Nutrient emissions to water from septic tank systems in rural catchments: Uncertainties and implications for policy

environmental science & policy 24 (2012) 71–82

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Nutrient emissions to water from septic tank systems in rural catchments: Uncertainties and implications for policy P.J.A. Withers a,*, L. May b, H.P. Jarvie c, P. Jordan d, D. Doody e, R.H. Foy e, M. Bechmann f, S. Cooksley g, R. Dils h, N. Deal i a

School of Environment, Natural Resources and Geography, Bangor University, Bangor, Gwynedd LL57 2UW, UK Centre for Ecology and Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB, UK c Centre for Ecology and Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB, UK d School of Environmental Sciences, University of Ulster, Coleraine BT52 1SA, UK e Agri-Food and Biosciences Institute, Belfast BT9 5PX, UK f Bioforsk – Norwegian Institute for Agricultural and Environmental Research, Fred. A. Dahls Vei 20, 1432 Aas, Norway g James Hutton Institute, Craigiebuckler, Aberdeen AB15 8QH, UK h Evidence Directorate, Environment Agency, Red Kite House, Howbery Park, Wallingford, Oxon OX10 8BD, UK i Department of Public Health Onsite Water Protection Branch, 1642 Mail Service Center, Raleigh, NC 27699-1642, USA b

abstract article info Keywords: Septic tank systems Nutrients Phosphorus Water pollution Registration Policy

Septic tank systems (STS) are widely used to treat domestic wastewater from individual dwellings in rural areas but are a potential source of water pollution. However, their contribution to freshwater eutrophication and impacts on human health are uncertain and difficult to quantify. Five case studies are presented to highlight the issues underpinning this problem. Uncertainty exists over the numbers and locations of STS because registration is not fully implemented in all regions. Underestimating the numbers of STS located in a catchment can lead to overestimation of the relative contribution from diffuse sources such as agriculture. In turn, this may lead to potential delays in meeting water quality limits due to disproportionate targeting of potential sources. System performance is uncertain due to a lack of information on factors (such as siting, design, age, nature and level of maintenance, proximity to a watercourse) that affect nutrient retention rates. Many systems still discharge directly to a watercourse, or to a drainage network closely connected to a watercourse despite prohibition of such discharges. Their effect on water quality is also uncertain because current nutrient abatement policies ignore the temporal variation in nutrient loading that can influence ecological response in streams and connecting ditches. These case studies show that although STS constitute a relatively small (often <10%) portion of total annual catchment nutrient loads, they can still significantly increase in-stream nutrient concentrations, especially during low flow periods in summer. STS may therefore be a greater risk to riverine eutrophication and human health than is currently assumed. More sophisticated resolution of source apportionment is needed to fully capture water quality impairment due to clustering of STS along stream networks. Targeted surveys and public awareness campaigns are useful tools for identifying failing STS, improving STS maintenance and highlighting alternative treatment options for use in catchment areas that are especially sensitive or valuable. These case studies support the need for a risk-based policy of STS registration and regulation governing maintenance to avoid further declines in the ecosystem services our freshwaters provide. # 2012 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +44 1248 382631; fax: +44 1248 354997. E-mail address: [email protected] (P.J.A. Withers). 1462-9011/$ – see front matter # 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envsci.2012.07.023

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1.

environmental science & policy 24 (2012) 71–82

Introduction

Individual and scattered rural dwellings not connected to a public sewerage system must rely on private individual wastewater treatment systems (IWWTS) to treat their wastewater, including pre-fabricated package plants, cesspits, reed beds and, most commonly, septic tank systems (STS). The latter have a common design with one or two treatment tanks that trap solids and allow limited anaerobic digestion of organic matter (Gill et al., 2009b). In order to attenuate organic content, nutrients (orthophosphate-P, ammonium-N) and pathogens in the effluent, the tank discharges to a soakaway or drainfield where effluent is further treated by percolating through the soil (Beal et al., 2005; EPA, 2009). Operating to design standards, STS can be an effective method of treating household wastewater in rural areas, but recent monitoring studies suggest they can contribute to diffuse pollution and increase risk of eutrophication (Ahmed et al., 2005; Arnscheidt et al., 2007; Palmer-Felgate et al., 2010; Withers et al., 2011). Jarvie et al. (2010) show that, in some circumstances, they can also operate as multiple point sources to surface waters. Understanding the contribution of STS to pollutant loads and concentrations in both flowing and standing freshwaters is critical to the development of catchment management plans that will meet the regulatory standards considered necessary to improve water quality. The European Court of Justice recently ruled against the Republic of Ireland (RoI), stating that despite their small size, IWWTS must be regulated under the EU Waste Framework Directives that have been enacted since 1975. These Directives require that each member state must enact legislation to ensure waste is recovered or disposed without endangering human health and does not use processes that harm the environment (C188/08, 2009). As a result of the court ruling, homeowners in the RoI must now register and maintain their STS according to legislative guidelines and to compliance standards issued by the Irish government (Water Services (Amendment) Bill, 2011). In the UK, different approaches to STS registration and regulation exist; for example a registration scheme for STS exists in England but is not fully implemented and is currently under government review. The number of consented septic tank discharges (ca. 51,000) is thought to be only ca. 10% of the total number of STS in England and Wales (Environment Agency, 2009a,b), but there is large uncertainty surrounding this estimate. The main argument against a registration scheme is that homeowners are responsible people who will ensure that their STS are maintained properly. However, it is uncertain whether homeowners understand how systems function, what maintenance is required and the costs associated with renovation or repair. There is a perception that STS are self-maintaining, that tank solids need not be regularly emptied and soakaway inspected for signs of failure (Butler and Payne, 1995; Moelants et al., 2008). There is also potential confusion over responsibility for maintenance of communal STS such as those linked to a community village hall or operated by water authorities. There is also much uncertainty about how STS affect water quality and risk of eutrophication. The contribution of STS to annual nutrient loads at the national scale is usually negligible

