There have been six sewage effluent discharges to streams feeding the Fleet and to the Fleet itself, listed below in order from west to east (and shown in the linked figure) and with level of treatment indicated:

Abbotsbury Swannery Restaurant (private) secondary;

Abbotsbury sewage treatment works (STW) (Wessex Water plc) secondary;

Langton Herring STW (Wessex Water plc) secondary;

Moonfleet Manor Hotel STW (private) secondary;

Royal Engineers Training Camp (RETC) Chickerell STW (private, managed by Wwplc) secondary (now revoked);

RETC Wyke Regis (Bridging Hard) (private) now revoked.

Discharges of sewage effluent have been monitored by the Environment Agency from 1990 to present. Frequency of monitoring is, however, low, due to the small size of the discharges involved. The sampling regime followed national criteria based on dry weather flow (DWF) of each works involved (EA 1997a). All concentrations below are expressed as µg/l-N or µg/l-P.

Although sparse, data obtained from 1990 to 1997 have been examined for trends in water quality. Due to the lack of long term data and infrequency of sampling, analysis for statistical significance was not performed, and the conclusions reached are therefore tentative.

Abbotsbury Swannery Restaurant has a DWF of 10.3 m³/day and is therefore sampled four times per year. It discharges into Abbotsbury Mill Stream (called Mill stream (Abbey Barn) in other EA reports), and has consent limits of 20 mg/l BOD, 10 mg/l ammonia and 30 mg/l suspended solids. No trend analysis of effluent monitoring results was carried out as this was a new consent, effective from February 1996.

Abbotsbury STW is the largest input, with a dry weather flow (DWF) of 140 m³/day, and is therefore sampled 12 times per year. It discharges into Portesham Mill Stream above Horsepool (called Mill Stream (Horsepool) in other EA reports). From 1990 to >96 a general decrease in concentrations of ammonia, nitrate, nitrite, TON and ortho-phosphate was observed in the final effluent from Abbotsbury STW.

Langton Herring STW has no numeric limits associated with its discharge consent, with an estimated DWF of <100 m³/day, and is therefore sampled four times per year. It discharges into Rodden Stream below Langton Herring village. Despite infrequent sampling, its effluent also showed a general decline in ammonia and ortho-phosphate concentrations from 1990 to >96. Nitrate concentrations have remained relatively constant.

Moonfleet Manor Hotel has a private STW with a DWF of 27 m³d, and is therefore sampled four times per year. It discharges direct to the Fleet, and has a history of poor consent compliance. Its consent is 50 mg/l BOD, 40 mg/l ammonia, 60 mg/l suspended solids. A reed bed has now been installed, which should provide improved effluent quality. No trend analysis was carried out due to lack of data.

RETC Chickerell STW had a DWF of 64 m³/day discharging direct to the Fleet, and is therefore sampled four times per year. Ammoniacal nitrogen concentrations of the effluent were very variable (probably due to short term variations in population at the camp). This discharge was incorporated into the mains sewer system by April >99.

RETC Wyke Regis was a small septic tank with a DWF of 9 m³/day and entered the Fleet at the Narrows where there are strong tidal currents and dilution is large. No elevation of nutrients at this point was detected during the surveys. This discharge has also been eliminated by transfer of flows to mains sewerage.

The relative contributions of these effluent inputs to nutrient concentrations within the Fleet lagoon itself has been modelled. The results of the nutrient load modelling are considered in the next section.

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