Tathra River Estate Development- Impacts on Bega River Estuary

Context

The South Coast Independent Review Panel has reported to Minister Sartor on development proposals for the Tathra River Estate.  The review has recommended that the scale of the development be limited to 300 dwellings, to enable the provision of sewerage services to it and the existing Tathra River Estate Stage 1.

The land is adjacent to the Bega River Estuary and Black Ada Swamp.  The Bega Estuary is an Intermittently Open and Closed Lake or Lagoon (ICOLL).  The Bega River mouth tends to be closed to the sea more often than it is open, notwithstanding the Bega Valley Shire Council’s practice of excavating the entrance when the road approaches to the bridge are at risk of damage due to a high water table.  Nutrients from sewerage pose a potential threat to the health of the estuary because they could stimulate a bloom of toxic marine algae, such as occurred in Wonboyn Lake in March 2002 (killing 90% of the oysters).

Current Sewerage Treatment

The Tathra Sewerage Treatment Plant (STP) has spare capacity for about 400 dwellings, having recently been upgraded.  Half of this spare capacity has been allocated to infilling of dwelling lots within the existing urban areas of Tathra.

The Tathra STP disposes of some effluent on the golf course.  However, due to limited requirements for water (in cool and wet weather and during the summer tourist peak), excess effluent is disposed of by dune exfiltration from storage ponds.  This effluent tends to end up in the estuary rather than directly out to sea, due to wave and tide action.  While phosphorous stripping with alum (aluminium sulphite) is feasible, chemical stripping of nitrogen is not undertaken due to the expense, and limited denitrification of the sewage stream occurs through biological activity.

There have not been dangerous toxic algal blooms in the Bega Estuary to date, although “nuisance” (malodorous and irritant) blooms have been recorded.  The cause of these may have been discharges from the Bega STP and agricultural runoff as well.

Sewerage Proposals for the Tathra River Estate expansion

One of the preferred options put forward by the proponents is to treat a proportion of the effluent using membrane technology to a standard suitable for reticulation as recycled water for reuse onsite.

This raises the question of the fate of excess effluent in prolonged wet or cool weather.  The report mentions “Identification of a site on the western end of the peninsula for recycled effluent disposal.”  

This means that a significant volume of treated effluent would end up in the estuary.  No information has been given in the report on the nutrient load from this effluent, and the sandy soils on the western peninsula are unlikely to have the capacity to absorb significant nutrient loads.

The Bega Estuary has remained remarkably healthy, in spite of extensive agricultural development in the catchment.  The Healthy Rivers Commission report on the Bega Catchment cited estimates that there is about 27 times the amount of nutrient produced from grazing activities compared to human waste.  However, the Bega River is tightly confined at Bottleneck Reach so that nutrient-rich flood waters spread out across Penuca Swamp and Horseshoe Lagoon, natural sinks for nutrients.  The velocity of flood flows in the lower Bega River has apparently been sufficient to carry a high proportion of bedload sediments through to the ocean. 

Discharge of further nutrient rich effluent into the Bega Estuary at times when it is closed to the ocean has the potential to cause eutrophication of the waters and blooms of noxious or toxic marine algae.    The Independent Panel Report contains insufficient information on the likely nutrient loads and the timing of their discharge into the Bega Estuary in order to make a robust judgement about the risks posed to the estuary health. 

Some of the recommendations in the report are welcome (such as revegetation and protection of foreshore and riparian corridors, treated effluent reuse on site, integrated stormwater management).  However, there is a clear need to identify the likely increase in nutrient loads and the potential to ameliorate this impact through the use of storages, for example, or irrigation dispersal further away from the sensitive impounded waters of this sensitive and remarkably intact ICOLL.