Remediation of a major acid mine drainage point source discharge restores headwater connectivity for a diadromous native fish

Figures & data

Figure 1. Fish survey streams and water quality monitoring sites within the Ngakawau, Mohikinui and Waimangaroa rivers. The Stockton mine and major waterways are shown along with the location of long-term water quality monitoring stations. Ngakawau sites devoid of fish are marked with an X. AMD discharges to to the Ngakawau River are labelled and the extent of the Ngakawau Gorge is highlighted.

Figure 2. Water pH recorded in the Mangatini Stream and lower Ngakawau River between 2006–2012. The vertical black line indicates the start date of limestone dosing.

Figure 3. Dissolved Al (mg/L) concentrations in the Mangatini Stream and Ngakawau River. The vertical black line indicates the start date of limestone dosing.

Figure 4. Turbidity (NTU) in the Mangatini Stream and Ngakawau River. The vertical dashed black line indicates the start date of limestone dosing, while instalment of the Mangatini sump is indicated by the dotted line.

Table 1. Comparison of water quality at reference sites and within the Ngakawau River system in 2005 and 2011–2012.

		Ngakawau River
		AMD	Unimpacted	Reference streams	Time	Time*treatment	Treatment Tukey HSD
n		5	13	7
Temperature (◦C)	2005	14 (10–16)	11 (9–15)	10 (9–10)	0.236	0.087	A:B:C
	2011–2012	13 (11–15)	12 (10–13)	8 (8–10)
pH	2005	3.7 (2.9–6.4)	7.09 (5.8–7.9)	7.6 (6.8–7.9)	0.946	0.609	A:B:B
	2011–2012	4.3 (2.9–6.3)	7.1 (4.8–7.8)	7.1 (7.8–6.4)
Conductivity (μS25/cm)	2005	372 (46–834)	50 (28–113)	50 (26–81)	0.001	0.169	A:B:B
	2011–2012	297 (33–840)	35.2 (19–100)	46 (31–69)

Notes: Data are the mean (min-max), except pH which is presented as a median. Ngakawau River sites are divided into those known to receive some quantity of AMD and those that do not (Harding et al. 2006). P values from repeated measures ANOVA are shown alongside Tukey HSD test results for pairwise comparisons of stream type. ANOVA used type III sums of squares to generate P values. Significance was assessed as P < 0.05 and is indicated by letters.

Figure 5. The standard error of the mean (±1 SEM) density of kōaro at sites within and adjacent to the Ngakawau River system in 2005 and 2012. Significant (P < 0.05) differences between groups derived from within treatment pairwise t-tests are indicated by letters.

Table 2. Repeated measures ANOVA of kōaro density at sites within and adjacent to the Ngakawau River system in 2005 and 2012.

	Type III sum of squares	df	F	P
Time	0.005	1	3.311	0.088
Time*treatment	0.043	1	30.459	<0.001
Error (time)	0.023	16

Figure 6. Length frequency distributions of kōaro within and adjacent to the Ngakawau River system in 2005 and 2011–2012.

Harding JS, McIntosh AR, Gray DP, Jellyman P, Eikaas H. 2006. Distribution of native fish in the Ngakawau and Waimangaroa rivers. Report for Solid Energy, School of Biological Sciences. Christchurch: University of Canterbury.

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