Lower Molonglo Water Quality Control Centre

Biological monitoring report
for the Molonglo and Murrumbudgee Rivers, 1998

This report was prepared by Ecowise Environmental for
ACTEW Corporation
August 1999
Report number: LMW ENV/99-1

Foreward

This is the 1998 Biological Monitoring Report, which describes the ACTEW Biological Monitoring Program in the Molonglo and Murrumbidgee Rivers in the vicinity of LMWQCC.

This study was undertaken by Ecowise Environmental on behalf of ACTEW Corporation, and was conducted from January to December 1998 to assess impacts on the Molonglo and Murrumbidgee Rivers from the discharge of tertiary treated effluent from the Lower Molonglo Water Quality Control Centre (LWMQCC). This is the fifth year of the survey, and it follows from the 1997 Annual Biomonitoring Report.

The objectives of this report are to:

• describe the monitoring program
• present and discuss results of the program relative to previous years
• analyse the data and make recommendations based on current years' results and historic data.

Executive summary

LMWQCC has been monitoring benthic macroinvertebrates as part of its river health biomonitoring program since 1993. The objective of this program is to assess the impact of discharge from LMWQCC on the benthic fauna of the Molonglo and Murrumbidgee River systems. This report summarises the results of monitoring in the period from January 1998 to December 1998.

For the last two years the sampling regime has been extended to include winter and summer data comparing sections of the Molonglo and Murrumbidgee rivers upstream and downstream of LMWQCC. The macroinvertebrate faunal composition and abundance varies with season, and although winter and summer are generally the least productive seasons, they may nevertheless play an important role in the life-cycle of some organisms.

A marginal decline in total number of taxa collected at all sites (upstream and downstream of LMWQCC) was observed over the last three years. One of the possible reasons could be an impoverishment of benthic organisms in the Molonglo and Murrumbidgee rivers. If this trend is confirmed in future monitoring programs, the reasons for its progression may need to be investigated further.

1998 was an extraordinary wet year in winter, preventing us from collecting macroinvertebrate data during that season. Extended period of high flows generally present unfavourable habitat conditions for collecting a representative data-set (National River Health Program, 1996) and present difficulties in sample collection. The high flows in the winter of 1998 coincided with high nutrient influxes in both rivers, derived from extra runoff from the catchments of both rivers.

Data analysis for 1998 indicated a clear difference in presence and abundance of macroinvertebrate taxa between the Murrumbidgee and Molonglo rivers, with the Molonglo containing fewer taxa and lower abundances than the Murrumbidgee. The Molonglo was in poorer biological condition, with the effluent discharge being one of several impacts on the river. This difference in condition between both rivers was also detected in 1997. In contrast to the previous year, however, in 1998 the water quality and habitat conditions at 'upstream' sites was poorer than at the 'downstream' sites, indicated by fewer taxa being detected at those sites.

The AusRivAS analysis, which uses the ratio of expected to observed taxa at a site to obtain a rating of its biological condition, detected a significant deterioration of 'upstream' sites compared to 1997, while the sites downstream of LMWQCC remained similar to the previous year. Habitat conditions ranged from fair to good, but were more likely to have impacted on the fauna of the Murrumbidgee sites, due to greater sedimentation of the streambed from erosion associated with the high flows during winter. In general, impacts of the effluent discharge on macroinvertebrate taxa appeared to be few, and were difficult to separate from the impacts already present on the Molonglo River from upstream sources. During summer and spring, taxon richness was higher below the treatment plant than above, indicating a beneficial effect of the effluent on the fauna of the Molonglo.

In the Murrumbidgee, total phosphorus and total phytoplankton were correlated throughout the year. This suggests that flows and velocities were sufficiently slow to enable riverine phytoplankton growth as a response to increased nutrient levels (Hötzel and Croome, 1998). This contrasts with the Molonglo river, where this correlation was not apparent. The narrower streambed, smaller size of the catchment, and difference in substrate composition of the Molonglo is most likely resulting in a different flow regime, less favourable for phytoplankton proliferation, compared to the Murrumbidgee. Higher turbidity levels detected in the Molonglo would also have impeded phytoplankton growth. Physical/chemical water quality attributes that were significantly altered by the effluent discharge for the 'downstream' sites were elevated nutrient levels, higher temperatures, elevated conductivity and increased pH. Some of these changes were restricted to particular sites or seasons only.

For some of the data, statistical analysis (CANOCO) was able to show a correlation between variability in benthic composition and physical/chemical variables. However, the analysis was limited by the small size of the data set. For example, elevated conductivity and nutrients appeared to correlate strongest with higher or lower abundances of a small number of taxa. Some other taxa that were present in significantly higher or lower numbers at downstream sites were not correlated with physical/chemical parameters, but masked correlations are likely to exist. The affected taxa ranged from being pollution tolerant to pollution sensitive.

An evaluation of the existing rapid assessment models (AusRivAS, Chessman biotic index) concluded that these models provide no information on the possible causes for the biological condition of a site; as a result they also lack predictive capability in this respect. In addition, important information (for example, abundance) is often lost and the robustness of both the AusRivAS and Chessman models is ambivalent for small data sets. Therefore, the possibility of developing a point source prediction impact model was contemplated, which would aim to have a predictive capability for impacts resulting from physical/chemical parameters changed by the effluent discharge. Such a model should be based on:

• parameters for which information has been collected to date, in order to enable a historical analysis of trends
• indicator taxa (pollution indicator taxa, "pest" taxa, "beneficial" taxa)
• possibly expanding existing models, indices, and the like
• only those physical/chemical variables of effluent that have significantly changed the characteristics of the receiving waters .

A point source prediction impact model would greatly assist managers with the optimisation of the effluent treatment and discharge processes, and could be conceived to have broader applications that go beyond the objectives of managing LMWQCC.

The continuous discharge of effluent with fairly uniform water quality characteristics provides a stable flow input throughout the year. This provides some 'buffering' effect on the fluctuations of the water quality of the receiving waters, with corresponding "positive" or "negative" effects on the benthic fauna downstream, relative to the waters upstream, but does not appear, from this study, to be of a major impact relative to the other impacts on the Molonglo River.

Return to top