relative to other sources such as large sewage treatment works (STW) and agriculture; for example they were ignored in the national assessment of total P (TP) loads in Great Britain (White and Hammond, 2009). When included as a source at the catchment scale, nutrient loads from STS are difficult to accurately estimate due not only to inadequate information on the number, location and condition of many unconsented STS discharges, but also due to uncertainties in rates of nutrient discharge. Densities of STS vary; for example from 3 to 14 km 2 in Ireland (Macintosh et al., 2011). Literature values for nutrient export from STS vary from 0.3 to 0.7 kg capita 1 yr 1 for P (Smith et al., 2005; Withers et al., 2011), and up to 4 kg capita 1 yr 1 for N (Gill et al., 2009b; Withers et al., 2011). Nutrient discharges from individual STS depend on the number of users, detergent use and other domestic habits, system type, age, condition and connectivity to the watercourse (Whelan and Titamnis, 1982; Canter and Knox, 1985). If these factors vary regionally or spatially as found by Patrick (1988), the effects on water quality will vary as well. In this paper, we explore the foundations of these uncertainties and use case studies to illustrate (a) how STS nutrient load contributions might be underestimated without full information on the numbers of STS in use (Nadder/Bure catchments); (b) that a high proportion of STS may not be maintained at a level and frequency needed to prevent negative water quality impacts (Lough Melvin and Jova catchments); (c) how STS load contributions based on an annual timestep might underestimate potential effects on eutrophication risk (Jova and Blackwater catchments), and; (d) evidence that STS can directly increase stream nutrient concentrations (Blackwater and Dee catchments). The case studies largely concentrate on P as this is the nutrient that drives targets for eutrophication control in Europe and the USA.

2.

Case studies

Case studies from England (Nadder/Bure), Scotland (Dee), Ireland (Lough Melvin, Blackwater) and Norway (Jova) are presented to show the potential contribution of STS to eutrophication risk in different river catchments and one lake catchment. The Nadder/Bure, Lough Melvin and Jova case studies quantify the potential contribution of STS to catchment P loads and potential consequences for water quality through survey and export coefficient modelling. The Blackwater case study compares measured stream P concentrations in two sub-catchments with varying STS density. The Dee case study demonstrates improvement in stream nutrient status when an STS in close proximity to a headwater stream was decommissioned as part of a catchment management plan and associated campaign to increase awareness of proper STS maintenance.

2.1.

Nadder and Bure catchments, England

To assess the significance of discrepancies in the number of consented and unconsented STS on catchment total phosphorus (TP) loads, the locations of all STS within two contrasting rural catchments (R. Nadder, Hampshire and

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the R. Bure, Norfolk Broads) were identified from aerial photography. Total numbers were then compared to the consented (‘known’) STS in terms of their influence on likely P discharges. The Nadder catchment, upstream of Wilton (NGR 409800, 130800), encompasses about 216 km2 with intensive mixed agriculture as the primary land use (67% of area, Jarvie et al., 2008). The Bure catchment, upstream of Horstead Mill (628800, 318700), encompasses about 328 km2 and is dominated by arable farming (77% of area, Johnes, 1996). Only about 17% of the Nadder and 6% of the Bure are connected to a mains sewer network according to maps of sewered areas supplied by the local water company (Wessex Water and Anglian Water). The number and location of STS within each catchment were derived from the location of residential properties outside of these sewered areas, as visually identified from a 1 m resolution aerial photograph supplied by Natural England. The photo-interpretation and digitisation of house locations suggested that there were about 1257 septic tanks within the Nadder and about 2515 within the Bure. Of these, EA records showed that only 18 (1%) and 56 (2%) of these, respectively, had discharge consents. The large difference in the numbers of consented and unconsented STS and the high proportion of STS close to the river network is illustrated for the Nadder in Fig. 1. The TP loss from each STS was estimated from average household size (2.4 people; www.neighbourhood.statistics. gov.uk), average per capita daily water usage (148 l; www. defra.gov.uk), and the most commonly reported concentration of P in raw septic tank effluent of 10 mg l 1 (Canter and Knox, 1985; Beal et al., 2005). These values were combined to give an

