Toxicity monitoring - Ranger Mine

Results of toxicity monitoring

Brief introduction to 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. In the 2009–10 wet season, Gulungul Creek was added to the toxicity monitoring program and testing is now conducted fortnightly, alternating with Magela Creek. Data collected from 1991–92 to 2007–08 using, primarily, the creekside monitoring procedure, are available in the archived results.

Each year's 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

2013–14 wet season

Flow was first recorded at the Magela Creek upstream and downstream monitoring stations on 28 November 2013. Three toxicity monitoring tests have subsequently been completed, for the four-day periods 5–9, 12–16 and and 19-23 December 2013. Upstream and downstream egg production and difference values for Magela Creek are displayed in Figure 1a. The similar upstream and downstream egg number differences observed for both sites reflect similar water quality differences at the two monitoring sites up until 23 December 2013, as reported in the water chemistry Commentary on Magela Creek 2013-14 monitoring data.

2012–13 wet season

The first of eight tests in Magela Creek commenced on 10 January 2013, six days after the establishment of continuous flow in the creek. Thereafter, tests in Magela Creek were conducted at fortnightly intervals, alternating with the similar pattern of testing in Gulungul Creek (i.e. Magela first week, then Gulungul second week, then Magela third week, etc). Eight tests were conducted in Magela Creek, the final test completed on 22 April 2013. Upstream and downstream egg production and difference values for Magela Creek are displayed in Figure 1a.

A marked increase in the mean number of eggs produced during each test for both sites of Magela Creek was observed over the 2012–13 wet season (Figure 1a). Similar high egg production had previously only been observed in the first three tests of the 2006–07 wet season (Figure 1a). A significant factor contributing to this increase in egg production appears to be a new and more effective culturing regime for the snails at the laboratory aquaculture facility. The potential influence of snail husbandry on egg production and wet season toxicity monitoring results is discussed in the 2012-13 Supervising Scientist Annual Report (chapter 4).

The positive mean difference value for the eight 2012–13 wet season tests, of 7.51, indicates higher upstream egg production than downstream, contrasting with the historical trend of greater downstream egg production (mean difference value across all wet seasons of -8.04). The only previous period for which a positive mean difference value was observed was for the 1995–96 wet season.

Analysis Of Variance (ANOVA) testing of the 2012–13 wet season data found no significant difference between the responses from this wet season (upstream-downstream difference values) and those from previous years (P = 0.083). However, the low ANOVA significance value (near the 5% level) highlights the unusually higher upstream egg production compared to downstream observed in this year Figure 1a.

In the Supervising Scientist's Annual Reports for 2009–2010 and 2010–2011 (Chapter 3 of both reports), the influence of water temperature and electrical conductivity (EC) on the snail egg laying response was described. In Chapter 4 of the Supervising Scientist's Annual Report for 2012–2013, these same water quality variables, together with snail husbandry conditions, were considered in the context of explaining the lower (compared to upstream) egg production observed in Magela Creek downstream of Ranger in 2012–13. It was concluded that the toxicity monitoring results for 2012–13 reflected patterns associated with natural water quality (water temperature, EC) conditions in Magela Creek. There was no evidence of mine-related effects upon snail egg production over the wet season.

2011–12 wet season

Continuous surface flow commenced in Magela on 22nd November 2011, with the first Magela test deployed on 1st December 2011. In total, nine four-day tests were completed at fortnightly intervals, thus: 1–5 December, 15–19 December, 12–16 January (2012), 30–3 February, 9–13 February 23–27 February, 8–12 March, 22–26 March and 5–9 April. The results from this wet season are displayed in Figure 1a.

The 2011–12 wet season results showed that, on average, slightly greater egg production occurred at the downstream site compared to the upstream site. This is reflected by the 2011–12 mean upstream-downstream difference value of -8.2 (Figure 1a). This result continues the trend of greater egg production downstream and is within previously recorded ranges. This has resulted in a new "all years" mean difference value of -8.0 (-5.8 pre-2009–10). The larger than expected difference values of 2009–10 and 2010–11 wet seasons (-22.3 and -12.8 respectively) are discussed in the respective wet season sections below.

Analysis Of Variance (ANOVA) testing of the 2011–12 wet season data found no significant difference between the responses from this wet season (upstream-downstream difference values) and those from previous years (P = 0.907).

