n d 38% o f the a n n u a l variation in S R P loading o f the Ballinderry a n d Six Mile W a t e r respectively. Increases in the river S R P loadings with time are evident from the u p w a r d trends with time of residuals from the two S R P vs l/flow regressions (Fig. 7). The high values of the

residuals from the Six Mile W a t e r multiple regression in 1974 and 1975, o f 6.0 a n d 7 . 0 t o n n e s P yr respectively, correspond closely to the S R P loadings from A n t r i m S T W of 6.2 a n d 6.9 tonnes P which entered the river at those years (Fig. 7b). Also in the Six Mile Water, the 1991 negative residual of - 3 . 0 1 tonnes P yr ~ corresponds well to the combined reduction in S R P load of 3 . 2 t o n n e s P y r - ' resulting from the closures of a creamery a n d M u c k a m o r e S T W (Fig. 7b). The slope coefficients of the time variables of the Ballinderry a n d Six Mile W a t e r multiple regression equations in Table 5 provide measures of the rates of increase in S R P loadings in each river over time. Normalised to a c a t c h m e n t area basis,

Table 6. Catchment SRP increase rates and changes in SRP loading from known point and non-point sources. All values expressed as kg P km-2 yr-t; +denotes 95% confidence limits P Reduction Ballinderry Six Mile Water Rivers River River Period 1981-91 1974-91 1976-87 Background SRP increase (a) 1.377 + 0.9671 1.850 _+0.6282 2.085 +_0.624-' Urban loading increase/decrease (b) - 0.045 0.185 0.790 Creamery loading increase (c) 0 0.121 0 Fish farm loading increase (d) 0.035 0.096 0 Unaccounted for (a-b~:-d) 1.387 1.448 1.295 Rural population loading increase/decrease - 0.190 + 0.164 0.344 tFrom equation (I). 2From Table 5.

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-10 1974 1976 1978 1980 1982 1984 1986 1988 1990 1975 1977 1979 1981 1983 1985 1987 1989 1991 Year Fig. 7. Time plots of residuals from SRP vs l/flow regression (m) and SRP vs I/flow and time multiple regression ( 0 ) for Ballinderry (a) and Six Mile Water (b). Bars denote changes in Six Mile Water SRP loading from sewage diversion and creamery closure. these two rates, together with the comparable rate of increase obtained for the remaining four rivers (equation 1) can be compared with variations in SRP loading due to changing urban and rural populations, p e r capita P usage and fish farming production together with a new creamery in the Ballinderry River catchement (Table 6). The periods of comparison were those of the respective regression analysis of SRP loading or areal loss rate vs time. Changes in the SRP loadings of towns not included in the P reduction programme were small as, although there was an increase in population, this was effectively counterbalanced by the declining SRP p e r capita due to the reduced P content of domestic detergents (Tables 1 and 6). The SRP loading from all minor STWs in the Lough Neagh catchment was computed to have declined from 49.1 tonnes P yr -~ in 1980 to 48.4tonnes P yr -~ in 1991. After the allowance was made for the changes in point source discharges, the three background increase rates for SRP varied by less than 0.15 kg P km -2 yr -~ and were much larger than potential changes in diffuse SRP load from the rural population (Table 6). Over the entire Lough Neagh catchment between 1988 and 1991, the reduction in SRP load from STWs

and creameries of 62.6 tonnes P year-~ was equivalent, when divided by the average annual inflow volume, to a potential reduction in the average inflow SRP concentration of 23.1/~gP1-L However the measured reduction in SRP inflow concentration was less than 50% of this value, as the mean SRP concentration from 1978 to 1981 was 100.4 ( + 5.7) pg P 1-~ compared to 90.2 ( + 5.6) pg P 1-~ from 1988 to 1991. When the potential reduction in SRP inflow concentration due to point source P reduction is added to the actual concentration, the resulting projected data for 1981-1991 combined with SRP concentrations from 1974 to 1980, are shown to increase steadily with time at a rate estimated, from linear regression with time, to be 1.54 ___0.54 pg P 1-~ year (Fig. 8). DISCUSSION

