Assessment of a novel development policy for the control of phosphorus losses from private sewage systems to the Loch Leven catchment, Scotland, UK
Introduction
The estimated annual total phosphorus (TP) load to British rivers is 41.6 kt yr−1. Households contribute 25.3 kt yr−1 (68.7%) of this, with 21.1 kt yr−1 being soluble reactive phosphorus (SRP), the most bioavailable form of phosphorus (P) in aquatic ecosystems (White and Hammond, 2006). Improved nutrient management practices associated with municipal waste water treatment works in recent decades have led to reductions in nutrient concentrations in receiving waters (Jeppesen et al., 2007). However, in many cases, ecological recovery lags behind chemical recovery (Jarvie et al., 2006, Jarvie et al., 2013). This is probably a result of legacy P release from bed sediments (Spears et al., 2011, Verdonschot et al., 2012) or insufficient reduction of P inputs from external sources.
It has been suggested that there are about 1.5 million private sewage systems (PSS) within the UK. Recent studies suggest that 80% of these are working inefficiently, potentially causing significant P pollution of freshwater bodies in rural Britain (Selyf-Consultancy, 2002, Kirk et al., 2003). A significant issue in monitoring P discharges from PSS is the lack of data on their location and state of repair (May et al., 2010). Under the revised Groundwater Directive (Directive, 2006/118/EC), discharges from PSS are no longer exempt from groundwater protection legislation. To reflect this, regulations introduced in 2010 outlined a need for registration of PSS in England and Wales and environmental permits for those located in areas vulnerable to groundwater pollution (Bennett, 2011).
In England there is still debate over legislation surrounding PSS and their registration, and environmental permits for PSS are not compulsory. In contrast, in Wales, registration of PSS is legally required. In Scotland, under The Water Environment (Controlled Activities) (Scotland) Regulations (2011), owners are obliged to register their PSS with the Scottish Environment Protection Agency (SEPA), although this is only legally imposed if the property is to be sold.
In catchment scale TP export calculations, PSS are rarely accounted for separately (Wood et al., 2005, White and Hammond, 2006) and, if they are, they are represented by simplified export coefficients (Smith et al., 2005). These approaches may underestimate the impacts of PSS and have limited use at a site specific level (May and Dudley, 2007). Limited evidence of PSS impacts on waterbody P concentrations exist in the literature. High frequency river sampling in a 5 km2 Irish rural sub-catchment within the Lough Neagh basin, that had no obvious industrial or municipal point sources, identified a chronic TP base-flow transfer of c. 0.25–0.50 mg l−1 that was characteristic of pollution from PSS (Jordan et al., 2007). Arnscheidt et al. (2007) reported a correlation between in stream TP concentrations and indicators of faecal and grey water from PSS during low-flow conditions in three Irish rural catchments. Spot sampling conducted in English rivers downstream of PSS have indicated increases of up to 700% in TP concentrations, with impacted concentrations of 0.4 mg l−1 being reported (May et al., 2010). The impact of PSS on P concentrations in receiving waters is expected to increase in rural catchments under low-flow conditions when dilution levels are reduced (Foy et al., 2003, May et al., 2010, Macintosh et al., 2011). Evidence suggests that, in some catchments, PSS may contribute significantly to the net P loading of their drainage waters, driving the need for legislation to address such potential impacts.
In east Scotland, UK, a novel planning policy has been put in place to address the potential increase in P discharges to the Loch Leven catchment from new developments with PSS. Under the Town and Country Planning (Scotland) Act 1997; as amended by the Planning etc. (Scotland) Act 2006 (amended in 2009) (Scottish Government, 2009), councils and national park authorities must construct a Development Plan (DP) to manage building development. The Loch Leven Catchment is covered by the TAYplan Strategic Development Plan (TAYplan, 2012), which provides guidance for an area of 8112 km2 with over half a million inhabitants. Local planning authorities must convert these broad DPs into a more detailed local development plan (LDP) that details land use policies and proposals for their area (Fig. 1, upper panel). DPs and LDPs may also accept supplementary guidance. For example, The Kinross Area Local Plan (2004) adopts the principles of the Loch Leven Catchment Management Plan (1999) (LLCMP) for the control of pollution to Loch Leven (Fig. 1).
The Kinross Area Local Plan (2004) contains novel rural policies that aim to ensure that new developments do not increase P loading to the Loch Leven catchment. The policies are aimed at individuals proposing any form of rural development within the catchment that require a PSS (policy 10). It states that the future P output from the PSS must be estimated (policy 11) and that measures to mitigate the estimated output to the catchment by 125% must be proposed (policy 12). This should be achieved by upgrading third party primary treatment PSS to systems with secondary or tertiary treatment (Loch Leven Special Protection Area and Ramsar Site, 2011). In the following text, these policies are, collectively, termed ‘the 125% rule’. The 125% rule assumes that PSS with secondary treatment (i.e. wetlands, reed beds and mechanical treatment plants) or tertiary treatment (i.e. sand filters, drum filters, membrane systems or chemical dosing) will produce lower TP discharge concentrations than PSS with primary treatment (single septic tank treatment, only) (SEPA, 2011), thereby reducing the P discharge to the environment. The efficacy of this new legislation relies on the accuracy of the desk based TP load estimation for proposed PSS and requires validation in the context of potential benefits or threats to the net TP load to the Loch.
In order to better understand the effectiveness of the 125% rule, we quantified potential uncertainty using the current desk based calculation procedure and compared it to actual measured TP concentrations from seven PSS within the Loch Leven catchment. The potential change in P output from projected developments over the next 90 years was forecast using both of these approaches. The results are compared and discussed in relation to potential policy appraisal.
Section snippets
Site description
Loch Leven is a large shallow lake (mean depth 3.9 m; surface area 13.3 km2) with a surface water catchment of 145 km2 that is dominated (80%) by agriculture (LLCMP, 1999). Due to its high conservation value, both nationally and internationally, it is recognised as a Special Site of Scientific Interest (SSSI), a Special Protected Area (SPA) (UK9004111), a RAMSAR site (UK13033) and is part of the Natura 2000 network. A detailed description of site characteristics is provided in May and Spears
Variation in P concentrations in PSS
No significant difference in TP concentration was observed between PSS or between treatment types of PSS in this study. The median TP concentration of all samples (9.28 mg l−1) most closely resembled concentrations expected from PSS with primary treatment (10 mg l−1) under the 125% rule assumptions (Loch Leven Special Protection Area and Ramsar Site, 2011, SEPA, 2011). These results indicate that secondary and tertiary treatments do not significantly reduce TP concentration in the sampled tanks,
Conclusions
The range of TP, SURP and PP in all seven PSS sampled were 1.91–18.01 mg l−1, 0.04–6.14 mg l−1 and 0.23–16.13 mg l−1, respectively.
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No significant differences in TP concentration between PSS with primary, secondary or tertiary treatment were observed in the PSS sampled in this study.
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Our results indicate that PSS treatment type may not be an accurate indicator of TP discharge.
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Policy changes should be made to encourage efficient and routine monitoring of all PSS.
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The importance of human domestic
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