As a consequence of the significantly different results reported below in both 2009-10 and 2010–11, analyses have been undertaken to better understand potential causative factors, and provide an improved understanding, of environmental conditions affecting the production of snail eggs during the toxicity monitoring tests. These analyses included:

  • Seeking correlates of variability in snail egg difference values using water quality variables recorded via the Surface Water Chemistry Monitoring Program;
  • The effects of electrical conductivity (EC) and water temperature upon snail egg production; and
  • Seeking relationships between snail egg responses and suspended inorganic and organic matter – as highlighted in previous website reports for 2009-10.

These results are discussed in detail in the 2011-12 Supervising Scientist Annual Report (Chapter 3).

Gulungul Creek

2013–14 wet season

So far for the 2013-14 wet season, all testing has focused on Magela Creek. Routine alternation between creeks will resume in 2014, with the first Gulungul Creek test due to be deployed on 9 January.

2012–13 wet season

On 17 January 2013 the first of seven Gulungul tests commenced. Thereafter, tests in Gulungul Creek were conducted at fortnightly intervals, alternating with the similar pattern of testing in Magela Creek. For the fifth and sixth Gulungul Creek tests, data arising from one of the two duplicate floating containers deployed at the upstream site were deemed invalid due to container misalignment and subsequent restricted through-flow of creek waters. As such, results from the remaining duplicate only were used for subsequent statistical analyses for these two tests. Upstream and downstream egg production and difference values for Gulungul Creek both creeks are displayed in Figure 1b.

The mean difference value across all Gulungul Creek tests for 2012–13, of -47.4, continues the trend of greater egg production downstream, as reported in previous years. Notable for this wet season were the particularly high egg numbers observed downstream in the first three tests compared to numbers upstream, giving rise to difference values much lower than previously recorded Figure 1b. While the wet season mean difference value is much lower than the running mean of -13.47, ANOVA testing found no significant difference between the 2012–13 difference values and those recorded in previous wet seasons (p = 0.228).

Apart from the primary Before/After factor and associated hypothesis, the particular two-factor ANOVA model used for toxicity monitoring also allows variation amongst years (or wet seasons) and among tests within a wet season to be estimated separately. The second 'Season' factor can be used to determine whether, within the Before and After periods, any set of difference values for a wet season are significantly different. For Gulungul Creek after both the 2011–12 and 2012–13 wet seasons, the season factor has been significant (p = 0.005 and 0.048 respectively), compared to Magela Creek where this factor has never been significant. A significant season factor does not in itself imply potential mine-related impact; in this (Gulungul) case, it highlights the high inter-annual variation observed in seasonal difference values, as shown in Figure 1b and as reported in the previous Supervising Scientist Annual Report (2011–2012).

In the Supervising Scientist's Annual Reports for 2009–2010 and 2010–2011 (Chapter 3 of both reports), the influence of water temperature and electrical conductivity on the snail egg laying response was described. In Chapter 4 of the Supervising Scientist's Annual Report for 2012–2013, these same water quality variables, together with snail husbandry conditions, were considered in the context of explaining the particularly high egg production observed in the first three tests in Gulungul Creek downstream of Ranger, in the 2012–13 wet season. It was concluded that the toxicity monitoring results for 2012–13 reflected patterns associated with natural water quality (water temperature, EC) conditions in Gulungul Creek. There was no evidence of mine-related effects upon snail egg production over the wet season.

2011–12 wet season

Continuous uninterrupted flow between the upstream and downstream sites commenced on 31st November 2011. In total, 9 four-day tests were completed in the 2011–12 wet season, thus: 8–12 December, 16–20 December, 5–9 January (2012), 19–23 January, 2–6 February, 16–20 February, 1–5 March, 15–19 March and 29 March–2 April. The second test deployed on 16th December was deemed invalid due to an excess amount of debris that had collected on the equipment, restricting water flow to the snails during the test period (Figure 1b).

Last year's (2010–11 wet season) consistently higher downstream egg production was also evident in the results of this wet season, with a mean upstream-downstream difference value of -16.49. The results of 2011–12 show a marked decrease in the variability of egg production and water quality, confirming last year's observation of higher variation in egg production being attributed to higher variability in water quality (Supervising Scientist Annual Report 2010–2011, Chapter 2 ).

These results are discussed in detail in the 2011–12 Supervising Scientist Annual Report.

Figure 1. . Toxicity (in situ) monitoring results for freshwater snail (Amerianna cumingi) egg production for wet seasons between 2006–07 and 2013–14. (a) Magela Creek; (b) Gulungul Creek. Chart last updated 23 December 2013