The river loadings of SRP to Lough Neagh have been shown to be increasing with time at rates sufficient to significantly reduce the impact on the SRP load to the Lough of reductions in the P discharges from creameries and STWs. This reduced impact appears to be due principally to background loadings increasing with time rather than any

Lough Neagh phosphorus reduction

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1974 1976 1978 1980 1982 1984 1986 1988 1990 1975 1977 1979 1981 1983 1985 1987 1989 1991 Year Fig. 8. Time plots of Lough Neagh inflow SRP concentration (ll) and predicted SRP concentration in the absence of P reduction from STWs and creameries (O). substantial underestimates in the point source SRP discharges presented in the Lough Neagh budget for 1979 by Foy et al. (1982) as subsequent data has tended to confirm the magnitude of point sources in the original budget. For example: the SRP per capita of 0.58 kg P person -~ yr -] in the 1979 budget compares to a 1974-1979 mean value of 0.56 kg P person -] yr-~ from dietary, detergent and dishwasher contributions to domestic P loadings (Table 1). Similarly, the measured reference TP loading of 125tonnes P year -~ for STWs serving an urban population of 141 k measured prior to P reduction by Gray (1984) equates to a TP per capita of 0.88 kg P p e r s o n - l y r -~ and a SRP per capita of 0.57kg P person -~ yr-L For creameries, the quantity of P employed in cleaning agents has been shown to be similar to the predicted creamery SRP loading of Foy et al. (1982). Annual SRP loads were highly flow dependent, so that small and gradual changes in loading may only be identified by reference to data collected over an extended number of years. In a separate analysis of the SRP loading data from the Main and Ballinderry rivers, Smith et al. (1994) using simulated flows and separate summer and winter load flow regressions showed that the winter increase in loadings was greater than in the summer. This finding may seem contrary to the concentration data of the Ballinderry River presented in Fig. 2 which shows a greater, if more erratic, increase in summer SRP concentrations than in the winter months. However as summer flows are usually much lower than winter flows, the summer increase in concentrations reflects quite small increases in SRP loading. The increased loadings and concentrations were measured close to the lower limit of each river and

therefore are net increases which do not take into account any changes in the internal SRP loading from the river bed. However, it is unlikely that the observed SRP loading increases could be due to a switch to a net flux of SRP from the river bed, given experimental data on the SRP adsorption kinetics of River Main bed sediment which measured an equilibrium SRP concentration of 40/~g P 1-~ (Smith and Jordan, 1985). This concentration is substantially lower than the River Main SRP concentrations shown in Fig. 2a, suggesting that the river bed sediment would continue to adsorb SRP even under the lowered SRP concentrations which followed P removal at STWs. The underlying cause(s) of the increases in river SRP loadings in the Lough Neagh catchment must remain a matter for speculation, but the results presented in Table 6 suggest that the increases were not due to changes in SRP loadings from known point sources. The changes in the SRP loadings from the rural population presented in Table 6 represent the maximum potential contribution, as the septic tanks which treat rural sewage are discharged to soakaways rather than directly to drains and watercourses. The results from the Ballinderry and Six Mile Water rivers indicate gradual increases in background loadings with time rather than sudden increases which could be attributed to new point sources. The quite uniform areal background increase rates of close to 1.5 kg P km 2 yr ~ measured for all six catchments indicates a process of change affecting all catchments equally, which, given the comparative uniformity of land use between catchments ( F o y e t al., 1982), suggests that the increased loadings originated from agricultural sources. In a predominantly arable Swedish catchment, Krug (1993) reported that,