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annual export from each STS of about 1.3 kg P household 1 yr 1. Phosphorus losses from these systems were scaled up to the catchment level by multiplying the annual export per household by the corresponding numbers of households. Delivery of STS effluent P to nearby rivers was then estimated according to the method proposed by Kraft and Turtumøygard (1997), which takes into account soil P retention capacity and travel distance to the receiving water. For purposes of this study, it was assumed that soils within both catchments had a ‘medium’ capacity to retain P. The P input to the R. Nadder from the three large STW within its catchment was derived from summary flow and concentration data given by Ash et al. (2006). Annual discharges of P from 11 smaller STWs within the Bure catchment were estimated on the basis of the population equivalent (p.e.) value for each works and per capita P export coefficients (0.053 kg y 1 for works without P stripping (Johnes et al., 2003) and 0.0053 kg y 1 for those with P removal (Anglian Water, pers. commun.). The contribution of P from agricultural sources was estimated as the difference between the combined inputs from upstream STS and STW and the measured in-river P load at the catchment outlet based on continuous flow and weekly, fortnightly or monthly sampling of P concentrations over a two-year period. The average annual P load in the Nadder at Wilton was estimated to be about 12.4 t yr 1. Of this, 2.6 t yr 1 (21%) was found to come from upstream STW discharges and the remainder (9.8 t y 1; 79%) from a combination of STS discharges and agricultural runoff. If consented STS discharges, only, were taken into account, the P load from these systems

Fig. 1 – The catchment of the River Nadder, Wiltshire showing sewered areas, consented and unconsented septic tank systems. # NERC (CEH); Includes mapping data based on Ordnance Survey 1:50,000 maps with the permission of HMSO # Crown copyright and/or database right 2006. Licence 100017572.

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Table 1 – Estimated source apportionment of the total P load in the Nadder and Bure catchments expressed as a percentage: sewage treatment works (STW), septic tank systems (STS) and agriculture according to whether P discharges from unconsented STS are either excluded or included. Nadder

STW STS Agriculture a

Bure a

Consented STS

All STS

Consented STS

All STSa

21 1 78

21 6 73

3 1 96

3 20 77

Consented and unconsented STS.

would be estimated to be 0.01 t yr 1 (<1%) and the remaining 9.79 t y 1 (78%) would be attributed to agricultural runoff. However, if all STS are taken into account, the estimated P discharge from these systems would be higher, i.e. 0.68 t y 1 (6%), and that from agricultural sources correspondingly lower, i.e. 9.12 t y 1 (73%), Table 1. The situation in the River Bure was found to be similar. The average annual P load in the Bure at Horstead Mill was estimated to be ca. 6.6 t y 1. Of this, the P input from STW was estimated to be about 0.2 t y 1 (3%) and that from consented STS approximately 0.03 t y 1 (<1%). By difference, this suggested that 6.37 t P y 1 (96%) was contributed by agriculture. However, if all STS within the catchment are taken into account, the P input would be about 1.35 t y 1 (20%) and that from agriculture would be correspondingly reduced to 5.05 t y 1 (77%), Table 1.

2.2.

Lough Melvin Septic Tank Survey, Ireland

Lough Melvin is a large mesotrophic lake draining a rural catchment (217 km2) in north-west Ireland. Between 1991 and 2007, the mean annual lake TP concentration increased from 19 to 29 mg P L 1, threatening the lake’s high conservation status (Girvan and Foy, 2006). An investigation of P sources revealed that low intensity agricultural grassland occupied 39% of the catchment area but contributed 62% of the annual TP load (11 tonnes). An urban population of ca. 1000 individuals contributed 7% via three small STW (Campbell and Foy, 2008). The rural population of ca. 2000 resides in dispersed single dwellings served by individual STS (or other IWWTS). With a decadal census derived average of 2.4 persons per dwelling, STS density was 3.8 km 2. Assuming a system P retention coefficient of 43% (i.e. 57% of the urban per capita value of 0.77 kg P person 1) used by Smith (1977), the contribution of STS was estimated to be 7% of the annual lake TP budget. However, retention estimates can be as low as 20% where there is a predominance of severely gleyed soil types and high rainfall as found in Lough Melvin, where 87% of soils are peats or gleys (Patrick, 1988). To resolve the uncertainty over STS retention, a survey of STS in the catchment in 2008 assessed the age and nature (if any) of a percolation system (soakaway), frequency of tank emptying, discharge point, distance to watercourses, soil type and slope. Fifty STS were randomly selected in the catchment, although the representativeness of the sample was depen-

