Accounting for phosphate inputs to water

Transcription

1 Accounting for phosphate inputs to water The issue of on site sewage treatment systems (septic tanks) Linda May, Paul Withers, Helen Jarvie & Phil Jordan

2 Outline of talk Numbers & locations Estimated P export to environment Impacts on water quality Remediation Conclusions

3 Outline of talk Numbers & locations Estimated P export to environment Impacts on water quality Remediation Conclusions

4 On site sewage treatment systems (STS) stream septic tank Approximately 1.4 million STS in UK ( official figures suggest 200,000) Many are close to watercourses Some discharge directly to the drainage network Exact numbers and locations are unknown

5 Sewerage Unconnected Properties (%) Estimates from large area statistics Large Area Analysis - All Data % Unsewered properties North West England Northern Ireland Scotland , ,000.0 Density of properties (no. km -2 ) Property Density (No. per km 2 ) Stephen Anthony, pers. comm.

6 Estimates from aerial photography Upper Hamshire Avon (Area: 664 km 2) 367 (11%) consented 2953 unconsented Equivalent to a sewage works serving 10,000 PE? May et al. (2012) Report to Natural England & Broads Authority

7 Estimates from map data May et al. (1999) Diffuse Pollution & Agriculture, vol. II

8 Improved knowledge from STS registration From April 2006, Scotland: All new domestic discharges require authorisation; > 15 PE require a licence Existing unconsented discharges must be registered during house sale England & Wales All discharges to be registered, but suspended in England pending review of the need Some planning authorities in Scotland apply the 125% rule when consenting new developments in P sensitive catchments

9 Outcome of improved registration in Wales River Conwy, North Wales

10 Outline of talk Numbers & locations Estimated P export to environment Impacts on water quality Remediation Conclusions

11 P sources in catchments No of STS P discharge (t P y -1 ) England & Wales 1.2M 1,560* Scotland 0.1M 130 N. Ireland 0.1M 130 Total UK 1.4M 1,810 TP Fertiliser derived P (UK) However, many STS discharge into areas with low agricultural intensity that are relatively close to sensitive freshwater ecosystems, e.g. headwater streams ~ 1,629 SRP** 123,200 TP ~ 61,600 SRP*** * P export = 1.3 kg tank -1 y -1 ** 90% is SRP *** 50% is SRP

12 Even more when things go wrong Poor design - system failure - direct discharge to water - storm runoff sensitive Incorrectly sited - hydraulic failure on impermeable soils - too close to a watercourse Poor management - leaks, blockages, chemicals - rarely emptied - biomat blockage Reduced processing Increased surface runoff Discharge to watercourses

13 Developing guidance to reduce risk System age (n = 48) Fildes (2011) Report to Shropshire Hills AONB Partnership

14 Hidden P sources within your catchment Typical rural scene with scattered houses; STS hidden, so impacts on water quality not immediately obvious! Direct discharge into stream; sewage fungus suggests a different story!

15 Outline of talk Numbers & locations Estimated P export to environment Impacts on water quality Remediation Conclusions

16 STS impacts on water quality, Wyre catchment Upstream Downstream Site no. Nicholson (2007) MRes. Thesis, Lancaster Univ.

17 SRP concentration (µg l -1 ) TP concentration (µg l -1 ) STS impacts on water quality, Eye Brook Upstream Downstream 0 Belton Bridge Loddington Upstream Downstream Belton Bridge Loddington Withers et al. (2011) Environment International

18 Catchment Hotspots - Hornsea Mere Outflow Maximum in-lake P concentration: 1.2 mg l -1 May et al. (2010) Report to Natural England

19 STS discharges, Chitterne Brook Load (kg d -1 ) Flow (m 3 s -1 ) Load (kg/d) Flow (m 3 /s) As some of you may be aware, certain houses in Chitterne have had their septic tanks flooded TP load (kg/d) B load (kg/d) flow (m3/s) Boron correlates with phosphorus STS source (detergent) 0 0 A S O N D J F J M A M J J A S O N D May et al. (2012) Report to Natural England & Broads Authority

20 Patterns of delivery, Chitterne Brook Isolated rural dwellings; near-river soakaways Initial dilution of point sources Mobilisation of SRP and B as groundwater levels rise and intercept septic tanks & soakaways Jarvie et al. (2008) Journal of Hydrology

21 STS contamination, Blackwater River Arnscheidt et al. (2007) Science Total Environment

22 OP load (g d -1 ) STS discharge during high rainfall, Loweswater SRP load (g P d -1 ) Input of P to lake from all septic tanks ~ 14% 0 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep 2004/2005 Maberly et al (2006) Report to Rural Development Service & National Trust

23 STS contribution to catchment P load Consented tanks 94% 6% Chitterne Chitterne 60% All tanks 40% Septic tanks Agriculture Often assumed to be negligible Correction for unregistered STS suggests otherwise! Consented tanks 42% 56% Consented tanks Wylye 21% 23% All tanks 56% All tanks Sewage works Septic tanks Agriculture May increase river TP concs. by 17 µg P l -1 (H._Avon) to 86 P µg l -1 (Norfolk Broads) Enough to fail WFD targets 2% May et al. (2012) Report to Natural England & Broads Authority

24 Outline of talk Numbers & locations Estimated P export to environment Impacts on water quality Local scale Catchment scale Remediation Conclusions

25 Better public information

26 Better public information Impact unknown

27 First time sewerage scheme, River Chew River Chew SRP concentration (µg P l -1) First time sewerage scheme Average SRP concentrations fell from 251μg P l -1 to 86 μg P l -1 after scheme introduced Barden (2007) Report to EA

28 Outline of talk Numbers & locations Estimated P export to environment Impacts on water quality Local scale Catchment scale Remediation Conclusions

29 Conclusions Mounting evidence that STS discharges are a problem at local and catchment scales Discharges can occur under high and low flows Affect stream P concentrations Pollution may be incorrectly attributed to agriculture Catchments with high STS densities are most vulnerable Removing/upgrading of STS appears to improve water quality but technical solutions to P discharge issues are deperately needed P discharged is highly bioavailable, so high risk of ecological impacts STS are poorly understood; need better process understanding to inform best design/management practices