Toxicity monitoring - Ranger Mine

Results of toxicity monitoring

Since their inception in 1991–92, toxicity monitoring tests have been performed approximately every other week (ie once a fortnight) during the wet season. Tests usually commence in December and cease in early April, the period of significant flow in Magela Creek. The results of the toxicity monitoring tests are plotted as part of a continuous time series of actual response and paired-site (upstream-downstream) 'difference' data. Figure 1 displays toxicity monitoring data for freshwater snail egg production acquired using the in situ testing procedure that has been deployed since the 2006–07 wet season. Data collected from 1991–92 to 2007–08 using, primarily, the creekside monitoring procedure, are available in the archived results.

Results are reported and discussed in detail on an annual basis (see respective Supervising Scientist Annual Reports). Results for 2010–11 are preliminary and will be analysed and discussed in further detail at the end of the wet season.

For a more detailed discussion about the history of the toxicity monitoring program and a detailed explanation of the testing methodology please see the Explanatory notes: history of toxicity monitoring.

Magela Creek

2010–11 wet season

For the 2010–11 wet season, continuous surface flow in Magela Creek commenced on November 15th 2010. However, creek flow remained very low for several weeks thereafter and water levels were too shallow for the deployment of continuous (water quality) and toxicity monitoring equipment for this period. The first toxicity monitoring test commenced in Magela Creek on 17 December, once moderate creek flows were established. Ensuing tests continued at fortnightly intervals, thus: between 6–10 January 2011, 20–24 January, 3–7 February, 18–22 February, 4–8 March and 17–21 March. With the exception of the sixth test for the tests conducted to date, results are consistent with data from previous years and, in particular, results recorded during the 2009–10 wet season where the trend towards higher egg production at the downstream, ‘exposed’ site was observed (see Figure 1 and description below). The sixth test (4–8 March) has been the only test so far this season to produce a positive difference value. Testing will continue fortnightly for the rest of the wet season.

2009–10 wet season

Toxicity monitoring results for the 2009-10 wet season, where consistently higher egg production at the downstream site was observed (see top panel of Figure 1), represented the first instance in which the difference values for the wet season were significantly different to results from all previous wet seasons, as determined by a Analysis Of Variance (ANOVA) testing (p=0.046). A number of factors have the potential to cause the different behaviour in snail egg production response observed for the 2009–10 wet season: methodological or systematic operator problems during the wet season; an unusual suppression in egg number upstream over the wet season; or enhancement of egg number downstream that may be associated with inputs of water (as measured by electrical conductivity or turbidity data) from the Ranger site.

Following the 2009–10 wet season, each of the above potential causative factors was assessed in detail, including an examination of the extensive available historical grab sampling and continuous water quality monitoring datasets. No correlation was found between any of these factors and the positive downstream effect on egg production. Other lines of evidence, including water physico-chemistry and results of the stream macroinvertebrate community studies that are conducted by SSD in the late wet season recessional flow period each year (reported below) also supported the conclusion of no mine-related effects upon freshwater snail egg production.

At this time it appears that the most probable explanation for the higher downstream egg production is an increase in food supply at this site as a result of increased settling-out of particulate matter. Field monitoring staff have noted that in recent times there has been a deepening of the channel at the downstream site. This deepening would result in a relative reduction in water velocity across the stream profile and hence an increased likelihood for deposition of suspended material. A visible increase, compared with previous years, in the amount of particulate material trapped inside the toxicity monitoring containers at the downstream site was noted during the 2009–10 wet season. In light of this, field monitoring staff are developing a method to quantify the nature and amount of particulate matter that is settling in the testing containers, with samples being collected after each of the test periods. These samples will be used to assess if there is any positive correlation between the amount and nature of the deposited material and egg production.

Gulungul Creek

Results for 2010–11 wet season

Seven tests have been completed to date in Gulungul Creek, thus: between 20–24 December 2010, 13–17 January, 27–31 January 2011, 10–14 February, 24–28 February, 10–14 March and 24–28 March. The results are shown in the bottom panel of Figure 1. Two observations are of note for Gulungul Creek results: (i) egg numbers are generally similar in magnitude for both wet seasons tested to date (whereas in Magela Creek generally higher egg production has so far been observed in the current (2010–11) wet season compared with the 2009–10 wet season); and (ii) downstream egg production has been higher than upstream egg production – similar to the pattern in Magela Creek for the past two wet seasons but in contrast to Gulungul Creek in 2009–10 when upstream egg production was generally higher than downstream. This higher egg production at the downstream Gulungul site was particularly marked in the fourth test of the season (Figure 1). Testing will continue fortnightly throughout the wet season.

2009–10 wet season

Five tests were conducted through the 2009–10 wet season, over a range of flow conditions, and in alternate weeks to the routine Magela Creek testing. Tests were conducted in the periods 25–29 January, 22–26 February, 22–26 March, 9–13 April and 19–23 April 2010. The results are shown in the bottom panel of Figure 1. The range of egg numbers produced over the wet season was similar to that recorded in Magela Creek.

Four out of the five tests resulted in positive difference values with more eggs produced upstream than downstream. This pattern was opposite to that observed in Magela Creek during the same period, where eight of the nine tests produced a negative difference value (Figure 1).

Figure 1. Toxicity (in situ) monitoring results for freshwater snail (Amerianna cumingi) egg production for wet seasons between 2006/07 and 2010/11. Magela Creek - top; Gulungul Creek - bottom. In situ testing for Gulungul Creek started in the 2009-10 wet season. Each four-day test is conducted at fortnightly intervals, with tests commencing in Magela Creek on December 17th 2010 and in Gulungul Creek on 20th December 2010. Testing will continue until the end of the wet season. Chart last updated Mar 25 2011.