dent on resident participation. Participants completed a questionnaire and a visual inspection of the area around the septic tank, soakaway/percolation area and discharge point was conducted. Full details of the survey can be found in Campbell and Foy (2008). Survey results are summarised in Table 2. Forty six % of STS were >20 yrs old and a majority of these lacked a proper percolation area or discharged to a cesspit instead, a standard practice at the time of installation. Only limited maintenance of STS had been performed. Seventy percent of tanks had not been emptied in more than 5 yrs and 54% of the respondents were unable to recall when their tanks had been last emptied. Due to an extensive natural drainage network, 60% of the catchment is within 200 m of a waterbody and 40% of STS were within 50 m of the lake or connecting rivers. This high connectivity is further enhanced by an extensive artificial drainage network, and 66% of STS were found to discharge directly into this drain network. A feature of the study was that newer systems (<10 years old) systems were not conventional STS but were categorised as ‘other’ (commonly, package plants with aeration systems) which accounted for 20% of all IWWTS. The results of the survey showed that a high proportion of STS discharges would reach either a river or the lake with little P retention due to the high connectivity and inadequate maintenance. Assuming a retention coefficient of 20%, the estimated contribution of STS to the TP load into the lake increased to 10% (Campbell and Foy, 2008).

2.3.

JOVA catchments, Norway

In a long term monitoring programme in Norway (JOVA), exports of nutrients (N and P) from a range of catchments have been quantified to examine trends in nutrient loads from agriculture as farm-based mitigation options become implemented in each catchment (Bechmann et al., 2008). Exports are based on continuous stream flow measurements and analyses of nutrient concentrations in flow-proportional composite water samples. Although there are no major STW within the catchments, there are a number of private dwellings with STS that may be contributing to the nutrient loads measured at the catchment outlet and therefore systematically increasing the load estimates from agriculture. To assess their relative importance as a nutrient source, the loads of P from STS were estimated in four of the JOVA catchments (Skuterud, Mørdre, Time and Heia) using a model called GIS-wastewater developed by Bioforsk (Kraft and Turtumøygard, 1997). The four catchments are representative of different climatic zones, farming systems and soil types in Norway (Table 3) and had similar proportions of the rural population with STS (16–19 p.e. km 2). GIS-wastewater uses a scoring system to calculate the load of P from STS from p.e. living in the catchment, reduced by the retention in the operating sanitary systems (depending on system type and age) and soil retention (depending on soil type) and connectivity to the stream. Information on the location, p.e., system type and site information was obtained from local government records and/or by field survey. The model assumes one p.e. generates 1.7 g P day 1 or 0.62 kg yr 1. Retention coefficients for different types of STS and their age are given in Table 4. Soil

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Table 2 – Site properties and maintenance of individual wastewater treatment systems (IWWTS) in the Lough Melvin catchment based on a survey in 2008. Details

Total number

Other (%)d

Two chamber tank

Overall (%)

With percolation trenches (%)a

With soak pit (%)b

With no percolation area (%)c

42

28

10

20

43 19 38

64 14 21

83 0 17

0 11 89

46 14 40

38 52 10

93 7 0

67 17 17

67 11 22

62 28 10

43 57

57 43

33 67

0 100

38 62

5 33 62

0 21 79

0 0 100

0 0 0

4 24 72

62 24 14

86 14 0

50 17 33

11 22 67

58 20 22

81 19

79 21

67 33

89 11

80 20

All IWWTS 50 Age (yrs) 23 >20 7 10–20 20 0–10 Discharges to 31 Drain 14 Field 5 Stream Desludged 19 Yes 31 No Nearest drain (m) 2 >50 12 10–50 36 <10 Nearest watercourse (m) 29 >50 10 10–50 11 <10 Soil drainage 40 Poor 10 Moderate

a Drainage channels in the percolation area assist effluent movement into the subsoil. This type may still be connected to a surface drain when soils are at field capacity. b A hole in the ground filled with stones through which effluent is discharged but the pit may still be connected to a stream or river either directly via a field drain. c Connected directly to a stream or river. d Puraflow systems with peat-based filter and percolation area, two chamber round tank with percolation area; package plant aeration systems.

retention coefficients can vary from 0.1 for large STS on thin stony soils very close to the stream to 0.8 for small STS on retentive sandy moraine soils that are >100 m from the stream. Further details are given by Kraft and Turtumøygard (1997).

Measured annual (May–April) loads at the outlet of each catchment in 2009/10 and 2010/11 and estimated loads from STS are shown in Table 3. There are large variations in P export between the four catchments due to area and amount of erosion, which was high in Skuterud and Mørdre. Per capita

Table 3 – Selected characteristics of the four monitored JOVA catchments in Norway together with the measured annual total (TP) loads, and estimated TP loads from STS, in each catchment in 2009/10 and 2010/11. Skuterud Area (ha) Agric. area (%) Soil type Dominant crop Mean temp. (8C) Mean annual rainfall (mm) No. of p.e.a TP load (kg yr 1) 2009/10 2010/11 TP load (kg yr 1) TP load (% of catchment) 2009/10 2010/11 a

p.e., person equivalents.