1062

R. H. Foy et al.

over a 28 year period, P from agriculture increased by a factor of 7.5 giving an areal increase rate of 1.5 kg P km 2 yr-~. This increase occurred despite a quite constant rate of fertiliser P usage with time. Although farmed land has higher P loss rates via drainage water than virgin land, little attention has been paid to precise relationships between farming practise and P loss rates and the nature of temporal changes in agricultural P loss rates (Ahl, 1988; Ryden et al., 1972; Cooke, 1976). Losses of P from agriculture have been expressed as a % of fertiliser P (e.g. Esser and Kohlmaier, 1991) but in the Lough Neagh catchment P fertiliser rates probably have remained relatively constant since 1942. A specific P balance for agriculture in the Lough Neagh catchment is not available, but it is likely to resemble that of Northern Ireland (NI), of which the Lough Neagh catchment forms approximately 30% of farmed area and has a similar farming economy. Fertiliser P levels in NI averaged of 9660 tonnes P year i between 1942 and 1952 compared to 10,074tonnes P year -~ between 1974 and 1991. This lack of change in P fertiliser usage compared with nitrogen over time has also been noted for other regions of the British Isles (CAS, 1978; Tunney, t990). A feature of Western European P fertiliser usage is that a large proportion of the P applied accumulates in the soil rather than being exported in crops and agricultural produce (Isermann, 1990). The proportion of P leaving NI farms as agricultural produce is small relative to inputs from fertiliser and imported foodstuffs. Expressed as a percentage of fertiliser input, 104% in 1949 and 160% in 1966 of P did not leave farms as produce; values in excess of 100% being due to the fact that total produce P was less than the P contained in imported animal foodstuffs (McConaghy and McA1lister, t955; McAllister, 1971). The comparable value for UK agriculture was 96% of fertiliser P in 1975 and, in the Republic of Ireland, values of 75% in 1968 and 74% in 1988 of fertiliser P have been calculated (CAS, 1978; Hanley and Murphy, 1973; Tunney, 1990). Given that the NI farm area is ca. 104 km 2 and loss rates of TP in drainage water of 50kg P km 2 y r - ' are typical for farmed land (Tunney, 1990; Foy et al., 1982), NI farm land would appear to have been accumulating P at a rate in the region of 1000 kg P km 2yr ~ for a period of 50 years which is consistent with an 8-fold increase in soil P between 1950 and 1990 reported by Tunney (1990) for the Republic of Ireland. Although Cooke (1976) emphasised the high capacity of clay mineral soils to retain fertiliser P, this property of soils is not necessarily at variance with the observed increasing background loadings of 1.5 kg P km 2yr ~ when they are considered in the context of soil P reserves increasing at a rate of around 1000 kg P km -2 yr -1 over a 50 year period. As a proportion of the soil P increase, the increasing SRP loss rates in drainage waters are therefore very small but are significant from a limnological perspective. Given a water yield of 0.7 m yr-~;

they reflect SRP concentration increases of close to 2/~g P l-~yr J which, over 10-20 years, would significantly change the trophic status of a receiving waterbody. Acknowledgements--We wish to thank the followingorganisations and individuals for their assistance in providing data employed in this paper: Department of Environment Northern Ireland (DOE NI) Water Service, Dr Bill Storey and Dr Tom Horridge for data relating to P reduction at STWs; DOE NI Environment Service and Mr Eamon Hagan for creamery data; DOE NI Town and Country Planning Service and Ms Donna Finlay for inter-census population data; DOE NI Water Data Unit and Mr Phillip Holland and Mr John Waterworth for river flows; Department of Agriculture Northern Ireland for fertiliser and fish production data; Dr Stuart Marshall, Unilever UK, supplied values for phosphorus consumption in UK fabric washing products (source UK Soap and Detergent Industry Association) for the years 1982-1991. REFERENCES

Anon. (1992) Northern Ireland Abstracts o f Statistics. Dept of Finance, Policy, Planning & Research Unit, BelfastNo 11, p. 18. Ahl T. (1988) Background yield of phosphorus from drainage area and atmosphere: an empirical approach. Hydrobiologia 170, 35-44. CAS (1978) Phosphorus: a Resource for UK Agriculture, pp. 64. Centre for Agricultural Strategy, Report 2, Reading. Clarke S., Towner J. V. and Yeoman S. (1992) Pollution impact of cleaning products. In: Freshwater Quality, Additional Reports undertaken for the Royal Commission on Environmental Pollution, pp. 133 191. HMSO,