Mørdre

Time

Heia

449 61 Silty clay Cereal 5.5 785 78 Catchment 392 1060 Septic tank systems 7.4

680 65 Silt and clay Cereal 4.3 665 106

91 94 Silty sand Grass 7.1 1189 18

160 62 Sand and silty clay Cereal, potatoes and vegetables 5.6 829 30

843 1807

96 75

– 235

51

5.4

12

1.9 0.7

6.0 2.8

5.6 7.2

5.1

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Table 4 – Retention coefficients for P used in the GIS-wastewater model for different types of STS under standard conditions (100 m distance to stream, new (2011) system, 1–5 p.e. in each dwelling). The numbers of the different types of STS in each of the four monitored JOVA catchments in Norway are also given, with those classed as sub-standard given in parenthesis. Wastewater system

Biological toilet Soil infiltration systema Package wastewater treatment plant (class 1: bio. and chem.)b Package wastewater treatment plant (class 2: bio.) Sandfilter systemc Septic tank direct to soil surface Septic tank direct to the watercourse Unknown Sealed tank Total a b c

Retention of P (%)

90 90 90 60 75 40–80 5

Numbers in each catchment Skuterud

Mørdre

Time

5 (0) 8 (0) 3 (3)

1 (0) 8 (0)

1 (0)

1 (1)

2 (0) 1 4 3 30 1

(1) (4) (3) (30) (1)

1 (1) 2 (2) 1 (1)

9 (9)

48 (39)

5 (4)

1 (0) 12 (9)

100 17 (4)

Heia

% retention reduced by 10% for each year older than 17 years. 75% for systems established before 2000. % retention reduced by 10% for each year older than 5 years.

loadings from STS were lowest in Skuterud (0.09 kg p.e. 1) and highest in Mørdre (0.48 kg p.e. 1), reflecting the large differences in the proportions of STS discharging directly into the stream. The contributions of STS to total annual P load (assuming no in-stream retention) never exceeded 8%, and were particularly low in Skuterud (2%) due to the higher standard of treatment (Table 3). In Mørdre and Skuterud, the relative contribution of STS was much lower in 2010/11 than in 2009/10 due to much larger rates of stream discharge and flowweighted TP concentrations. In contrast, STS contributions were similar in both years in the Time catchment. Total catchment P export varied considerably from month to month depending on discharge and farming practice. Large stream discharges and P fluxes were recorded due to high precipitation and runoff during autumn months and due to snowmelt in spring (Fig. 2). Low stream discharge and low P fluxes were observed during summer months (May–July) and

during winter when streams were frozen (December–February). The contribution of P from STS (which can be assumed to be near constant throughout the year) as a proportion of the total catchment P load therefore varied widely. In autumn and spring, STS P loads were relatively unimportant but in summer when there is little runoff, STS become a much more significant source of P (Fig. 3). Maximum STS contributions to catchment P loads in summer were 34% in Skuterud (June 2009), 127% in Mørdre (July 2009), 76% in Time (June 2010) and 40% in Heia (May 2010). The value of over 100% for Mørdre suggests that retention of P occurs within the stream system between the point of discharge (a first order tributary) and the catchment outlet. During winter when the soil is often frozen and runoff is minimal, STS loads are also apparently greater than the monthly catchment P load, especially in Mørdre. However it is likely that the discharge of P from STS also becomes frozen and will then contribute to P export during the subsequent snowmelt in spring.

Fig. 2 – Total measured P loss from the four JOVA catchments in 2009/10 and 2010/11 and the calculated source contributions of P from point-source STS (the total P loss includes STS sources).

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Fig. 3 – Monthly export of P from point-source STS expressed as a percentage of the monthly total P export from each catchment in 2009/10 and 2010/11.

2.4.

Ulster Blackwater catchment, Ireland

In the border region of Ireland, Arnscheidt et al. (2007) correlated the number and condition of STS with low flow P concentrations in small streams during the summer. Jordan et al. (2007) indicated that low flows in these streams exhibited diurnal signals that could be attributed to waste water discharges; as measured by high-resolution bank-side P analysers (Cassidy and Jordan, 2011). To further assess the effects of STS discharges on water quality, a 2007–2008 (1st April–31st March) high resolution P and discharge dataset reported in Macintosh et al. (2011) was analysed with reference to total and low percentile flows and P loads in two of the same small (5 km2) agricultural sub-catchments in Co. Tyrone and Co. Monaghan, part of the 1500 km2 Blackwater River which flows into Lough Neagh. Here, sub-hourly data (three and four samples per hour, respectively) were collated to hourly averages for the two catchments (see Supplementary information). Macintosh et al. (2011) reported that STS densities in the Tyrone and Monaghan catchments were 3.4 km 2 (17 in total) and 13.8 km 2 (69 in total), respectively in 2007–2008 and with no central STW. As rural Ireland household densities in these counties are approximately 3 people and average human P emissions are approximately 0.7 kg person yr 1 in the EU (CEEP, 2008 – citing de Madariaga, 2007 and Schmid Neset et al., 2008), the daily human P loading in these two catchments approximates to 35 kg yr 1 and 143 kg yr 1, respectively. While these figures refer to potential P loading to STS it is often more difficult to estimate P loading from the same systems due to the variability in treatment technology and soil attenuation mechanisms. A measured total of 586 kg TP was exported from the Tyrone catchment and 1869 kg from the Monaghan catchment over the year and related mostly to a series of storm runoff events during the summer and autumn/winter periods. The

magnitude of these loads compared with the estimated potential export of TP from STS highlights the overwhelming dominance of diffuse P transfers during wet weather in this environment, a process reported extensively in the literature (e.g. Heathwaite and Dils, 2000; Sharpley et al., 2008; Doody et al., 2010). At worst, STS discharges (for example direct discharges of 0.7 kg person yr 1) represent only 6% and 8% of total annual export from the two catchments, respectively. This figure is in close agreement with the 5% septic tank contribution to the total P budget to all inland and coastal waters estimated by Smith et al. (2005) for the whole of Northern Ireland and is likely to be even closer when considering at least some soil attenuation. The hourly TP time series (Supplementary information) shows the variability in concentration over a year and, in the 2007–2008 year, a wet summer period precluded a detailed comparison of summer and winter STS loads. However, periods of stable low flow were observed during the ranked percentile Q90–Q95 flows in April, May and September (Fig. 4 and Supplementary information). Extracting these data shows that TP load varied during these months from 2 to 3 g h 1 and 4 to 6 g h 1 (2nd quartiles) in the Tyrone and Monaghan catchments, respectively. The corresponding concentration ranges are also shown in Fig. 4 and, for Monaghan, the TP concentration approached 0.2 mg L 1 for sustained low flow periods due to the higher low flow TP loading. The hourly TP load in Monaghan was twice that of Tyrone during these flows, despite four times the potential wastewater loading, and this disproportionality is most likely due to the increased drainage density over lower gradients and the potential this gives for stream bed attenuation in Monaghan (Arnscheidt et al., 2007).

2.5.

Dee catchment, Scotland

The River Dee in North East Scotland is a major river of exceptional conservation value due to its populations of

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Fig. 4 – Discharge TP concentration and TP load for two Irish border catchments during low flow Q90–Q95 flows taken from hourly average time series. Sample sizes were 92, 31 and 143 for April, May and September in Tyrone, respectively; and 178, 114, 120 in Monaghan, respectively.

Atlantic salmon (Salmo salar), otter (Lutra lutra) and freshwater pearl mussel (Margaritifera margaritifera). It is a gravel-bed river, rising at 1220 m in the Cairngorms National Park and flowing 136 km east to enter the sea at Aberdeen. The main stem, along with 17 major tributaries, drains a catchment of 2100 km2 with an upland area (montane, moorland and forest) in the south and west of the catchment and more intensively managed lowland arable and improved pasture in the east (Cooksley, 2007). The total population of the Dee catchment is 146,177 with 99,268 living in Aberdeen City. Water quality in the lower half of the catchment is downgraded due to a range of nutrient and pathogen source pressures that threaten the large number of private water supplies and the sensitive condition of the pearl mussel population. This includes STS located in clusters near tributaries (hotspot areas). It is estimated there are close to 5000 individual dwellings with private STS (2179 were registered by 2011) in the catchment (SEPA, 2010 characterisation data), representing a density of >2 km 2 and 25% of the catchment population outside of Aberdeen. The majority are located in the lower catchment, with around 92% discharging to a soakaway and 8% discharging directly to watercourses. Reducing pollution from STS was consequently identified as a priority for the area’s catchment management planning group, the Dee Catchment Partnership (DCP, Cooksley, 2007). Potential hotspots were identified by linking the locations of STS with existing water quality data and the locations of private water supplies. Properties not served by mains sewerage were assumed to be served by a STS, only half of which were registered. Five of the catchment’s tributaries were identified as being impacted by P according to UK targets