London. Cooke G. W. (1976) A review of the effects of agriculture on the chemical composition and quality of surface and underground waters. Tech. Bull. Ministr. Agric. Fish. Fd., 32, 5-57. D.M.R. (1991) Home Hygiene. Data Monitor Research Report, Sept 1, 1991, 6 pp. (Accessed using Dialog Online Computer Search, File 761). Esser G. and Kohlmaier G. H. (1991) Modelling terrestrial sources of nitrogen, phosphorus, sulphur and organic carbon to rivers. Biogeochemistry of Major Worm Rivers (Edited by Degen E. T., Kenfe S. and Ridley S. E.), pp. 297 322. Wiley, Chichester. Foy R. H., Smith R. V., Stevens R. J. and Stewart D. A. (1982) Identification of factors affecting nitrogen and phosphorus loadings to Lough Neagh. J. Environ. Mgt. 15, 109 129. Gray A. V. (1984) The Lough Neagh Rivers--Monitoring and Water Quality, Part II. Sewage Treatment Works. Paper presented to conference: Lough Neagh and its Rivers, pp. 6. Ulster Anglers Federation. Hanley P. K. and Murphy M. D. (1973) Soil and fertilizer phosphorus in the Irish ecosystem. Wat. Res. 7, 197-210. Isermann K. (1990) Share of agriculture and phosphorus emissions into the surface waters of Western Europe against the background of their eutrophication. Fert. Res. 26, 253-269. Jordan C. and Dinsmore P. (1985) Determination of biologicallyavailable phosphorus using a radiobioassay technique. Freshwat. Biol. 15, 597-603. Krug A. (1993) Drainage history and land use pattern of a Swedish river system--their importance for understanding nitrogen and phosphorus load. Hydrobiologia 251, 285-296.

Lough Neagh phosphorus reduction McAllister J. V. S. (1971) Nutrient balance on livestock farms. 1st Colloquium of the Potassium Inst. ll3-121. McConaghy S. and McAllister J. V. S. (1955) The use of fertilisers in Northern Ireland. Res. exp. Rec. Min. Agric. NI 1952-1953, pp. 1-15. Murphy J. and Riley J. B. (1962) A modified single solution method for the determination of phosphate in natural waters. Analytic. Chim. Acta. 27, 31-36. Patrick S. (1983) The Calculation of Per Capita Phosphorus Outputs from Detergents in the Lough Erne Catchment, pp. 23. Working Paper No 4, Palaeoecology Dept., University Coll. London. Ryden J. C., Syers J. K. and Harris R. F. (1972) Phosphorus in runoff and streams. Adv. Agron. 25, 1-45. Sas H. (1989) Lake Restoration by Reduction of NutrientLoading, pp. 497 + xxl. Academia, St Augustin. Smith R. V. (1977) Domestic and agricultural contributions to the inputs of phosphorus and nitrogen to Lough Neagh. Wat. Res. 11, 453-459. Smith R. V. and Jordan C. (1985) The fate of nitrogen and phosphorus in a rural ecosystem. Annual Report on Research and Technical Work of the Department of Agriculture for Northern Ireland, p. 124. Smith R. V. and Stewart D. A. (1977) Statistical models of

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river loading of nitrogen and phosphorus in Lough Neagh system. Wat. Res. 11, 631-636. Smith R. V., Foy R. H. and Lennox S. D, (1994) Mathematical modelling techniques to evaluate the impact of phosphorus reduction on phosphate loads to Lough Neagh. In Statistics for the Environment: Water Related Issues (Edited by Barnett V. and Turkman K. F.), pp. 271-284. Wiley, London. Stevens R. J. and Smith R. V. (1978) A comparison of discrete and intensive sampling for measuring the loads of nitrogen and phosphorus in the River Main, County Antrim. War. Res. 12, 823-830. Stevens R. J. and Stewart D. A. (1981) The effect of sampling interval and method of calculation on the accuracy of estimated phosphorus and nitrogen loads in drainage water from two different sized catchment areas. Rec. Agric. Res. (Dept of Agriculture, Northern Ireland) 29, 29-38. Storey W. C. (1990) Operational aspects of phosphorus removal from sewage by chemical treatment. Inst, Wat. and Environ. Mgt., Seminar on Eutrophication, Dundalk, Ireland, 3 October 1990. Tunney, H. (1990) A note on a balance sheet approach to estimating the phosphorus fertiliser needs of agriculture. It. J. Agric. Res. 29, 149-154.