set under the Water Framework Directive (WFD); all had STS in close proximity (Fig. 5). Modelling studies indicated that STS were contributing ca. 10–14% of the soluble reactive P (SRP) load in these sub-catchments (Anthony et al., 2005). One subcatchment (Tarland) where STS were contributing 13% of the SRP load also had a high number of private water supplies. This area was selected for a targeted awareness raising campaign and for the removal of a large STS as part of a suite of mitigation options implemented in this sub-catchment (Bergfur et al., 2012). A two-year catchment-wide campaign was initiated to raise awareness of good STS management, specifically targeting the hotspot areas, and to help establish mechanisms for long-term provision of information and advice. To raise awareness, a new leaflet was produced as part of a roadshow and distributed across the catchment at community information points. The STS was decommissioned in September–October 2002 because the condition of the Tarland stream indicated that it was failing. The STS served a care home with about 20 residents and was discharging effluent directly into the stream. Instantaneous stream flow volumes and nutrient concentrations have been monitored (3–14 times per year) at a point ca. 1 km downstream of the STS since 2000. Average flow in the Tarland stream over the monitoring period was 0.11 m3 s 1 (range 0.02–0.66 m3 s 1). Annual mean concentrations of SRP decreased significantly (P < 0.01) from 0.08 to 0.02 mg L 1, and annual mean NH4N concentrations decreased from 0.08 to 0.04 mg L 1 (P < 0.05), after the STS was decommissioned (Fig. 6). The reduction in SRP concentrations represents a shift from moderate to high ecological status.

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Fig. 5 – Septic tanks occurring in close proximity to nutrient-impacted watercourses in the River Dee catchment. # Crown copyright and database right (2012). All rights reserved. The James Hutton Institute, Ordnance Survey Licence Number 100019294. Septic tanks locations derived by SEPA.

3.

Discussion

According to the US Environmental Protection Agency (USEPA), adequately managed decentralised wastewater systems (i.e. such as STS) are a cost-effective and longterm option for meeting public health and water quality goals, particularly in less densely populated areas. The use of these technologies ‘‘can achieve significant cost savings while recharging local aquifers and providing other water reuse opportunities close to points of wastewater generation’’ (USEPA, 1997). In view of the very high costs of installing and maintaining centralised systems (i.e. STW), then it seems logical and appropriate that judicious use of

Fig. 6 – Removal of a large STS in September/October 2002 achieved significant reductions in the concentrations of soluble reactive P (SRP) and ammonium-N in the Tarland tributary of the River Dee, Scotland. Error bars represent the standard error of the mean.

STS (or other IWWTS) must remain part of the infrastructure of rural wastewater treatment. Their effectiveness is clearly dependent on proper siting, design, installation, operation, maintenance, inspection and monitoring but this is considered achievable. However, many STS have been historically installed for expediency and without proper design considerations relating to pollution risk creating a number of subsequent problems (Butler and Payne, 1995; Day, 2004). The surveys in the Lough Melvin and Norwegian catchments suggest that many STS are not being adequately maintained and are discharging directly to the stream or to a drainage network connected to the stream. Direct discharge is now illegal and prohibited but still clearly occurs in older systems which do not have a soakaway or where the soakaway is no longer adequately functioning. Other surveys in the literature report very similar findings. In a survey of 48 STS in Australia, Ahmed et al. (2005) found that only 15% were well maintained, 67% of septic tanks needed solids removal, 72% of soakaways were soggy and (by inference) ineffective, 8% needed structural repairs, 4% were incorrectly sited and 6% had insufficient capacity. Using two pathogen strains as indicators, they found a direct link between STS failure and surface water contamination. In a survey of 23 IWWTs in Belgium, 52% did not meet legal water quality standards, largely due to a lack of adequate maintenance (Moelants et al., 2008). In Ireland, Arnscheidt et al. (2007) conducted a risk analysis of 113 STS and found that 35% were at high risk of causing pollution and over 70% were at medium risk of causing pollution. Among the main causes of failure based on the scoring system used were insufficient soakaways and direct discharges. The highest failure rate was in the Monaghan catchment where STS density was highest (Fig. 4). Targeted survey work is clearly an important tool for investigating STS condition and connectivity to watercourses.

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Both the modelling predictions (JOVA catchments) and stream monitoring (Blackwater catchments) results illustrate the apparent ambiguity of STS impacts in some freshwaters. Despite being a small part of an overall P budget leaving catchments at small or large scales, the continued discharge of STS effluents and loss of dilution capacity in receiving streams is likely to maintain a trophic effect which manifests during periods of stable low flow. Using the contribution to annual catchment loads to estimate eutrophication risk associated with STS is clearly misleading as STS discharges can dominate nutrient inputs to streams during periods when the loading from agriculture is low or absent. Increased stream nutrient concentrations (as opposed to annual load) are likely to have a much larger impact on stream ecology, especially as a high proportion (70–80%) of the P discharged from STS is in a dissolved and highly bioavailable form (Edwards et al., 2000; Withers and Jarvie, 2008; Withers et al., 2011). During the summer months, these STS inputs are likely to have much greater ecological influence than when discharged during the winter months when aquatic biota are not so biologically active (Jarvie et al., 2006). Stream data from the Tarland after decommissioning of one large STS discharging effluent directly to the stream suggests that STS also increase annual average nutrient concentrations in headwaters and not just during the summer period. Withers et al. (2011) found that large increases in stream nutrient concentrations downstream of STS in an English village during low flow increased annual average concentrations by up to tenfold depending on dilution capacity. As the court ruling against Ireland demonstrates, the risks that STS pose to water quality and human health should be controlled through a supervised registration system and regulations governing the maintenance of STS. The case studies presented here support this view and the need for registration. At present, registration and regulation of STS varies considerably across the EU and USA. While the Irish were found to have a system of controls, they were not being consistently implemented. Similarly in the UK, registration and regulation is only now being considered for widespread implementation. Without an adequate registration scheme that can capture unconsented discharges, it is difficult to know the precise contribution that STS make to catchment nutrient loads. In the Nadder and Bure catchments, over 95% of STS were unconsented and the contribution of agriculture was consequently overestimated. This may lead to potential failures or delays in achieving water quality targets and farmers may also become disillusioned that the right sources are not being adequately targeted. Accurate source load apportionment is central to cost-effective implementation of eutrophication control measures. Attenuation of nutrient discharges from septic tanks via soakaways in areas with slowly permeable clay soils, extensive field underdrainage, a dense river network and high rainfall is clearly a challenge, as reported by Patrick (1988). In such circumstances, alternative systems that do not rely on soakaways, including localised centralised infrastructures, must be more rigorously evaluated. Research into alternative systems on impermeable soils is on-going in the RoI (Gill et al., 2007, 2009a,b) and detailed tests on soil properties are now mandatory at the planning stage before

choosing an appropriate system (EPA, 2009). The Lough Melvin survey suggested package plants are becoming more prevalent (Table 2), but while this alternative system may control organic discharges, they do not effectively remove P but rather solubilise and mineralise particulate and organic P (Gill et al., 2007). Further research into alternative systems that reduce the risks to water quality and human health is still urgently required. The awareness raising initiative in the Dee catchment proved extremely popular (8000+ leaflets distributed; 100+ roadshow visitors) and has been extended to the wider northeast area of Scotland with the aim of eventual Scotland-wide coverage. The leaflet is now sent out (a) by the Local Authority when building warrants are issued in relation to STS installation, (b) by the water regulators (Scottish Environment Protection Agency, SEPA) when tanks are registered, and (c) by local STS maintenance companies. The STS roadshow was a useful means of increasing awareness in other catchments. Campaigns such as this are inexpensive, public response is good and they can be used to highlight other issues such as mitigating nutrient loss from agriculture, farmyards and public STW. However, there is uncertainty regarding their effectiveness – do they reach the right people, how do people respond in the long term, and are the changes in behaviour advocated effective in achieving measurable improvements?

4.

Conclusions

Five case studies from across Europe have highlighted that the contribution of STS to water quality deterioration is greater than currently assumed in catchment management planning. The contribution of STS to total nutrient loading at catchment scale is highly dependent on STS density, and the relative contribution of STS to catchment nutrient loads can be underestimated if not all STS in the catchment area are included. Although their contribution to total annual nutrient loads at the catchment scale is often relatively modest (<10% for total P), STS discharges can lead to highly elevated stream nutrient concentrations, especially during summer months with potentially major impacts on river or stream ecology. A more sophisticated resolution of source apportionment is needed to accurately assess the risk of localised eutrophication arising from clusters of STS in close proximity to watercourses. This is equally important for characterization of temporal variations in nutrient loading from STS (and other rural point sources) that elevate eutrophication risk in headwater streams. The catchment surveys of STS reported here suggest a large proportion of them are not maintained adequately, are discharging directly to streams, or are by-passing ineffective soakaway areas to dense drainage networks in areas with impermeable soils. Targeted surveys are an effective tool for highlighting such STS failures and should be considered as part of effective catchment management. Raising awareness amongst catchment stakeholders may be a cost-effective method of encouraging and facilitating improved management of STS and highlighting alternative systems that might be implemented in high-risk situations. In the EU, the recent Irish experience has set a precedent for a formal and comprehen-

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sive registration and implementation policy backed by legislation to control discharges from STS and other IWWTS. A greater challenge, however, will be developing effective mitigation policies, especially where the impact site or catchment is very sensitive or valuable.

Acknowledgments Natural England funded the source apportionment work in the Nadder and Bure catchments. The Lough Melvin Nutrient Reduction Programme was funded by the EU INTERREG IIIA Programme with support from Northern Regional Fisheries Board; Agri-Food and Biosciences Institute (AFBI) Northern Ireland; Queen’s University Belfast and Teagasc. Marc Stutter and Julian Dawson (both James Hutton Institute) provided the Tarland data. Graham Stephen produced the Dee catchment map, using septic tank locations derived by SEPA. The Dee Catchment Partnership is funded by Aberdeenshire Council, Aberdeen City Council, Aberdeen Harbour Board, Cairngorms National Park Authority, James Hutton Institute, SEPA, and Scottish Natural Heritage.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.envsci.2012.07.023.

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